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The experimental investigation of refrigerant distribution and leaking characteristics of R290 in split type household air conditioner
Applied Thermal Engineering 115 (2017) 72–80
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Research Paper
The experimental investigation of refrigerant distribution and leaking characteristics of R290 in split type household air conditioner Weier Tang, Guogeng He ⇑, Dehua Cai, Yihao Zhu, Aoni Zhang, Qiqi Tian School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s A new quasi-liquid nitrogen method (QLNM) was proposed and firstly applied in experiments. The R290 distribution was investigated by QLNM and the results proved the validation. A solenoid valve was proposed to install near the capillary in STHAC in order to reduce risk factor. R290 leaking rate was firstly measured by QLNM before and after the installation a solenoid valve.
a r t i c l e
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Article history: Received 5 May 2016 Revised 20 November 2016 Accepted 20 December 2016 Available online 22 December 2016 Keywords: R290 STHAC Refrigerant distribution Leaking rate Building safety
a b s t r a c t As a high-profile replacement for R22 split type household air conditioner (STHAC), R290 has several advantages in terms of thermodynamic properties, environmental characteristics, and cost. However, the obvious shortcoming of R290 is its flammability, which has a potential fire risk to the building. At present, the most important measure to ensure the safety of a R290 STHAC is to limit the refrigerant charge by domestic and international standards. But in fact, when the leakage of R290 occur from a STHAC, the distribution of R290 in STHAC, and the leaking rate also will seriously affect the safety of an R290 STHAC. In this study, a new quasi-liquid nitrogen method (QLNM) has been proposed in order to investigate the refrigerant distribution in R290 STHACs and the leaking rate under various conditions, and the experiments have been conducted. The experimental results of distribution proved the validation of the QLNM and showed that a large portion of the refrigerant distributed in the condenser when the air conditioner is on running stage and the refrigerant will migrate from the condenser to the evaporator when the air conditioner is on closed stage. Based on this, the installation of a solenoid valve near the capillary has been proposed. The comparison of experimental results of R290 leaking rate before and after the installation of a solenoid valve showed it will obviously reduce the leaking rate and thereby improve the safety of the R290 STHACs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The split type household air conditioner (STHAC) is an important type of household air conditioner and has been widely used all over the world for the air conditioning of homes, offices, etc. [1]. Especially in developing countries such as China, India, Brazil, etc., with its tremendous sales, the STHAC has become an important household appliance. In the past few decades, conventional refrigerants such as R22 have predominantly been used in STHACs, chest freezer and heat pumps owing to their excellent thermodynamic properties and safety features such as being nontoxic, non-flammable, and nonexplosive [2]. However, R22 still contains ⇑ Corresponding author. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.applthermaleng.2016.12.083 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
chlorine and causes environmental problems in spite of its lower ozone depletion potential (ODP) value [3]. The Montreal Protocol on Substances that Deplete the Ozone Layer has decided to phase out R22 as well as other HCFCs which are harmful to the ozone layer before 2030 for developing countries [4]. Now many developed countries such as USA, EU and Japan have used R410a as the alternative for R22 in the STHACs. R410a is a mixture of HFCs and does no harm to the ozone layer. But R410a has a higher global warming potential (GWP = 2020) than R22, it makes R410a facing elimination in the long-term use since global warming has attracted the worldwide attention [5]. Thus for the alternative of HCFCs in STHACs, refrigeration field must not only focus on the technical problems, but also to address the GWP criterion. Compared with the R410a, R290 offer zero ODP and negligible GWP [6]. Besides, as a natural refrigerant, R290 do not cause any harm
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Nomenclature STHAC HCFC HFC HC GWP ODP LFL LNM OMM QLNM G
split type household air conditioner hydrochlorofluorocarbon hydrofluorocarbon hydrocarbon global warming potential ozone depletion potential Lower Flammability Limit, kg m3 liquid nitrogen method online measurement method quasi liquid nitrogen method weight of the refrigerant
to the environment and can achieve a higher cooling capacity and COP than R22. From the perspective of long-term environmental safety, the use of HCs is a practical choice. Propane (R290), as a representative HC refrigerant, offers higher efficiency, better heat transfer in heat exchangers, lower refrigerant charge quantity [7], and lower compressor discharge temperatures [8]. It has been used in heat pumps [9], organic Rankine cycle [10], and he application of R290 in large capacity chest freezer was also discussed [11]. The Chinese air-conditioning system manufacturer Gree Electric Appliances Inc., one of the world’s largest manufacturers of STHACs, has started pilot production of STHACs using R290 in 2009 [12]. However, owing to R290’s flammability and explosiveness, when used in STHACs, it has a potential fire risk resulting from leakage, especially from indoor units. Thus, the safety of the STHACs has a great influence on the safety of buildings. The most common approach to mitigating the flammability risk is restricting the maximum charge of refrigerant. For example, criteria such as EN 378 and IEC 60335-2-40 [13] include the requirement of limiting the quantity of refrigerant used in STHACs (typically with respect to room size). Recently, several researchers have carried out studies on the fire hazards from flammable refrigerants. D. Colbourne [14] assessed the flammability risk of HCs within horizontal type ice cream cabinets. Tingxun Li [15] and Wang Zhang [16] analyzed the impact of indoor leakage of R290 on room safety. Li assessed different leakage situations and found that the maximum concentration seldom exceeded two-thirds of the LFL, and the leaking rate has a major influence on the refrigerant concentration distribution. Zhang researched the flammability hazards of R290 STHAC and found that the flammable range is limited to the space close to the indoor unit. However, the researchers in the aforementioned studies focused on the safety issues in the case of refrigerant leakage, and provided suggestions to reduce the R290 fire hazards based on their results. They did not study to reduce the refrigerant mass leakage in the system, or reduce the leaking rate to enhance the safety of R290 application. Although the charge amount of R290 in STHACs is one key factor on the safety of R290 and the domestic standards and international standards all have given clear limit, in fact, the different leaking rate with same amount charge of R290 also will result in various degrees of safe problem for R290 STHACs. For example, comparing the 300 g R290 leak out in 4 min and the 300 g R290 leak out in 10 min, the concentration of R290 in indoor for the later will be lower than that for the former and the R290 leaked from the later will have much time to diffuse. Moreover, a lower R290 leaking rate can also improve room safety. Thus, a further study to analyze the refrigerant mass distribution in STHACs by measuring the refrigerant distribution in the various components of an R290 STHAC and an experiment on R290 leaking rate from a STHAC can provide support in the design of safety measures in the R290 STHAC.
