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Assessment of leakage and risk reduction of R290 in a split type household air conditioner
Accepted Manuscript
Assessment of leakage and risk reduction of R290 in a split type household air conditioner Weier Tang , Guogeng He , Wei Sun , Sai Zhou , Dehua Cai , Yihao Zhu PII: DOI: Reference:
S0140-7007(18)30091-4 10.1016/j.ijrefrig.2018.03.012 JIJR 3925
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
8 November 2017 31 January 2018 14 March 2018
Please cite this article as: Weier Tang , Guogeng He , Wei Sun , Sai Zhou , Dehua Cai , Yihao Zhu , Assessment of leakage and risk reduction of R290 in a split type household air conditioner, International Journal of Refrigeration (2018), doi: 10.1016/j.ijrefrig.2018.03.012
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Highlights 1. R290 concentration distribution in the room was measured by the detectors. 2. R290 leakage from the air conditioner with different diameter hole was presented. 3. To install a solenoid valve to reduce the fire risk was proposed and verified.
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4. Proposal for risk detection in rooms by the noise of huge gas leaks was presented.
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Assessment of leakage and risk reduction of R290 in a split type household air conditioner Weier Tang, Guogeng He*, Wei Sun, Sai Zhou, Dehua Cai, Yihao Zhu School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074,
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China
Abstract:
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R290 (propane) is a famous hydrocarbon substance, but the flammable and explosive
characteristics require safety precautions for its use. In fact, R290 is also a refrigerant with low global warming potential (GWP) and excellent thermodynamic properties. However, the
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application of R290 in split type household air conditioners (STHAC) is hindered by unpredictable gas leakages into the room. This paper evaluates the fire hazard of R290 as a
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refrigerant in STHAC and provides a new method of installing a solenoid valve near the
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capillary to reduce the fire risk by decreasing the R290 leaking rate. The approach uses a numerical model to analyze the major parameters effect on the concentration distribution of
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R290 in the room after leakage from the STHAC indoor unit, as well as a series of
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experimental tests for verification. The influence of different charge quantities, leakage hole diameters and air flows on the R290 concentration distribution in the room were compared and discussed. The new method was also validated by experiments. The results showed that the fire hazard only occurs during the fast-leaking period and that the most likely flammable region is underneath the location of the leak. In addition, the flammable hazards can be assessed by the huge noise generated by the R290 leakage. 2
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Key words:
R290; Air conditioner; Gas leakage; Concentration distribution; Safety
Corresponding author. Tel.: +86 27 87542718.
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E-mail address: [email protected] (G. He).
Nomenclature global warming potential
LFL
lower flammable limit
ODP
ozone depleting potential
STHAC
split type household air conditioner
UFL
upper flammable limit
bs
local characteristic width of the plume, m
c*
concentration on the plume axis, kg/m3
P1
R290 pressure in the machine, Pa
Pa
R290 pressure in the room, Pa
r
radial distance to the plume axis in a normal section of the plume, m
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s
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GWP
distance along the plume axis from the origin to a certain point on the plume
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axis, m
Greek symbols μw
wind speed, m/s
μv
leaking rate from the leakage position, m/s
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theoretical value of leaking rate, m/s
ρa
density of the atmosphere, kg/m3
ρr290
density on the plume axis, kg/m3
ρ1
R290 density in the machine, kg/m3
λ
turbulent Schmidt number
κ
adiabatic exponent
ξ
coefficient of friction loss
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μ0
1. Introduction
As a part of the liquified natural gas, R290 (propane) has the characteristics of flammable
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and explosive, it needs a series of safety precautions to avoid leakage and ensure the safety.
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For instance, the lower minimum ignition energy is only 0.48mJ at an R290 concentration of 5.2 VOL% (Eckhoff et al., 2010); and the lower and upper flammability limits (LFL and UFL)
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of R290 in the air at 25℃ and 1atm is at very low concentration of 2.1 and 9.5 VOL%,
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(Kuchta, 1986), respectively. It makes the gas mixture of R290 and air could be easily ignited, any leakage or diffusion of R290 in an insufficiently ventilated space has significant
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possibility to cause a hazardous event and subsequent combustion or explosion by an accidental ignition or spark. Thus, the hazard assessment and controlment of R290 or other hydrocarbon gas leakage must be paid full attention.
Not only a fuel, R290 is also a suitable refrigerant for split type household air conditioner (STHAC) because it could provide higher energy efficiency (Wu et al., 2012) and reduce the 4
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energy consumption (Padalkar et al., 2014). Most importantly, it is a natural refrigerant with zero ozone depleting potential (ODP) value and negligible global warming potential (GWP) values, which is friendly to the environment. In history, HCFC-22 is the traditional refrigerant in STHAC, but its ODP values (0.055) will bring the damage of the ozone layer (Wang et al.,
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2016). It had been classified into the phase-out list by Montreal Protocol in 1987, and the phase-out progress had been accelerated (Calm, 2008). The high GWP values for current
HFCs alternative refrigerants to HCFC-22, such as HFC-410A (GWP=2090) and HFC-32
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(GWP=675), will contribute to accelerating the global warming (Yang and Wu, 2013). Hence, the Kigali Amendment to the Montreal Protocol in 2016 announced that the HFCs would be phased-down in the future (Höglund-Isaksson et al., 2017). The environmental friendly
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refrigerants in STHACs.
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refrigerant, R290, is therefore the most promising alternative to replace high-GWP
While applied in STHAC, the inflammable and explosive characteristics of R290 is still
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the biggest obstacle to its promotion. It was classed as an A3 flammable refrigerant by
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ASHARE, the utilization and maximum charge quantity of R290 in STHAC are strictly restricted by the international criterions, i.e., EN 378 and IEC 60335-2-40 (Corberán et al.,
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2008). Besides, the method of adding another non-flammable refrigerant in the STHAC is also considered to avoid the hazardous events (Tian et al., 2015). Nonetheless, the unpredictable leakage accident of R290 from STHAC is unavoidable, which might cause combustion or explosion once the leaked R290 reaches to the LFL concentration. For this reason, this paper examined the indoor safety concerns and measured concentration distribution in the room after R290 leaked from STHAC indoor unit into the room under 5
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different scenarios.
Up to now, several studies have reported on the leakage and diffusion properties of various refrigerants. For example, Cheong and Riffat (1996) monitored the indoor leakage and estimated the emission rate of R134a by using tracer gas SF6. Blackwell et al. (2004)
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developed an independent risk assessment for R152a and carbon dioxide mobile air
conditioner systems. Gigiel (2004) analyzed the system pressure and the refrigerant
concentration distribution when R600a leaked from a refrigerator according to the methods
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specified in the safety Standard, IEC/EN 60335-2-24. Jia et al. (2017) tested the R32 leakage and diffusion characteristic of STHACs under different operating conditions, the combustible zone only appears near the leakage hole, and the duration of the combustible zone is very
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short. Nagaosa (2014) presented a new numerical formulation to simulate the R290 leakage
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and spread into an enclosed room and examined the effect of leakage rate on concentration profiles. D. Colbourne carried out the quantitative risk assessment to estimate the likelihood
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of ignition and severity of the consequences after the R290 leaked from the ice cream cabinet
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(Colbourne and Espersen, 2013), refrigerator and STHAC (Colbourne and Suen, 2015).
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However, there is a lack of experimental results of R290 concentration distribution in the residential space following an actual leakage from the STHAC. Although Zhang evaluated the distribution of R290 in the room (Zhang et al., 2013) and measured the explosion characteristics in the indoor and outdoor units of an R290 STHAC (Zhang et al., 2016). Li (2014) also tested the R290 distributions inside a room under different scenarios. The leaking rate is constant both in their experiments since the R290 was released by the vessel and 6
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controlled by a valve. It is different with the actual leakage condition, which the leaking rate will gradually decrease since the pressure in the STHAC drops after R290 leaked out. The simulated (Nagaosa, 2014) and experimental (Li, 2014) results illustrated that the leaking rate could affect the R290 concentration distribution in the room, so the constant leaking rate
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cannot reveal the actual leakage condition. Considering that the global environmental problem has made it an urgent issue to develop the R290 STHAC, and there will be a huge amount of the R290 STHACs in the future. It is necessary to investigate the R290
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concentration distribution in the room following with an actual leakage under different
scenarios. This paper used the numerical model to analyze major factors impact on the R290 indoor leakage and measured the R290 concentration distribution in the room by series gas
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detectors. The influence of charge quantity, the diameter of leak hole and airflow on the R290 concentration distribution was discussed and compared. A new method of using a solenoid
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valve near the capillary to reduce the leaking rate, proposed by R290 leaking rate experiment
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(Tang et al., 2017), was investigated and verified. The results could provide valuable guidance for improving the indoor safety and accident prevention when using R290 as working fluid in
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the STHACs.
2. Numerical analysis of R290 leakage and diffusion The plume model proposed by Ooms (1972) has been successfully used to simulate the plume path of “light” gases (whose density is same as or less than that of air) leaking into the atmosphere at atmospheric temperature and pressure. The model was extended to “heavy” 7
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gases (whose density is higher than air) with a number of assumptions (Ooms et al., 1974).
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Fig. 1- Schematic of the Plume model
In the Plume model (Fig. 1), the plume concentration is assumed to be cylindrically
c( s, r ) c* exp(
r 2 ) 2bs2
(1)
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symmetric and it can be calculated by:
Where c (s, r) represents the values of the concentration at an arbitrary point in the plume;
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c* denotes the concentration of gas on the plume axis; r is the radial distance to plume axis in a normal section of plume, m; λ2(≈1.35) is the so-called turbulent Schmidt number; bs represents the local characteristic width of the plume and is one of the unknown quantities.
Khan and Abbasi (1999, 2000) developed some empirical equations and defined the dimensionless number Q f to solve the parameters c*, bs and predicted the behaviour of the 8
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gas concentration in the plume, then the Eq. (1) can be calculated by:
Qf (
uw a )( ) uv r 290
(2)
0.382484Q f
(3)
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c* (0.0161965 0.00543481 Q f 0.00128631 Q2f ) ln( s) 0.183894 e
bs (0.0157719 0.104969 Q f 0.0347085 Q2f ) ln( s) 0.953469 Q0.22422 f
(4)
Where uw is the wind speed, uv represents the gas leaking rate from the leaking position.
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ρa is the density of the atmosphere and ρr290 is the density of the plume gas, s is the distance along the plume axis from the leaking position to a certain point.
Considering the leakage as an adiabatic process, the theoretical value of leaking rate u0 is
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determined by:
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P0 u02 dP / 0 P 1 2
(6)
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P / P1 / 1 Pa / a
(5)
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Where P1 and Pa is the pressure of R290 in the STHAC and room, respectively. ρ1 is the
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density of R290 in the STHAC, respectively. 𝜅 is the adiabatic exponent.
The actual leaking rate uv=𝜉 u0 due to the friction loss while R290 through the leakage
hole. ξ means the coefficient of friction loss which has a correlation with the shape and diameter of the leakage hole. By solving eq. (5) and (6), the expression of uv is as follows:
Pa 1 2 P1 uv = u0 1 ( ) ( 1) 1 P1 9
(7)
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From Eq. (3) and (7), it can be known that the R290 concentration in the same position was decided by 𝜅, Pa, ξ, P1, ρ1 and uw, ρa, ρr290. The R290 leakage experiments were carried out in an enclosed room at 25℃ and 1 atm in this study, so the 𝜅, uw, Pa is constant and ρa =1.184kg/m3 and ρr290 =1.832kg/m3. In the STHAC, the ρ1 was determined by P1 since the
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inner volume is constant. Therefore, the P1, ξ, and uw are three major parameters to affect the R290 concentration. In the following chapters, the impact of different charge quantity on P1, different diameter of leakage hole on ξ will be discussed and verified through the various
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experiments. In addition, some measures to reduce the R290 concentration in the room, such as installing a solenoid valve near the capillary to reduce the uv and opening the window and
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door to increase the uw, will also be conducted and verified.
