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Performance of ejector cooling systems using low ecological impact refrigerants
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 7 0 7 e1 7 1 6
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Performance of ejector cooling systems using low ecological impact refrigerants Raul Roman a, Jorge I. Hernandez b,* a b
Posgrado en Ingenierı´a, Energı´a, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 34, Temixco, Morelos 62580, Mexico Centro de Investigacio´n en Energı´a, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 34, Temixco, Morelos 62580, Mexico
article info
abstract
Article history:
The theoretical behaviour of an ejector cooling system, using as working fluids propane,
Received 16 March 2010
butane, isobutane, R152a and R134a, is obtained. The ejector works as a thermo-
Received in revised form
compressor that is simulated with a validated one-dimensional mathematical model,
18 February 2011
whose errors are lower than 6%. For a system unitary cooling capacity, a parametric study
Accepted 12 March 2011
is carried out varying the generation, condensation and evaporation temperatures. From
Available online 21 March 2011
the obtained data, a complete analysis of the system performance can be achieved when the ejector and system operation parameters are considered. The best performance
Keywords:
corresponds to the system using propane, because has the highest system coefficient of
Cooling
performance and its ejector has the maximum entrainment ratio value, the least area ratio
Ejector system
value and the highest efficiency value. The considered generation temperature ranging
Performance
from 70 C to 95 C is appropriate for low-grade energy sources assisting thermal cooling
Hydrocarbon
systems. After this system performance, come those in which R152a and R134a are
R134a
employed, with isobutane and butane at the end. The obtained results represent potential
R152a
design points of an efficient ejector cooling system operation, because to each combination of the above mentioned temperatures corresponds one and only one ejector geometry. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Performance des syste`mes de refroidissement a` e´jecteur employant des frigorige`nes exerc¸ant un faible impact sur l’environnement Mots cle´s : Refroidissement ; Syste`me a` e´jecteur ; Performance ; Hydrocarbure ; R134a ; R152a
1.
Introduction
The current searching of new refrigerants to operate Ejector Cooling Systems, ECS, has a dual purpose: to improve its performance and decrease its ecological impact (Sun, 1995;
Chunnanond and Aphornratana, 2004; Abdulateef et al., 2009). ECS employing halocarbon refrigerants have reached good efficiencies as well as appropriate levels of operation versatility. Unfortunately, the phasing-out of ozone-damaging refrigerants
* Corresponding author. Tel./fax: þ55 56229791. E-mail addresses: [email protected] (R. Roman), [email protected] (J.I. Hernandez). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.03.006
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Nomenclature COP d ECS Ff F0f G h k l L _ m M* MFPM p Q_ T U V _ W f G h U j
coefficient of performance (dimensionless) diameter (m) ejector cooling system evaluated friction factor (dimensionless) friction factor datum (dimensionless) mathematical function specific enthalpy (kJ kg1) specific heat ratio, cp/cv, (dimensionless) distance, length (m) length ratio (dimensionless) mass flow rate (kg s1) critical Mach number (dimensionless) modified false position method pressure (MPa) heat flow rate (kW) temperature ( C) _ 1 (dimensionless) _ 2 =m entrainment ratio m velocity (m s1) mechanical power or work rate (kW) ejector area ratio, (dm/dt)2, (dimensionless) ejector expansion ratio, p1/p2, (dimensionless) efficiency (dimensionless) diffuser area ratio, (dd/dm)2, (dimensionless) main nozzle area ratio, (de/dt)2, (dimensionless)
has made necessary the research of alternatives, among which, the hydrocarbons are advisable candidates in small refrigeration systems. They are environmentally friendly with null or negligible ozone depletion and global warming potentials, but are explosive as well as flammable and require special manipulation in practical usage. In order to know how the ECS behaves thermodynamically in typical cooling applications employing some hydrocarbons as refrigerant, certain theoretical research has been carried out (Haidar et al., 1995; Selvaraju and Mani, 2004; Pridasawas and Lundqvist, 2007; Nehdi et al., 2008; Boumaraf and Lallemand, 2009). The main information available in literature for some of the hydrocarbon and nowadays accepted halocarbon refrigerants is shown in Table 1. As observed, the temperature difference TGE TCO and TCO TEV is an important parameter related to the expansion and compression processes carried out internally in the ejector, in which the first has to be higher as a work provider for the compression process. So, for an ECS employing isobutane were obtained entrainment ratio values between 0.12 and 0.45 for generator, condenser and evaporator temperatures varying from 82 C to 90 C; 25 C to 52 C and 4 C to 10 C, respectively (Haidar et al., 1995; Selvaraju and Mani, 2004; Pridasawas and Lundqvist, 2007). The entrainment ratio value of 0.12 resulted to be high for an expansion and compression temperature difference of 30 C and 48 C, whose values have a contrary tendency to the other reported results. For the butane only one study was found with an entrainment ratio value of 0.31 for generator and evaporator temperatures of 90 C and 15 C with a condenser temperature of 35 C (Nehdi et al., 2008). A similar situation of inconsistency was also found for the propane and halocarbon refrigerants, according to the results shown in
q x
ejector temperature ratio, T2/T1, (dimensionless) ejector driving pressure ratio, p1/p3, (dimensionless)
Subscripts a ideal condition at the main nozzle exit a’ actual condition at the main nozzle exit c mixing chamber exit CO condenser comp compression d diffuser e main nozzle exit E ejector exp expansion EV evaporator GE generator i ideal m mixing chamber n main nozzle, between main nozzle exit and mixing chamber inlet reversible pump pr s system t main nozzle throat 1,.,6 thermodynamic cycle states or mathematical function increasing numeration
Table 1. As seen, these data do not agree and certainly the experimental information available by that time was insufficient to validate the employed ejector mathematical models. On the other hand, and from a practical point of view, some domestic mechanical compression systems employing isobutane are nowadays available commercially, because operation risks have been lessened with appropriate designs. In order to clarify the discrepancies above mentioned, a careful study on an ECS with low ecological impact refrigerants is necessary. Therefore, the purpose of the present work is to obtain theoretically the performance parameters of an ECS, for the system and ejector, through a parametric study in which the temperatures of the generator, TGE, condenser, TCO, and evaporator, TEV, are varied. A one-dimensional ejector mathematical model performing under secondary flow choking shall be used with a previous validation with experimental data available in the recent literature. As well, the refrigerants with low ecological impact, propane, butane, isobutene and R152a are employed. Refrigerant R134a is also included because it has appropriate thermodynamic properties, in spite of being considered as a refrigerant in transition as a consequence of its medium global warming potential value.