Fn F V1 SLS SHS
test weight of the refrigerant from the balance buoyancy force of the air volume of plastic bag. system low-pressure side system high-pressure side
Greek symbols qr density of R290 qa density of air
In this paper, 1P and 1.5P STHACs have been investigated. The refrigerant distribution regularities during the running and closed stage of the STHACs were investigated through experiments by QLNM. Based on this, the installation of a solenoid valve near the capillary has been proposed. Furthermore, the experiments of R290 leaking rate before and after the installation of a solenoid valve under different situations have been conducted. 2. Quasi liquid nitrogen method (QLNM) for measuring refrigerant mass distribution 2.1. Analysis of existing methods for measuring refrigerant mass The famous methods for measuring refrigerant mass in components of a refrigeration system are liquid nitrogen method (LNM) [17,18] and online measurement method (OMM). LNM, in which the cylinder is weighed after the refrigerant is drawn from the component into the sampling cylinder by the low temperature environment generated by liquid nitrogen, has the advantage of high accuracy. OMM, in which the components are placed on the balance and weighed directly, is convenient but less accurate. The measurement error while using LNM is 0.07–0.11% and that of OMM is 3–8% [19]. Furthermore, the maximal uncertainty of OMM may exceed 10% because of the refrigerant flow, fan vibration, and hosepipe support force. As mentioned earlier, the experiment on R290 distribution and leaking rate is to be carried out. However, LNM cannot measure the actual leaking rate because the vacuum environment in the sampling cylinder caused by liquid nitrogen will increase the leaking rate. Moreover, the test system of LNM is very complicated and the liquid nitrogen (which is dangerous for experimenters) is needed during the test process. OMM can test the leaking rate, however, the refrigerant flow and hosepipe support force may increase the error to exceed 10%. Therefore, a method called quasi-liquid nitrogen method (QLNM) was proposed, which is expected to be more convenient than LNM and more accurate than OMM, especially can measure the leaking rate in real-time. Thus, QLNM can overcome the shortcomings of LNM and OMM during leaking rate tests. 2.2. The principle of QLNM In QLNM, the refrigerant flows into a large plastic bag because of the higher pressure in the components. However, the actual mass of the refrigerant is different from the measurement result because of air buoyancy. The actual refrigerant mass in the plastic bag based on the Archimedes buoyancy formula is given by Eqs. (1) and (2).
G ¼ qr V1 g ¼ Fn þ F ¼ Fn þ qa V1 g
ð1Þ
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Table 1 The data of verification QLNM experiment. M1 (g)
M2 (g)
M3 (g)
M4 (g)
M1-M2 (g)
2.83 (M4-M3) (g)
e (%)
4943.5 4869.6
4869.8 4807.5
26.6 27.2
53.0 49.3
73.7 62.1
74.7 62.5
1.36 0.64
G G qr V1 g qr ¼ ¼ ¼ Fn G F qr V1 g qa V1 g qr qa
ð2Þ
where G is the actual weight of the refrigerant, Fn is the weight of the refrigerant measured by using the balance, F is the buoyancy force by the air, and V1 is the volume of the plastic bag. Moreover, qr is the density of R290 and qa is the density of air. Fn can be read from the balance. The ambient temperature of the experimental situation is 32 °C, and the atmospheric pressure is 1 atm. Thus, qr = 1.7879 kg m3 and qa = 1.1568 kg m3, 1:7879 G ¼ 1:78791:1568 Fn ¼ 2:83Fn. In order to investigate the error brought in by QLNM, the refrigerant was released from the refrigerant container into the plastic bag. The initial weight of the refrigerant container and the final weight of the plastic bag were measured by a precision electronic balance with a 15 kg range and ±0.1 g precision. The deviation of QLNM is presented by Table 1. In Table 1, M1 and M2 are the initial and final weights of the refrigerant container, respectively, and M3 and M4 are the initial and final weights of the plastic bag, respectively. Thus, M4 is the weight of the plastic bag containing the refrigerant. In Table 1, the deviation of QLNM is shown to be less than 2%, demonstrating that QLNM is reliable. In this experiment, there were refrigerant residues in the components because the pressure of the refrigerant caused it to flow into the plastic bag. The process continued until the refrigerant pressure in the component was equal to that in the plastic bag. The experimental uncertainties will be analyzed in the following chapters. 3. Experimental apparatus and procedures 3.1. Experimental apparatus The widely used 1P and 1.5P STHACs were used in the experiment. As shown in Fig. 1, STHAC system was reconstructed as
follows. (1) The compressor was replaced with a R290 compressor and the condenser tube was replaced with diameter 5 mm—as researchers have used the 5 mm tube for reducing the heat exchanger’s inner volume to reduce the charge quantity. (2) Solenoid valve and stop valves were installed on the two sides of the condenser, evaporator, and accumulator. The diameter of the valves should be the same as those of the tubes to reduce pressure drop across the valves. (3) The capillary, hand valve D, and solenoid valves were installed on the delivery tube because of the limitations of size set by the STHAC’s structure. The actual experimental apparatus is shown as Fig. 2. The solenoid valves I, II, III, and IV and hand valves A, B, and C were installed in the experimental system. The system was divided into five sections (evaporator, vapor delivery tube, liquid delivery tube, condenser, and accumulator) by these valves. The accumulator section referred to the section between solenoid valve I and the compressor including the compressor’s low-pressure side. The condenser section was the section between the compressor and the solenoid valve II including the compressor’s high-pressure side. During the experiment, by adjusting the switches of solenoid valves and hand valves, the refrigerant quantity in each section was measured by using QLNM. 3.2. Experimental procedure When QLNM is applied for measuring refrigerant distribution, the operational steps are as follows: (1) Run the STHAC and ensure it is at the test condition. (2) Weigh the empty plastic bag (i.e., measure M3). (3) Connect the plastic bag and the STHAC with a soft pipe, open hand valve and let the refrigerant in the component flow into the plastic bag until the pressure is stable. Then, weigh the plastic bag (i.e., measure M4). The refrigerant mass in the component is given by 2.83 ⁄ (M4 M3).
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. The actual experimental apparatus.
Table 2 The running stage results. 1P machine Charge/g Evaporator/g Vapor delivery tube/g Liquid delivery tube/g Condenser/g Accumulator/g The experimental total weight/g Deviation/%
260 64.8 0.9 24.9 104.7 59.1 254.4 2.15
1.5P machine 300 92.1 0.9 21.6 117.9 60.9 293.4 2.19
350 120 1.2 25.5 128.1 66.6 341.4 2.46
300 195 1.2 2.1 20.7 66.9 285.9 4.71
350 242.7 0.9 2.4 21.0 69.9 336.9 3.74
300 66.6 0.9 28.2 122.4 71.7 289.8 3.41
350 77.4 0.9 31.2 150.0 80.7 340.2 2.80
400 74.7 0.9 30.6 191.4 91.5 389.1 2.73
350 243.9 2.4 3.9 27 66.3 343.5 1.86
400 284.7 2.4 2.7 26.7 77.1 393.6 1.60
Table 3 The closed stage results. 1P machine Charge/g Evaporator/g Vapor delivery tube/g Liquid delivery tube/g Condenser/g Accumulator/g The experimental total weight/g Deviation/%
260 159.3 1.8 2.1 20.4 66 249.6 4.00
1.5P machine
(4) Repeat steps 2–3 for the testing of each component of the STHAC. The experimental procedure is to be followed with care to prevent the functional failure of the solenoid valves—as they are normally closed one-direction valves—and to ensure that the pressure across the solenoid valves does not exceed the critical pressure. The experimental procedures were carried out in the following sequence, and all the valves were closed after each step. a. Close all valves including the two stop valves. Then, open hand valve C to measure the refrigerant quantity in the evaporator section. b. Open solenoid valve III and hand valve C to measure the refrigerant quantity in the vapor delivery tube. c. Open the solenoid valve IV and hand valve C to measure the refrigerant quantity in the liquid delivery tube. d. Open hand valve A to measure the refrigerant quantity in the condenser section. e. Finally, open hand valve B to measure the refrigerant quantity in the accumulator section.
300 183.9 1.5 2.4 27.9 68.4 284.1 5.29
It should be noted that in order to accurately simulate a domestic environment and explore the distribution regularities, the STHAC was installed in a room and not in an enthalpy-difference room. The ambient temperature was 31–32 °C and the room temperature was 26 °C while STHAC was stable. The test conditions required the STHAC to be brought to a stable operation state and the temperature in the room to be kept constant. 4. Results and discussion The STHAC was started and operated in the cooling mode until it was operating stably. Then, the STHAC and all solenoid valves were simultaneously shut off. The refrigerant quantity was measured using the procedures a–e mentioned earlier. The results are shown in Table 2. The experimental error, e, can be estimated by Eqs. (3) and (4).
e ¼ eQLNM þ 2:83 em þ ee em ¼
dðM 4 M 3 Þ dM4 þ dM 3 ¼ M4 M3 M4 M3
ð3Þ ð4Þ
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Fig. 3. R290 mass distribution in components affected by charge of 1P machine running stage.