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3. Experimental apparatus and procedures
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3.1 Experimental apparatus and set-up
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Fig. 2 and 3 illustrated the schematic and locations of the indoor leakage experimental apparatus. As shown in Fig. 3, the experimental room dimensions are 4.5 (L) ×3.2 (W) ×2.6
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(H) m, giving a volume of 37.44m3 and floor area of 14.4m2, with a door (2.3 (H) ×0.9 (W) m) and a window (1.6 (W) ×1.2 (H) m).
Based on the experimental room dimensions, a 1P R290 STHAC with a nominal cooling capacity of 2500W was chosen for the test, the size of the pipes in the STHAC was shown in Table 1. Besides, it could be found in Fig.2 that for imitating the R290 leaking inside the 10
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indoor unit and simultaneously not affect the STHAC system, another same indoor unit was employed as the experimental indoor unit 7 and separately mounted at the center of the 4.5 m (L) wall with 1.8m height. The soft pipe 6 with 0.9 m length and 6.35 mm inner diameter was used to deliver the R290 from the STHAC to experimental indoor unit 7. One of the ports of
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soft pipe 6 was put inside the experimental indoor unit 7 to imitate the leakage point. Based on a record by STHAC service engineers, the position of u-tube junctions of the evaporator is the most likely leakage position inside the indoor unit, the leakage point was therefore located
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at that position for imitating the actual leakage accident. Another port of soft pipe 6 was
installed on hand valve 5 with the replaceable copper tube, those replaceable copper tubes with different diameter hole (as shown in Fig. 4) were employed to imitate the different
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diameter leakage hole. To exclude the impact of the liquid refrigerant, the three-way tube 4 and hand valve 5 was installed on the outlet pipe of the STHAC indoor unit. In addition, a
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solenoid valve 2 was installed near the capillary to investigate its effects on reducing the
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leaking rate. The indoor leakage experiments were carried out by opening the hand valve 5, the R290 gas in STHAC will flow into the experimental indoor unit 7 through soft pipe 6 and
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then diffuse in the room through the indoor unit vent.
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Fig. 2- Schematic diagram of the indoor leakage experimental apparatus
Fig. 3- Locations of indoor leakage experimental apparatus
Table 1- Tube length and diameter in the 1P STHAC
Heat Exchanges Condenser (In outdoor unit)
Evaporator (In indoor unit)
Tube length (mm)
695
600
Tube inner diameter (mm)
4.37
6.56
Tube spacing (mm)
19.5
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Number of tube row
2
2
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Numbers of tubes 3
Inner volume (cm )
52
28
566
599
Liquid pipe (From outdoor to
Gas pipe (From indoor to
indoor unit)
outdoor unit)
3
3
4.37
8.02
45.0
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Length (m) Tube inner diameter (mm) 3
Inner volume (cm )
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Remark: the inner volumes were determined by calculation
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Connect pipes
Fig. 4- Photograph of the replaceable copper tubes with different leakage hole
Five infrared flammable gas detectors (A-E) were placed at proper locations under the experimental indoor unit 7, because the CFD simulation results and previous studies (Zhang et al., 2013) (Li, 2014) showed that R290 will concentrate in the region underneath the leak 13
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position while concentration of R290 in other regions seldom approach the LFL. Detectors A and B were located at the height of 0.8 m to measure the R290 concentration in mid-air and the distance between them is 0.5 m. Detectors C, D, E were located at the height of 0.1 m to measure the R290 concentration in floor level, and they are 0.3 m away from each other
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because the width of STHAC indoor unit vent is 0.6m. In addition, the distance from STHAC indoor unit vent to the wall is 0.1 m so all detectors were 0.1 m away from the wall. The room is empty except for the experimental equipment. Fig. 5 is the photograph of the experiment
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equipment, and the plume axis was speculated by the soft pipe and leak position.
Fig. 5- Photograph of the experimental apparatus
The experimental instruments are detailed in Table 2, which were all fully calibrated. The gas detectors were calibrated by a standard R290 gas of 2.1 VOL % concentration, so the LFL 14
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in this study is equal to 2.1 VOL %.
Table 2- Apparatus specifications Name
Type
Indoor gas detector
Honeywell (Searchpoint optima plus)
Specifications Range: 0-100% LFL Accuracy: ±1% LFL
Agilent
20 channels
(34970A with 34901A module)
Speed: 60 chs /s
3.2 Experimental procedures
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Data acquisition instrument
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The experiments were carried out under STHAC cooling mode because the results of R290 distribution in STHAC (Tang et al., 2017) indicated that R290 would migrate from
outdoor to the indoor unit in cooling mode, which is the dangerous case for R290 STHAC.
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Besides, the temperature of STHAC was set at 26 ℃, and it is non-operating during the
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leaking process to eliminate the impact of airflow caused by the indoor unit fan. The door and window were closed throughout the experimental process if there is no special explanation.
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The experimental procedures were carried out according to the following sequence:
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a) Ensure there is no residual R290 in the room (the background concentration is less
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than 1% of LFL).
b) Install the appropriate replaceable copper tube at hand valve 5 and charge the
corresponding R290 quantity into STHAC, then operate the STHAC into experimental conditions. c) Start the data acquisition system, measuring and recording the R290 concentration in the room, after a few minutes shut down the STHAC. 15
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d) Open the hand valve 5 immediately after the STHAC powered off, release the R290 into the room. e) If the experiments with solenoid valve 2 closed were carried out, it needs to close the solenoid valve 2 simultaneously with the STHAC powered off.
stable.
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4. Experimental results and analysis
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f) Terminate the data acquisition system once all the concentration measurements remain
4.1 Influence of charge quantity
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According to the results of R290 mass distribution in the components of STHAC (Tang
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et al., 2017), the evaporator will hold more liquid R290 in it with the increasing of charge quantity since the constant inner volume and approximately the same saturation evaporating
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temperature in this study. When R290 leakage occurs, the initial P1 is the saturation
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evaporating pressure and it drops with the R290 leaking out. However, the liquid R290 in the evaporator will evaporate and maintain the saturation pressure P1 when R290 gas has leaked
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out. The more liquid R290 in the evaporator, the longer P1 will remain at saturation pressure at the beginning of the leakage process, and the leaking rate will also remain at a higher value. To explore the relationship between different R290 charge quantity and concentration distribution in the room, three R290 charge quantity with 260 g, 300 g, and 350 g were selected in this study. The reason is according to the results of the performance measurement,
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the optimal charge quantity for this 1P STHAC is 300 g, the minimum and maximum charge quantity to maintain its designed performance are 260 g and 350 g, respectively. The results of R290 concentration distribution with different charge quantity and 0.5 mm leakage hole were shown in Fig. 6-8. Since the range of the gas detectors is 0-100% LFL, and their output
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data unit is 4-20 mA, which is corresponding to 0-100% LFL, respectively. The relationship between LFL % and time is presented in the result figures. It could be convenient to find that the concentration of which detectors could exceed LFL in those figures.
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fast-leaking buffering
The process of R290 concentration falling and rising again
80 70
B D
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60
A C E
50 40
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Concentration/ LFL%
90
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diffusing
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30
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20 10
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0
0
200
400
600
800
1000
1200
1400
1600
1800
Time/s
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Fig. 6- Concentration distribution results of 260 g charge quantity and 0.5 mm leakage hole
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fast-leaking buffering
diffusing
100
A C E
80 70
B D
60 50
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Concentration/ LFL%
90
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 7- Concentration distribution results of 300 g charge quantity and 0.5 mm leakage hole
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fast-leaking buffering
100
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80
B D
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70
A C E
60 50
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Concentration/ LFL%
90
diffusing
40
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30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 8- Concentration distribution results of 350 g charge quantity and 0.5 mm leakage hole
The whole leakage process in Fig. 6-8 was summarized in three periods, which is the 18
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fast-leaking period, buffering period and diffusing period. As shown in Fig. 6, the fast-leaking period is at the beginning of the leakage process. In this period, the R290 in the evaporator leaks quickly and results in the R290 concentration of all detectors rise rapidly. Then is the buffering period, it could be observed in Fig .6 that after the fast-leaking period, the R290
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concentration of all detectors has a process of falling and rising again. The reason is that the leaked R290 in fast-leaking period reduced the pressure in the evaporator and leaking rate is also decreased according to Eq. (7), so the R290 concentration falling at the beginning of
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buffering period. After that the high pressure in the condenser will drive the R290 to migrate from condenser to evaporator through the capillary and liquid connect pipe then leaks off, the migrated R290 from condenser will raise the pressure in the evaporator and the leaking rate,
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and the R290 concentration in the room rising again. Finally, it is the diffusing period. After the most of R290 in the STAHC leaks off and disperses in the room, the R290 concentration
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in the room will gradually decline and reach a stabilized level in the last. The stabilized R290
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concentration level can be verified by the charge quantity. For the charge quantity of 260 g, it will bring the 0.146 m3 volume of R290 into the room because of the density of R290 is 1.78
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kg/m3 under the experimental conditions. Hence, the R290 volume fraction in the room should
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be 0.39 VOL % because the volume of the room is 37.44m3, and the final concentration level will be 18.5 LFL% as the LFL of R290 is 2.1 VOL %. The final stabilized concentration results in Fig. 6 are approximately equal to this value, which proved that the results in Fig. 6 are effective, other experimental results can also be validated by this method.
Fig. 7 and 8 are the results of 300 g and 350 g R290 charge quantity in 0.5 mm leakage hole, the trend of the R290 concentration curve is similar to Fig. 6 and there are also three
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periods in those results. However, the duration time of fast-leaking period is extended since the evaporator gathers more liquid R290 in it, the concentration of detector D also reached LFL level in the fast-leaking period. The biggest difference between the three figures is the duration time of concentration exceeds the LFL, which means the time of combustible zone
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appears in the room. The combustible zone’s residence time in Fig. 6-8 are 53s, 82s and 99s, respectively. It is increasing with the charge quantity, which implies the higher R290 charge
quantity will bring the longer dangerous time in the room when R290 leakage occurs. Hence,
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it needs to strictly restrict the R290 charge quantity to ensure the safety.
The results of Fig. 6-8 show that only detectors B and D reached the LFL level, and the concentration of detector E is higher than C while A always keep the lowest R290
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concentration. As shown in Fig. 2 and 5, the R290 plume axis is most likely to be diagonal
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from the right side of the indoor unit vent to the ground since the leak position was located at the right of experimental indoor unit 7. Eqs. (1), (3) and (4) indicated that the plume axis
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distance, s, and radial distance, r, could affect the concentration. It could be speculated from
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Fig. 5 that the plume axis is near to detectors B and D, the r of B and D is shorter than others, and detector B is closer to the leak position. That is the reason that only detectors B and D
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reached the LFL level and D cannot reach the LFL level in the fast-leaking period of Fig. 6. In addition, the distance from detectors A and B is 0.5 m while detectors C, D, E are placed at floor level and 0.3 m away from each other. The r of E is shorter than C since detector E was placed at the right side, it could help to explain why the concentration of detector E is higher than C. Detector A always keep the lowest R290 concentration because it has the longest r. This phenomenon implies that the combustible zone is more likely appear in the region 20
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underneath and near to the leak position. Hence, the electrical components, such as the socket, should not install underneath the R290 STHAC to prevent the generation of accidental short circuit electric sparks in the combustible zone.
