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Experimental evaluation of an ejector as liquid re-circulator in an overfeed NH3 system with a plate evaporator
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
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Experimental evaluation of an ejector as liquid re-circulator in an overfeed NH3 system with a plate evaporator J. Alberto Dopazo, Jose´ Ferna´ndez-Seara* A´rea de Ma´quinas y Motores Te´rmicos, E.T.S. de Ingenieros Industriales. University of Vigo, Campus Lagoas-Marcosende No 9, 36310 Vigo, Spain
article info
abstract
Article history:
This paper deals with the experimental performance evaluation of an ejector, linked to
Received 13 August 2010
a manual expansion valve, working as a liquid re-circulator component in an overfeed NH3
Received in revised form
plate evaporator. The evaporator was tested in a single stage system belonging to a cascade
22 December 2010
refrigeration system prototype. The evaporator is an ALFANOVA HP76 plate heat exchanger
Accepted 23 December 2010
with 50 plates. A Phillips ejector with a 1/200 diameter throat and 1.4 mm diameter nozzle was
Available online 4 January 2011
used. The recirculation rate was experimentally determined for different operating conditions. Experimental data are reported for volumetric flow rate at the manual expansion valve
Keywords:
inlet from 0.8 to 1.6 l min1, evaporating pressure from 0.14 to 0.22 MPa and condensing
Ejector
pressure from 0.85 to 1 MPa. The experimental result showed recirculation rates between 2
Evaporator
and 4. The evaporating capacity varied from 9.48 kW to 18.37 kW. In addition, another two
System
nozzles were tested and the results are also presented and discussed. ª 2010 Elsevier Ltd and IIR. All rights reserved.
Overfeed Ammonia
Evaluation expe´rimentale d’un e´jecteur utilise´ pour recirculer l’ammoniac liquide dans un syste`me avec un e´vaporateur a` plaques suralimente´ Motscle´s : E´jecteur ; E´vaporateur ; Syste`me ; Suralimentation ; Ammoniac
1.
Introduction
Refrigeration evaporators can be classified according to the liquid feed method employed, as direct-expansion evaporators, flooded evaporators and overfeed evaporators. Directexpansion evaporators are usually fed by using an expansion
valve that regulates the flow of liquid through the evaporator. In this case, the amount of liquid used to feed the evaporator is limited by the amount of refrigerant that can be vaporized in it, so that the refrigerant leaves the evaporator superheated and only vapour is suctioned off by the compressor. The second concept is the flooded evaporator, whereby the
* Corresponding author. Tel.: þ34 986 812605; fax: þ34 986 811995. E-mail address: [email protected] (J. Ferna´ndez-Seara). 0140-7007/$ e see front matter ª 2010 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2010.12.023
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
Nomenclature A d h m P Q R PR T V v X
2
area, m diameter, mm specific enthalpy, kJ kg1 mass flow rate, kg s1 pressure, MPa heat transfer rate, kW recirculation rate pressure ratio temperature, C volumetric flow rate, l min1 velocity , m s1 vapour quality, %
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Subscripts c condenser, condensation carbon dioxide CO2 dis discharge e evaporator, evaporation ejec ejector m mean, average main main ejector inlet mev manual expansion valve in inlet ammonia NH3 out outlet rec re-circulated, recirculation sat saturated, saturation
Greek symbols r density, kg m3
evaporator is completely filled with liquid refrigerant so that the entire evaporator inner surface is wet thus improving the heat transfer coefficient (Dossat, 1980). The flooded evaporator is coupled to a liquid receiver from which the liquid is recirculated taking advantage of gravity action, and the vapour is suctioned off by the compressor. The liquid level in the receiver is controlled by using a float device. In liquid overfeed systems much more liquid is fed into the evaporator than actually vaporizes. Some liquid boils in the evaporator and the remainder floods out of the outlet. As a result, the refrigerant leaving the evaporator is always saturated with a vapour quality less than one. The mass flow rate flowing through the evaporator (low pressure side) is higher than the compressor and condenser (high pressure side) rates. The ratio of the total mass flowing through the evaporator compared to the refrigerant vapour mass flow generated in the evaporator is defined as the recirculation rate. There is an ideal circulation rate for each evaporator which will result in the minimum temperature difference and the best evaporator efficiency (Lorentzen, 1968). Several parameters have to be considered to obtain the optimum recirculation rate, such as the heat load, pipe diameter, circuit length, top and bottom feed evaporators and number of parallel circuits in order to achieve the best performance. Usually, the evaporator manufactures specify the ideal recirculation rates for their equipment. For ammonia, ASHRAE (2002) recommends recirculation rate using larger diameter tubes as 6e7 for top deed evaporators and using smaller tubes as 2e4 for bottom feed evaporators. A liquid overfeed system includes a liquid/vapour separator to separate the liquid and supply vapour to the compressor and saturated liquid to the evaporator. The compressor is fed using the vapour from the top of the tank. The vapour coming out of the separator is close to saturation conditions, which mean lower compressor inlet gas temperatures and consequently, lower discharge gas temperatures, which are a critical factor for ammonia systems working at low temperature applications. To feed the evaporator with saturated liquid re-circulated from the separator, a common practice is to use a pump as a liquid re-circulator component, described by Stoecker (1988), Bivens et al. (1997) and Giuliani et al. (1999). However,
the use of pumps increases the initial investment of the facility and the operation and maintenance costs, especially in low capacity refrigeration systems. On the other hand, Gac (1974), in his publication “Automatisme des systemes frigorifiques”, comments that an alternative to the use of pumps in liquid overfeed systems is the use of ejectors. The principal advantage of this option is its simplicity, without mobiles parts, and strong construction. In addition, ejectors are more economical compared to pumps. Nevertheless, Gac (1974) stated that the effectiveness of ejector performance is strongly linked to the quality of its construction, pointing out that in practice the use of ejectors had been consigned to auxiliary refrigeration systems in which the refrigeration capacity remained constant or with few changes. One liquid overfeed system using R-22, in which an ejector was used to feed a plate freezer of eight freezing stations was studied by Radchenko (1985). His results included data of the liquid collected in the liquid/vapour separator. However, recirculation rate data were not shown. In recent years, ejectors and their applications in jet refrigeration cycles have been widely studied. As examples of this latter, the researches of Sarkar (2008), Elbel and Hrnjak (2008) and Nakagawa et al. (2009), and the reviews performed by Chunnanond and Aphornratana (2004) and Abdulateef et al. (2009), can be cited. The authors have not found any other references about the use of ejectors in liquid overfeed systems. The main purpose of this work was to experimentally evaluate the performance of an ejector working as a liquid re-circulator component in an overfeed plate evaporator with NH3. The experimental tests were performed using an experimental prototype cascade refrigeration system with CO2 and NH3. In this paper, the experimental setup and procedure are described, the reduction data process detailed, and the results shown and discussed.
2.