Fig. 4. R290 mass distribution in components affected by charge of 1.5P machine running stage.
where eQLNM is the error brought in by the QLNM, em is the measurement error. Furthermore, ee is the error brought by the residue and other experimental factors, which means the difference of charged and collected refrigerant mass. As shown in Table 1, eQLNM is 0.64–1.36%. A precision electronic balance with 15 kg range and ±0.1 g precision was used to measure the weight of the plastic bag. Thus, the error brought in by measurement, em , is 0.07–0.2%. ee , as calculated using the figures in Tables 2 and 3 is 1.60–5.29%. Therefore, the experimental error is 2.44–7.22% and the mean error is 4.46%. By comparing the ee to the results of LNM on R290 distribution [20], it can be observed that the deviation (1.60 to 5.29%) of QLNM is slightly more than that (0.64 to 2.31%) of LNM. Therefore, QLNM is validation in measuring the distribution of R290 in STHACs.
charge, the heat transfer area of condenser was redundant, and the supply refrigerant will accrue in the condenser by condensing into the liquid form. Hence, as shown in Table 2, by comparing the 1P machine with the 1.5P machine at equal charges of 300 g and 350 g, it can be concluded that the evaporator held a higher quantity of R290 in the 1P machine, which was excessive charge, while the condenser held a higher quantity of R290 in the 1.5P machine, which was insufficient charge. Thus, reducing the R290 charge quantity is an effective method to improve STHAC safety because the refrigerant will not accumulate in the evaporator when STHAC is insufficient charge.
4.1. The influence of refrigerant charge quantity From Table 2, it can be concluded that a large portion of the refrigerant is distributed in the heat exchanger (accounts for approximately 65–72%). It should be noted that the accumulator held another large portion of the R290 distribution because the R290 distributed in the accumulator is in the liquid form. As can be observed from Fig. 3, when the refrigerant charge quantity in the 1P machine is increased, the R290 distribution in the condenser and evaporator follows a linear relationship with the increase in charge. On the other hand, the distributions in the other components are almost constant regardless of the increasing R290 charge quantity. A possible explanation for this is that the pressures in those components have reached their respective saturation points owing to their internal volume limits. Moreover, it can be observed from Fig. 4 that the quantity of R290 in the evaporator changed little in the 1.5P machine. This outcome is expected since the internal volume of 1.5P machine is larger than that of the 1P machine. This indicates that while 1P machine is excessive charge, 1.5P machine may be insufficient charge by the same charge quantity. When the system was excessive charge, because of the limitation of the evaporator heat transfer area, the refrigerant in the evaporator could not obtain enough heat and therefore, was in vapor-liquid two-phase at the outlet of the evaporator. Thus, the excess refrigerant will accrue in the evaporator in the liquid form. Conversely, if the system was insufficient
4.2. The influence of STHAC working state In order to investigate the regularities of refrigerant distribution in the STHAC in closed stage, the STHAC was started and operated on cooling mode until it attained stability. Then, the STHAC was shut off while all valves were kept open. The valves were closed after 1.5 h. Then, the refrigerant quantity was measured by QLNM following the procedures a–e. The results are shown in Table 3. Figs. 5 and 6 show the R290 mass distribution affected by charge quantity during the closed stage. As these results differ from that of the running stage, it shows that the quantity of refrigerant in the evaporator is higher (accounts for approximately 60%– 70% of the charge), whilst the quantity of refrigerant in the condenser is lower and the accumulator is still occupied another large portion of R290. It is clear that in both 1P machine and 1.5P machine, the evaporator holds the largest portion of the refrigerant, whilst the quantity of refrigerant in the other components remains constant. The results indicate that the refrigerant migrates from the condenser to the evaporator after the STHAC is shut down. This outcome is expected since the pressure in the condenser is higher than that in the evaporator when the STHAC is shut down. Hence, the refrigerant migrates from condenser to evaporator until the equilibrium pressure is reached. The phenomenon of refrigerant migration after STHAC shutdown raises safety concerns since the STHAC closed stage occupied most of time in the room. R290 will gather in the indoor unit after the STHAC cooling mode powered off. On the other hand, we can infer that in the heating mode, the potential indoor fire risk of
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As shown in Figs. 7 and 8, by comparing the results of SLS during running and closed stage, it was observed that the refrigerant distribution changed from 50–60% to 85–90%. This indicates that approximately 30% of charge quantity migrates to the SLS in closed stage. Moreover, the R290 quantity in SHS during closed stage is approximately constant and is not affected by charge because the pressure within the system was only affected by the environment temperature while reach the equilibrium. Thus, the quantity in SHS is constant at equivalent pressure and condenser structure. It can be speculated that if a solenoid valve is installed close to the capillary, the migration of 30% of the refrigerant charge would be cut off when the STHAC was shut down. This is equivalent to reducing the charge quantity by 30%, and will enhance the R290 STHAC safety. 5. Experiment to determine leaking characteristics
Fig. 5. R290 mass distribution in components during closed stage of 1P split type air-conditioner.
To verify the feasibility of the solenoid valve, several R290 leaking rate experiments were carried out, whereby the R290 in the 1P STHAC machine under steady cooling condition was released through the leak hole. Thereafter, the leaked refrigerant mass was measured by QLNM. 5.1. Leaking experimental apparatus and procedures
STHAC will be reduced because R290 will gather in the outdoor unit. 4.3. The comparing of system high-pressure and low-pressure sides It was found that the refrigerant migrates from condenser to evaporator because of the pressure difference, which is caused by capillary tube. Therefore, in order to investigate the variation of refrigerant distribution on both sides of the capillary tube, the system was divided into high-pressure and low-pressure sides. The system high-pressure side contains the condenser section; the low-pressure side contains the vapor delivery tube, evaporator, liquid delivery tube, and accumulator sections. The refrigerant distribution in system high-pressure (SHS) and low-pressure side (SLS) of 1P and 1.5P machines are shown in Figs. 7 and 8.
Fig. 6. R290 mass distribution in components during closed stage of 1.5P split type air-conditioner.
During the leakage experiment, a camera was positioned close to the balance to record the weighing results. The refrigerant charge was 260 g because the plastic bag could not carry a quantity of gaseous R290 that was equal to or higher than 300 g. Solenoid valve II was closed and hand valve C was opened after the machine was brought to stable cooling mode. QLNM was used to measure the refrigerant leakage quantity. When QLNM is applied for measuring the leaking rate, the operational steps are as follows: (1) Run the STHAC and bring it to the test condition. (2) Weigh the empty plastic bag (i.e., measure M3). (3) Connect the plastic bag and the STHAC with soft pipe; shut down the STHAC and close the solenoid valve II while other solenoid and stop valves keep opening.
Fig. 7. R290 mass distribution in high and low pressure side affected by charge of 1P machine.
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Fig. 9. R290 leaking rate affected by different diameter with solenoid valve II closed. Fig. 8. R290 mass distribution in high and low pressure side affected by charge of 1.5P machine.