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4.2 Influence of diameter of leakage hole
The coefficient of friction loss 𝜉 in Eq. (7) was determined by the shape and area of
leakage hole. In the same area, the circular leakage hole has the largest 𝜉. Besides, the larger
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area results in larger 𝜉 in the same shape. Three kinds of circular leakage hole with a diameter of 0.5 mm, 1.0 mm and 4.37 mm were chosen in this study to investigate the influence of different leakage hole on R290 concentration distribution in the room. The diameter of the
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leakage hole is determined by the corresponding driller and it has been remeasured to ensure accuracy. The leakage hole with 0.5 mm diameter was to imitate the situation that tiny rupture
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on the tube, because it is difficult to drill a smaller diameter hole in the copper tube due to
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limitations imposed by process. At the same time, the 1.0 mm was to imitate the partial
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rupture on the tube while 4.37 mm was corresponding to a breakage of the pipe in this study. The 4.37 mm leakage hole was exactly a tube with 5.0 mm external diameter (as shown in Fig.
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4) since the researchers in China have applied it to reduce the heat exchanger’s inner volume for reducing the R290 charge quantity. Fig. 6-8 illustrated that the trend of the R290 concentration curve is similar in the whole leakage process and detector B always have the highest R290 concentration. Hence, R290 concentration in detector B under different situations were discussed and the results were shown in Fig. 9.
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260g, 0.5mm 300g, 0.5mm 350g, 0.5mm 300g, 1.0mm 300g, 4.37mm
90
70 60 50
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Concentration/ LFL%
80
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 9- R290 concentration of detector B under different charge quantity and diameter of leakage
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hole
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The results described in Fig. 9 show that the combustible zone’s residence time will increase with the charge quantity, but not with the diameter of leakage hole. For example, in
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the condition of 300 g R290 charge quantity, the combustible zone’s residence time under 0.5
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mm, 1.0 mm and 4.37 mm diameter of leakage hole is 82 s, 178 s, and 115 s, respectively. The longest combustible zone’s residence time appeared in the condition of 1.0 mm diameter
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of leakage hole, which is not 4.37 mm. The larger diameter of leakage hole brings the faster leaking rate and leads to more expansive combustible zone in the room, but it also leads to shorter leaking time in the same charge quantity at the same time. The combustible zone of 4.37 mm is more expansive than 1.0 mm (all detectors exceeded the LFL in 4.37 mm while detectors A and C cannot reach LFL level in 1.0 mm), but a shorter leaking time leads to the combustible zone’s residence time of 4.37 mm (115s) is shorter than 1.0 mm (178s). The 22
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result of 0.5mm leakage hole remains the shortest combustible zone’s residence time (82s) because its leaking rate is too low to maintain the R290 concentration exceed LFL for a long time. The longer leaking time and fast enough leaking rate results in the longest combustible zone’s residence time under 1.0 mm diameter of leakage hole. Therefore, it is noteworthy that
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the size of the leakage hole is not positively related to the degree of risk, the certain leakage hole with long leaking time and fast enough leaking rate will bring the most dangerous
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situation in the room.
4.3 Influence of solenoid valve
From the above results, it can be concluded that the leaking rate uv, has a major influence
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on R290 concentration distribution, it is therefore necessary to find a way to reduce the
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leaking rate. The results of leaking rate test (Tang et al., 2017) indicated that installing a solenoid valve near to the capillary could reduce the leaking rate by delay the R290 migrate
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from condenser to evaporator. Hence, the experiments with solenoid valve closed were
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carried out to investigate its effects on reducing the leaking rate. Since the 350 g R290 charge
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quantity under 1.0 mm leakage diameter is the most dangerous situation according to the former discussion, the results of solenoid valve opening (Fig. 10) and closed (Fig. 11) under this condition were shown to compare and discuss.
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110
fast-leaking buffering
diffusing
100
A C E
80 70
B D
60 50
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Concentration/ LFL%
90
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
110
fast-leaking
buffering
100
The process of R290 concentration falling and rising again
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90 80
A C E
B D
PT
70
diffusing
60 50
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Concentration/ LFL%
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Fig. 10- Concentration distribution results of 350 g charge quantity and 1.0 mm leakage hole
40
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30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 11- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with solenoid valve closed
24
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Fig. 10 illustrates that all detectors have reached the LFL level and the longest combustible zone’s residence time (186s) since it has the largest charge quantity (350 g) and the longest leaking time (1.0 mm). However, the result of the situation that solenoid valve closed (Fig. 11) shows that the region and residence time of the combustible zone were
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reduced, only detectors B and D have reached the LFL and the residence time was shortened to 85s. It could be explained by the following reasons: The closed solenoid valve 2 will hinder the R290 migrate from condenser to evaporator through the capillary and liquid connect pipe
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(Fig. 2) in the buffering period. However, the high pressure in the condenser will drive the
R290 through the compressor to gas connect pipe then leaks out since there exist clearances in the rotary compressor of STHAC (Cai et al., 2015). Comparing to through the capillary and
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liquid connect pipe, the migration process will be slowed down by the clearances in the compressor and the leaking rate will be reduced in buffering period. Therefore, it could be
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found in Fig. 11 that the duration of buffering period has been extended and the process of
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concentration rising in the buffering period was distinct.
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Fig. 10 and 11 show the comparison of solenoid valve opening or closed under the most dangerous situation in this study. As a contrast, Fig. 12 illustrates the safest situation, which is
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the results of 260 g charge quantity, 0.5 mm diameter of leakage hole with solenoid valve closed. By comparing Fig.12 to Fig. 6, it is obvious that all detectors cannot reach the LFL and the highest R290 concentration is under 50 %LFL with solenoid valve closed. Therefore, it could be concluded that closing the solenoid valve could greatly improve the safety of R290 STHAC, and the combustible zone maybe disappears in the situation that tiny tube rupture with the low leaking rate. 25
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110
A C E
100
80 70 60
fast-leaking
buffering
diffusing
50
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Concentration/ LFL%
90
B D
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 12- Concentration distribution results of 260 g charge quantity, 0.5 mm leakage hole with
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4.4 Influence of airflow
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solenoid valve closed
The above experimental results and conclusions are obtained in an enclosed room with
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the door and window closed, the velocity of air convection uw in the enclosed room is about
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0.1m/s or less by measurement of anemoscope. However, according to the Eqs. (1)- (4), the concentration will decrease as the wind speed, uw, increases in the condition that the other
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coefficients are constant. The experiments with window and door opening were conducted to analyze the influence of airflow. Fig. 13 shows the concentration results of 350 g charge quantity, 1.0 mm leakage hole, solenoid valve closed with window and door opening. Comparing Fig.13 against Fig. 11, it could be found that the combustible zone’s residence time was further reduced from 85s to 64s after the window and door opening. In addition, the
26
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R290 concentration in Fig. 13 is close to zero after 400 s. The reason is that the higher air convection velocity could help to R290 disperse, and the leaking rate is less than the diffusing rate after 400 s. The R290 is directly diffused in the environment and not gather in the room since the window and door opening, so the R290 concentration in the room is close to zero. In
R290 STHAC.
110
fast-leaking
buffering and diffusing
100
A C E
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80 70 60
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50
B D
40 30
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Concentration/ LFL%
90
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short, this result indicates that open window and door could effectively improve the safety of
20
0
200
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0
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10
400
600
800
1000
1200
1400
1600
1800
Time/s
Fig. 13- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with
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solenoid valve closed and window and door opening
4.5 Summary of indoor leakage results In addition to those representative experimental results to analyze the influence of the various factors, overall 20 different experiments were carried and the results were
27
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summarized in Table 3. It can be seen from Table 3 that detector B and D always exceed the LFL level since they are closer to the speculated plume axis as shown in Fig. 5. The regularity of the combustible zone’s residence time under different experimental conditions in Table 3 as shown in Fig. 14. Fig .14 and the experimental results involved in Table 3 demonstrate the
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similar results as indicated above. At the same condition, the combustible zone’s residence time increasing with the higher R290 charge quantity, and the longest combustible zone’s
residence time appears in the situation of 350 g R290 charge quantity and 1.0 mm diameter of
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leakage hole whether solenoid valve opening or closed. While the solenoid valve is closed,
the combustible zone’s residence time and the number of gas detectors which exceed LFL will significantly decrease, and there may be no combustible zone appear in the room at the condition of 0.5 mm leakage hole and solenoid valve closed. In addition, opening window and
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door could further reduce the combustible zone’s residence time.
Table 3- Indoor leakage tests conditions and results
Charge
Leak
PT
Solenoid valve
Combustible zone’s
Sensor NO. of
Sensors NO. of
Close
residence time /s
maximum time
exceeding LFL
Window and door
quantity/g
hole/mm
260
4.37
*
38
D
A, B, C, D
300
4.37
*
41
D
A, B, C, D
350
4.37
*
54
D
A, B, C, D, E
*
260
4.37
*
110
B
A, B, C, D, E
*
300
4.37
*
115
D
A, B, C, D, E
*
350
4.37
*
145
B
A, B, C, D, E
*
260
1.0
*
28
D
B, D
*
300
1.0
*
68
B
B, D
*
350
1.0
*
85
B
B, D
*
260
1.0
*
161
B
B, D, E
*
300
1.0
*
178
B
B, D, E
*
350
1.0
*
186
B
A, B, C, D, E
260
0.5
*
Off
On
*
AC
*
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*
*
Open
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300
0.5
*
*
350
0.5
*
*
260
0.5
*
53
B
B, D
*
300
0.5
*
82
D
B, D
*
350
0.5
*
99
B
B, D
*
260
1.0
*
*
350
1.0
*
64
B
B, D
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*
200
0.5 mm hole with soleniod valve opening 0.5 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve opening 1.0 mm hole with soleniod valve closed 4.37 mm hole with soleniod valve opening 4.37 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve closed; window and door opening
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160
120
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80
40
0
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260
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Combustible zone’s residence time /s
240
300
350
Charge quantity /g
conditions
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Fig. 14- Relationship of the combustible zone’s residence time with different experimental
Particular, there is an important observation cannot be ignored that within the
fast-leaking period, the R290 leakage is accompanied by a huge sound. This huge sound is similar to the sound at the start of the STHAC switching to the defrost mode from the heating mode. The difference is the huge sound is transitory in the defrost mode but continues in the whole leakage process, it will be quieter in the buffering period but still could be clearly heard 29
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in the room. In addition, this huge sound can also be heard by the experimenter even in the tests that no detectors reach the LFL level such as under the condition that 0.5 mm leakage hole and solenoid valve closed. Hence, this observation could help the consumers to identify the flammable risk in the room where using R290 STHAC: if they heard the huge leakage
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sound, there probably exists fire risk in the room, and if not, the room is safe.
5. Conclusions:
Based on the above experimental results, the following conclusions can be reached:
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1) Refrigerant charge quantity has a major influence on the R290 concentration distribution in the room. A higher R290 charge quantity will lead to a more extensive combustible zone and longer residence time in the room. The charge quantity of R290 in STHAC
2)
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needs to be strictly restricted, as mentioned in EN 378 and IEC 60335-2-40. The leakage hole can also affect the R290 concentration distribution. The results of this
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study reveal that the 1.0 mm diameter of leakage hole has the longest combustible zone’s
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residence time. The larger leakage hole (4.37 mm) leads to the combustible zone more extensive but shorter residence time of it, the smaller leakage hole (0.5 mm) makes the
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lower leaking rate and the R290 concentration is harder to approach the LFL. The certain
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leakage hole with a long leaking time and a high enough leaking rate will be the most dangerous situation while using R290 STHAC within the room.