2.
Parametric study
2.1.
System characteristics
The ECS is made up by a generator, condenser, evaporator, ejector, pump and expansion valve, as indicated in Fig. 1, where the system thermodynamic states are also included.
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Table 1 e Principal characteristics of the ECS behaviour obtained from literature. Working fluid
U
f
R152a R134a R123 R600a
0.235 0.23 0.20 0.12
5.32 5.37 7.19 2.29
55 55 55 30
30 30 30 48
R290 R600a R152a R134a R600a
2
R152a R134a R290 R600 R134a
0.365 0.29 0.28 0.275 0.02 0.04 0.075 0.105 0.385 0.38 0.37 0.31 0.35
4.42 4.95 6.14 7.5 6.06 6.12 5.13 6.41 6.68
16 14 14 14.5 55 60 70 80 55 55 55 55 60
7 9 9 9.5 45 45 45 45 20 20 20 20 25.4
10
R134a R600a R134a
0.48 0.45 0.27
6.63 6.42 5.64
55 55 51
20 20 19
10
R600a
0.225
3.63
45
35
Type of study
2nd flow choking
ln (mm)
Ff
TGE ( C)
TCO ( C)
TEV ( C)
Sun, 1999
Th.
Yes
e
e
90
35
5
Haidar et al., 1995 Kairouani et al., 2009
Th.
e
e
e
82
52
4
Th.
Yes
s0
0.06
5
18
Boumaraf and Lallemand, 2009
Th.
Yes
e
e
Nehdi et al., 2008 a
Th.
Yes
e
e
100 105 115 125 90
11 9 9 8.5 45
35
15
Cizungu et al., 2001 a Selvaraju and Mani, 2004 Selvaraju and Mani, 2006 Pridasawas and Lundqvist, 2007
Th.
Yes
0.03
90
30
4.6
Th.
Yes
e
e
80
25
5
Ex.
e
1.8
e
80
29
Th.
Yes
e
e
90
45
Author
0
0
TGE TCO ( C)
TCO TEV ( C)
a Values in the U column correspond to COPs.
2 de dt
The ejector is shown in Fig. 2 with its main geometric variables, where subscripts 1 and 2 correspond to the inlet of primary and secondary flows and 3 to the exit flow. From the cross-sectional areas belonging to the throat and exit of the ejector’s main nozzle, inlet of the cylindrical mixing chamber and exit of the diffuser, the following ejector dimensionless geometrical parameters result: the ejector area ratio
j¼
2 dm f¼ dt
where lm is the mixing chamber length; and the main nozzle separation distance ratio
(1)
in which dm is the mixing chamber diameter and dt is the main nozzle throat diameter; the main nozzle area ratio
being de the main nozzle exit diameter. Other important ejector geometrical relations are the mixing chamber length ratio Lm ¼
Ln ¼
lm dm
(3)
ln dm
(4)
in which ln is the distance between the mixing chamber inlet and the main nozzle exit. The considered ejector’s thermodynamic parameters are its entrainment ratio U¼
Fig. 1 e Ejector cooling system configuration.
(2)
_2 m _1 m
(5)
Fig. 2 e Ejector configuration.
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_ 1 and m _ 1 the mass flow rates for the primary and being m secondary fluids; and its efficiency _ com W hE ¼ _ exp W
(6)
_ com is the work rate required in the secondary in which W _ exp the work rate fluid’s isentropic compression process and W produced in the main fluid’s isentropic expansion process (Mooney David, 1955). Considering the conservation equations for the actual and ideal processes, the resulting efficiency is hE ¼ hcom hexp
(6a)
where hcom and hexp are the efficiencies for the compression and expansion processes, respectively. As these efficiencies behave contrary, a couple of these parameters shall maximize the ejector efficiency. A more common form of the ejector efficiency is found when the equations of conservation and production of entropy for the ideal and actual processes are employed, resulting hE ¼
U Ui
(6b)
where Ui is the ideal entrainment ratio for the ejector’s reversible processes of expansion and compression (Chen and Hsu, 1987). In regard to the ECS, its coefficient of performance is Q_ EV (7) COPs ¼ _ _ r Q GE þ Wp
Ma ¼ G1 ðk; GÞ
(11)
1 f ¼ G2 k; G; Uq =2 ; Ma
(12)
1 M3 ¼ G3 k; f; U; x; Uq =2
(13)
Mc ¼ G4 k; hd ; U; M3
1 Ff ¼ G5 k; Lm ; Mc ; Ma ; Uq =2
(14) (15)
where k is the refrigerant specific heat ratio, cp/cv, G is the ejector expansion ratio, p1/p2, q the ejector temperature ratio, T2/T1, x the ejector driving pressure ratio, p1/p3, hd the diffuser efficiency, U the diffuser area ratio, (dd/dm)2, Ff the friction factor and Ma ; Mc andM3 are the exit critical Mach numbers for the main nozzle, mixing chamber and ejector, respectively. Lu found from experimental results a friction factor of 0.07 for an ECS operating with refrigerant R11. The algorithm employed to solve the mathematical model of Lu is shown in Fig. 3. The input values to find G converging to F0f datum are T2, T3, U, f, hd and k. Firstly, G values enclosing its converging solution are found. Then, with the Modified
whose terms are the heat flow rate transferred at the evaporator and generator as well as the rate of mechanical work required by the reversible pump, which are defined as _ 1 ðh1 h5r Þ Q_ GE ¼ m
(8)
_ 2 ðh2 h6 Þ Q_ EV ¼ m
(9)
_ r¼m _ 1 ðh5r h4 Þ Wp
(10)
and subscripts are indicated in Fig. 1, where the reversible pump exit enthalpy is given by h5r. Substitution of Eqs. (5) and (8) to (10) into Eq. (7), gives ðh2 h6 Þ COPs ¼ U ðh1 h4 Þ
2.2.
(7a)
Validation of the ejector mathematical model
The ejector simulation is performed with the one-dimensional mathematical model of Lu (Lu, 1986), in which the secondary fluid choking is considered in conjunction with the following assumptions: 1. 2. 3. 4.
Steady state performance; Fluids behave as perfect gases; Flow along the main nozzle is isentropic; Friction losses in mixing chamber are considered as pipeline minor losses; 5. Null main nozzle separation distance; 6. Complete fluid mixture at mixing chamber exit which gave rise to the next system of dimensionless nonlinear explicit relations.
Fig. 3 e Algorithm to solve the Lu’s mathematical model (Lu, 1986), for the ejector operating under secondary flow choking.