(4) Open hand valve C, making the refrigerant in the component flow into the plastic bag. Simultaneously weigh the plastic bag online as M4 and record using the camera. The leakage quantity is 2.83 ⁄ (M4 M3). (5) Using the camera recordings, leaking rate can be calculated by the relationship of leakage quantity and time. Leak holes of different diameters were drilled as part of the experiment to simulate different conditions of R290 leakage in STHACs. For example, a 7 mm external diameter with 6.3 mm internal diameter tube was used to simulate the situation of evaporator tube whole rupture. As mentioned earlier, the condenser tube was replaced with 5 mm external diameter and 4.37 mm internal diameter, thus the 4.37 mm diameter hole was used to simulate the situation of condenser tube whole rupture. Holes of other diameters were drilled to simulate the tube tiny rupture situation, and explore the regularities with different leakage hole diameters.
in the outdoor unit increases until it exceeds the valve’s critical pressure. In addition, the refrigerant in the accumulator leaks out. Therefore, after 240 s, the leaking rate increases, although to a value as high as that in the first phase. Finally, the leakage process ends after 1200 s because the leaking rate was stable. Furthermore, the result shows that for a larger leak hole, the leaking rate is higher. Besides, the results of the leak holes with 1 mm and 0.5 mm diameters show an absence of the three phases because of the small diameter. The results under the condition where solenoid valve II and hand valve C were open are shown in Fig. 10. In Fig. 10, it can be observed that the refrigerant in the system leaks out at a high rate. The high pressure in the system causes the liquid refrigerant to flow directly into the plastic bag. Then, the water vapor in the air would form frost on the outside of the plastic bag. This explains why the curves of the 1 mm and 7 mm diameter holes indicate that the leakage mass exceeds the charge amount. This proves that the QLNM is unsuitable for online measurement
5.2. Leaking experimental results Several leaking rate experiments with solenoid valve II kept closed were carried out and a summary of the results is shown in Fig. 9. Unlike the assumption that the leakage mass is linear with respect to the time, the results of 2 mm, 4.37 mm, and 6.3 mm diameter holes show that there are three phases in the leakage process. Initially, approximately 100 g of refrigerant leaked at a high rate in the first 60 s. According to the operational state results, the refrigerant mass in the evaporator and liquid delivery tube are 64.8 g and 24.9 g, and their sum is approximately 90 g. In the first phase, the refrigerant in the evaporator and liquid delivery tube leaked accompanied by a huge sound created by the refrigerant flowing through the pipeline. Then, there is refrigerant leakage at a lower rate from 60 s to 240 s because the pressure across solenoid valve II exceeds the critical pressure. After the indoor unit leakage, the difference between the refrigerant pressure of the outdoor unit and that of the indoor unit exceeds solenoid valve II’s critical pressure. Therefore, the leakage took place at a low rate. A certain quantity of liquid refrigerant remained in the condenser, and it evaporated from liquid to gas. Then, the pressure
Fig. 10. R290 leaking rate affected by different diameter with solenoid valve II opened.
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Fig. 11. R290 leaking rate of 0.5 mm diameter with solenoid valve II opened and closed.
of high leaking rates because the frost causes error in the QLNM. However, from Fig. 10, it can be found that the refrigerant leaking rate is high and it does not reduce when the solenoid valve II is opened. Furthermore, the results of the 0.5 mm diameter hole reveal an absence of a high leaking rate. The results of the 0.5 mm diameter hole with solenoid valve II kept both opened and closed is shown in Fig. 11. In Fig. 11, compared with the 0.5 mm diameter hole, the leaking rate with solenoid valve II kept open is higher than that with the valve kept closed. Thus, it can be argued that installing a solenoid valve installed near the capillary can improve the safety of R290 STHACs. However, the actual effect needs to be verified through further studies. 6. Conclusions R290 is one of the most competitive alternative of R22 in split type household air conditioner (STHAC) due to several advantages in terms of thermodynamic properties, environmental characteristics, and cost. However, the obvious shortcoming of R290 is its flammability. This paper proposed an a new quasi-liquid nitrogen method (QLNM) in order to investigate the refrigerant distribution in R290 STHACs and the leaking rate under various conditions, and the various experiments have been conducted. The conclusions are as follows: (1) QLNM was used to measure the R290 distribution and the maximal deviation of QLNM was 5.29%. The deviation of QLNM is slightly more than that of LNM. This proves the validation of QLNM. (2) The results of the R290 distribution experiments by using QLNM show that a large portion of the refrigerant mass is distributed in the heat exchanger (especially in the condenser) when the STHAC is running in cooling mode. With the increase of the refrigerant charge, the refrigerant mass in the heat exchangers increases while those in the other components remain almost constant, and the evaporator will hold more refrigerant when the STHAC is excessive charge, the condenser will hold more charge refrigerant when the STHAC is insufficient charge. (3) The refrigerant will migrate from the high-pressure side to the low-pressure side when the STHAC is on closed stage in the cooling mode until reaching the pressure balance of
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whole system. As a result, the quantity of refrigerant in the evaporator is greater than that in the other components, and the entire low-pressure side accounts for 85–92% of the total refrigerant charge. The refrigerant migration will increase the flammable risk while using R290 STHAC in buildings due to a large portion of the refrigerant in the indoor unit. On the contrary, the flammable risk can be reduced since in the heating mode, the refrigerant accumulates in the outdoor unit. (4) Based on the experimental results of the R290 distribution, a solenoid valve was proposed to install near the capillary in order to prevent the refrigerant migration from the highpressure side to the low-pressure side considering to reducing the refrigerant mass in indoor unit when the STHAC is on closed stage. (5) The experimental results of the leaking rate under various conditions before and after the installation a solenoid valve near the capillary in STHAC showed that the refrigerant leaking rates after the installation a solenoid valve near the capillary in STHAC are obviously less than that before the installation a solenoid valve. This means that the flammable risk brought by the R290 leaking from STHAC also will be effectively reduced and the safety of R290 STHACs will improve.
Acknowledgements This present study was supported by the Foreign Economic Cooperation Office, Ministry of Environmental Protection of China. References [1] K. Sumeru, S. Sulaimon, H. Nasution, F.N. Ani, Numerical and experimental study of an ejector as an expansion device in split-type air conditioner for energy savings, Energy Build. 79 (2014) 98–105. [2] S. Devotta, A.V. Waghmare, N.N. Sawant, B.M. Domkundwar, Alternatives to HCFC-22 for air conditioners, Appl. Therm. Eng. 21 (2001) 703–715. [3] B.O. Bolaji, Z. Huan, Ozone depletion and global warming: case for the use of natural refrigerant – a review, Renew. Sustain. Energy Rev. 18 (2013) 49–54. [4] J.M. Corberán, J. Segurado, D. Colbourne, J. Gonzálvez, Review of standards for the use of hydrocarbon refrigerants in A/C, heat pump and refrigeration equipment, Int. J. Refrig. 31 (2008) 748–756. [5] T.-P. Teng, H.-E. Mo, H. Lin, Y.-H. Tseng, R.-H. Liu, Y.-F. Long, Retrofit assessment of window air conditioner, Appl. Therm. Eng. 32 (2012) 100–107. [6] V. La Rocca, G. Panno, Experimental performance evaluation of a vapour compression refrigerating plant when replacing R22 with alternative refrigerants, Appl. Energy 88 (2011) 2809–2815. [7] A.S. Padalkar, K.V. Mali, S. Devotta, Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22, Appl. Therm. Eng. 62 (2014) 277–284. [8] S. Cheng, S. Wang, Z. Liu, Cycle performance of alternative refrigerants for domestic air-conditioning system based on a small finned tube heat exchanger, Appl. Therm. Eng. 64 (2014) 83–92. [9] J. Blanco Castro, J.F. Urchueguía, J.M. Corberán, J. Gonzálvez, Optimized design of a heat exchanger for an air-to-water reversible heat pump working with propane (R290) as refrigerant: modelling analysis and experimental observations, Appl. Therm. Eng. 25 (2005) 2450–2462. [10] H. Wang, J. Xu, X. Yang, Z. Miao, C. Yu, Organic Rankine cycle saves energy and reduces gas emissions for cement production, Energy 86 (2015) 59–73. [11] M.-G. He, X.-Z. Song, H. Liu, Y. Zhang, Application of natural refrigerant propane and propane/isobutane in large capacity chest freezer, Appl. Therm. Eng. 70 (2014) 732–736. [12] J. Karin, Propane replacing R-22, Ashrae J. 52 (2010) 74–76. [13] I.E. Commission, IEC 60335-2-40: 2013, Household and similar electrical appliances-Safety-part, 2–40. [14] D. Colbourne, L. Espersen, Quantitative risk assessment of R290 in ice cream cabinets, Int. J. Refrig. 36 (2013) 1208–1219. [15] T. Li, Indoor leakage test for safety of R-290 split type room air conditioner, Int. J. Refrig. 40 (2014) 380–389. [16] W. Zhang, Z. Yang, J. Li, C.-X. Ren, D. Lv, J. Wang, X. Zhang, W. Wu, Research on the flammability hazards of an air conditioner using refrigerant R-290, Int. J. Refrig. 36 (2013) 1483–1494. [17] W. Mulroy, D. Didion, Refrigerant migration in a split-unit air conditioner, ASHRAE Trans. 91 (1985) 193–206 (Discussion).