Experimental facility
The experimental facility consists of a cascade refrigeration system prototype made of two single stage systems connected by a heat exchanger. CO2 is used as the refrigerant in the low
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NH Evaporator 3 (cascade exchanger) CO2 low temperature system
(3)
Air condenser (1) P
(6)
T
01
01
P
02
T
02
(2)
NH3 Compressor
900 mm
(7) V
T 03
03
02
P (4) (5)
P
Ejector
04
Liquid receiver V01
200 mm
1150 mm
Liquid/vapour separator
Manual expansion valve
Fig. 1 e Schematic diagram of the experimental facility test section.
temperature system and NH3 for the high temperature system. Fuller information related to the cascade system prototype can be found in Dopazo and Ferna´ndez-Seara (2011). The test section is the high temperature system with NH3. The schematic diagram of the test section is shown in Fig. 1, and a real view of the experimental facility can be appreciated in Fig. 2. In the high temperature system, the flow of NH3 coming from the liquid/vapour separator is compressed in a fourcylinder reciprocating open type compressor coupled to a variable speed electric motor with a pulley system. The superheated flow of NH3 enters into the air condenser and ejects the heat into the atmospheric air. The condensed stream of NH3 coming from the air condenser is channelled into a liquid
Fig. 2 e Photograph of the experimental prototype of the cascade refrigeration system.
receiver. The NH3 liquid leaves the receiver and flows through a manual expansion valve and enters into the NH3 ejector (used to overfeed the evaporator) where it is expanded until it reaches evaporating pressure and mixed with a secondary flow of liquid NH3 coming from the liquid/vapour separator. The resulting flow mixture enters into the cascade heat exchanger, in which it evaporates and returns to the separator. The cascade heat exchanger, where the NH3 is evaporated, consists of an ALFANOVA HP76 counter flow plate heat exchanger, with 50 plates of Alloy 316 SS, and dimensions 191 191 618 mm. The diameters of the refrigerant lines at the main and re-circulated liquid ejector’s inlets and outlet, refer to Fig. 1, are d4 ¼ 10 mm, d5 ¼ 25.4 mm and d7 ¼ 25.4 mm, respectively. The ejector and the manual expansion valve were located at the lowest level of the experimental facility. The level difference between the outlet of the liquid receiver and inlet of the manual expansion valve is 200 mm, the level difference between the secondary outlet of the vapour/liquid separator and the secondary ejector’s inlet is 900 mm and the level difference between the ejector’s outlet and the NH3 inlet of the cascade heat exchanger is 1150 mm. Fig. 1 includes the height levels of the separator re-circulated liquid outlet, liquid receiver outlet and the NH3 evaporator inlet with regard to the ejector location. The experimental facility was equipped with a data acquisition system based on a 16-bit data acquisition card and a PC. The NH3 suction and discharge temperatures are measured by using the sensors T01 and T02, located in the suction and discharge lines of the NH3 compressor, respectively (refer Fig. 1). The NH3 condenser temperature is measured by using the sensor T03, located in the NH3 liquid receiver. The suction pressure is measured by using the pressure transducer P01, located in the liquid/vapour separator. The discharge pressure of the NH3 compressor is measured by using the pressure transducer P02, located in the discharge line of the NH3 compressor. The condensing
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
pressure of NH3 is measured by using the pressure transducer P03, installed in the NH3 liquid receiver. The pressure at the NH3 ejector main inlet is measured by using the pressure transducer P04. The main volumetric flow rate of NH3 is measured at the manual expansion valve inlet by using the volumetric flow-meter V01 (refer Fig. 1). The re-circulated liquid flow rate of NH3 used to overfeed the NH3 evaporator, is measured by using the volumetric flow-meter V02, located at the auxiliary outlet of the NH3 liquid/vapour separator. All the temperature sensors used in the experimental facility are A Pt100 inserted in 3 mm diameter stainless steel pockets, with an accuracy of (0.15 þ 0.002 T) ( C) in the measurement range (25/130 C). The pressure transducers are WIKA ECO 1 type, with an accuracy of 0.5% of the full scale (2.6 MPa). The NH3 volumetric flows were measured by using electromagnetic flow-meters, with an accuracy of 0.25% of the measured value.
3. Experimental procedure and data reduction
Table 1 e System operating conditions considered in the experimental parametric study. Vmev,in (V01) (l min1) 0.8, 1.0, 1.2, 1.4, 1.6
Pe;NH3 (MPa)
Pc;NH3 (MPa)
0.14, 0.16, 0.18, 0.2, 0.22 0.85, 0.9, 0.95, 1
resistances regulated independently by using electric power regulators were used in the low temperature system with CO2 to establish the refrigeration capacity of the cascade system. The NH3 compressor capacity was established by using a frequency converter, which permitted the modification of the electric motor velocity, and consequently the gradual variation of the NH3 compressor capacity. The mass flow of the liquid refrigerant was controlled by means of the manual expansion valve located at the main inlet line of the ejector (Fig. 1). The condensing pressure of NH3 was controlled by modifying the air flow in the condenser. Once all temperature, pressure and flow measurements were stabilized, the experimental data were collected during a time frame of five minutes, at intervals of approximately 5.4 s.
3.2. 3.1.
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Data reduction
Experimental procedure
The NH3 ejector used in the experimental evaluation was a Phillips ejector with a 1/200 diameter throat and 1.4 mm diameter nozzle, as is shown in Fig. 3. All the tests were performed under stable operating conditions, i.e. when the variations in all temperature and pressure measurements were within 1% range. The experimental evaluation was initiated by fixing the actual operating condition of the cascade system prototype. The NH3 evaporating pressure was 0.2 MPa, the condenser pressure was 0.85 MPa and the volumetric flow rate at the manual expansion valve inlet was 1.2 l min1. Subsequently, the ejector behaviour as a liquid re-circulator component was evaluated at several NH3 volumetric flow rates, keeping constant the NH3 evaporating and condensing pressures at 0.2 MPa and 0.85 MPa, respectively. Finally, the influence of the evaporating and condensing NH3 pressures on the ejector performance was also evaluated. The operating conditions of the facility during the experimental evaluation carried out are shown in Table 1. The low temperature system with CO2 was used to set the evaporating NH3 pressure and to supply the refrigeration charge in the cascade heat exchanger (NH3 evaporator). Electric
The recirculation rate, the NH3 vapour quality at the main inlet and outlet of the ejector, the refrigerant flow mean velocities and the NH3 system refrigeration capacity were calculated from the data experimentally measured. The calculations of the previously mentioned parameters were formulated from the mass and energy balances applied to the NH3 system components. Thermodynamic properties of NH3 were obtained using the REFPROP Database (Lemmon et al., 2008). The following assumptions were made in the analysis: (a) Pressure losses in connecting pipes and heat exchangers have been neglected. (b) The ejector was perfectly isolated. (c) Vapour and liquid phases were in thermodynamic equilibrium. (d) Changes in kinetic and potential energy were neglected. (e) There was saturated liquid at the re-circulated liquid inlet (line 7, Fig. 1) of the ejector. Taking into account the above assumptions the calculated parameters are detailed as follows, according to the nomenclature in Fig. 1.
Fig. 3 e Schematic of the Phillips ejector tested.
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m4 ¼ m3
(1)
h4 ¼ h3
(2)
m5 ¼ m4 þ m7
(3)
components in any available literature. Therefore, an energy balance was performed in the cascade heat exchanger to compare the heat transfer rate obtained from the NH3 evaporation to the heat transfer rate obtained from the CO2 condensation of the low temperature system of the experimental facility. Results shown in Fig. 4 reveal that all the experimental data remain within an error band of 5%.
m4 m7 þ h7 $ h5 ¼ h4 $ m5 m5
(4)
4.2.