3)
A practical and achievable approach to significantly reduce the leaking rate and enhance the room safety is installing a solenoid valve behind the capillary, and simultaneously closing it as the STHAC shutdown. The results demonstrate that both range and residence time of the combustible zone were significantly reduced under the condition of 30
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solenoid valve closed. Furthermore, the results of opening the door and window indicate that it could further reduce the R290 concentration and improve the safety of R290 STHAC, enhancing the ventilation in the room would help to accelerate the dispersion of leaked R290 and alleviate its flammable risks. Potential ignition sources, such as switches and other electrical components should be
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4)
avoided installed underneath the indoor unit of STHAC because it is the most probable
region where combustible zone appears. The huge sound accompanied with the leakage
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might be the judgment whether the flammable risk appeared in the room. The
combustible zone might appear in the room if the huge sound could be heard by the
Acknowledgements:
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consumers; and if not, it is not necessary to worry about the fire risk in the room.
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This present study was supported by the Foreign Economic Cooperation Office, Ministry
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of Environmental Protection of China.
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References
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EN 378: 2008 Refrigerating systems and heat pumps. Safety and environmental requirements, 2008. Blackwell, N., Bendixen, L., Birgfeld, E., 2004. Risk Assessment of HFC 152a and Carbon Dioxide Mobile Air Conditioning Systems, Proc. Earth Technologies Forum, Washington, USA. Cai, D., He, G., Yokoyama, T., Tian, Q., Yang, X., Pan, J., 2015. Simulation and comparison of leakage characteristics of R290 in rolling piston type rotary compressor. International Journal of Refrigeration 53, 42-54. Calm, J.M., 2008. The next generation of refrigerants – Historical review, considerations, and outlook. International Journal of Refrigeration 31, 1123-1133. Cheong, K.W., Riffat, S.B., 1996. Monitoring hydrofluorocarbon refrigerant leakage from air-conditioning systems in buildings. Applied Energy 53, 341-347. Colbourne, D., Espersen, L., 2013. Quantitative risk assessment of R290 in ice cream cabinets.
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International Journal of Refrigeration 36, 1208-1219. Colbourne, D., Suen, K.O., 2015. Comparative evaluation of risk of a split air conditioner and refrigerator using hydrocarbon refrigerants. International Journal of Refrigeration 59, 295-303. Corberán, J.M., Segurado, J., Colbourne, D., Gonzálvez, J., 2008. Review of standards for the use of hydrocarbon refrigerants in A/C, heat pump and refrigeration equipment. International Journal of Refrigeration 31, 748-756. Eckhoff, R.K., Ngo, M., Olsen, W., 2010. On the minimum ignition energy (MIE) for propane/air. Journal of Hazardous Materials 175, 293-297. Gigiel, A., 2004. Safety testing of domestic refrigerators using flammable refrigerants. International
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Journal of Refrigeration 27, 621-628.
Höglund-Isaksson, L., Purohit, P., Amann, M., Bertok, I., Rafaj, P., Schöpp, W., Borken-Kleefeld, J., 2017. Cost estimates of the Kigali Amendment to phase-down hydrofluorocarbons. Environmental Science & Policy 75, 138-147.
Jia, L., Jin, W., Zhang, Y., 2017. Experimental study on R32 leakage and diffusion characteristic of wall-mounted air conditioners under different operating conditions. Applied Energy 185, Part 2,
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2127-2133.
Khan, F.I., Abbasi, S., 1999. Modelling and control of the dispersion of hazardous heavy gases. Journal of loss prevention in the process industries 12, 235-244.
Khan, F.I., Abbasi, S., 2000. Modelling and simulation of heavy gas dispersion on the basis of modifications in plume path theory. Journal of hazardous materials 80, 15-30.
Kuchta, J.M., 1986. Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries: A Manual. Technical Report Archive & Image Library 680. Journal of Refrigeration 40, 380-389.
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Li, T., 2014. Indoor leakage test for safety of R-290 split type room air conditioner. International Nagaosa, R.S., 2014. A new numerical formulation of gas leakage and spread into a residential space in
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terms of hazard analysis. Journal of Hazardous Materials 271, 266-274. Ooms, G., 1972. A new method for the calculation of the plume path of gases emitted by a stack. Atmospheric Environment (1967) 6, 899-909.
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Ooms, G., Mahieu, A., Zelis, F., 1974. The plume path of vent gases heavier than air, First International-Symposium on Loss Prevention and Safety Promotion and Safety Promotion in the
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Process Industries, The Hague.
Padalkar, A.S., Mali, K.V., Devotta, S., 2014. Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22. Applied Thermal
AC
Engineering 62, 277-284. Tang, W., He, G., Cai, D., Zhu, Y., Zhang, A., Tian, Q., 2017. The experimental investigation of refrigerant distribution and leaking characteristics of R290 in split type household air conditioner. Applied Thermal Engineering 115, 72-80. Tian, Q., Cai, D., Ren, L., Tang, W., Xie, Y., He, G., Liu, F., 2015. An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners. International Journal of Refrigeration 57, 216-228. Wang, Z., Fang, X., Li, L., Bie, P., Li, Z., Hu, J., Zhang, B., Zhang, J., 2016. Historical and projected emissions of HCFC-22 and HFC-410A from China's room air conditioning sector. Atmospheric Environment 132, 30-35. Wu, J.H., Yang, L.D., Hou, J., 2012. Experimental performance study of a small wall room air
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conditioner retrofitted with R290 and R1270. International Journal of Refrigeration 35, 1860-1868. Yang, Z., Wu, X., 2013. Retrofits and options for the alternatives to HCFC-22. Energy 59, 1-21. Zhang, W., Yang, Z., Li, J., Ren, C.-x., Lv, D., Wang, J., Zhang, X., Wu, W., 2013. Research on the flammability hazards of an air conditioner using refrigerant R-290. International Journal of Refrigeration 36, 1483-1494. Zhang, W., Yang, Z., Zhang, X., Lv, D., Jiang, N., 2016. Experimental research on the explosion characteristics in the indoor and outdoor units of a split air conditioner using the R290 refrigerant.
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International Journal of Refrigeration 67, 408-417.
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Table 1- Tube length and diameter in the 1P STHAC Heat Exchanges Evaporator (In indoor unit)
Tube length (mm)
695
600
Tube inner diameter (mm)
4.37
6.56
Tube spacing (mm)
19.5
21
Number of tube row
2
2
Numbers of tubes
52
28
Inner volume (cm3)
566
599
Liquid pipe (From outdoor to
Gas pipe (From indoor to
indoor unit)
outdoor unit)
3
3
Length (m) Tube inner diameter (mm)
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Connect pipes
4.37
3
Inner volume (cm )
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Condenser (In outdoor unit)
8.02
45.0
151
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Remark: the inner volumes were determined by calculation
Name
Type
Specifications
Honeywell (Searchpoint optima plus)
Range: 0-100% LFL
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Indoor gas detector
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Table 2- Apparatus specifications
Accuracy: ±1% LFL
Agilent
20 channels
instrument
(34970A with 34901A module)
Speed: 60 chs /s
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Data acquisition
Table 3- Indoor leakage tests conditions and results Solenoid valve Off
On
Charge
Leak
quantity/g
hole/mm
Window and door Open
Close
Combustible
Sensor NO. of
Sensors NO. of
zone’s residence
maximum time
exceeding LFL
time /s *
260
4.37
*
38
D
A, B, C, D
*
300
4.37
*
41
D
A, B, C, D
34
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*
54
D
A, B, C, D, E
*
260
4.37
*
110
B
A, B, C, D, E
*
300
4.37
*
115
D
A, B, C, D, E
*
350
4.37
*
145
B
A, B, C, D, E
*
260
1.0
*
28
D
B, D
*
300
1.0
*
68
B
B, D
*
350
1.0
*
85
B
B, D
*
260
1.0
*
161
B
B, D, E
*
300
1.0
*
178
B
B, D, E
*
350
1.0
*
186
*
260
0.5
*
*
300
0.5
*
*
350
0.5
*
*
260
0.5
*
53
*
300
0.5
*
82
*
350
0.5
*
99
*
260
1.0
*
*
350
1.0
*
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350
64
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PT
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*
35
B
A, B, C, D, E
B
B, D
D
B, D
B
B, D
B
B, D
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Fig. 1- Schematic of the Plume model
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Fig. 2- Schematic diagram of the indoor leakage experimental apparatus
Fig. 3- Locations of indoor leakage experimental apparatus
36
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Fig. 4- Photograph of the replaceable copper tubes with different leakage hole
37
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110
fast-leaking buffering
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100 90
A C E
The process of R290 concentration falling and rising again
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80 70 60
diffusing
B D
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Concentration/ LFL%
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Fig. 5- Photograph of the experimental apparatus
50
AC
40 30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 6- Concentration distribution results of 260 g charge quantity and 0.5 mm leakage
38
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hole
110
fast-leaking buffering
diffusing
100
70 60 50
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Concentration/ LFL%
80
B D
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A C E
90
40 30 20
0 0
200
400
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10
600
800
1000
1200
1400
1600
1800
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Time/s
hole
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Fig. 7- Concentration distribution results of 300 g charge quantity and 0.5 mm leakage
39
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110
fast-leaking buffering
diffusing
100
A C E
80 70
B D
60 50
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Concentration/ LFL%
90
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 8- Concentration distribution results of 350 g charge quantity and 0.5 mm leakage
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hole
40
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100
260g, 0.5mm 300g, 0.5mm 350g, 0.5mm 300g, 1.0mm 300g, 4.37mm
90
70 60 50
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Concentration/ LFL%
80
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 9- R290 concentration of detector B under different charge quantity and diameter of
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leakage hole
110
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fast-leaking buffering
100
PT
A C E
80 70
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Concentration/ LFL%
90
diffusing
B D
60
AC
50 40 30 20 10
0 0
200
400
600
800
1000
Time/s 41
1200
1400
1600
1800
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Fig. 10- Concentration distribution results of 350 g charge quantity and 1.0 mm leakage hole
110
fast-leaking
buffering
diffusing
100
A C E
80 70 60 50 40 30 20 10 0 0
200
400
600
B D
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Concentration/ LFL%
90
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The process of R290 concentration falling and rising again
800
1000
1200
1400
1600
1800
M
Time/s
Fig. 11- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with
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solenoid valve closed
42
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110
A C E
100 90
Concentration/ LFL%
80
B D
70 60
fast-leaking
buffering
diffusing
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50 40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 12- Concentration distribution results of 260 g charge quantity, 0.5 mm leakage hole with
110
fast-leaking
100
buffering and diffusing
PT
A C E
80 70
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Concentration/ LFL%
90
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solenoid valve closed
B D
60
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50 40 30 20 10
0 0
200
400
600
800
1000
Time/s 43
1200
1400
1600
1800
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Fig. 13- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with solenoid valve closed and window and door opening
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200
0.5 mm hole with soleniod valve opening 0.5 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve opening 1.0 mm hole with soleniod valve closed 4.37 mm hole with soleniod valve opening 4.37 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve closed; window and door opening
160
120
80
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Combustible zone’s residence time /s
240
40
0 260
300
350
M
Charge quantity /g
conditions
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PT
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Fig. 14- Relationship of the combustible zone’s residence time with different experimental
44
Assessment of leakage and risk reduction of R290 in a split type household air conditioner Weier Tang , Guogeng He , Wei Sun , Sai Zhou , Dehua Cai , Yihao Zhu PII: DOI: Reference:
S0140-7007(18)30091-4 10.1016/j.ijrefrig.2018.03.012 JIJR 3925
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
8 November 2017 31 January 2018 14 March 2018
Please cite this article as: Weier Tang , Guogeng He , Wei Sun , Sai Zhou , Dehua Cai , Yihao Zhu , Assessment of leakage and risk reduction of R290 in a split type household air conditioner, International Journal of Refrigeration (2018), doi: 10.1016/j.ijrefrig.2018.03.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights 1. R290 concentration distribution in the room was measured by the detectors. 2. R290 leakage from the air conditioner with different diameter hole was presented. 3. To install a solenoid valve to reduce the fire risk was proposed and verified.