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[18] W.J. Mulroy, D. Didion, A Laboratory Investigation of Refrigerant Migration in a Split Unit Air Conditioner, US Department of Commerce, National Bureau of Standards, 1983. [19] G. Ding, X. Ma, P. Zhang, W. Han, S. Kasahara, T. Yamaguchi, Practical methods for measuring refrigerant mass distribution inside refrigeration system, Int. J. Refrig. 32 (2009) 327–334.
[20] T. Li, J. Lu, L. Chen, D. He, X. Qiu, H. Li, Z. Liu, Measurement of refrigerant mass distribution within a R290 split air conditioner, Int. J. Refrig. 57 (2015) 163– 172.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
The experimental investigation of refrigerant distribution and leaking characteristics of R290 in split type household air conditioner Weier Tang, Guogeng He ⇑, Dehua Cai, Yihao Zhu, Aoni Zhang, Qiqi Tian School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s A new quasi-liquid nitrogen method (QLNM) was proposed and firstly applied in experiments. The R290 distribution was investigated by QLNM and the results proved the validation. A solenoid valve was proposed to install near the capillary in STHAC in order to reduce risk factor. R290 leaking rate was firstly measured by QLNM before and after the installation a solenoid valve.
a r t i c l e
i n f o
Article history: Received 5 May 2016 Revised 20 November 2016 Accepted 20 December 2016 Available online 22 December 2016 Keywords: R290 STHAC Refrigerant distribution Leaking rate Building safety
a b s t r a c t As a high-profile replacement for R22 split type household air conditioner (STHAC), R290 has several advantages in terms of thermodynamic properties, environmental characteristics, and cost. However, the obvious shortcoming of R290 is its flammability, which has a potential fire risk to the building. At present, the most important measure to ensure the safety of a R290 STHAC is to limit the refrigerant charge by domestic and international standards. But in fact, when the leakage of R290 occur from a STHAC, the distribution of R290 in STHAC, and the leaking rate also will seriously affect the safety of an R290 STHAC. In this study, a new quasi-liquid nitrogen method (QLNM) has been proposed in order to investigate the refrigerant distribution in R290 STHACs and the leaking rate under various conditions, and the experiments have been conducted. The experimental results of distribution proved the validation of the QLNM and showed that a large portion of the refrigerant distributed in the condenser when the air conditioner is on running stage and the refrigerant will migrate from the condenser to the evaporator when the air conditioner is on closed stage. Based on this, the installation of a solenoid valve near the capillary has been proposed. The comparison of experimental results of R290 leaking rate before and after the installation of a solenoid valve showed it will obviously reduce the leaking rate and thereby improve the safety of the R290 STHACs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The split type household air conditioner (STHAC) is an important type of household air conditioner and has been widely used all over the world for the air conditioning of homes, offices, etc. [1]. Especially in developing countries such as China, India, Brazil, etc., with its tremendous sales, the STHAC has become an important household appliance. In the past few decades, conventional refrigerants such as R22 have predominantly been used in STHACs, chest freezer and heat pumps owing to their excellent thermodynamic properties and safety features such as being nontoxic, non-flammable, and nonexplosive [2]. However, R22 still contains ⇑ Corresponding author. E-mail address: [email protected] (G. He). http://dx.doi.org/10.1016/j.applthermaleng.2016.12.083 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
chlorine and causes environmental problems in spite of its lower ozone depletion potential (ODP) value [3]. The Montreal Protocol on Substances that Deplete the Ozone Layer has decided to phase out R22 as well as other HCFCs which are harmful to the ozone layer before 2030 for developing countries [4]. Now many developed countries such as USA, EU and Japan have used R410a as the alternative for R22 in the STHACs. R410a is a mixture of HFCs and does no harm to the ozone layer. But R410a has a higher global warming potential (GWP = 2020) than R22, it makes R410a facing elimination in the long-term use since global warming has attracted the worldwide attention [5]. Thus for the alternative of HCFCs in STHACs, refrigeration field must not only focus on the technical problems, but also to address the GWP criterion. Compared with the R410a, R290 offer zero ODP and negligible GWP [6]. Besides, as a natural refrigerant, R290 do not cause any harm
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Nomenclature STHAC HCFC HFC HC GWP ODP LFL LNM OMM QLNM G
split type household air conditioner hydrochlorofluorocarbon hydrofluorocarbon hydrocarbon global warming potential ozone depletion potential Lower Flammability Limit, kg m3 liquid nitrogen method online measurement method quasi liquid nitrogen method weight of the refrigerant
to the environment and can achieve a higher cooling capacity and COP than R22. From the perspective of long-term environmental safety, the use of HCs is a practical choice. Propane (R290), as a representative HC refrigerant, offers higher efficiency, better heat transfer in heat exchangers, lower refrigerant charge quantity [7], and lower compressor discharge temperatures [8]. It has been used in heat pumps [9], organic Rankine cycle [10], and he application of R290 in large capacity chest freezer was also discussed [11]. The Chinese air-conditioning system manufacturer Gree Electric Appliances Inc., one of the world’s largest manufacturers of STHACs, has started pilot production of STHACs using R290 in 2009 [12]. However, owing to R290’s flammability and explosiveness, when used in STHACs, it has a potential fire risk resulting from leakage, especially from indoor units. Thus, the safety of the STHACs has a great influence on the safety of buildings. The most common approach to mitigating the flammability risk is restricting the maximum charge of refrigerant. For example, criteria such as EN 378 and IEC 60335-2-40 [13] include the requirement of limiting the quantity of refrigerant used in STHACs (typically with respect to room size). Recently, several researchers have carried out studies on the fire hazards from flammable refrigerants. D. Colbourne [14] assessed the flammability risk of HCs within horizontal type ice cream cabinets. Tingxun Li [15] and Wang Zhang [16] analyzed the impact of indoor leakage of R290 on room safety. Li assessed different leakage situations and found that the maximum concentration seldom exceeded two-thirds of the LFL, and the leaking rate has a major influence on the refrigerant concentration distribution. Zhang researched the flammability hazards of R290 STHAC and found that the flammable range is limited to the space close to the indoor unit. However, the researchers in the aforementioned studies focused on the safety issues in the case of refrigerant leakage, and provided suggestions to reduce the R290 fire hazards based on their results. They did not study to reduce the refrigerant mass leakage in the system, or reduce the leaking rate to enhance the safety of R290 application. Although the charge amount of R290 in STHACs is one key factor on the safety of R290 and the domestic standards and international standards all have given clear limit, in fact, the different leaking rate with same amount charge of R290 also will result in various degrees of safe problem for R290 STHACs. For example, comparing the 300 g R290 leak out in 4 min and the 300 g R290 leak out in 10 min, the concentration of R290 in indoor for the later will be lower than that for the former and the R290 leaked from the later will have much time to diffuse. Moreover, a lower R290 leaking rate can also improve room safety. Thus, a further study to analyze the refrigerant mass distribution in STHACs by measuring the refrigerant distribution in the various components of an R290 STHAC and an experiment on R290 leaking rate from a STHAC can provide support in the design of safety measures in the R290 STHAC.