The main and re-circulated mass flow rates at both inlets of the ejector are given by Eqs. (5) and (6). In these equations the NH3 volumetric flow rates V01 and V02 are experimentally measured and the densities r3 and r7, are obtained as a function of the pressure and temperature measurements at the liquid receiver P03 and T03, and the liquid/vapour separator P01, respectively. m4 ¼ V01 $r3 ¼ mmain
(5)
m7 ¼ V02 $r7 ¼ mrec
(6)
The recirculation rate RNH3 , and the pressure ratio PRejec evaluated between the main inlet and the outlet of the ejector were determined according to Eqs. (7) and (8), respectively. m5 (7) RNH3 ¼ m4 PRejec ¼
P4 P5
(8)
In Eq. (8), P5 was considered equal to the evaporating pressure, measured at the liquid/vapour separator (P01). The evaporating capacity of the NH3 system is calculated in Eq. (9) Qe;NH3 ¼ m3 $ðh1;sat h3 Þ
(9)
The NH3 mean flow velocities at the inlets and outlet of the ejector were obtained from Eq. (10). m (10) v¼ r$A The NH3 qualities at the main inlet and outlet of the ejector (states 4 and 5, Fig. 1) were calculated as a function of the NH3 specific enthalpies and the pressures experimental measurements P04 and P01, respectively. On the other hand, a detailed analysis of the experimental uncertainties was carried out. This analysis takes into account the uncertainties in the determination of the fluid properties from the Refprop database. These uncertainties are 0.2% in density, 2% in heat capacity and 0.2% in vapour pressure. The experimental uncertainties were determined according to ISO (1995). The maximum for typical uncertainties was estimated to be 0.25% for mmain, 0.30% for mrec, 0.31% for RNH3 , 4.07% for PRejec, 1.84% for Qe;NH3 , and 14.39% for v.
Experimental results
The experimental results obtained at the initial operating condition established previously (Pe;NH3 ¼ 0:2 MPa, Pc;NH3 ¼ 0:85 MPa and Vmev,in ¼ 1.2 l min1) showed a NH3 liquid recirculation rate of 3.1, which indicates that the ejector performance working as a liquid re-circulator component was satisfactory. The evaporating power of the NH3 obtained in this case was 13.79 kW. The results obtained from the experimental parametric study performed at the operating conditions described in Table 1 are shown and discussed below. Figs. 5 and 6 depict the experimental measurements of pressures and re-circulated liquid volumetric flow rates Vrec, obtained for each value of NH3 volumetric flow rate at the manual expansion valve inlet Vmev,in specified in Table 1. The instantaneous values of the variables measured during five minutes are shown. The condensing and evaporating pressures remained constant at 0.85 and 0.2 MPa, respectively. In Fig. 5 the pressure at the main inlet of the ejector P04 and the main NH3 volumetric flow rate Vmev,in for different positions of the manual expansion valve can be appreciated. As the manual expansion valve opening becomes higher, the pressure drop in this valve decreases. As a result, the pressure at the main inlet of the ejector is higher and the NH3 volumetric flow rate at manual expansion valve inlet increases. Related to the experimental measurements of both main and re-circulated NH3 volumetric flow rates shown in Fig. 6, stable values can be appreciated for the main volumetric flow rates. Nevertheless, low oscillations can be observed in the NH3 re-circulated liquid volumetric flow rate measurements for NH3 main volumetric flow rate of 1.0, 1.2, 1.4 and
20 18 + 5%
16 14
Qc,CO2 (kW)
The mass and energy balances on the manual expansion valve and on the ejector are given by Eqs. (1)e(4), respectively.
- 5%
12 10 8 6 4 2
4. 4.1.
Results and discussion Experimental data validation
The authors did not find experimental data for recirculation rates in overfeed evaporators using ejectors as recirculation
0 0
2
4
6
8
10 12 14 16 18 20
Qe,NH3 (kW) Fig. 4 e Comparison of the heat transfer rates determined from the experimental data through the energy balances on the cascade heat exchanger.
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1.2
20 -1
1.0
Vmev,in (l·min ) 1.0
1.2
1.4
18
1.6
0.8
Qe,NH3 (kW)
Pressure (MPa)
0.8
0.6 0.4 0.2
14 12 10
PPA separator NH3 0.0 0
16
P comp,dis PD NH3
Preceiver PL NH3
P ejec,in PE NH3
8 300║0
300║0
300║0
300║0
25
300
30
35
40
45
50
55
60
65
-1
mmain (kg·h )
Time (s)
Fig. 5 e Experimental pressures at several NH3 main volumetric flow rates (Vmev,in).
Fig. 7 e NH3 evaporating power as a function of the NH3 main mass flow rate.
1.6 l min1. The oscillations corresponding to the lower NH3 main volumetric flow rate of 0.8 l min1 are especially significant. The results indicate that, for the fixed evaporating and condensing pressures, there would seem to exist a minimum main mass flow rate value which assures the stabilities of the dragging process taking place inside the ejector. Once this main mass flow limited value is reached, high instabilities occur at the ejector’s secondary inlet having an adverse effect on the liquid recirculation. However, it is fitting to point out the fact that in spite of this latter, liquid recirculation was present. On the other hand, as the NH3 mass flow rate at the main inlet of the ejector increases, the evaporating power of the system also increases as can be observed in Fig. 7. The evaporating power obtained varied from 9.48 kW within the main mass flow rate of 29.8 kg h1 to 18.37 kW within the main mass flow rate of 58.3 kg h1. Fig. 8 shows the results of the NH3 vapour quality values calculated at the main inlet and outlet of the ejector. The tendency observed that the NH3 vapour quality at the main inlet of the ejector follows the behaviour traced by the manual expansion valve opening. As the valve opening increases, the mass flow rate and the pressure at the main inlet of the ejector P04 increase. The pressure rise causes a decreasing trend in the NH3 vapour quality as the flow of NH3 at the main inlet of the
ejector draws closer to a saturated liquid condition. However, the results obtained from the NH3 vapour quality at the ejector’s outlet showed the opposite trend. In this case, as the main mass flow rate increases, the NH3 vapour quality calculated at the outlet of the ejector tends to rise lightly. Fig. 9 shows the NH3 flow mean velocity values calculated at the main and re-circulated liquid inlets and outlet of the ejector. Despite the increase in the NH3 main mass flow rate, a decreasing trend of the flow mean velocity values obtained at the main inlet of the ejector as a result of the NH3 density rise caused by the NH3 vapour quality decrease, can be appreciated. On the other hand, the NH3 flow mean velocity values calculated at the outlet of the ejector tend to increase, whereas the flow mean velocity values obtained at the recirculated liquid inlet of the ejector show a constant trend. All the NH3 flow mean velocity values observed in the experimental evaluation performed are included between 0.88 and 1.71 m s1. In Fig. 10 the experimental results of the re-circulated liquid volumetric flow rate Vrec and the NH3 liquid recirculation rate are represented as a function of the NH3 mass flow rate at the main inlet of the ejector. A decreasing tendency of the recirculation rate while the re-circulated liquid volumetric flow rate tends to remain constant can be appreciated. It is
Vmev,in
5.5
Vrec
Xin,main
5.0
5
Xout
4.5 4
Quality (%)
Volumetric flow rate (l·min-1)
6
3 2
4.0 3.5 3.0 2.5
1 0
2.0 1.5 0
300║0
300║0
300║0
300║0
Time (s)
Fig. 6 e Experimental main and re-circulated liquid volumetric flow rate.
300
25
30
35
40
45
50
55
60
65
-1
mmain (kg·h )
Fig. 8 e NH3 vapour quality as a function of the NH3 main mass flow rate.