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False Position Method, MFPM, converging G is determined as well as the respective entrainment ratio U. In order to validate Lu’s model with other ejector geometries and refrigerants, the experimental results of Yapici et al. (2008) were employed. Using refrigerant R123, they considered the influence of the ejector area ratio on optimum ejector performance. Six different ejectors were used with a fixed separation distance of the main nozzle of 5 mm and a constant mixing chamber length ratio of 7.5. This separation allows the ejector to reach higher U values which render in a system improvement. Therefore, original Lu’s model was modified and a similar algorithm was employed having T1, T2, T3, f, U, hd, lm and U as inputs. Eqs. (11) and (12) are uncoupled and the ideal critical Mach number, Ma , is determined with the first equation and the actual critical Mach number, Ma0 , with the second one. The rest of the unknowns are found sequentially until Ff. With these critical Mach numbers, the main nozzle efficiency, hn, is evaluated as hn ¼
2 Ma0 Ma
(16)
The results obtained are shown in Fig. 4, where the friction factor and nozzle efficiency are plotted against dimensionless abscissa Ln and j, respectively. A straight line was adjusted to every group of results and the extrapolation of Ff for a null nozzle separation distance reached a value of 0.066 which agrees with that obtained by Lu for R11. With these distributions, a generalised algorithm was employed to validate the ejector performance giving T1, T2, T3, U, hd, lm, k, Ff like function of Ln and hn as function of j in order to obtain the theoretical values of U and f, whose errors with Yapici data were lower than 6%, as Table 2 shows. So, a reliable ejector model with such main nozzle geometry and mixing chamber characteristics is available. As well, it is
Fig. 4 e Friction factor Ff and nozzle efficiency hn against the double dimensionless abscissa, main nozzle separation distance ratio Ln and main nozzle area ratio j.
Table 2 e Errors obtained in the validation of the Lu’s ejector model with experimental data of Yapici et al. (2008). TEV ( C)
TCO ( C)
TGE ( C)
10 10 10 10 10
33.75 33.75 33.75 33.75 33.75
83 90 91 98.2 103.3
f Lu
Exp.
6.54 8 8.25 10.09 11.39
6.56 7.86 8.32 9.97 11.45
Error % 0.305 1.781 0.841 1.204 0.524
U Lu
Exp.
0.398 0.441 0.472 0.516 0.548
0.376 0.454 0.448 0.524 0.555
Error % 5.851 2.863 5.357 1.527 1.261
important to point out that this algorithm allows obtaining an efficient ejector for every group of inlet data. Therefore, the results found correspond to the characteristics of a particular ejector and one ejector obtained in this way shall never satisfy two different groups of inlet data.
2.3.
Selected refrigerants characteristics
The working fluids chosen for the ECS operation are the hydrocarbon refrigerants: propane, isobutane and butane, as well as halocarbon refrigerants R152a and R134a, in which R123 is also included because it was employed in the ejector model validation. Table 3 shows their main physical, environmental and safety characteristics. In the first column is indicated the refrigerant name. The second column contains the refrigerant molecular mass which has been related to the ejector size. From the third to the fifth column are indicated the critical values of temperature, pressure and density. The first two parameters are related to the higher conditions at which the generator can operate in a subcritical system. As well, the ratio of this pressure to its temperature is a parameter related to the level of pressure in which the system will operate. Thus, an ECS employing R290, R152a and R134a will experience the higher pressures and will have the more robust construction. The sixth and seventh columns contain the typical saturation conditions at which the generator and evaporator can operate. So, when higher the saturation pressure is at the generator, the more robust the generator will be. While, higher the latent heat ratio is, the higher the system COP will be. The eighth column is indicating the normal boiling point, and as lower this temperature is higher the pressures in the system will be. The next column is related to the quality the refrigerant has when falling or not into the saturation curve at the main nozzle exit in its expansion process. Finally, the environmental and safety characteristics of the refrigerants are shown in the last four columns. In regard to the ODP, all the refrigerants have null values and for the GWP only the R134a has a high value for which is transition refrigerant. In relation to the flammability, only two halocarbon refrigerants have null values and the hydrocarbon have medium values, the highest value corresponds to R152a. Regarding the toxicity, the propane and halocarbon R152a and R134a have the highest values. So, the selected refrigerants, excepting partially the R134a, satisfy the condition of being environmental friendly and only their thermodynamic behaviour in the operation of an ECS needs to be study.
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Table 3 e Properties of the refrigerants used as working fluid in an ECS, obtained partially from Calm and Hourahan (2001). Critical Critical Saturation Ratio of latent NBPa Wet/ ODPb GWPb LFLc (%) Toxicity Name Molecular Critical dry (ppm) mass temperature pressure density pressure at heats at 10 C ( C) (MPa) (kg/m3) 95 C (MPa) and 95 C vapour (kg/kmol) ( C) R290 R600 R600a R134a R152a R123
44.10 58.12 58.12 102.03 66.05 152.93
96.7 152.0 134.7 101.1 113.3 183.8
4.25 3.80 3.64 4.06 4.52 3.66
222.22 227.83 222.22 507.87 364.96 554.94
4.12 1.38 1.80 3.59 3.17 0.70
4.51 1.40 1.55 2.92 1.97 1.30
42.2 0.5 11.7 26.50 25.0 27.33
Wet Dry Dry Wet Wet Dry
0 0 0 0 0 0.012
20 20 20 1300 120 120
2.1 1.5 1.7 None 3.7 None
2500 800 800 1000 1000 50
a Normal boiling point. b Ozone depletion potential relative to R11 (ODP). Global warming potential relative to CO2 (GWP); integration time ¼ 100 years. c Lower flammability limit in % concentration ambient air.
2.4.
Obtention of results
In order to determine the ejector and ECS performance with different hydrocarbon and halocarbon refrigerants, a parametric study with lowest Ff and highest hn was carried out for a constant cooling capacity of 1 kW. Temperatures for the generator, condenser and evaporator varied from 70 C to 100 C; 25 C to 35 C and 5 C to 15 C, respectively. A superheating of 5 C was considered at the generator exit while the saturation at condenser and evaporator exits. Corresponding results are shown from Figs. 5e13.