Fn F V1 SLS SHS
test weight of the refrigerant from the balance buoyancy force of the air volume of plastic bag. system low-pressure side system high-pressure side
Greek symbols qr density of R290 qa density of air
In this paper, 1P and 1.5P STHACs have been investigated. The refrigerant distribution regularities during the running and closed stage of the STHACs were investigated through experiments by QLNM. Based on this, the installation of a solenoid valve near the capillary has been proposed. Furthermore, the experiments of R290 leaking rate before and after the installation of a solenoid valve under different situations have been conducted. 2. Quasi liquid nitrogen method (QLNM) for measuring refrigerant mass distribution 2.1. Analysis of existing methods for measuring refrigerant mass The famous methods for measuring refrigerant mass in components of a refrigeration system are liquid nitrogen method (LNM) [17,18] and online measurement method (OMM). LNM, in which the cylinder is weighed after the refrigerant is drawn from the component into the sampling cylinder by the low temperature environment generated by liquid nitrogen, has the advantage of high accuracy. OMM, in which the components are placed on the balance and weighed directly, is convenient but less accurate. The measurement error while using LNM is 0.07–0.11% and that of OMM is 3–8% [19]. Furthermore, the maximal uncertainty of OMM may exceed 10% because of the refrigerant flow, fan vibration, and hosepipe support force. As mentioned earlier, the experiment on R290 distribution and leaking rate is to be carried out. However, LNM cannot measure the actual leaking rate because the vacuum environment in the sampling cylinder caused by liquid nitrogen will increase the leaking rate. Moreover, the test system of LNM is very complicated and the liquid nitrogen (which is dangerous for experimenters) is needed during the test process. OMM can test the leaking rate, however, the refrigerant flow and hosepipe support force may increase the error to exceed 10%. Therefore, a method called quasi-liquid nitrogen method (QLNM) was proposed, which is expected to be more convenient than LNM and more accurate than OMM, especially can measure the leaking rate in real-time. Thus, QLNM can overcome the shortcomings of LNM and OMM during leaking rate tests. 2.2. The principle of QLNM In QLNM, the refrigerant flows into a large plastic bag because of the higher pressure in the components. However, the actual mass of the refrigerant is different from the measurement result because of air buoyancy. The actual refrigerant mass in the plastic bag based on the Archimedes buoyancy formula is given by Eqs. (1) and (2).
G ¼ qr V1 g ¼ Fn þ F ¼ Fn þ qa V1 g
ð1Þ
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Table 1 The data of verification QLNM experiment. M1 (g)
M2 (g)
M3 (g)
M4 (g)
M1-M2 (g)
2.83 (M4-M3) (g)
e (%)
4943.5 4869.6
4869.8 4807.5
26.6 27.2
53.0 49.3
73.7 62.1
74.7 62.5
1.36 0.64
G G qr V1 g qr ¼ ¼ ¼ Fn G F qr V1 g qa V1 g qr qa
ð2Þ
where G is the actual weight of the refrigerant, Fn is the weight of the refrigerant measured by using the balance, F is the buoyancy force by the air, and V1 is the volume of the plastic bag. Moreover, qr is the density of R290 and qa is the density of air. Fn can be read from the balance. The ambient temperature of the experimental situation is 32 °C, and the atmospheric pressure is 1 atm. Thus, qr = 1.7879 kg m3 and qa = 1.1568 kg m3, 1:7879 G ¼ 1:78791:1568 Fn ¼ 2:83Fn. In order to investigate the error brought in by QLNM, the refrigerant was released from the refrigerant container into the plastic bag. The initial weight of the refrigerant container and the final weight of the plastic bag were measured by a precision electronic balance with a 15 kg range and ±0.1 g precision. The deviation of QLNM is presented by Table 1. In Table 1, M1 and M2 are the initial and final weights of the refrigerant container, respectively, and M3 and M4 are the initial and final weights of the plastic bag, respectively. Thus, M4 is the weight of the plastic bag containing the refrigerant. In Table 1, the deviation of QLNM is shown to be less than 2%, demonstrating that QLNM is reliable. In this experiment, there were refrigerant residues in the components because the pressure of the refrigerant caused it to flow into the plastic bag. The process continued until the refrigerant pressure in the component was equal to that in the plastic bag. The experimental uncertainties will be analyzed in the following chapters. 3. Experimental apparatus and procedures 3.1. Experimental apparatus The widely used 1P and 1.5P STHACs were used in the experiment. As shown in Fig. 1, STHAC system was reconstructed as
follows. (1) The compressor was replaced with a R290 compressor and the condenser tube was replaced with diameter 5 mm—as researchers have used the 5 mm tube for reducing the heat exchanger’s inner volume to reduce the charge quantity. (2) Solenoid valve and stop valves were installed on the two sides of the condenser, evaporator, and accumulator. The diameter of the valves should be the same as those of the tubes to reduce pressure drop across the valves. (3) The capillary, hand valve D, and solenoid valves were installed on the delivery tube because of the limitations of size set by the STHAC’s structure. The actual experimental apparatus is shown as Fig. 2. The solenoid valves I, II, III, and IV and hand valves A, B, and C were installed in the experimental system. The system was divided into five sections (evaporator, vapor delivery tube, liquid delivery tube, condenser, and accumulator) by these valves. The accumulator section referred to the section between solenoid valve I and the compressor including the compressor’s low-pressure side. The condenser section was the section between the compressor and the solenoid valve II including the compressor’s high-pressure side. During the experiment, by adjusting the switches of solenoid valves and hand valves, the refrigerant quantity in each section was measured by using QLNM. 3.2. Experimental procedure When QLNM is applied for measuring refrigerant distribution, the operational steps are as follows: (1) Run the STHAC and ensure it is at the test condition. (2) Weigh the empty plastic bag (i.e., measure M3). (3) Connect the plastic bag and the STHAC with a soft pipe, open hand valve and let the refrigerant in the component flow into the plastic bag until the pressure is stable. Then, weigh the plastic bag (i.e., measure M4). The refrigerant mass in the component is given by 2.83 ⁄ (M4 M3).
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. The actual experimental apparatus.