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6
5
5
4
4
3
1.0
3
2
0.5
2
vin,main vin,rec vout
1.5
0.0 25
30
35
40
45
50
55
60
1 0.80
65
-1
mmain (kg·h )
R
1
V
0.85
0.90
0.95
0 1.05
1.00
Pc,NH3 (MPa)
Fig. 9 e Flow mean velocities as a function of the NH3 main mass flow rate.
worth pointing out the special behaviour observed when the measured value of main volumetric flow rate was 0.8 l min1. At this point a considerable reduction of the recirculation rate was found and a tendency change was appreciated. The drop of this value can be linked with the high amplitude of oscillations observed in the NH3 re-circulated liquid volumetric flow rate measurements, depicted in Fig. 6. As previously indicated, instabilities at the ejector’s secondary inlet have an adverse effect on the liquid recirculation. Despite this adverse condition, the mean recirculation rate obtained was 3.41. Results depicted in Fig. 10 also show that for the evaluated experimental operating conditions, the lowest main mass flow rate which assures a stable process in the ejector is within the range from 30 to 36 kg h1, approximately. Fig. 11 shows the experimental measurements of recirculated liquid volumetric flow rates and the recirculation rate calculated as a function of the NH3 condensing pressure. The evaporating pressure remained constant at 0.2 MPa. Increasing trends were observed in both cases when the condensing pressure increased. The recirculation rate rose 18.4%, from 3.1 with condensing pressure at 0.85 MPa, to 3.67 with condensing pressure at 1 MPa, while the increment observed in the re-circulated liquid mean volumetric flow rate was up 17%, from 3.67 to 4.31 l min1.
Fig. 11 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 condensing pressure.
The variation of the evaporating pressure in the values range described in Table 1 did not show a clear influence on the ejector behaviour, as can be observed in Fig. 12. The condensing pressure was fixed at 0.85 MPa. In this case, the recirculated liquid volumetric flow rate, as well as the NH3 recirculation rate showed fairly constant tendencies. On the other hand, two other nozzles with diameters of 1.2 and 1.6 mm, E1 and E3 respectively, were used to evaluate the ejector behaviour. The experimental results of the recirculation rate and the ejector pressure ratio are depicted in Fig. 13. A similar behaviour of the ejector using all three nozzles can also be appreciated. The highest recirculation rate values, around 3.16 of mean value, were obtained using the nozzle E2 (initial case). The mean value of recirculation rate obtained by using the nozzle E3 was 2.56, which represents a reduction of 19% compared to nozzle E2. In both cases the main NH3 volumetric flow rate Vmev,in was 1.2 l min1. With nozzle E1, the highest NH3 main volumetric flow rate Vmev,in obtained during the experimental evaluation was 0.8 l min1, with the manual expansion valve positioned completely open. The mean value of the recirculation rate was 2.19, 30% lower than the mean recirculation rate obtained with nozzle E2.
10
6
6.0
R
9
V
4.0
4
3.0
-1
2.0 2.0 R 1.0 25
35
40
45
-1
6
3
5
2
4 1
3
V 1.0
30
RNH3
RNH3
3.0
Vrec (l·min )
7 4.0
50
55
60
5
8
5.0
65
-1
mmain (kg·h )
Fig. 10 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 main mass flow rate.
Vrec (l·min )
2.0
RNH3
-1
Flow mean velocity (m·s )
2.5
Vrec (l·min-1)
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
2 0 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 Pe,NH3 (MPa)
Fig. 12 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 evaporating pressure.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
re-circulated liquid volumetric flow rate. Both variables tend to remain constant.
5 4
E3
3 2
RNH3
4
PRejec
E1 E2
1
3 2 1 0.13
0.14
0.15
0.16
0.17 0.18 0.19 Pe,NH3 (MPa)
0.20
0.21
0.22
0.23
Fig. 13 e Recirculation rate and ejector’s pressure ratio as a function of the NH3 evaporating pressure.
In addition, as the evaporating pressure increases, the ejector’s pressure ratio decreases as can be appreciated in Fig. 13. However, a clear relationship between the ejector’s pressure ratio and the recirculation rate values is not apparent.
5.
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Conclusions
The experimental results obtained in this work confirm that an ejector linked to a manual expansion valve can be used as a liquid re-circulator component in liquid overfeed systems with NH3, under different operating conditions and with variable refrigeration capacity. The initial test showed a recirculation rate of 3.1. The refrigeration capacity was 13.79 kW, the evaporating and condensing pressures 0.2 and 0.85 MPa, respectively, and the liquid main volumetric flow rate measured at the manual expansion valve inlet was 1.2 l min1. The ejector tested has a nozzle with a diameter of 1.4 mm and a throat with a diameter of 1/200 . An experimental parametric study was performed to evaluate the behaviour of the tested ejector working as a liquid recirculator component. The experimental results showed that, as the main liquid mass flow rate increases, the re-circulated liquid volumetric flow rate tends to remain constant and the recirculation rate tends to decrease. The evaporating power obtained from the experimental measurements varied from 9.48 kW using a main mass flow rate of 29.8 kg h1 to 18.37 kW using a main mass flow rate of 58.3 kg h1. In addition, it can be observed from the experimental parametric study results that the liquid re-circulated volumetric flow rate, as well as the recirculation rate, clearly showed increased trends when the NH3 condensing pressure rose. The increase of the recirculation rate was 18.4%, from 3.1 at condensing pressure of 0.85 MPa, to 3.67 at a condensing pressure of 1 MPa, while the rise observed in the re-circulated liquid mean volumetric flow rate was 17.4%, from 3.67 to 4.31 l min1. With regard to the NH3 evaporating pressure no clear influence was observed on the recirculation rate nor on the
Acknowledgements The authors would like to acknowledge the technical support provided by the KINARCA S.A.u. Company in designing and building the prototype. The prototype has been built and tested at the KINARCA facilities in Vigo (Spain). The authors also acknowledge the financial support received from the “Xunta de Galicia” (Project PGIDIT05DPI006E).
references
Abdulateef, J.M., Sopian, K., Alghoul, M.A., Sulaiman, M.Y., 2009. Review on solar-driven ejector refrigeration technologies. Renewable Sustainable Energy Rev. 13, 1338e1349. ASHRAE, 2002. Liquid Overfeed Systems. ASHRAE Applications Handbook, pp. 1.4 (Chapter 1, Circulating Rate). Bivens, D.B., Patron, D.M., Yokozeki, A., 1997. Performance of R32/ R125/R134a mixture in systems with accumulators or flooded evaporators. ASHRAE Trans. 103 (1), 777e780. Chunnanond, K., Aphornratana, S., 2004. Ejectors: applications in refrigeration technology. Renewable Sustainable Energy Rev. 8, 129e155. Dossat, Roy, 1980. Principles of Refrigeration. John Wiley & Sons, Inc. ISBN 0-471-03550-5. Dopazo, J.A., Ferna´ndez-Seara, J., 2011. Experimental evaluation of a cascade refrigeration system prototype with CO2 and NH3 for freezing process applications. Int. J. Refrigeration 34 (1), 257e267. Elbel, S., Hrnjak, P., 2008. Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation. Int. J. Refrigeration 31 (3), 411e422. Gac, A., 1974. Automatisme des systemes frigorifiques. Association Francaise du Froid, Supplement a` la Revue Ge`ne`rale du Froid Janvier. Giuliani, G., Hewitt, N.J., Marchesi-Donati, F., Polonara, F., 1999. Composition shift in liquid-recirculation refrigerating systems: an experimental investigation for the pure fluid R134a and the mixture R32/134a. Int. J. Refrigeration 22, 486e498. ISO, 1995. Guide to the expression of uncertainty in measurements. International Organization for Standardization (ISO), 9e27. Lemmon, E.W., McLinden, M.O., Huber, M.L., 2008. NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP), Version 8.0. National Institute of Standards and Technology (NIST). Lorentzen, G., 1968. Evaporator design and liquid feed regulation. Int. J. Refrigeration 160 (NovembereDecember). Nakagawa, M., Serrano-Berana, M., Kishine, A., 2009. Supersonic twophase flow of CO2 through convergingediverging nozzles for the ejector refrigeration cycle. Int. J. Refrigeration 32 (6), 1195e1202. Radchenko, N.I., 1985. On reducing the size of liquid separators for injector circulation plate freezers. Int. J. Refrigeration 8 (5), 267e269. Sarkar, J., 2008. Optimization of ejector-expansion transcritical CO2 heat pump cycle. Energy 33 (9), 1399e1406. Stoecker, W.F., 1988. Industrial of Refrigeration. Business News Publishing Co, Troy (MI).