_ 2 remains constant. As TGE capacity is unitary, consequently, m increases, the primary fluid pressure and enthalpy also grow _ 1 to be entrained, _ 2 requires a lower m and unvarying m resulting thus an increase in U. In regard to the working fluid influence on the ejector behaviour, the highest U values correspond to a device operating with R290 and the lowest with R123. The ejectors using R152a, R134a and R600a have almost the same U values and are immediately below the ejector employing R290. Finally, the ejector using R600 is beneath the tendency before mentioned as well as above that for the ejector operating with R123. In regard to f, it increases _1 at higher generation temperatures because a reduction in m
The behaviour of U and f against TGE is shown in Fig. 5 for the five refrigerants chosen. In this case, the condenser and evaporator temperatures are fixed and the system cooling
entails a decrease in dt, while dm remains almost constant. On the other hand, an ejector operating with R290 has lower f values than using R123. This behaviour is caused by the larger _ 1 and m _ 2 required to dm required with R123, due to the higher m operate at the given temperatures and cooling capacity. This means that an ejector using R290 is smaller than another operating with R123. The ejectors using R600a, R152a, R134a and R600 have larger dimensions than the one employing R290 with very similar f values.
Fig. 5 e Entrainment ratio U and ejector area ratio f against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 6 e Entrainment ratio U and ejector area ratio f against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
3.
Analysis of results
3.1.
Ejector Performance
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Fig. 7 e Entrainment ratio U and ejector area ratio f against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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Fig. 9 e Ejector efficiency hE against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
According to Fig. 5, it is important to point out that an ejector operating with R290 reaches a maximum generator temperature of 95 C, because it operates nearly and under its critical point, which is around 96 C as Table 3 shows. For the other refrigerants, this limitation is not present and it is possible to attain generator temperatures of 100 C. Also, as the TGE of any refrigerant has a corresponding saturation pressure and considering its critical point shown in Table 3, an
ECS working with R290, R152a and R134a will experience the higher pressures and will have the more robust construction. The trend of U and f against TCO is shown in Fig. 6 for a unitary capacity of the cooling system and constant generator and evaporator temperatures. In this plot, U decreases as TCO increases because any increase in this temperature results _ 1 and m _ 2 in order to provide in a pCO growth, causing a rise in m
Fig. 8 e Ejector efficiency hE against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 10 e Ejector efficiency hE against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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Fig. 11 e System coefficient of performance COPs against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 13 e System coefficient of performance COPs against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
a higher secondary fluid recompression and to maintain _ 1 increases a fixed cooling capacity, respectively. Then, as m _ 2 does, U decreases. In relation to the much more than m refrigerant effect on the ejector operation, lower U values are obtained for the ejector using R123 and higher with R290. For the other refrigerants, the ejectors have intermediate U values and follow the tendency above mentioned. Regarding f, the _ 2 as TCO grows, causes an increase in dt and dm _ 1 and m rise in m
in which the growing rate is higher for the first one, giving that f decreases. Once again, the order in the magnitude of f above mentioned in the feTGE plot is repeated and the lowest f values correspond to an ejector operating with R290. For a system unitary cooling capacity with constant generator and condenser temperatures, the change of U and f against TEV is shown in Fig. 7 in which U experiences a growth tendency with TEV increments. As any increase in tempera_ 2 remains almost ture goes with a pressure raise, for which m _ 1 is required as the reduction effect in invariable and a lower m the secondary fluid recompression, U increases. Again, the lower U values are obtained for an ejector employing R123 and the higher using R290. Also, ejectors employing the other refrigerants have intermediate U values and follow the tendency already mentioned. Regarding f, as there is a higher _ 2 as TEV grows, a lower decreasing rate _ 1 than m reduction of m of dm than dt is found and gives rise to a f growth. Again, the tendency already mentioned is repeated with the lower f values belonging to an ejector using R290. From the above U plots, an ejector operating at the same temperatures and using hydrocarbon refrigerants has the highest U values for R290, intermediate for R600a and lower for R600. In regard to the halocarbon refrigerants, ejectors employing R152a and R134a are immediately below that one using R290 with R123 at the end of all. In relation to f, the highest values are given for an ejector using R123 and the lowest for R290. For the other refrigerants, intermediate f values are found. Finally, the last ejector parameter considered is its efficiency, hE, which is shown from Figs. 8 to 10. In the plot of hE against TGE shown in Fig. 8, as this temperature increases hE has a growing tendency with the higher value, for three of the employed refrigerants that according to Table 3 are wet. For the ejectors
Fig. 12 e System coefficient of performance COPs against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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using the other refrigerants, which are dry, hE reaches a maximum value and starts decreasing. The behaviour above mentioned is given by the growth rate of U and Ui with TGE, regarding the wet refrigerants is lower for Ui than U and higher for the dry refrigerants. Also, the change in ejector efficiency growth tendency for the wet refrigerants is caused by the ejector ideal exit, which falls inside the saturation curve. So, the higher efficiency is obtained for the ejector operating with R290 and the lower when R123 is used. In a descending order come the ejectors using R152a and R134a, with very similar values, then comes an ejector using R600a and finally that employing R600. The plot of hE against TCO is shown in Fig. 9. As this temperature grows, a maximum efficiency is reached by the ejector operating with R290 and then starts decreasing. For the other refrigerants, the ejectors have efficiencies with a decreasing tendency. Again, the better performance is for an ejector using R290 and the poorer when employing R123, while the ejectors operating with the rest of refrigerants repeat the above mentioned order in the magnitude of hE. The variation of hE against TEV is shown in Fig. 10. As this temperature increases, a maximum efficiency is reached once more for the ejector employing R290 to start decreasing. Ejectors operating with the rest of refrigerants have an increasing tendency. The higher and lower hE values are given by the ejector using R290 and R123, respectively. For ejectors employing the other refrigerants, the above indicated order in the magnitude of hE is followed. From above hE plots, the highest values are obtained for an ejector operating with R290, which also has higher U and lowest f values. In regard to the ejector using refrigerants R134a and R152a, they have similar hE values, which are lower than those of R290, followed by ejectors employing R600a and R600 with R123 at the end.
3.2.