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4. Proposal for risk detection in rooms by the noise of huge gas leaks was presented.
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Assessment of leakage and risk reduction of R290 in a split type household air conditioner Weier Tang, Guogeng He*, Wei Sun, Sai Zhou, Dehua Cai, Yihao Zhu School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074,
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China
Abstract:
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R290 (propane) is a famous hydrocarbon substance, but the flammable and explosive
characteristics require safety precautions for its use. In fact, R290 is also a refrigerant with low global warming potential (GWP) and excellent thermodynamic properties. However, the
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application of R290 in split type household air conditioners (STHAC) is hindered by unpredictable gas leakages into the room. This paper evaluates the fire hazard of R290 as a
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refrigerant in STHAC and provides a new method of installing a solenoid valve near the
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capillary to reduce the fire risk by decreasing the R290 leaking rate. The approach uses a numerical model to analyze the major parameters effect on the concentration distribution of
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R290 in the room after leakage from the STHAC indoor unit, as well as a series of
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experimental tests for verification. The influence of different charge quantities, leakage hole diameters and air flows on the R290 concentration distribution in the room were compared and discussed. The new method was also validated by experiments. The results showed that the fire hazard only occurs during the fast-leaking period and that the most likely flammable region is underneath the location of the leak. In addition, the flammable hazards can be assessed by the huge noise generated by the R290 leakage. 2
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Key words:
R290; Air conditioner; Gas leakage; Concentration distribution; Safety
Corresponding author. Tel.: +86 27 87542718.
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E-mail address: [email protected] (G. He).
Nomenclature global warming potential
LFL
lower flammable limit
ODP
ozone depleting potential
STHAC
split type household air conditioner
UFL
upper flammable limit
bs
local characteristic width of the plume, m
c*
concentration on the plume axis, kg/m3
P1
R290 pressure in the machine, Pa
Pa
R290 pressure in the room, Pa
r
radial distance to the plume axis in a normal section of the plume, m
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s
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GWP
distance along the plume axis from the origin to a certain point on the plume
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axis, m
Greek symbols μw
wind speed, m/s
μv
leaking rate from the leakage position, m/s
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theoretical value of leaking rate, m/s
ρa
density of the atmosphere, kg/m3
ρr290
density on the plume axis, kg/m3
ρ1
R290 density in the machine, kg/m3
λ
turbulent Schmidt number
κ
adiabatic exponent
ξ
coefficient of friction loss
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μ0
1. Introduction
As a part of the liquified natural gas, R290 (propane) has the characteristics of flammable
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and explosive, it needs a series of safety precautions to avoid leakage and ensure the safety.
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For instance, the lower minimum ignition energy is only 0.48mJ at an R290 concentration of 5.2 VOL% (Eckhoff et al., 2010); and the lower and upper flammability limits (LFL and UFL)
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of R290 in the air at 25℃ and 1atm is at very low concentration of 2.1 and 9.5 VOL%,
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(Kuchta, 1986), respectively. It makes the gas mixture of R290 and air could be easily ignited, any leakage or diffusion of R290 in an insufficiently ventilated space has significant
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possibility to cause a hazardous event and subsequent combustion or explosion by an accidental ignition or spark. Thus, the hazard assessment and controlment of R290 or other hydrocarbon gas leakage must be paid full attention.
Not only a fuel, R290 is also a suitable refrigerant for split type household air conditioner (STHAC) because it could provide higher energy efficiency (Wu et al., 2012) and reduce the 4
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energy consumption (Padalkar et al., 2014). Most importantly, it is a natural refrigerant with zero ozone depleting potential (ODP) value and negligible global warming potential (GWP) values, which is friendly to the environment. In history, HCFC-22 is the traditional refrigerant in STHAC, but its ODP values (0.055) will bring the damage of the ozone layer (Wang et al.,
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2016). It had been classified into the phase-out list by Montreal Protocol in 1987, and the phase-out progress had been accelerated (Calm, 2008). The high GWP values for current
HFCs alternative refrigerants to HCFC-22, such as HFC-410A (GWP=2090) and HFC-32
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(GWP=675), will contribute to accelerating the global warming (Yang and Wu, 2013). Hence, the Kigali Amendment to the Montreal Protocol in 2016 announced that the HFCs would be phased-down in the future (Höglund-Isaksson et al., 2017). The environmental friendly
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refrigerants in STHACs.
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refrigerant, R290, is therefore the most promising alternative to replace high-GWP
While applied in STHAC, the inflammable and explosive characteristics of R290 is still
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the biggest obstacle to its promotion. It was classed as an A3 flammable refrigerant by
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ASHARE, the utilization and maximum charge quantity of R290 in STHAC are strictly restricted by the international criterions, i.e., EN 378 and IEC 60335-2-40 (Corberán et al.,
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2008). Besides, the method of adding another non-flammable refrigerant in the STHAC is also considered to avoid the hazardous events (Tian et al., 2015). Nonetheless, the unpredictable leakage accident of R290 from STHAC is unavoidable, which might cause combustion or explosion once the leaked R290 reaches to the LFL concentration. For this reason, this paper examined the indoor safety concerns and measured concentration distribution in the room after R290 leaked from STHAC indoor unit into the room under 5
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different scenarios.
Up to now, several studies have reported on the leakage and diffusion properties of various refrigerants. For example, Cheong and Riffat (1996) monitored the indoor leakage and estimated the emission rate of R134a by using tracer gas SF6. Blackwell et al. (2004)
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developed an independent risk assessment for R152a and carbon dioxide mobile air
conditioner systems. Gigiel (2004) analyzed the system pressure and the refrigerant
concentration distribution when R600a leaked from a refrigerator according to the methods
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specified in the safety Standard, IEC/EN 60335-2-24. Jia et al. (2017) tested the R32 leakage and diffusion characteristic of STHACs under different operating conditions, the combustible zone only appears near the leakage hole, and the duration of the combustible zone is very
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short. Nagaosa (2014) presented a new numerical formulation to simulate the R290 leakage
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and spread into an enclosed room and examined the effect of leakage rate on concentration profiles. D. Colbourne carried out the quantitative risk assessment to estimate the likelihood
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of ignition and severity of the consequences after the R290 leaked from the ice cream cabinet
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(Colbourne and Espersen, 2013), refrigerator and STHAC (Colbourne and Suen, 2015).
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However, there is a lack of experimental results of R290 concentration distribution in the residential space following an actual leakage from the STHAC. Although Zhang evaluated the distribution of R290 in the room (Zhang et al., 2013) and measured the explosion characteristics in the indoor and outdoor units of an R290 STHAC (Zhang et al., 2016). Li (2014) also tested the R290 distributions inside a room under different scenarios. The leaking rate is constant both in their experiments since the R290 was released by the vessel and 6
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controlled by a valve. It is different with the actual leakage condition, which the leaking rate will gradually decrease since the pressure in the STHAC drops after R290 leaked out. The simulated (Nagaosa, 2014) and experimental (Li, 2014) results illustrated that the leaking rate could affect the R290 concentration distribution in the room, so the constant leaking rate
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cannot reveal the actual leakage condition. Considering that the global environmental problem has made it an urgent issue to develop the R290 STHAC, and there will be a huge amount of the R290 STHACs in the future. It is necessary to investigate the R290
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concentration distribution in the room following with an actual leakage under different
scenarios. This paper used the numerical model to analyze major factors impact on the R290 indoor leakage and measured the R290 concentration distribution in the room by series gas
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detectors. The influence of charge quantity, the diameter of leak hole and airflow on the R290 concentration distribution was discussed and compared. A new method of using a solenoid
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valve near the capillary to reduce the leaking rate, proposed by R290 leaking rate experiment
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(Tang et al., 2017), was investigated and verified. The results could provide valuable guidance for improving the indoor safety and accident prevention when using R290 as working fluid in
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the STHACs.
2. Numerical analysis of R290 leakage and diffusion The plume model proposed by Ooms (1972) has been successfully used to simulate the plume path of “light” gases (whose density is same as or less than that of air) leaking into the atmosphere at atmospheric temperature and pressure. The model was extended to “heavy” 7
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gases (whose density is higher than air) with a number of assumptions (Ooms et al., 1974).
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Fig. 1- Schematic of the Plume model
In the Plume model (Fig. 1), the plume concentration is assumed to be cylindrically
c( s, r ) c* exp(
r 2 ) 2bs2
(1)
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symmetric and it can be calculated by:
Where c (s, r) represents the values of the concentration at an arbitrary point in the plume;
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c* denotes the concentration of gas on the plume axis; r is the radial distance to plume axis in a normal section of plume, m; λ2(≈1.35) is the so-called turbulent Schmidt number; bs represents the local characteristic width of the plume and is one of the unknown quantities.
Khan and Abbasi (1999, 2000) developed some empirical equations and defined the dimensionless number Q f to solve the parameters c*, bs and predicted the behaviour of the 8
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gas concentration in the plume, then the Eq. (1) can be calculated by:
Qf (
uw a )( ) uv r 290
(2)
0.382484Q f
(3)
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c* (0.0161965 0.00543481 Q f 0.00128631 Q2f ) ln( s) 0.183894 e
bs (0.0157719 0.104969 Q f 0.0347085 Q2f ) ln( s) 0.953469 Q0.22422 f
(4)
Where uw is the wind speed, uv represents the gas leaking rate from the leaking position.
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ρa is the density of the atmosphere and ρr290 is the density of the plume gas, s is the distance along the plume axis from the leaking position to a certain point.
Considering the leakage as an adiabatic process, the theoretical value of leaking rate u0 is
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determined by:
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P0 u02 dP / 0 P 1 2
(6)
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P / P1 / 1 Pa / a
(5)
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Where P1 and Pa is the pressure of R290 in the STHAC and room, respectively. ρ1 is the
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density of R290 in the STHAC, respectively. 𝜅 is the adiabatic exponent.
The actual leaking rate uv=𝜉 u0 due to the friction loss while R290 through the leakage
hole. ξ means the coefficient of friction loss which has a correlation with the shape and diameter of the leakage hole. By solving eq. (5) and (6), the expression of uv is as follows:
Pa 1 2 P1 uv = u0 1 ( ) ( 1) 1 P1 9
(7)
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From Eq. (3) and (7), it can be known that the R290 concentration in the same position was decided by 𝜅, Pa, ξ, P1, ρ1 and uw, ρa, ρr290. The R290 leakage experiments were carried out in an enclosed room at 25℃ and 1 atm in this study, so the 𝜅, uw, Pa is constant and ρa =1.184kg/m3 and ρr290 =1.832kg/m3. In the STHAC, the ρ1 was determined by P1 since the
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inner volume is constant. Therefore, the P1, ξ, and uw are three major parameters to affect the R290 concentration. In the following chapters, the impact of different charge quantity on P1, different diameter of leakage hole on ξ will be discussed and verified through the various
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experiments. In addition, some measures to reduce the R290 concentration in the room, such as installing a solenoid valve near the capillary to reduce the uv and opening the window and
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door to increase the uw, will also be conducted and verified.