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Experimental evaluation of an ejector as liquid re-circulator in an overfeed NH3 system with a plate evaporator J. Alberto Dopazo, Jose´ Ferna´ndez-Seara* A´rea de Ma´quinas y Motores Te´rmicos, E.T.S. de Ingenieros Industriales. University of Vigo, Campus Lagoas-Marcosende No 9, 36310 Vigo, Spain
article info
abstract
Article history:
This paper deals with the experimental performance evaluation of an ejector, linked to
Received 13 August 2010
a manual expansion valve, working as a liquid re-circulator component in an overfeed NH3
Received in revised form
plate evaporator. The evaporator was tested in a single stage system belonging to a cascade
22 December 2010
refrigeration system prototype. The evaporator is an ALFANOVA HP76 plate heat exchanger
Accepted 23 December 2010
with 50 plates. A Phillips ejector with a 1/200 diameter throat and 1.4 mm diameter nozzle was
Available online 4 January 2011
used. The recirculation rate was experimentally determined for different operating conditions. Experimental data are reported for volumetric flow rate at the manual expansion valve
Keywords:
inlet from 0.8 to 1.6 l min1, evaporating pressure from 0.14 to 0.22 MPa and condensing
Ejector
pressure from 0.85 to 1 MPa. The experimental result showed recirculation rates between 2
Evaporator
and 4. The evaporating capacity varied from 9.48 kW to 18.37 kW. In addition, another two
System
nozzles were tested and the results are also presented and discussed. ª 2010 Elsevier Ltd and IIR. All rights reserved.
Overfeed Ammonia
Evaluation expe´rimentale d’un e´jecteur utilise´ pour recirculer l’ammoniac liquide dans un syste`me avec un e´vaporateur a` plaques suralimente´ Motscle´s : E´jecteur ; E´vaporateur ; Syste`me ; Suralimentation ; Ammoniac
1.
Introduction
Refrigeration evaporators can be classified according to the liquid feed method employed, as direct-expansion evaporators, flooded evaporators and overfeed evaporators. Directexpansion evaporators are usually fed by using an expansion
valve that regulates the flow of liquid through the evaporator. In this case, the amount of liquid used to feed the evaporator is limited by the amount of refrigerant that can be vaporized in it, so that the refrigerant leaves the evaporator superheated and only vapour is suctioned off by the compressor. The second concept is the flooded evaporator, whereby the
* Corresponding author. Tel.: þ34 986 812605; fax: þ34 986 811995. E-mail address: [email protected] (J. Ferna´ndez-Seara). 0140-7007/$ e see front matter ª 2010 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2010.12.023
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
Nomenclature A d h m P Q R PR T V v X
2
area, m diameter, mm specific enthalpy, kJ kg1 mass flow rate, kg s1 pressure, MPa heat transfer rate, kW recirculation rate pressure ratio temperature, C volumetric flow rate, l min1 velocity , m s1 vapour quality, %
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Subscripts c condenser, condensation carbon dioxide CO2 dis discharge e evaporator, evaporation ejec ejector m mean, average main main ejector inlet mev manual expansion valve in inlet ammonia NH3 out outlet rec re-circulated, recirculation sat saturated, saturation
Greek symbols r density, kg m3
evaporator is completely filled with liquid refrigerant so that the entire evaporator inner surface is wet thus improving the heat transfer coefficient (Dossat, 1980). The flooded evaporator is coupled to a liquid receiver from which the liquid is recirculated taking advantage of gravity action, and the vapour is suctioned off by the compressor. The liquid level in the receiver is controlled by using a float device. In liquid overfeed systems much more liquid is fed into the evaporator than actually vaporizes. Some liquid boils in the evaporator and the remainder floods out of the outlet. As a result, the refrigerant leaving the evaporator is always saturated with a vapour quality less than one. The mass flow rate flowing through the evaporator (low pressure side) is higher than the compressor and condenser (high pressure side) rates. The ratio of the total mass flowing through the evaporator compared to the refrigerant vapour mass flow generated in the evaporator is defined as the recirculation rate. There is an ideal circulation rate for each evaporator which will result in the minimum temperature difference and the best evaporator efficiency (Lorentzen, 1968). Several parameters have to be considered to obtain the optimum recirculation rate, such as the heat load, pipe diameter, circuit length, top and bottom feed evaporators and number of parallel circuits in order to achieve the best performance. Usually, the evaporator manufactures specify the ideal recirculation rates for their equipment. For ammonia, ASHRAE (2002) recommends recirculation rate using larger diameter tubes as 6e7 for top deed evaporators and using smaller tubes as 2e4 for bottom feed evaporators. A liquid overfeed system includes a liquid/vapour separator to separate the liquid and supply vapour to the compressor and saturated liquid to the evaporator. The compressor is fed using the vapour from the top of the tank. The vapour coming out of the separator is close to saturation conditions, which mean lower compressor inlet gas temperatures and consequently, lower discharge gas temperatures, which are a critical factor for ammonia systems working at low temperature applications. To feed the evaporator with saturated liquid re-circulated from the separator, a common practice is to use a pump as a liquid re-circulator component, described by Stoecker (1988), Bivens et al. (1997) and Giuliani et al. (1999). However,
the use of pumps increases the initial investment of the facility and the operation and maintenance costs, especially in low capacity refrigeration systems. On the other hand, Gac (1974), in his publication “Automatisme des systemes frigorifiques”, comments that an alternative to the use of pumps in liquid overfeed systems is the use of ejectors. The principal advantage of this option is its simplicity, without mobiles parts, and strong construction. In addition, ejectors are more economical compared to pumps. Nevertheless, Gac (1974) stated that the effectiveness of ejector performance is strongly linked to the quality of its construction, pointing out that in practice the use of ejectors had been consigned to auxiliary refrigeration systems in which the refrigeration capacity remained constant or with few changes. One liquid overfeed system using R-22, in which an ejector was used to feed a plate freezer of eight freezing stations was studied by Radchenko (1985). His results included data of the liquid collected in the liquid/vapour separator. However, recirculation rate data were not shown. In recent years, ejectors and their applications in jet refrigeration cycles have been widely studied. As examples of this latter, the researches of Sarkar (2008), Elbel and Hrnjak (2008) and Nakagawa et al. (2009), and the reviews performed by Chunnanond and Aphornratana (2004) and Abdulateef et al. (2009), can be cited. The authors have not found any other references about the use of ejectors in liquid overfeed systems. The main purpose of this work was to experimentally evaluate the performance of an ejector working as a liquid re-circulator component in an overfeed plate evaporator with NH3. The experimental tests were performed using an experimental prototype cascade refrigeration system with CO2 and NH3. In this paper, the experimental setup and procedure are described, the reduction data process detailed, and the results shown and discussed.