Ejector cooling system performance
Figs. 11e13 show the tendency of the system coefficient of performance, COPs. Its variation with TGE is shown in Fig. 11 and follows a similar U growing path, due to its dependence with this parameter as well as the refrigerant properties, according to Eq. (7a). The best performance corresponds to the system using R290 and R152a. So, at a TGE of 95 C an ECS employing the first refrigerant has a COP around 0.72 and of 0.69 for the second, while the system operating with R134a comes immediately below them. As well, COPs values for an ECS operating with R600a and R600 are lower. At a temperature of 100 C, the system operating with R152a reaches the higher COPs and if the tendencies for the ECS using refrigerants R134a, R600, R600a and R123 are continued, they can operate at higher temperatures and efficiencies, having only as limit their critical temperatures. Therefore, the best performance using a hydrocarbon refrigerant is obtained for an ECS employing R290 at TGE not higher than 95 C. The change of COPs with TCO is shown in Fig. 12. As this temperature grows the COPs follows the U decreasing tendency, repeating the trend shown in Fig. 6. The variation of COPs against TEV is shown in Fig. 13. As this temperature is varied, a similar pattern between COPs and U is observed. The highest COPs values belong to the system using R290 and R152a with the lowest for R123. In between, higher
1715
values belong to the system employing R134a and then follow those for R600a and R600, which are similar. As observed from Figs. 11e13, the upper COPs is achieved at higher TGE and TEV with lower TCO. For an ECS using hydrocarbon refrigerants, the highest COPs are obtained with a system employing R290, the intermediate values for that with R600a and the lower for the one with R600. In regard to the halocarbon refrigerants, the system with R152a is below and very close to that with R290, the system with R134a is under them and the one with R123 is at the end of all.
4.
Conclusions
The theoretical behaviour of an ECS is obtained with a validated ejector one-dimensional mathematical model with secondary fluid choking. The refrigerants considers are hydrocarbons R290, R600a and R600 as well as halocarbons R152a and R134a, among which only the last one is partially environmentally friendly. For a system constant unitary capacity, a parametric study is carried out varying the temperatures of the generator, condenser and evaporator. The ejector parameters found are: the entrainment ratio; area ratio and efficiency. The system parameter found is the coefficient of performance. In regard to the ejector, the best performance is obtained when the hydrocarbon refrigerant R290 is used because has the maximum U values, which mean lowest primary fluid mass flow rates; least f values, meaning smallest sizes; upper hE values denoting proficiency. Below this outstanding behaviour, come those for ejectors employing halocarbon refrigerants R152a and R134a with hydrocarbon refrigerants R600a and R600 at the end. For the ECS, upper COPs values are achieved at higher TGE and TEV with lower TCO. An ECS operating with hydrocarbon refrigerants has the highest COPs using R290, the intermediate values when R600a is employed and the smallest ones when operating with R600. In regard to the halocarbon refrigerants, the COPs for a system using R152a is below but very close to that of R290 with R134a under them and R123 at the end of all. Therefore, the systems operating with R290 or R152a are good options. Also, an ECS working with R290, R152a and R134a will experience the higher pressures and will have the more robust construction. From the above mentioned data, it is important to consider the system and ejector operation parameters to have a complete sight of the system performance. So, the best system is that using hydrocarbon R290 because it has the highest COPs, meaning the most efficient system; and its ejector has the highest U, connotation of the minimum primary mass flow rate; lower f, denotation of the smallest ejector; and the highest hE, implying the most efficient device. Therefore, the use of R290 in an ECS gives the best performance for TGE ranging from 70 C to 95 C, which are appropriate temperatures for thermal cooling systems. As to each combination of the generation, condensation and evaporator temperatures corresponds one and only one ejector geometry, the obtained data represent possible design points of the efficient operation of an ECS.
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Acknowledgements We thank to the Consejo Nacional de Ciencia y Tecnologı´a, CONACYT, the financial support given to this research through Project U-44764-Y.
references
Abdulateef, J.M., Sopian, K., Alghoul, M.A., Sulaiman, M.Y., 2009. Review on solar-driven ejector refrigeration technologies. Renew. Sust. Energ. Rev. 13 (6e7), 1338e1349. Boumaraf, L., Lallemand, A., 2009. Modeling of an ejector refrigerating system operating in dimensioning and offdimensioning conditions with the working fluids R142b and R600a. Appl. Therm. Eng. 29 (2e3), 265e274. Calm, J.M., Hourahan, G.C., 2001. Refrigerant data summary. Engineered Systems 18 (11), 74e88. Chen, F.C., Hsu, C., 1987. Performance of ejector heat pumps. Int. J. Energ. Res. 11, 289e300. Chunnanond, K., Aphornratana, S., 2004. Ejectors: applications in refrigeration technology. Renew. Sust. Energ. Rev. 8, 129e155. Cizungu, K., Mani, A., Groll, M., 2001. Performance comparison of vapour jet refrigeration system with environment friendly working fluids. Appl. Therm. Eng. 21 (5), 585e598. Haidar, J., Kowalski, G., Foster, A., 1995. Examination of ejector parameters in solar powered refrigeration systems, Proceedings of the International Mechanical Engineering
Congress, Thermodynamics and Design, Analysis, and Improvements of Energy Systems, AESD. ASME 35, 205e215. Kairouani, L., Elakhdar, M., Nehdi, E., Bouaziz, N., 2009. Use of ejectors in a multi-evaporator refrigeration system for performance enhancement. Int. J. Refrigeration 32 (6), 1173e1185. Lu L.-T.: Etudes the´orique et expe´rimentale de la production de froid par machine tritherme a ejecteur de fluide frigorige`ne, Doctoral thesis from Institut National Polytechnique de Grenoble, France, 1986. Mooney David, A., 1955. Introduction to Thermodynamics and Heat Transfer. Prentice Hall. Nehdi, E., Kairouani, L., Elakhdar, M., 2008. A solar ejector airconditioning system using environment-friendly working fluids. Int. J. Energ. Res. 32 (13), 1194e1201. Pridasawas, W., Lundqvist, P., 2007. A year-round dynamic simulation of solar-driven ejector refrigeration system with iso-butane as a refrigerant. Int. J. Refrigeration 30, 840e850. Selvaraju, A., Mani, A., 2004. Appl. Therm. Eng. 24 (5e6), 827e838. Selvaraju, A., Mani, A., 2006. Experimental investigation on R134a vapour ejector refrigeration system. Int. J. Refrigeration 29 (7), 1160e1166. Sun, D.-W., 1995. Recent developments in the design theories and applications of ejectors-a review. J. I. Energy 68 (June), 65e79. Sun, D., 1999. Comparative study of the performance of an ejector refrigeration cycle operating with various refrigerants. Energ. Convers. Manage. 40 (8), 873e884. Yapici, R., Ersoy, H.K., Aktoprakoglu, A., Halkaci, H.S., Yigit, O., 2008. Experimental determination of the optimum performance of ejector refrigeration system depending on ejector area ratio. Int. J. Refrigeration 31, 1183e1189.