Table 2 The running stage results. 1P machine Charge/g Evaporator/g Vapor delivery tube/g Liquid delivery tube/g Condenser/g Accumulator/g The experimental total weight/g Deviation/%
260 64.8 0.9 24.9 104.7 59.1 254.4 2.15
1.5P machine 300 92.1 0.9 21.6 117.9 60.9 293.4 2.19
350 120 1.2 25.5 128.1 66.6 341.4 2.46
300 195 1.2 2.1 20.7 66.9 285.9 4.71
350 242.7 0.9 2.4 21.0 69.9 336.9 3.74
300 66.6 0.9 28.2 122.4 71.7 289.8 3.41
350 77.4 0.9 31.2 150.0 80.7 340.2 2.80
400 74.7 0.9 30.6 191.4 91.5 389.1 2.73
350 243.9 2.4 3.9 27 66.3 343.5 1.86
400 284.7 2.4 2.7 26.7 77.1 393.6 1.60
Table 3 The closed stage results. 1P machine Charge/g Evaporator/g Vapor delivery tube/g Liquid delivery tube/g Condenser/g Accumulator/g The experimental total weight/g Deviation/%
260 159.3 1.8 2.1 20.4 66 249.6 4.00
1.5P machine
(4) Repeat steps 2–3 for the testing of each component of the STHAC. The experimental procedure is to be followed with care to prevent the functional failure of the solenoid valves—as they are normally closed one-direction valves—and to ensure that the pressure across the solenoid valves does not exceed the critical pressure. The experimental procedures were carried out in the following sequence, and all the valves were closed after each step. a. Close all valves including the two stop valves. Then, open hand valve C to measure the refrigerant quantity in the evaporator section. b. Open solenoid valve III and hand valve C to measure the refrigerant quantity in the vapor delivery tube. c. Open the solenoid valve IV and hand valve C to measure the refrigerant quantity in the liquid delivery tube. d. Open hand valve A to measure the refrigerant quantity in the condenser section. e. Finally, open hand valve B to measure the refrigerant quantity in the accumulator section.
300 183.9 1.5 2.4 27.9 68.4 284.1 5.29
It should be noted that in order to accurately simulate a domestic environment and explore the distribution regularities, the STHAC was installed in a room and not in an enthalpy-difference room. The ambient temperature was 31–32 °C and the room temperature was 26 °C while STHAC was stable. The test conditions required the STHAC to be brought to a stable operation state and the temperature in the room to be kept constant. 4. Results and discussion The STHAC was started and operated in the cooling mode until it was operating stably. Then, the STHAC and all solenoid valves were simultaneously shut off. The refrigerant quantity was measured using the procedures a–e mentioned earlier. The results are shown in Table 2. The experimental error, e, can be estimated by Eqs. (3) and (4).
e ¼ eQLNM þ 2:83 em þ ee em ¼
dðM 4 M 3 Þ dM4 þ dM 3 ¼ M4 M3 M4 M3
ð3Þ ð4Þ
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Fig. 3. R290 mass distribution in components affected by charge of 1P machine running stage.
Fig. 4. R290 mass distribution in components affected by charge of 1.5P machine running stage.
where eQLNM is the error brought in by the QLNM, em is the measurement error. Furthermore, ee is the error brought by the residue and other experimental factors, which means the difference of charged and collected refrigerant mass. As shown in Table 1, eQLNM is 0.64–1.36%. A precision electronic balance with 15 kg range and ±0.1 g precision was used to measure the weight of the plastic bag. Thus, the error brought in by measurement, em , is 0.07–0.2%. ee , as calculated using the figures in Tables 2 and 3 is 1.60–5.29%. Therefore, the experimental error is 2.44–7.22% and the mean error is 4.46%. By comparing the ee to the results of LNM on R290 distribution [20], it can be observed that the deviation (1.60 to 5.29%) of QLNM is slightly more than that (0.64 to 2.31%) of LNM. Therefore, QLNM is validation in measuring the distribution of R290 in STHACs.
charge, the heat transfer area of condenser was redundant, and the supply refrigerant will accrue in the condenser by condensing into the liquid form. Hence, as shown in Table 2, by comparing the 1P machine with the 1.5P machine at equal charges of 300 g and 350 g, it can be concluded that the evaporator held a higher quantity of R290 in the 1P machine, which was excessive charge, while the condenser held a higher quantity of R290 in the 1.5P machine, which was insufficient charge. Thus, reducing the R290 charge quantity is an effective method to improve STHAC safety because the refrigerant will not accumulate in the evaporator when STHAC is insufficient charge.
4.1. The influence of refrigerant charge quantity From Table 2, it can be concluded that a large portion of the refrigerant is distributed in the heat exchanger (accounts for approximately 65–72%). It should be noted that the accumulator held another large portion of the R290 distribution because the R290 distributed in the accumulator is in the liquid form. As can be observed from Fig. 3, when the refrigerant charge quantity in the 1P machine is increased, the R290 distribution in the condenser and evaporator follows a linear relationship with the increase in charge. On the other hand, the distributions in the other components are almost constant regardless of the increasing R290 charge quantity. A possible explanation for this is that the pressures in those components have reached their respective saturation points owing to their internal volume limits. Moreover, it can be observed from Fig. 4 that the quantity of R290 in the evaporator changed little in the 1.5P machine. This outcome is expected since the internal volume of 1.5P machine is larger than that of the 1P machine. This indicates that while 1P machine is excessive charge, 1.5P machine may be insufficient charge by the same charge quantity. When the system was excessive charge, because of the limitation of the evaporator heat transfer area, the refrigerant in the evaporator could not obtain enough heat and therefore, was in vapor-liquid two-phase at the outlet of the evaporator. Thus, the excess refrigerant will accrue in the evaporator in the liquid form. Conversely, if the system was insufficient
4.2. The influence of STHAC working state In order to investigate the regularities of refrigerant distribution in the STHAC in closed stage, the STHAC was started and operated on cooling mode until it attained stability. Then, the STHAC was shut off while all valves were kept open. The valves were closed after 1.5 h. Then, the refrigerant quantity was measured by QLNM following the procedures a–e. The results are shown in Table 3. Figs. 5 and 6 show the R290 mass distribution affected by charge quantity during the closed stage. As these results differ from that of the running stage, it shows that the quantity of refrigerant in the evaporator is higher (accounts for approximately 60%– 70% of the charge), whilst the quantity of refrigerant in the condenser is lower and the accumulator is still occupied another large portion of R290. It is clear that in both 1P machine and 1.5P machine, the evaporator holds the largest portion of the refrigerant, whilst the quantity of refrigerant in the other components remains constant. The results indicate that the refrigerant migrates from the condenser to the evaporator after the STHAC is shut down. This outcome is expected since the pressure in the condenser is higher than that in the evaporator when the STHAC is shut down. Hence, the refrigerant migrates from condenser to evaporator until the equilibrium pressure is reached. The phenomenon of refrigerant migration after STHAC shutdown raises safety concerns since the STHAC closed stage occupied most of time in the room. R290 will gather in the indoor unit after the STHAC cooling mode powered off. On the other hand, we can infer that in the heating mode, the potential indoor fire risk of
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As shown in Figs. 7 and 8, by comparing the results of SLS during running and closed stage, it was observed that the refrigerant distribution changed from 50–60% to 85–90%. This indicates that approximately 30% of charge quantity migrates to the SLS in closed stage. Moreover, the R290 quantity in SHS during closed stage is approximately constant and is not affected by charge because the pressure within the system was only affected by the environment temperature while reach the equilibrium. Thus, the quantity in SHS is constant at equivalent pressure and condenser structure. It can be speculated that if a solenoid valve is installed close to the capillary, the migration of 30% of the refrigerant charge would be cut off when the STHAC was shut down. This is equivalent to reducing the charge quantity by 30%, and will enhance the R290 STHAC safety. 5. Experiment to determine leaking characteristics
Fig. 5. R290 mass distribution in components during closed stage of 1P split type air-conditioner.