2.
Experimental facility
The experimental facility consists of a cascade refrigeration system prototype made of two single stage systems connected by a heat exchanger. CO2 is used as the refrigerant in the low
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NH Evaporator 3 (cascade exchanger) CO2 low temperature system
(3)
Air condenser (1) P
(6)
T
01
01
P
02
T
02
(2)
NH3 Compressor
900 mm
(7) V
T 03
03
02
P (4) (5)
P
Ejector
04
Liquid receiver V01
200 mm
1150 mm
Liquid/vapour separator
Manual expansion valve
Fig. 1 e Schematic diagram of the experimental facility test section.
temperature system and NH3 for the high temperature system. Fuller information related to the cascade system prototype can be found in Dopazo and Ferna´ndez-Seara (2011). The test section is the high temperature system with NH3. The schematic diagram of the test section is shown in Fig. 1, and a real view of the experimental facility can be appreciated in Fig. 2. In the high temperature system, the flow of NH3 coming from the liquid/vapour separator is compressed in a fourcylinder reciprocating open type compressor coupled to a variable speed electric motor with a pulley system. The superheated flow of NH3 enters into the air condenser and ejects the heat into the atmospheric air. The condensed stream of NH3 coming from the air condenser is channelled into a liquid
Fig. 2 e Photograph of the experimental prototype of the cascade refrigeration system.
receiver. The NH3 liquid leaves the receiver and flows through a manual expansion valve and enters into the NH3 ejector (used to overfeed the evaporator) where it is expanded until it reaches evaporating pressure and mixed with a secondary flow of liquid NH3 coming from the liquid/vapour separator. The resulting flow mixture enters into the cascade heat exchanger, in which it evaporates and returns to the separator. The cascade heat exchanger, where the NH3 is evaporated, consists of an ALFANOVA HP76 counter flow plate heat exchanger, with 50 plates of Alloy 316 SS, and dimensions 191 191 618 mm. The diameters of the refrigerant lines at the main and re-circulated liquid ejector’s inlets and outlet, refer to Fig. 1, are d4 ¼ 10 mm, d5 ¼ 25.4 mm and d7 ¼ 25.4 mm, respectively. The ejector and the manual expansion valve were located at the lowest level of the experimental facility. The level difference between the outlet of the liquid receiver and inlet of the manual expansion valve is 200 mm, the level difference between the secondary outlet of the vapour/liquid separator and the secondary ejector’s inlet is 900 mm and the level difference between the ejector’s outlet and the NH3 inlet of the cascade heat exchanger is 1150 mm. Fig. 1 includes the height levels of the separator re-circulated liquid outlet, liquid receiver outlet and the NH3 evaporator inlet with regard to the ejector location. The experimental facility was equipped with a data acquisition system based on a 16-bit data acquisition card and a PC. The NH3 suction and discharge temperatures are measured by using the sensors T01 and T02, located in the suction and discharge lines of the NH3 compressor, respectively (refer Fig. 1). The NH3 condenser temperature is measured by using the sensor T03, located in the NH3 liquid receiver. The suction pressure is measured by using the pressure transducer P01, located in the liquid/vapour separator. The discharge pressure of the NH3 compressor is measured by using the pressure transducer P02, located in the discharge line of the NH3 compressor. The condensing
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
pressure of NH3 is measured by using the pressure transducer P03, installed in the NH3 liquid receiver. The pressure at the NH3 ejector main inlet is measured by using the pressure transducer P04. The main volumetric flow rate of NH3 is measured at the manual expansion valve inlet by using the volumetric flow-meter V01 (refer Fig. 1). The re-circulated liquid flow rate of NH3 used to overfeed the NH3 evaporator, is measured by using the volumetric flow-meter V02, located at the auxiliary outlet of the NH3 liquid/vapour separator. All the temperature sensors used in the experimental facility are A Pt100 inserted in 3 mm diameter stainless steel pockets, with an accuracy of (0.15 þ 0.002 T) ( C) in the measurement range (25/130 C). The pressure transducers are WIKA ECO 1 type, with an accuracy of 0.5% of the full scale (2.6 MPa). The NH3 volumetric flows were measured by using electromagnetic flow-meters, with an accuracy of 0.25% of the measured value.
3. Experimental procedure and data reduction
Table 1 e System operating conditions considered in the experimental parametric study. Vmev,in (V01) (l min1) 0.8, 1.0, 1.2, 1.4, 1.6
Pe;NH3 (MPa)
Pc;NH3 (MPa)
0.14, 0.16, 0.18, 0.2, 0.22 0.85, 0.9, 0.95, 1
resistances regulated independently by using electric power regulators were used in the low temperature system with CO2 to establish the refrigeration capacity of the cascade system. The NH3 compressor capacity was established by using a frequency converter, which permitted the modification of the electric motor velocity, and consequently the gradual variation of the NH3 compressor capacity. The mass flow of the liquid refrigerant was controlled by means of the manual expansion valve located at the main inlet line of the ejector (Fig. 1). The condensing pressure of NH3 was controlled by modifying the air flow in the condenser. Once all temperature, pressure and flow measurements were stabilized, the experimental data were collected during a time frame of five minutes, at intervals of approximately 5.4 s.
3.2. 3.1.
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Data reduction
Experimental procedure
The NH3 ejector used in the experimental evaluation was a Phillips ejector with a 1/200 diameter throat and 1.4 mm diameter nozzle, as is shown in Fig. 3. All the tests were performed under stable operating conditions, i.e. when the variations in all temperature and pressure measurements were within 1% range. The experimental evaluation was initiated by fixing the actual operating condition of the cascade system prototype. The NH3 evaporating pressure was 0.2 MPa, the condenser pressure was 0.85 MPa and the volumetric flow rate at the manual expansion valve inlet was 1.2 l min1. Subsequently, the ejector behaviour as a liquid re-circulator component was evaluated at several NH3 volumetric flow rates, keeping constant the NH3 evaporating and condensing pressures at 0.2 MPa and 0.85 MPa, respectively. Finally, the influence of the evaporating and condensing NH3 pressures on the ejector performance was also evaluated. The operating conditions of the facility during the experimental evaluation carried out are shown in Table 1. The low temperature system with CO2 was used to set the evaporating NH3 pressure and to supply the refrigeration charge in the cascade heat exchanger (NH3 evaporator). Electric
The recirculation rate, the NH3 vapour quality at the main inlet and outlet of the ejector, the refrigerant flow mean velocities and the NH3 system refrigeration capacity were calculated from the data experimentally measured. The calculations of the previously mentioned parameters were formulated from the mass and energy balances applied to the NH3 system components. Thermodynamic properties of NH3 were obtained using the REFPROP Database (Lemmon et al., 2008). The following assumptions were made in the analysis: (a) Pressure losses in connecting pipes and heat exchangers have been neglected. (b) The ejector was perfectly isolated. (c) Vapour and liquid phases were in thermodynamic equilibrium. (d) Changes in kinetic and potential energy were neglected. (e) There was saturated liquid at the re-circulated liquid inlet (line 7, Fig. 1) of the ejector. Taking into account the above assumptions the calculated parameters are detailed as follows, according to the nomenclature in Fig. 1.