available at www.sciencedirect.com
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journal homepage: www.elsevier.com/locate/ijrefrig
Performance of ejector cooling systems using low ecological impact refrigerants Raul Roman a, Jorge I. Hernandez b,* a b
Posgrado en Ingenierı´a, Energı´a, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 34, Temixco, Morelos 62580, Mexico Centro de Investigacio´n en Energı´a, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 34, Temixco, Morelos 62580, Mexico
article info
abstract
Article history:
The theoretical behaviour of an ejector cooling system, using as working fluids propane,
Received 16 March 2010
butane, isobutane, R152a and R134a, is obtained. The ejector works as a thermo-
Received in revised form
compressor that is simulated with a validated one-dimensional mathematical model,
18 February 2011
whose errors are lower than 6%. For a system unitary cooling capacity, a parametric study
Accepted 12 March 2011
is carried out varying the generation, condensation and evaporation temperatures. From
Available online 21 March 2011
the obtained data, a complete analysis of the system performance can be achieved when the ejector and system operation parameters are considered. The best performance
Keywords:
corresponds to the system using propane, because has the highest system coefficient of
Cooling
performance and its ejector has the maximum entrainment ratio value, the least area ratio
Ejector system
value and the highest efficiency value. The considered generation temperature ranging
Performance
from 70 C to 95 C is appropriate for low-grade energy sources assisting thermal cooling
Hydrocarbon
systems. After this system performance, come those in which R152a and R134a are
R134a
employed, with isobutane and butane at the end. The obtained results represent potential
R152a
design points of an efficient ejector cooling system operation, because to each combination of the above mentioned temperatures corresponds one and only one ejector geometry. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Performance des syste`mes de refroidissement a` e´jecteur employant des frigorige`nes exerc¸ant un faible impact sur l’environnement Mots cle´s : Refroidissement ; Syste`me a` e´jecteur ; Performance ; Hydrocarbure ; R134a ; R152a
1.
Introduction
The current searching of new refrigerants to operate Ejector Cooling Systems, ECS, has a dual purpose: to improve its performance and decrease its ecological impact (Sun, 1995;
Chunnanond and Aphornratana, 2004; Abdulateef et al., 2009). ECS employing halocarbon refrigerants have reached good efficiencies as well as appropriate levels of operation versatility. Unfortunately, the phasing-out of ozone-damaging refrigerants
* Corresponding author. Tel./fax: þ55 56229791. E-mail addresses: [email protected] (R. Roman), [email protected] (J.I. Hernandez). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.03.006
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Nomenclature COP d ECS Ff F0f G h k l L _ m M* MFPM p Q_ T U V _ W f G h U j
coefficient of performance (dimensionless) diameter (m) ejector cooling system evaluated friction factor (dimensionless) friction factor datum (dimensionless) mathematical function specific enthalpy (kJ kg1) specific heat ratio, cp/cv, (dimensionless) distance, length (m) length ratio (dimensionless) mass flow rate (kg s1) critical Mach number (dimensionless) modified false position method pressure (MPa) heat flow rate (kW) temperature ( C) _ 1 (dimensionless) _ 2 =m entrainment ratio m velocity (m s1) mechanical power or work rate (kW) ejector area ratio, (dm/dt)2, (dimensionless) ejector expansion ratio, p1/p2, (dimensionless) efficiency (dimensionless) diffuser area ratio, (dd/dm)2, (dimensionless) main nozzle area ratio, (de/dt)2, (dimensionless)
has made necessary the research of alternatives, among which, the hydrocarbons are advisable candidates in small refrigeration systems. They are environmentally friendly with null or negligible ozone depletion and global warming potentials, but are explosive as well as flammable and require special manipulation in practical usage. In order to know how the ECS behaves thermodynamically in typical cooling applications employing some hydrocarbons as refrigerant, certain theoretical research has been carried out (Haidar et al., 1995; Selvaraju and Mani, 2004; Pridasawas and Lundqvist, 2007; Nehdi et al., 2008; Boumaraf and Lallemand, 2009). The main information available in literature for some of the hydrocarbon and nowadays accepted halocarbon refrigerants is shown in Table 1. As observed, the temperature difference TGE TCO and TCO TEV is an important parameter related to the expansion and compression processes carried out internally in the ejector, in which the first has to be higher as a work provider for the compression process. So, for an ECS employing isobutane were obtained entrainment ratio values between 0.12 and 0.45 for generator, condenser and evaporator temperatures varying from 82 C to 90 C; 25 C to 52 C and 4 C to 10 C, respectively (Haidar et al., 1995; Selvaraju and Mani, 2004; Pridasawas and Lundqvist, 2007). The entrainment ratio value of 0.12 resulted to be high for an expansion and compression temperature difference of 30 C and 48 C, whose values have a contrary tendency to the other reported results. For the butane only one study was found with an entrainment ratio value of 0.31 for generator and evaporator temperatures of 90 C and 15 C with a condenser temperature of 35 C (Nehdi et al., 2008). A similar situation of inconsistency was also found for the propane and halocarbon refrigerants, according to the results shown in
q x
ejector temperature ratio, T2/T1, (dimensionless) ejector driving pressure ratio, p1/p3, (dimensionless)
Subscripts a ideal condition at the main nozzle exit a’ actual condition at the main nozzle exit c mixing chamber exit CO condenser comp compression d diffuser e main nozzle exit E ejector exp expansion EV evaporator GE generator i ideal m mixing chamber n main nozzle, between main nozzle exit and mixing chamber inlet reversible pump pr s system t main nozzle throat 1,.,6 thermodynamic cycle states or mathematical function increasing numeration
Table 1. As seen, these data do not agree and certainly the experimental information available by that time was insufficient to validate the employed ejector mathematical models. On the other hand, and from a practical point of view, some domestic mechanical compression systems employing isobutane are nowadays available commercially, because operation risks have been lessened with appropriate designs. In order to clarify the discrepancies above mentioned, a careful study on an ECS with low ecological impact refrigerants is necessary. Therefore, the purpose of the present work is to obtain theoretically the performance parameters of an ECS, for the system and ejector, through a parametric study in which the temperatures of the generator, TGE, condenser, TCO, and evaporator, TEV, are varied. A one-dimensional ejector mathematical model performing under secondary flow choking shall be used with a previous validation with experimental data available in the recent literature. As well, the refrigerants with low ecological impact, propane, butane, isobutene and R152a are employed. Refrigerant R134a is also included because it has appropriate thermodynamic properties, in spite of being considered as a refrigerant in transition as a consequence of its medium global warming potential value.
2.
Parametric study
2.1.
System characteristics
The ECS is made up by a generator, condenser, evaporator, ejector, pump and expansion valve, as indicated in Fig. 1, where the system thermodynamic states are also included.