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3. Experimental apparatus and procedures
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3.1 Experimental apparatus and set-up
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Fig. 2 and 3 illustrated the schematic and locations of the indoor leakage experimental apparatus. As shown in Fig. 3, the experimental room dimensions are 4.5 (L) ×3.2 (W) ×2.6
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(H) m, giving a volume of 37.44m3 and floor area of 14.4m2, with a door (2.3 (H) ×0.9 (W) m) and a window (1.6 (W) ×1.2 (H) m).
Based on the experimental room dimensions, a 1P R290 STHAC with a nominal cooling capacity of 2500W was chosen for the test, the size of the pipes in the STHAC was shown in Table 1. Besides, it could be found in Fig.2 that for imitating the R290 leaking inside the 10
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indoor unit and simultaneously not affect the STHAC system, another same indoor unit was employed as the experimental indoor unit 7 and separately mounted at the center of the 4.5 m (L) wall with 1.8m height. The soft pipe 6 with 0.9 m length and 6.35 mm inner diameter was used to deliver the R290 from the STHAC to experimental indoor unit 7. One of the ports of
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soft pipe 6 was put inside the experimental indoor unit 7 to imitate the leakage point. Based on a record by STHAC service engineers, the position of u-tube junctions of the evaporator is the most likely leakage position inside the indoor unit, the leakage point was therefore located
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at that position for imitating the actual leakage accident. Another port of soft pipe 6 was
installed on hand valve 5 with the replaceable copper tube, those replaceable copper tubes with different diameter hole (as shown in Fig. 4) were employed to imitate the different
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diameter leakage hole. To exclude the impact of the liquid refrigerant, the three-way tube 4 and hand valve 5 was installed on the outlet pipe of the STHAC indoor unit. In addition, a
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solenoid valve 2 was installed near the capillary to investigate its effects on reducing the
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leaking rate. The indoor leakage experiments were carried out by opening the hand valve 5, the R290 gas in STHAC will flow into the experimental indoor unit 7 through soft pipe 6 and
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then diffuse in the room through the indoor unit vent.
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Fig. 2- Schematic diagram of the indoor leakage experimental apparatus
Fig. 3- Locations of indoor leakage experimental apparatus
Table 1- Tube length and diameter in the 1P STHAC
Heat Exchanges Condenser (In outdoor unit)
Evaporator (In indoor unit)
Tube length (mm)
695
600
Tube inner diameter (mm)
4.37
6.56
Tube spacing (mm)
19.5
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Number of tube row
2
2
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Numbers of tubes 3
Inner volume (cm )
52
28
566
599
Liquid pipe (From outdoor to
Gas pipe (From indoor to
indoor unit)
outdoor unit)
3
3
4.37
8.02
45.0
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Length (m) Tube inner diameter (mm) 3
Inner volume (cm )
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Remark: the inner volumes were determined by calculation
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Connect pipes
Fig. 4- Photograph of the replaceable copper tubes with different leakage hole
Five infrared flammable gas detectors (A-E) were placed at proper locations under the experimental indoor unit 7, because the CFD simulation results and previous studies (Zhang et al., 2013) (Li, 2014) showed that R290 will concentrate in the region underneath the leak 13
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position while concentration of R290 in other regions seldom approach the LFL. Detectors A and B were located at the height of 0.8 m to measure the R290 concentration in mid-air and the distance between them is 0.5 m. Detectors C, D, E were located at the height of 0.1 m to measure the R290 concentration in floor level, and they are 0.3 m away from each other
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because the width of STHAC indoor unit vent is 0.6m. In addition, the distance from STHAC indoor unit vent to the wall is 0.1 m so all detectors were 0.1 m away from the wall. The room is empty except for the experimental equipment. Fig. 5 is the photograph of the experiment
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equipment, and the plume axis was speculated by the soft pipe and leak position.
Fig. 5- Photograph of the experimental apparatus
The experimental instruments are detailed in Table 2, which were all fully calibrated. The gas detectors were calibrated by a standard R290 gas of 2.1 VOL % concentration, so the LFL 14
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in this study is equal to 2.1 VOL %.
Table 2- Apparatus specifications Name
Type
Indoor gas detector
Honeywell (Searchpoint optima plus)
Specifications Range: 0-100% LFL Accuracy: ±1% LFL
Agilent
20 channels
(34970A with 34901A module)
Speed: 60 chs /s
3.2 Experimental procedures
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Data acquisition instrument
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The experiments were carried out under STHAC cooling mode because the results of R290 distribution in STHAC (Tang et al., 2017) indicated that R290 would migrate from
outdoor to the indoor unit in cooling mode, which is the dangerous case for R290 STHAC.
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Besides, the temperature of STHAC was set at 26 ℃, and it is non-operating during the
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leaking process to eliminate the impact of airflow caused by the indoor unit fan. The door and window were closed throughout the experimental process if there is no special explanation.
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The experimental procedures were carried out according to the following sequence:
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a) Ensure there is no residual R290 in the room (the background concentration is less
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than 1% of LFL).
b) Install the appropriate replaceable copper tube at hand valve 5 and charge the
corresponding R290 quantity into STHAC, then operate the STHAC into experimental conditions. c) Start the data acquisition system, measuring and recording the R290 concentration in the room, after a few minutes shut down the STHAC. 15
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d) Open the hand valve 5 immediately after the STHAC powered off, release the R290 into the room. e) If the experiments with solenoid valve 2 closed were carried out, it needs to close the solenoid valve 2 simultaneously with the STHAC powered off.
stable.
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4. Experimental results and analysis
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f) Terminate the data acquisition system once all the concentration measurements remain
4.1 Influence of charge quantity
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According to the results of R290 mass distribution in the components of STHAC (Tang
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et al., 2017), the evaporator will hold more liquid R290 in it with the increasing of charge quantity since the constant inner volume and approximately the same saturation evaporating
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temperature in this study. When R290 leakage occurs, the initial P1 is the saturation
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evaporating pressure and it drops with the R290 leaking out. However, the liquid R290 in the evaporator will evaporate and maintain the saturation pressure P1 when R290 gas has leaked
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out. The more liquid R290 in the evaporator, the longer P1 will remain at saturation pressure at the beginning of the leakage process, and the leaking rate will also remain at a higher value. To explore the relationship between different R290 charge quantity and concentration distribution in the room, three R290 charge quantity with 260 g, 300 g, and 350 g were selected in this study. The reason is according to the results of the performance measurement,
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the optimal charge quantity for this 1P STHAC is 300 g, the minimum and maximum charge quantity to maintain its designed performance are 260 g and 350 g, respectively. The results of R290 concentration distribution with different charge quantity and 0.5 mm leakage hole were shown in Fig. 6-8. Since the range of the gas detectors is 0-100% LFL, and their output
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data unit is 4-20 mA, which is corresponding to 0-100% LFL, respectively. The relationship between LFL % and time is presented in the result figures. It could be convenient to find that the concentration of which detectors could exceed LFL in those figures.
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fast-leaking buffering
The process of R290 concentration falling and rising again
80 70
B D
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60
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50 40
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Concentration/ LFL%
90
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diffusing
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30
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20 10
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0
200
400
600
800
1000
1200
1400
1600
1800
Time/s
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Fig. 6- Concentration distribution results of 260 g charge quantity and 0.5 mm leakage hole
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fast-leaking buffering
diffusing
100
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80 70
B D
60 50
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Concentration/ LFL%
90
40 30 20
0 0
200
400
600
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800
1000
1200
1400
1600
1800
Time/s
Fig. 7- Concentration distribution results of 300 g charge quantity and 0.5 mm leakage hole
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fast-leaking buffering
100
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B D
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Concentration/ LFL%
90
diffusing
40
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30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 8- Concentration distribution results of 350 g charge quantity and 0.5 mm leakage hole
The whole leakage process in Fig. 6-8 was summarized in three periods, which is the 18
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fast-leaking period, buffering period and diffusing period. As shown in Fig. 6, the fast-leaking period is at the beginning of the leakage process. In this period, the R290 in the evaporator leaks quickly and results in the R290 concentration of all detectors rise rapidly. Then is the buffering period, it could be observed in Fig .6 that after the fast-leaking period, the R290
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concentration of all detectors has a process of falling and rising again. The reason is that the leaked R290 in fast-leaking period reduced the pressure in the evaporator and leaking rate is also decreased according to Eq. (7), so the R290 concentration falling at the beginning of
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buffering period. After that the high pressure in the condenser will drive the R290 to migrate from condenser to evaporator through the capillary and liquid connect pipe then leaks off, the migrated R290 from condenser will raise the pressure in the evaporator and the leaking rate,
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and the R290 concentration in the room rising again. Finally, it is the diffusing period. After the most of R290 in the STAHC leaks off and disperses in the room, the R290 concentration
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in the room will gradually decline and reach a stabilized level in the last. The stabilized R290
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concentration level can be verified by the charge quantity. For the charge quantity of 260 g, it will bring the 0.146 m3 volume of R290 into the room because of the density of R290 is 1.78
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kg/m3 under the experimental conditions. Hence, the R290 volume fraction in the room should
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be 0.39 VOL % because the volume of the room is 37.44m3, and the final concentration level will be 18.5 LFL% as the LFL of R290 is 2.1 VOL %. The final stabilized concentration results in Fig. 6 are approximately equal to this value, which proved that the results in Fig. 6 are effective, other experimental results can also be validated by this method.
Fig. 7 and 8 are the results of 300 g and 350 g R290 charge quantity in 0.5 mm leakage hole, the trend of the R290 concentration curve is similar to Fig. 6 and there are also three
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periods in those results. However, the duration time of fast-leaking period is extended since the evaporator gathers more liquid R290 in it, the concentration of detector D also reached LFL level in the fast-leaking period. The biggest difference between the three figures is the duration time of concentration exceeds the LFL, which means the time of combustible zone
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appears in the room. The combustible zone’s residence time in Fig. 6-8 are 53s, 82s and 99s, respectively. It is increasing with the charge quantity, which implies the higher R290 charge
quantity will bring the longer dangerous time in the room when R290 leakage occurs. Hence,
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it needs to strictly restrict the R290 charge quantity to ensure the safety.
The results of Fig. 6-8 show that only detectors B and D reached the LFL level, and the concentration of detector E is higher than C while A always keep the lowest R290
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concentration. As shown in Fig. 2 and 5, the R290 plume axis is most likely to be diagonal
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from the right side of the indoor unit vent to the ground since the leak position was located at the right of experimental indoor unit 7. Eqs. (1), (3) and (4) indicated that the plume axis
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distance, s, and radial distance, r, could affect the concentration. It could be speculated from
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Fig. 5 that the plume axis is near to detectors B and D, the r of B and D is shorter than others, and detector B is closer to the leak position. That is the reason that only detectors B and D
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reached the LFL level and D cannot reach the LFL level in the fast-leaking period of Fig. 6. In addition, the distance from detectors A and B is 0.5 m while detectors C, D, E are placed at floor level and 0.3 m away from each other. The r of E is shorter than C since detector E was placed at the right side, it could help to explain why the concentration of detector E is higher than C. Detector A always keep the lowest R290 concentration because it has the longest r. This phenomenon implies that the combustible zone is more likely appear in the region 20
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underneath and near to the leak position. Hence, the electrical components, such as the socket, should not install underneath the R290 STHAC to prevent the generation of accidental short circuit electric sparks in the combustible zone.