Fig. 3 e Schematic of the Phillips ejector tested.
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
m4 ¼ m3
(1)
h4 ¼ h3
(2)
m5 ¼ m4 þ m7
(3)
components in any available literature. Therefore, an energy balance was performed in the cascade heat exchanger to compare the heat transfer rate obtained from the NH3 evaporation to the heat transfer rate obtained from the CO2 condensation of the low temperature system of the experimental facility. Results shown in Fig. 4 reveal that all the experimental data remain within an error band of 5%.
m4 m7 þ h7 $ h5 ¼ h4 $ m5 m5
(4)
4.2.
The main and re-circulated mass flow rates at both inlets of the ejector are given by Eqs. (5) and (6). In these equations the NH3 volumetric flow rates V01 and V02 are experimentally measured and the densities r3 and r7, are obtained as a function of the pressure and temperature measurements at the liquid receiver P03 and T03, and the liquid/vapour separator P01, respectively. m4 ¼ V01 $r3 ¼ mmain
(5)
m7 ¼ V02 $r7 ¼ mrec
(6)
The recirculation rate RNH3 , and the pressure ratio PRejec evaluated between the main inlet and the outlet of the ejector were determined according to Eqs. (7) and (8), respectively. m5 (7) RNH3 ¼ m4 PRejec ¼
P4 P5
(8)
In Eq. (8), P5 was considered equal to the evaporating pressure, measured at the liquid/vapour separator (P01). The evaporating capacity of the NH3 system is calculated in Eq. (9) Qe;NH3 ¼ m3 $ðh1;sat h3 Þ
(9)
The NH3 mean flow velocities at the inlets and outlet of the ejector were obtained from Eq. (10). m (10) v¼ r$A The NH3 qualities at the main inlet and outlet of the ejector (states 4 and 5, Fig. 1) were calculated as a function of the NH3 specific enthalpies and the pressures experimental measurements P04 and P01, respectively. On the other hand, a detailed analysis of the experimental uncertainties was carried out. This analysis takes into account the uncertainties in the determination of the fluid properties from the Refprop database. These uncertainties are 0.2% in density, 2% in heat capacity and 0.2% in vapour pressure. The experimental uncertainties were determined according to ISO (1995). The maximum for typical uncertainties was estimated to be 0.25% for mmain, 0.30% for mrec, 0.31% for RNH3 , 4.07% for PRejec, 1.84% for Qe;NH3 , and 14.39% for v.
Experimental results
The experimental results obtained at the initial operating condition established previously (Pe;NH3 ¼ 0:2 MPa, Pc;NH3 ¼ 0:85 MPa and Vmev,in ¼ 1.2 l min1) showed a NH3 liquid recirculation rate of 3.1, which indicates that the ejector performance working as a liquid re-circulator component was satisfactory. The evaporating power of the NH3 obtained in this case was 13.79 kW. The results obtained from the experimental parametric study performed at the operating conditions described in Table 1 are shown and discussed below. Figs. 5 and 6 depict the experimental measurements of pressures and re-circulated liquid volumetric flow rates Vrec, obtained for each value of NH3 volumetric flow rate at the manual expansion valve inlet Vmev,in specified in Table 1. The instantaneous values of the variables measured during five minutes are shown. The condensing and evaporating pressures remained constant at 0.85 and 0.2 MPa, respectively. In Fig. 5 the pressure at the main inlet of the ejector P04 and the main NH3 volumetric flow rate Vmev,in for different positions of the manual expansion valve can be appreciated. As the manual expansion valve opening becomes higher, the pressure drop in this valve decreases. As a result, the pressure at the main inlet of the ejector is higher and the NH3 volumetric flow rate at manual expansion valve inlet increases. Related to the experimental measurements of both main and re-circulated NH3 volumetric flow rates shown in Fig. 6, stable values can be appreciated for the main volumetric flow rates. Nevertheless, low oscillations can be observed in the NH3 re-circulated liquid volumetric flow rate measurements for NH3 main volumetric flow rate of 1.0, 1.2, 1.4 and
20 18 + 5%
16 14
Qc,CO2 (kW)
The mass and energy balances on the manual expansion valve and on the ejector are given by Eqs. (1)e(4), respectively.
- 5%
12 10 8 6 4 2
4. 4.1.
Results and discussion Experimental data validation
The authors did not find experimental data for recirculation rates in overfeed evaporators using ejectors as recirculation
0 0
2
4
6
8
10 12 14 16 18 20
Qe,NH3 (kW) Fig. 4 e Comparison of the heat transfer rates determined from the experimental data through the energy balances on the cascade heat exchanger.
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
1.2
20 -1
1.0
Vmev,in (l·min ) 1.0
1.2
1.4
18
1.6
0.8
Qe,NH3 (kW)
Pressure (MPa)
0.8
0.6 0.4 0.2
14 12 10
PPA separator NH3 0.0 0
16
P comp,dis PD NH3
Preceiver PL NH3
P ejec,in PE NH3
8 300║0
300║0
300║0
300║0
25
300
30
35
40
45
50
55
60
65
-1
mmain (kg·h )
Time (s)
Fig. 5 e Experimental pressures at several NH3 main volumetric flow rates (Vmev,in).
Fig. 7 e NH3 evaporating power as a function of the NH3 main mass flow rate.
1.6 l min1. The oscillations corresponding to the lower NH3 main volumetric flow rate of 0.8 l min1 are especially significant. The results indicate that, for the fixed evaporating and condensing pressures, there would seem to exist a minimum main mass flow rate value which assures the stabilities of the dragging process taking place inside the ejector. Once this main mass flow limited value is reached, high instabilities occur at the ejector’s secondary inlet having an adverse effect on the liquid recirculation. However, it is fitting to point out the fact that in spite of this latter, liquid recirculation was present. On the other hand, as the NH3 mass flow rate at the main inlet of the ejector increases, the evaporating power of the system also increases as can be observed in Fig. 7. The evaporating power obtained varied from 9.48 kW within the main mass flow rate of 29.8 kg h1 to 18.37 kW within the main mass flow rate of 58.3 kg h1. Fig. 8 shows the results of the NH3 vapour quality values calculated at the main inlet and outlet of the ejector. The tendency observed that the NH3 vapour quality at the main inlet of the ejector follows the behaviour traced by the manual expansion valve opening. As the valve opening increases, the mass flow rate and the pressure at the main inlet of the ejector P04 increase. The pressure rise causes a decreasing trend in the NH3 vapour quality as the flow of NH3 at the main inlet of the
ejector draws closer to a saturated liquid condition. However, the results obtained from the NH3 vapour quality at the ejector’s outlet showed the opposite trend. In this case, as the main mass flow rate increases, the NH3 vapour quality calculated at the outlet of the ejector tends to rise lightly. Fig. 9 shows the NH3 flow mean velocity values calculated at the main and re-circulated liquid inlets and outlet of the ejector. Despite the increase in the NH3 main mass flow rate, a decreasing trend of the flow mean velocity values obtained at the main inlet of the ejector as a result of the NH3 density rise caused by the NH3 vapour quality decrease, can be appreciated. On the other hand, the NH3 flow mean velocity values calculated at the outlet of the ejector tend to increase, whereas the flow mean velocity values obtained at the recirculated liquid inlet of the ejector show a constant trend. All the NH3 flow mean velocity values observed in the experimental evaluation performed are included between 0.88 and 1.71 m s1. In Fig. 10 the experimental results of the re-circulated liquid volumetric flow rate Vrec and the NH3 liquid recirculation rate are represented as a function of the NH3 mass flow rate at the main inlet of the ejector. A decreasing tendency of the recirculation rate while the re-circulated liquid volumetric flow rate tends to remain constant can be appreciated. It is
Vmev,in
5.5
Vrec
Xin,main
5.0
5
Xout
4.5 4
Quality (%)
Volumetric flow rate (l·min-1)
6
3 2
4.0 3.5 3.0 2.5
1 0
2.0 1.5 0
300║0
300║0
300║0
300║0
Time (s)
Fig. 6 e Experimental main and re-circulated liquid volumetric flow rate.