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Table 1 e Principal characteristics of the ECS behaviour obtained from literature. Working fluid
U
f
R152a R134a R123 R600a
0.235 0.23 0.20 0.12
5.32 5.37 7.19 2.29
55 55 55 30
30 30 30 48
R290 R600a R152a R134a R600a
2
R152a R134a R290 R600 R134a
0.365 0.29 0.28 0.275 0.02 0.04 0.075 0.105 0.385 0.38 0.37 0.31 0.35
4.42 4.95 6.14 7.5 6.06 6.12 5.13 6.41 6.68
16 14 14 14.5 55 60 70 80 55 55 55 55 60
7 9 9 9.5 45 45 45 45 20 20 20 20 25.4
10
R134a R600a R134a
0.48 0.45 0.27
6.63 6.42 5.64
55 55 51
20 20 19
10
R600a
0.225
3.63
45
35
Type of study
2nd flow choking
ln (mm)
Ff
TGE ( C)
TCO ( C)
TEV ( C)
Sun, 1999
Th.
Yes
e
e
90
35
5
Haidar et al., 1995 Kairouani et al., 2009
Th.
e
e
e
82
52
4
Th.
Yes
s0
0.06
5
18
Boumaraf and Lallemand, 2009
Th.
Yes
e
e
Nehdi et al., 2008 a
Th.
Yes
e
e
100 105 115 125 90
11 9 9 8.5 45
35
15
Cizungu et al., 2001 a Selvaraju and Mani, 2004 Selvaraju and Mani, 2006 Pridasawas and Lundqvist, 2007
Th.
Yes
0.03
90
30
4.6
Th.
Yes
e
e
80
25
5
Ex.
e
1.8
e
80
29
Th.
Yes
e
e
90
45
Author
0
0
TGE TCO ( C)
TCO TEV ( C)
a Values in the U column correspond to COPs.
2 de dt
The ejector is shown in Fig. 2 with its main geometric variables, where subscripts 1 and 2 correspond to the inlet of primary and secondary flows and 3 to the exit flow. From the cross-sectional areas belonging to the throat and exit of the ejector’s main nozzle, inlet of the cylindrical mixing chamber and exit of the diffuser, the following ejector dimensionless geometrical parameters result: the ejector area ratio
j¼
2 dm f¼ dt
where lm is the mixing chamber length; and the main nozzle separation distance ratio
(1)
in which dm is the mixing chamber diameter and dt is the main nozzle throat diameter; the main nozzle area ratio
being de the main nozzle exit diameter. Other important ejector geometrical relations are the mixing chamber length ratio Lm ¼
Ln ¼
lm dm
(3)
ln dm
(4)
in which ln is the distance between the mixing chamber inlet and the main nozzle exit. The considered ejector’s thermodynamic parameters are its entrainment ratio U¼
Fig. 1 e Ejector cooling system configuration.
(2)
_2 m _1 m
(5)
Fig. 2 e Ejector configuration.
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_ 1 and m _ 1 the mass flow rates for the primary and being m secondary fluids; and its efficiency _ com W hE ¼ _ exp W
(6)
_ com is the work rate required in the secondary in which W _ exp the work rate fluid’s isentropic compression process and W produced in the main fluid’s isentropic expansion process (Mooney David, 1955). Considering the conservation equations for the actual and ideal processes, the resulting efficiency is hE ¼ hcom hexp
(6a)
where hcom and hexp are the efficiencies for the compression and expansion processes, respectively. As these efficiencies behave contrary, a couple of these parameters shall maximize the ejector efficiency. A more common form of the ejector efficiency is found when the equations of conservation and production of entropy for the ideal and actual processes are employed, resulting hE ¼
U Ui
(6b)
where Ui is the ideal entrainment ratio for the ejector’s reversible processes of expansion and compression (Chen and Hsu, 1987). In regard to the ECS, its coefficient of performance is Q_ EV (7) COPs ¼ _ _ r Q GE þ Wp
Ma ¼ G1 ðk; GÞ
(11)
1 f ¼ G2 k; G; Uq =2 ; Ma
(12)
1 M3 ¼ G3 k; f; U; x; Uq =2
(13)
Mc ¼ G4 k; hd ; U; M3
1 Ff ¼ G5 k; Lm ; Mc ; Ma ; Uq =2
(14) (15)
where k is the refrigerant specific heat ratio, cp/cv, G is the ejector expansion ratio, p1/p2, q the ejector temperature ratio, T2/T1, x the ejector driving pressure ratio, p1/p3, hd the diffuser efficiency, U the diffuser area ratio, (dd/dm)2, Ff the friction factor and Ma ; Mc andM3 are the exit critical Mach numbers for the main nozzle, mixing chamber and ejector, respectively. Lu found from experimental results a friction factor of 0.07 for an ECS operating with refrigerant R11. The algorithm employed to solve the mathematical model of Lu is shown in Fig. 3. The input values to find G converging to F0f datum are T2, T3, U, f, hd and k. Firstly, G values enclosing its converging solution are found. Then, with the Modified
whose terms are the heat flow rate transferred at the evaporator and generator as well as the rate of mechanical work required by the reversible pump, which are defined as _ 1 ðh1 h5r Þ Q_ GE ¼ m
(8)
_ 2 ðh2 h6 Þ Q_ EV ¼ m
(9)
_ r¼m _ 1 ðh5r h4 Þ Wp
(10)
and subscripts are indicated in Fig. 1, where the reversible pump exit enthalpy is given by h5r. Substitution of Eqs. (5) and (8) to (10) into Eq. (7), gives ðh2 h6 Þ COPs ¼ U ðh1 h4 Þ
2.2.
(7a)
Validation of the ejector mathematical model
The ejector simulation is performed with the one-dimensional mathematical model of Lu (Lu, 1986), in which the secondary fluid choking is considered in conjunction with the following assumptions: 1. 2. 3. 4.
Steady state performance; Fluids behave as perfect gases; Flow along the main nozzle is isentropic; Friction losses in mixing chamber are considered as pipeline minor losses; 5. Null main nozzle separation distance; 6. Complete fluid mixture at mixing chamber exit which gave rise to the next system of dimensionless nonlinear explicit relations.
Fig. 3 e Algorithm to solve the Lu’s mathematical model (Lu, 1986), for the ejector operating under secondary flow choking.