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4.2 Influence of diameter of leakage hole
The coefficient of friction loss 𝜉 in Eq. (7) was determined by the shape and area of
leakage hole. In the same area, the circular leakage hole has the largest 𝜉. Besides, the larger
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area results in larger 𝜉 in the same shape. Three kinds of circular leakage hole with a diameter of 0.5 mm, 1.0 mm and 4.37 mm were chosen in this study to investigate the influence of different leakage hole on R290 concentration distribution in the room. The diameter of the
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leakage hole is determined by the corresponding driller and it has been remeasured to ensure accuracy. The leakage hole with 0.5 mm diameter was to imitate the situation that tiny rupture
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on the tube, because it is difficult to drill a smaller diameter hole in the copper tube due to
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limitations imposed by process. At the same time, the 1.0 mm was to imitate the partial
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rupture on the tube while 4.37 mm was corresponding to a breakage of the pipe in this study. The 4.37 mm leakage hole was exactly a tube with 5.0 mm external diameter (as shown in Fig.
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4) since the researchers in China have applied it to reduce the heat exchanger’s inner volume for reducing the R290 charge quantity. Fig. 6-8 illustrated that the trend of the R290 concentration curve is similar in the whole leakage process and detector B always have the highest R290 concentration. Hence, R290 concentration in detector B under different situations were discussed and the results were shown in Fig. 9.
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260g, 0.5mm 300g, 0.5mm 350g, 0.5mm 300g, 1.0mm 300g, 4.37mm
90
70 60 50
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Concentration/ LFL%
80
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 9- R290 concentration of detector B under different charge quantity and diameter of leakage
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hole
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The results described in Fig. 9 show that the combustible zone’s residence time will increase with the charge quantity, but not with the diameter of leakage hole. For example, in
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the condition of 300 g R290 charge quantity, the combustible zone’s residence time under 0.5
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mm, 1.0 mm and 4.37 mm diameter of leakage hole is 82 s, 178 s, and 115 s, respectively. The longest combustible zone’s residence time appeared in the condition of 1.0 mm diameter
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of leakage hole, which is not 4.37 mm. The larger diameter of leakage hole brings the faster leaking rate and leads to more expansive combustible zone in the room, but it also leads to shorter leaking time in the same charge quantity at the same time. The combustible zone of 4.37 mm is more expansive than 1.0 mm (all detectors exceeded the LFL in 4.37 mm while detectors A and C cannot reach LFL level in 1.0 mm), but a shorter leaking time leads to the combustible zone’s residence time of 4.37 mm (115s) is shorter than 1.0 mm (178s). The 22
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result of 0.5mm leakage hole remains the shortest combustible zone’s residence time (82s) because its leaking rate is too low to maintain the R290 concentration exceed LFL for a long time. The longer leaking time and fast enough leaking rate results in the longest combustible zone’s residence time under 1.0 mm diameter of leakage hole. Therefore, it is noteworthy that
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the size of the leakage hole is not positively related to the degree of risk, the certain leakage hole with long leaking time and fast enough leaking rate will bring the most dangerous
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situation in the room.
4.3 Influence of solenoid valve
From the above results, it can be concluded that the leaking rate uv, has a major influence
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on R290 concentration distribution, it is therefore necessary to find a way to reduce the
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leaking rate. The results of leaking rate test (Tang et al., 2017) indicated that installing a solenoid valve near to the capillary could reduce the leaking rate by delay the R290 migrate
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from condenser to evaporator. Hence, the experiments with solenoid valve closed were
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carried out to investigate its effects on reducing the leaking rate. Since the 350 g R290 charge
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quantity under 1.0 mm leakage diameter is the most dangerous situation according to the former discussion, the results of solenoid valve opening (Fig. 10) and closed (Fig. 11) under this condition were shown to compare and discuss.
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100
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Concentration/ LFL%
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Fig. 10- Concentration distribution results of 350 g charge quantity and 1.0 mm leakage hole
40
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30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 11- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with solenoid valve closed
24
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Fig. 10 illustrates that all detectors have reached the LFL level and the longest combustible zone’s residence time (186s) since it has the largest charge quantity (350 g) and the longest leaking time (1.0 mm). However, the result of the situation that solenoid valve closed (Fig. 11) shows that the region and residence time of the combustible zone were
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reduced, only detectors B and D have reached the LFL and the residence time was shortened to 85s. It could be explained by the following reasons: The closed solenoid valve 2 will hinder the R290 migrate from condenser to evaporator through the capillary and liquid connect pipe
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(Fig. 2) in the buffering period. However, the high pressure in the condenser will drive the
R290 through the compressor to gas connect pipe then leaks out since there exist clearances in the rotary compressor of STHAC (Cai et al., 2015). Comparing to through the capillary and
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liquid connect pipe, the migration process will be slowed down by the clearances in the compressor and the leaking rate will be reduced in buffering period. Therefore, it could be
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found in Fig. 11 that the duration of buffering period has been extended and the process of
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concentration rising in the buffering period was distinct.
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Fig. 10 and 11 show the comparison of solenoid valve opening or closed under the most dangerous situation in this study. As a contrast, Fig. 12 illustrates the safest situation, which is
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the results of 260 g charge quantity, 0.5 mm diameter of leakage hole with solenoid valve closed. By comparing Fig.12 to Fig. 6, it is obvious that all detectors cannot reach the LFL and the highest R290 concentration is under 50 %LFL with solenoid valve closed. Therefore, it could be concluded that closing the solenoid valve could greatly improve the safety of R290 STHAC, and the combustible zone maybe disappears in the situation that tiny tube rupture with the low leaking rate. 25
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110
A C E
100
80 70 60
fast-leaking
buffering
diffusing
50
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Concentration/ LFL%
90
B D
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 12- Concentration distribution results of 260 g charge quantity, 0.5 mm leakage hole with
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4.4 Influence of airflow
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solenoid valve closed
The above experimental results and conclusions are obtained in an enclosed room with
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the door and window closed, the velocity of air convection uw in the enclosed room is about
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0.1m/s or less by measurement of anemoscope. However, according to the Eqs. (1)- (4), the concentration will decrease as the wind speed, uw, increases in the condition that the other
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coefficients are constant. The experiments with window and door opening were conducted to analyze the influence of airflow. Fig. 13 shows the concentration results of 350 g charge quantity, 1.0 mm leakage hole, solenoid valve closed with window and door opening. Comparing Fig.13 against Fig. 11, it could be found that the combustible zone’s residence time was further reduced from 85s to 64s after the window and door opening. In addition, the
26
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R290 concentration in Fig. 13 is close to zero after 400 s. The reason is that the higher air convection velocity could help to R290 disperse, and the leaking rate is less than the diffusing rate after 400 s. The R290 is directly diffused in the environment and not gather in the room since the window and door opening, so the R290 concentration in the room is close to zero. In
R290 STHAC.
110
fast-leaking
buffering and diffusing
100
A C E
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80 70 60
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50
B D
40 30
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Concentration/ LFL%
90
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short, this result indicates that open window and door could effectively improve the safety of
20
0
200
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0
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10
400
600
800
1000
1200
1400
1600
1800
Time/s
Fig. 13- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with
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solenoid valve closed and window and door opening
4.5 Summary of indoor leakage results In addition to those representative experimental results to analyze the influence of the various factors, overall 20 different experiments were carried and the results were
27
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summarized in Table 3. It can be seen from Table 3 that detector B and D always exceed the LFL level since they are closer to the speculated plume axis as shown in Fig. 5. The regularity of the combustible zone’s residence time under different experimental conditions in Table 3 as shown in Fig. 14. Fig .14 and the experimental results involved in Table 3 demonstrate the
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similar results as indicated above. At the same condition, the combustible zone’s residence time increasing with the higher R290 charge quantity, and the longest combustible zone’s
residence time appears in the situation of 350 g R290 charge quantity and 1.0 mm diameter of
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leakage hole whether solenoid valve opening or closed. While the solenoid valve is closed,
the combustible zone’s residence time and the number of gas detectors which exceed LFL will significantly decrease, and there may be no combustible zone appear in the room at the condition of 0.5 mm leakage hole and solenoid valve closed. In addition, opening window and
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door could further reduce the combustible zone’s residence time.
Table 3- Indoor leakage tests conditions and results
Charge
Leak
PT
Solenoid valve
Combustible zone’s
Sensor NO. of
Sensors NO. of
Close
residence time /s
maximum time
exceeding LFL
Window and door
quantity/g
hole/mm
260
4.37
*
38
D
A, B, C, D
300
4.37
*
41
D
A, B, C, D
350
4.37
*
54
D
A, B, C, D, E
*
260
4.37
*
110
B
A, B, C, D, E
*
300
4.37
*
115
D
A, B, C, D, E
*
350
4.37
*
145
B
A, B, C, D, E
*
260
1.0
*
28
D
B, D
*
300
1.0
*
68
B
B, D
*
350
1.0
*
85
B
B, D
*
260
1.0
*
161
B
B, D, E
*
300
1.0
*
178
B
B, D, E
*
350
1.0
*
186
B
A, B, C, D, E
260
0.5
*
Off
On
*
AC
*
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*
*
Open
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300
0.5
*
*
350
0.5
*
*
260
0.5
*
53
B
B, D
*
300
0.5
*
82
D
B, D
*
350
0.5
*
99
B
B, D
*
260
1.0
*
*
350
1.0
*
64
B
B, D
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*
200
0.5 mm hole with soleniod valve opening 0.5 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve opening 1.0 mm hole with soleniod valve closed 4.37 mm hole with soleniod valve opening 4.37 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve closed; window and door opening
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160
120
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80
40
0
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260
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Combustible zone’s residence time /s
240
300
350
Charge quantity /g
conditions
AC
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Fig. 14- Relationship of the combustible zone’s residence time with different experimental
Particular, there is an important observation cannot be ignored that within the
fast-leaking period, the R290 leakage is accompanied by a huge sound. This huge sound is similar to the sound at the start of the STHAC switching to the defrost mode from the heating mode. The difference is the huge sound is transitory in the defrost mode but continues in the whole leakage process, it will be quieter in the buffering period but still could be clearly heard 29
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in the room. In addition, this huge sound can also be heard by the experimenter even in the tests that no detectors reach the LFL level such as under the condition that 0.5 mm leakage hole and solenoid valve closed. Hence, this observation could help the consumers to identify the flammable risk in the room where using R290 STHAC: if they heard the huge leakage
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sound, there probably exists fire risk in the room, and if not, the room is safe.
5. Conclusions:
Based on the above experimental results, the following conclusions can be reached:
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1) Refrigerant charge quantity has a major influence on the R290 concentration distribution in the room. A higher R290 charge quantity will lead to a more extensive combustible zone and longer residence time in the room. The charge quantity of R290 in STHAC
2)
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needs to be strictly restricted, as mentioned in EN 378 and IEC 60335-2-40. The leakage hole can also affect the R290 concentration distribution. The results of this
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study reveal that the 1.0 mm diameter of leakage hole has the longest combustible zone’s
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residence time. The larger leakage hole (4.37 mm) leads to the combustible zone more extensive but shorter residence time of it, the smaller leakage hole (0.5 mm) makes the
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lower leaking rate and the R290 concentration is harder to approach the LFL. The certain
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leakage hole with a long leaking time and a high enough leaking rate will be the most dangerous situation while using R290 STHAC within the room.