300
25
30
35
40
45
50
55
60
65
-1
mmain (kg·h )
Fig. 8 e NH3 vapour quality as a function of the NH3 main mass flow rate.
1682
6
5
5
4
4
3
1.0
3
2
0.5
2
vin,main vin,rec vout
1.5
0.0 25
30
35
40
45
50
55
60
1 0.80
65
-1
mmain (kg·h )
R
1
V
0.85
0.90
0.95
0 1.05
1.00
Pc,NH3 (MPa)
Fig. 9 e Flow mean velocities as a function of the NH3 main mass flow rate.
worth pointing out the special behaviour observed when the measured value of main volumetric flow rate was 0.8 l min1. At this point a considerable reduction of the recirculation rate was found and a tendency change was appreciated. The drop of this value can be linked with the high amplitude of oscillations observed in the NH3 re-circulated liquid volumetric flow rate measurements, depicted in Fig. 6. As previously indicated, instabilities at the ejector’s secondary inlet have an adverse effect on the liquid recirculation. Despite this adverse condition, the mean recirculation rate obtained was 3.41. Results depicted in Fig. 10 also show that for the evaluated experimental operating conditions, the lowest main mass flow rate which assures a stable process in the ejector is within the range from 30 to 36 kg h1, approximately. Fig. 11 shows the experimental measurements of recirculated liquid volumetric flow rates and the recirculation rate calculated as a function of the NH3 condensing pressure. The evaporating pressure remained constant at 0.2 MPa. Increasing trends were observed in both cases when the condensing pressure increased. The recirculation rate rose 18.4%, from 3.1 with condensing pressure at 0.85 MPa, to 3.67 with condensing pressure at 1 MPa, while the increment observed in the re-circulated liquid mean volumetric flow rate was up 17%, from 3.67 to 4.31 l min1.
Fig. 11 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 condensing pressure.
The variation of the evaporating pressure in the values range described in Table 1 did not show a clear influence on the ejector behaviour, as can be observed in Fig. 12. The condensing pressure was fixed at 0.85 MPa. In this case, the recirculated liquid volumetric flow rate, as well as the NH3 recirculation rate showed fairly constant tendencies. On the other hand, two other nozzles with diameters of 1.2 and 1.6 mm, E1 and E3 respectively, were used to evaluate the ejector behaviour. The experimental results of the recirculation rate and the ejector pressure ratio are depicted in Fig. 13. A similar behaviour of the ejector using all three nozzles can also be appreciated. The highest recirculation rate values, around 3.16 of mean value, were obtained using the nozzle E2 (initial case). The mean value of recirculation rate obtained by using the nozzle E3 was 2.56, which represents a reduction of 19% compared to nozzle E2. In both cases the main NH3 volumetric flow rate Vmev,in was 1.2 l min1. With nozzle E1, the highest NH3 main volumetric flow rate Vmev,in obtained during the experimental evaluation was 0.8 l min1, with the manual expansion valve positioned completely open. The mean value of the recirculation rate was 2.19, 30% lower than the mean recirculation rate obtained with nozzle E2.
10
6
6.0
R
9
V
4.0
4
3.0
-1
2.0 2.0 R 1.0 25
35
40
45
-1
6
3
5
2
4 1
3
V 1.0
30
RNH3
RNH3
3.0
Vrec (l·min )
7 4.0
50
55
60
5
8
5.0
65
-1
mmain (kg·h )
Fig. 10 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 main mass flow rate.
Vrec (l·min )
2.0
RNH3
-1
Flow mean velocity (m·s )
2.5
Vrec (l·min-1)
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
2 0 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 Pe,NH3 (MPa)
Fig. 12 e Recirculation rate and re-circulated liquid volumetric flow rate as a function of the NH3 evaporating pressure.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 6 7 6 e1 6 8 3
re-circulated liquid volumetric flow rate. Both variables tend to remain constant.
5 4
E3
3 2
RNH3
4
PRejec
E1 E2
1
3 2 1 0.13
0.14
0.15
0.16
0.17 0.18 0.19 Pe,NH3 (MPa)
0.20
0.21
0.22
0.23
Fig. 13 e Recirculation rate and ejector’s pressure ratio as a function of the NH3 evaporating pressure.
In addition, as the evaporating pressure increases, the ejector’s pressure ratio decreases as can be appreciated in Fig. 13. However, a clear relationship between the ejector’s pressure ratio and the recirculation rate values is not apparent.
5.
1683
Conclusions
The experimental results obtained in this work confirm that an ejector linked to a manual expansion valve can be used as a liquid re-circulator component in liquid overfeed systems with NH3, under different operating conditions and with variable refrigeration capacity. The initial test showed a recirculation rate of 3.1. The refrigeration capacity was 13.79 kW, the evaporating and condensing pressures 0.2 and 0.85 MPa, respectively, and the liquid main volumetric flow rate measured at the manual expansion valve inlet was 1.2 l min1. The ejector tested has a nozzle with a diameter of 1.4 mm and a throat with a diameter of 1/200 . An experimental parametric study was performed to evaluate the behaviour of the tested ejector working as a liquid recirculator component. The experimental results showed that, as the main liquid mass flow rate increases, the re-circulated liquid volumetric flow rate tends to remain constant and the recirculation rate tends to decrease. The evaporating power obtained from the experimental measurements varied from 9.48 kW using a main mass flow rate of 29.8 kg h1 to 18.37 kW using a main mass flow rate of 58.3 kg h1. In addition, it can be observed from the experimental parametric study results that the liquid re-circulated volumetric flow rate, as well as the recirculation rate, clearly showed increased trends when the NH3 condensing pressure rose. The increase of the recirculation rate was 18.4%, from 3.1 at condensing pressure of 0.85 MPa, to 3.67 at a condensing pressure of 1 MPa, while the rise observed in the re-circulated liquid mean volumetric flow rate was 17.4%, from 3.67 to 4.31 l min1. With regard to the NH3 evaporating pressure no clear influence was observed on the recirculation rate nor on the
Acknowledgements The authors would like to acknowledge the technical support provided by the KINARCA S.A.u. Company in designing and building the prototype. The prototype has been built and tested at the KINARCA facilities in Vigo (Spain). The authors also acknowledge the financial support received from the “Xunta de Galicia” (Project PGIDIT05DPI006E).
references
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