To verify the feasibility of the solenoid valve, several R290 leaking rate experiments were carried out, whereby the R290 in the 1P STHAC machine under steady cooling condition was released through the leak hole. Thereafter, the leaked refrigerant mass was measured by QLNM. 5.1. Leaking experimental apparatus and procedures
STHAC will be reduced because R290 will gather in the outdoor unit. 4.3. The comparing of system high-pressure and low-pressure sides It was found that the refrigerant migrates from condenser to evaporator because of the pressure difference, which is caused by capillary tube. Therefore, in order to investigate the variation of refrigerant distribution on both sides of the capillary tube, the system was divided into high-pressure and low-pressure sides. The system high-pressure side contains the condenser section; the low-pressure side contains the vapor delivery tube, evaporator, liquid delivery tube, and accumulator sections. The refrigerant distribution in system high-pressure (SHS) and low-pressure side (SLS) of 1P and 1.5P machines are shown in Figs. 7 and 8.
Fig. 6. R290 mass distribution in components during closed stage of 1.5P split type air-conditioner.
During the leakage experiment, a camera was positioned close to the balance to record the weighing results. The refrigerant charge was 260 g because the plastic bag could not carry a quantity of gaseous R290 that was equal to or higher than 300 g. Solenoid valve II was closed and hand valve C was opened after the machine was brought to stable cooling mode. QLNM was used to measure the refrigerant leakage quantity. When QLNM is applied for measuring the leaking rate, the operational steps are as follows: (1) Run the STHAC and bring it to the test condition. (2) Weigh the empty plastic bag (i.e., measure M3). (3) Connect the plastic bag and the STHAC with soft pipe; shut down the STHAC and close the solenoid valve II while other solenoid and stop valves keep opening.
Fig. 7. R290 mass distribution in high and low pressure side affected by charge of 1P machine.
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Fig. 9. R290 leaking rate affected by different diameter with solenoid valve II closed. Fig. 8. R290 mass distribution in high and low pressure side affected by charge of 1.5P machine.
(4) Open hand valve C, making the refrigerant in the component flow into the plastic bag. Simultaneously weigh the plastic bag online as M4 and record using the camera. The leakage quantity is 2.83 ⁄ (M4 M3). (5) Using the camera recordings, leaking rate can be calculated by the relationship of leakage quantity and time. Leak holes of different diameters were drilled as part of the experiment to simulate different conditions of R290 leakage in STHACs. For example, a 7 mm external diameter with 6.3 mm internal diameter tube was used to simulate the situation of evaporator tube whole rupture. As mentioned earlier, the condenser tube was replaced with 5 mm external diameter and 4.37 mm internal diameter, thus the 4.37 mm diameter hole was used to simulate the situation of condenser tube whole rupture. Holes of other diameters were drilled to simulate the tube tiny rupture situation, and explore the regularities with different leakage hole diameters.
in the outdoor unit increases until it exceeds the valve’s critical pressure. In addition, the refrigerant in the accumulator leaks out. Therefore, after 240 s, the leaking rate increases, although to a value as high as that in the first phase. Finally, the leakage process ends after 1200 s because the leaking rate was stable. Furthermore, the result shows that for a larger leak hole, the leaking rate is higher. Besides, the results of the leak holes with 1 mm and 0.5 mm diameters show an absence of the three phases because of the small diameter. The results under the condition where solenoid valve II and hand valve C were open are shown in Fig. 10. In Fig. 10, it can be observed that the refrigerant in the system leaks out at a high rate. The high pressure in the system causes the liquid refrigerant to flow directly into the plastic bag. Then, the water vapor in the air would form frost on the outside of the plastic bag. This explains why the curves of the 1 mm and 7 mm diameter holes indicate that the leakage mass exceeds the charge amount. This proves that the QLNM is unsuitable for online measurement
5.2. Leaking experimental results Several leaking rate experiments with solenoid valve II kept closed were carried out and a summary of the results is shown in Fig. 9. Unlike the assumption that the leakage mass is linear with respect to the time, the results of 2 mm, 4.37 mm, and 6.3 mm diameter holes show that there are three phases in the leakage process. Initially, approximately 100 g of refrigerant leaked at a high rate in the first 60 s. According to the operational state results, the refrigerant mass in the evaporator and liquid delivery tube are 64.8 g and 24.9 g, and their sum is approximately 90 g. In the first phase, the refrigerant in the evaporator and liquid delivery tube leaked accompanied by a huge sound created by the refrigerant flowing through the pipeline. Then, there is refrigerant leakage at a lower rate from 60 s to 240 s because the pressure across solenoid valve II exceeds the critical pressure. After the indoor unit leakage, the difference between the refrigerant pressure of the outdoor unit and that of the indoor unit exceeds solenoid valve II’s critical pressure. Therefore, the leakage took place at a low rate. A certain quantity of liquid refrigerant remained in the condenser, and it evaporated from liquid to gas. Then, the pressure
Fig. 10. R290 leaking rate affected by different diameter with solenoid valve II opened.
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Fig. 11. R290 leaking rate of 0.5 mm diameter with solenoid valve II opened and closed.
of high leaking rates because the frost causes error in the QLNM. However, from Fig. 10, it can be found that the refrigerant leaking rate is high and it does not reduce when the solenoid valve II is opened. Furthermore, the results of the 0.5 mm diameter hole reveal an absence of a high leaking rate. The results of the 0.5 mm diameter hole with solenoid valve II kept both opened and closed is shown in Fig. 11. In Fig. 11, compared with the 0.5 mm diameter hole, the leaking rate with solenoid valve II kept open is higher than that with the valve kept closed. Thus, it can be argued that installing a solenoid valve installed near the capillary can improve the safety of R290 STHACs. However, the actual effect needs to be verified through further studies. 6. Conclusions R290 is one of the most competitive alternative of R22 in split type household air conditioner (STHAC) due to several advantages in terms of thermodynamic properties, environmental characteristics, and cost. However, the obvious shortcoming of R290 is its flammability. This paper proposed an a new quasi-liquid nitrogen method (QLNM) in order to investigate the refrigerant distribution in R290 STHACs and the leaking rate under various conditions, and the various experiments have been conducted. The conclusions are as follows: (1) QLNM was used to measure the R290 distribution and the maximal deviation of QLNM was 5.29%. The deviation of QLNM is slightly more than that of LNM. This proves the validation of QLNM. (2) The results of the R290 distribution experiments by using QLNM show that a large portion of the refrigerant mass is distributed in the heat exchanger (especially in the condenser) when the STHAC is running in cooling mode. With the increase of the refrigerant charge, the refrigerant mass in the heat exchangers increases while those in the other components remain almost constant, and the evaporator will hold more refrigerant when the STHAC is excessive charge, the condenser will hold more charge refrigerant when the STHAC is insufficient charge. (3) The refrigerant will migrate from the high-pressure side to the low-pressure side when the STHAC is on closed stage in the cooling mode until reaching the pressure balance of
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whole system. As a result, the quantity of refrigerant in the evaporator is greater than that in the other components, and the entire low-pressure side accounts for 85–92% of the total refrigerant charge. The refrigerant migration will increase the flammable risk while using R290 STHAC in buildings due to a large portion of the refrigerant in the indoor unit. On the contrary, the flammable risk can be reduced since in the heating mode, the refrigerant accumulates in the outdoor unit. (4) Based on the experimental results of the R290 distribution, a solenoid valve was proposed to install near the capillary in order to prevent the refrigerant migration from the highpressure side to the low-pressure side considering to reducing the refrigerant mass in indoor unit when the STHAC is on closed stage. (5) The experimental results of the leaking rate under various conditions before and after the installation a solenoid valve near the capillary in STHAC showed that the refrigerant leaking rates after the installation a solenoid valve near the capillary in STHAC are obviously less than that before the installation a solenoid valve. This means that the flammable risk brought by the R290 leaking from STHAC also will be effectively reduced and the safety of R290 STHACs will improve.
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