3)
A practical and achievable approach to significantly reduce the leaking rate and enhance the room safety is installing a solenoid valve behind the capillary, and simultaneously closing it as the STHAC shutdown. The results demonstrate that both range and residence time of the combustible zone were significantly reduced under the condition of 30
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solenoid valve closed. Furthermore, the results of opening the door and window indicate that it could further reduce the R290 concentration and improve the safety of R290 STHAC, enhancing the ventilation in the room would help to accelerate the dispersion of leaked R290 and alleviate its flammable risks. Potential ignition sources, such as switches and other electrical components should be
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4)
avoided installed underneath the indoor unit of STHAC because it is the most probable
region where combustible zone appears. The huge sound accompanied with the leakage
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might be the judgment whether the flammable risk appeared in the room. The
combustible zone might appear in the room if the huge sound could be heard by the
Acknowledgements:
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consumers; and if not, it is not necessary to worry about the fire risk in the room.
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This present study was supported by the Foreign Economic Cooperation Office, Ministry
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of Environmental Protection of China.
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References
ANSI/ASHRAE Standard. Designation and safety classification of refrigerants, 2013.
AC
EN 378: 2008 Refrigerating systems and heat pumps. Safety and environmental requirements, 2008. Blackwell, N., Bendixen, L., Birgfeld, E., 2004. Risk Assessment of HFC 152a and Carbon Dioxide Mobile Air Conditioning Systems, Proc. Earth Technologies Forum, Washington, USA. Cai, D., He, G., Yokoyama, T., Tian, Q., Yang, X., Pan, J., 2015. Simulation and comparison of leakage characteristics of R290 in rolling piston type rotary compressor. International Journal of Refrigeration 53, 42-54. Calm, J.M., 2008. The next generation of refrigerants – Historical review, considerations, and outlook. International Journal of Refrigeration 31, 1123-1133. Cheong, K.W., Riffat, S.B., 1996. Monitoring hydrofluorocarbon refrigerant leakage from air-conditioning systems in buildings. Applied Energy 53, 341-347. Colbourne, D., Espersen, L., 2013. Quantitative risk assessment of R290 in ice cream cabinets.
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International Journal of Refrigeration 36, 1208-1219. Colbourne, D., Suen, K.O., 2015. Comparative evaluation of risk of a split air conditioner and refrigerator using hydrocarbon refrigerants. International Journal of Refrigeration 59, 295-303. Corberán, J.M., Segurado, J., Colbourne, D., Gonzálvez, J., 2008. Review of standards for the use of hydrocarbon refrigerants in A/C, heat pump and refrigeration equipment. International Journal of Refrigeration 31, 748-756. Eckhoff, R.K., Ngo, M., Olsen, W., 2010. On the minimum ignition energy (MIE) for propane/air. Journal of Hazardous Materials 175, 293-297. Gigiel, A., 2004. Safety testing of domestic refrigerators using flammable refrigerants. International
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Journal of Refrigeration 27, 621-628.
Höglund-Isaksson, L., Purohit, P., Amann, M., Bertok, I., Rafaj, P., Schöpp, W., Borken-Kleefeld, J., 2017. Cost estimates of the Kigali Amendment to phase-down hydrofluorocarbons. Environmental Science & Policy 75, 138-147.
Jia, L., Jin, W., Zhang, Y., 2017. Experimental study on R32 leakage and diffusion characteristic of wall-mounted air conditioners under different operating conditions. Applied Energy 185, Part 2,
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2127-2133.
Khan, F.I., Abbasi, S., 1999. Modelling and control of the dispersion of hazardous heavy gases. Journal of loss prevention in the process industries 12, 235-244.
Khan, F.I., Abbasi, S., 2000. Modelling and simulation of heavy gas dispersion on the basis of modifications in plume path theory. Journal of hazardous materials 80, 15-30.
Kuchta, J.M., 1986. Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries: A Manual. Technical Report Archive & Image Library 680. Journal of Refrigeration 40, 380-389.
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Li, T., 2014. Indoor leakage test for safety of R-290 split type room air conditioner. International Nagaosa, R.S., 2014. A new numerical formulation of gas leakage and spread into a residential space in
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terms of hazard analysis. Journal of Hazardous Materials 271, 266-274. Ooms, G., 1972. A new method for the calculation of the plume path of gases emitted by a stack. Atmospheric Environment (1967) 6, 899-909.
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Ooms, G., Mahieu, A., Zelis, F., 1974. The plume path of vent gases heavier than air, First International-Symposium on Loss Prevention and Safety Promotion and Safety Promotion in the
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Process Industries, The Hague.
Padalkar, A.S., Mali, K.V., Devotta, S., 2014. Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22. Applied Thermal
AC
Engineering 62, 277-284. Tang, W., He, G., Cai, D., Zhu, Y., Zhang, A., Tian, Q., 2017. The experimental investigation of refrigerant distribution and leaking characteristics of R290 in split type household air conditioner. Applied Thermal Engineering 115, 72-80. Tian, Q., Cai, D., Ren, L., Tang, W., Xie, Y., He, G., Liu, F., 2015. An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners. International Journal of Refrigeration 57, 216-228. Wang, Z., Fang, X., Li, L., Bie, P., Li, Z., Hu, J., Zhang, B., Zhang, J., 2016. Historical and projected emissions of HCFC-22 and HFC-410A from China's room air conditioning sector. Atmospheric Environment 132, 30-35. Wu, J.H., Yang, L.D., Hou, J., 2012. Experimental performance study of a small wall room air
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conditioner retrofitted with R290 and R1270. International Journal of Refrigeration 35, 1860-1868. Yang, Z., Wu, X., 2013. Retrofits and options for the alternatives to HCFC-22. Energy 59, 1-21. Zhang, W., Yang, Z., Li, J., Ren, C.-x., Lv, D., Wang, J., Zhang, X., Wu, W., 2013. Research on the flammability hazards of an air conditioner using refrigerant R-290. International Journal of Refrigeration 36, 1483-1494. Zhang, W., Yang, Z., Zhang, X., Lv, D., Jiang, N., 2016. Experimental research on the explosion characteristics in the indoor and outdoor units of a split air conditioner using the R290 refrigerant.
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International Journal of Refrigeration 67, 408-417.
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Table 1- Tube length and diameter in the 1P STHAC Heat Exchanges Evaporator (In indoor unit)
Tube length (mm)
695
600
Tube inner diameter (mm)
4.37
6.56
Tube spacing (mm)
19.5
21
Number of tube row
2
2
Numbers of tubes
52
28
Inner volume (cm3)
566
599
Liquid pipe (From outdoor to
Gas pipe (From indoor to
indoor unit)
outdoor unit)
3
3
Length (m) Tube inner diameter (mm)
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Connect pipes
4.37
3
Inner volume (cm )
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Condenser (In outdoor unit)
8.02
45.0
151
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Remark: the inner volumes were determined by calculation
Name
Type
Specifications
Honeywell (Searchpoint optima plus)
Range: 0-100% LFL
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Indoor gas detector
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Table 2- Apparatus specifications
Accuracy: ±1% LFL
Agilent
20 channels
instrument
(34970A with 34901A module)
Speed: 60 chs /s
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Data acquisition
Table 3- Indoor leakage tests conditions and results Solenoid valve Off
On
Charge
Leak
quantity/g
hole/mm
Window and door Open
Close
Combustible
Sensor NO. of
Sensors NO. of
zone’s residence
maximum time
exceeding LFL
time /s *
260
4.37
*
38
D
A, B, C, D
*
300
4.37
*
41
D
A, B, C, D
34
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*
54
D
A, B, C, D, E
*
260
4.37
*
110
B
A, B, C, D, E
*
300
4.37
*
115
D
A, B, C, D, E
*
350
4.37
*
145
B
A, B, C, D, E
*
260
1.0
*
28
D
B, D
*
300
1.0
*
68
B
B, D
*
350
1.0
*
85
B
B, D
*
260
1.0
*
161
B
B, D, E
*
300
1.0
*
178
B
B, D, E
*
350
1.0
*
186
*
260
0.5
*
*
300
0.5
*
*
350
0.5
*
*
260
0.5
*
53
*
300
0.5
*
82
*
350
0.5
*
99
*
260
1.0
*
*
350
1.0
*
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350
64
AC
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PT
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M
*
35
B
A, B, C, D, E
B
B, D
D
B, D
B
B, D
B
B, D
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Fig. 1- Schematic of the Plume model
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Fig. 2- Schematic diagram of the indoor leakage experimental apparatus
Fig. 3- Locations of indoor leakage experimental apparatus
36
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Fig. 4- Photograph of the replaceable copper tubes with different leakage hole
37
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110
fast-leaking buffering
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100 90
A C E
The process of R290 concentration falling and rising again
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80 70 60
diffusing
B D
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Concentration/ LFL%
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Fig. 5- Photograph of the experimental apparatus
50
AC
40 30 20 10
0 0
200
400
600
800
1000
1200
1400
1600
1800
Time/s Fig. 6- Concentration distribution results of 260 g charge quantity and 0.5 mm leakage
38
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hole
110
fast-leaking buffering
diffusing
100
70 60 50
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Concentration/ LFL%
80
B D
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A C E
90
40 30 20
0 0
200
400
M
10
600
800
1000
1200
1400
1600
1800
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Time/s
hole
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Fig. 7- Concentration distribution results of 300 g charge quantity and 0.5 mm leakage
39
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110
fast-leaking buffering
diffusing
100
A C E
80 70
B D
60 50
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Concentration/ LFL%
90
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 8- Concentration distribution results of 350 g charge quantity and 0.5 mm leakage
AC
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hole
40
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100
260g, 0.5mm 300g, 0.5mm 350g, 0.5mm 300g, 1.0mm 300g, 4.37mm
90
70 60 50
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Concentration/ LFL%
80
40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 9- R290 concentration of detector B under different charge quantity and diameter of
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leakage hole
110
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fast-leaking buffering
100
PT
A C E
80 70
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Concentration/ LFL%
90
diffusing
B D
60
AC
50 40 30 20 10
0 0
200
400
600
800
1000
Time/s 41
1200
1400
1600
1800
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Fig. 10- Concentration distribution results of 350 g charge quantity and 1.0 mm leakage hole
110
fast-leaking
buffering
diffusing
100
A C E
80 70 60 50 40 30 20 10 0 0
200
400
600
B D
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Concentration/ LFL%
90
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The process of R290 concentration falling and rising again
800
1000
1200
1400
1600
1800
M
Time/s
Fig. 11- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with
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solenoid valve closed
42
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110
A C E
100 90
Concentration/ LFL%
80
B D
70 60
fast-leaking
buffering
diffusing
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50 40 30 20
0 0
200
400
600
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10
800
1000
1200
1400
1600
1800
Time/s
Fig. 12- Concentration distribution results of 260 g charge quantity, 0.5 mm leakage hole with
110
fast-leaking
100
buffering and diffusing
PT
A C E
80 70
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Concentration/ LFL%
90
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solenoid valve closed
B D
60
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50 40 30 20 10
0 0
200
400
600
800
1000
Time/s 43
1200
1400
1600
1800
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Fig. 13- Concentration distribution results of 350 g charge quantity, 1.0 mm leakage hole with solenoid valve closed and window and door opening
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200
0.5 mm hole with soleniod valve opening 0.5 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve opening 1.0 mm hole with soleniod valve closed 4.37 mm hole with soleniod valve opening 4.37 mm hole with soleniod valve closed 1.0 mm hole with soleniod valve closed; window and door opening
160
120
80
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Combustible zone’s residence time /s
240
40
0 260
300
350
M
Charge quantity /g
conditions
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PT
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Fig. 14- Relationship of the combustible zone’s residence time with different experimental
44