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False Position Method, MFPM, converging G is determined as well as the respective entrainment ratio U. In order to validate Lu’s model with other ejector geometries and refrigerants, the experimental results of Yapici et al. (2008) were employed. Using refrigerant R123, they considered the influence of the ejector area ratio on optimum ejector performance. Six different ejectors were used with a fixed separation distance of the main nozzle of 5 mm and a constant mixing chamber length ratio of 7.5. This separation allows the ejector to reach higher U values which render in a system improvement. Therefore, original Lu’s model was modified and a similar algorithm was employed having T1, T2, T3, f, U, hd, lm and U as inputs. Eqs. (11) and (12) are uncoupled and the ideal critical Mach number, Ma , is determined with the first equation and the actual critical Mach number, Ma0 , with the second one. The rest of the unknowns are found sequentially until Ff. With these critical Mach numbers, the main nozzle efficiency, hn, is evaluated as hn ¼
2 Ma0 Ma
(16)
The results obtained are shown in Fig. 4, where the friction factor and nozzle efficiency are plotted against dimensionless abscissa Ln and j, respectively. A straight line was adjusted to every group of results and the extrapolation of Ff for a null nozzle separation distance reached a value of 0.066 which agrees with that obtained by Lu for R11. With these distributions, a generalised algorithm was employed to validate the ejector performance giving T1, T2, T3, U, hd, lm, k, Ff like function of Ln and hn as function of j in order to obtain the theoretical values of U and f, whose errors with Yapici data were lower than 6%, as Table 2 shows. So, a reliable ejector model with such main nozzle geometry and mixing chamber characteristics is available. As well, it is
Fig. 4 e Friction factor Ff and nozzle efficiency hn against the double dimensionless abscissa, main nozzle separation distance ratio Ln and main nozzle area ratio j.
Table 2 e Errors obtained in the validation of the Lu’s ejector model with experimental data of Yapici et al. (2008). TEV ( C)
TCO ( C)
TGE ( C)
10 10 10 10 10
33.75 33.75 33.75 33.75 33.75
83 90 91 98.2 103.3
f Lu
Exp.
6.54 8 8.25 10.09 11.39
6.56 7.86 8.32 9.97 11.45
Error % 0.305 1.781 0.841 1.204 0.524
U Lu
Exp.
0.398 0.441 0.472 0.516 0.548
0.376 0.454 0.448 0.524 0.555
Error % 5.851 2.863 5.357 1.527 1.261
important to point out that this algorithm allows obtaining an efficient ejector for every group of inlet data. Therefore, the results found correspond to the characteristics of a particular ejector and one ejector obtained in this way shall never satisfy two different groups of inlet data.
2.3.
Selected refrigerants characteristics
The working fluids chosen for the ECS operation are the hydrocarbon refrigerants: propane, isobutane and butane, as well as halocarbon refrigerants R152a and R134a, in which R123 is also included because it was employed in the ejector model validation. Table 3 shows their main physical, environmental and safety characteristics. In the first column is indicated the refrigerant name. The second column contains the refrigerant molecular mass which has been related to the ejector size. From the third to the fifth column are indicated the critical values of temperature, pressure and density. The first two parameters are related to the higher conditions at which the generator can operate in a subcritical system. As well, the ratio of this pressure to its temperature is a parameter related to the level of pressure in which the system will operate. Thus, an ECS employing R290, R152a and R134a will experience the higher pressures and will have the more robust construction. The sixth and seventh columns contain the typical saturation conditions at which the generator and evaporator can operate. So, when higher the saturation pressure is at the generator, the more robust the generator will be. While, higher the latent heat ratio is, the higher the system COP will be. The eighth column is indicating the normal boiling point, and as lower this temperature is higher the pressures in the system will be. The next column is related to the quality the refrigerant has when falling or not into the saturation curve at the main nozzle exit in its expansion process. Finally, the environmental and safety characteristics of the refrigerants are shown in the last four columns. In regard to the ODP, all the refrigerants have null values and for the GWP only the R134a has a high value for which is transition refrigerant. In relation to the flammability, only two halocarbon refrigerants have null values and the hydrocarbon have medium values, the highest value corresponds to R152a. Regarding the toxicity, the propane and halocarbon R152a and R134a have the highest values. So, the selected refrigerants, excepting partially the R134a, satisfy the condition of being environmental friendly and only their thermodynamic behaviour in the operation of an ECS needs to be study.
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Table 3 e Properties of the refrigerants used as working fluid in an ECS, obtained partially from Calm and Hourahan (2001). Critical Critical Saturation Ratio of latent NBPa Wet/ ODPb GWPb LFLc (%) Toxicity Name Molecular Critical dry (ppm) mass temperature pressure density pressure at heats at 10 C ( C) (MPa) (kg/m3) 95 C (MPa) and 95 C vapour (kg/kmol) ( C) R290 R600 R600a R134a R152a R123
44.10 58.12 58.12 102.03 66.05 152.93
96.7 152.0 134.7 101.1 113.3 183.8
4.25 3.80 3.64 4.06 4.52 3.66
222.22 227.83 222.22 507.87 364.96 554.94
4.12 1.38 1.80 3.59 3.17 0.70
4.51 1.40 1.55 2.92 1.97 1.30
42.2 0.5 11.7 26.50 25.0 27.33
Wet Dry Dry Wet Wet Dry
0 0 0 0 0 0.012
20 20 20 1300 120 120
2.1 1.5 1.7 None 3.7 None
2500 800 800 1000 1000 50
a Normal boiling point. b Ozone depletion potential relative to R11 (ODP). Global warming potential relative to CO2 (GWP); integration time ¼ 100 years. c Lower flammability limit in % concentration ambient air.
2.4.
Obtention of results
In order to determine the ejector and ECS performance with different hydrocarbon and halocarbon refrigerants, a parametric study with lowest Ff and highest hn was carried out for a constant cooling capacity of 1 kW. Temperatures for the generator, condenser and evaporator varied from 70 C to 100 C; 25 C to 35 C and 5 C to 15 C, respectively. A superheating of 5 C was considered at the generator exit while the saturation at condenser and evaporator exits. Corresponding results are shown from Figs. 5e13.
_ 2 remains constant. As TGE capacity is unitary, consequently, m increases, the primary fluid pressure and enthalpy also grow _ 1 to be entrained, _ 2 requires a lower m and unvarying m resulting thus an increase in U. In regard to the working fluid influence on the ejector behaviour, the highest U values correspond to a device operating with R290 and the lowest with R123. The ejectors using R152a, R134a and R600a have almost the same U values and are immediately below the ejector employing R290. Finally, the ejector using R600 is beneath the tendency before mentioned as well as above that for the ejector operating with R123. In regard to f, it increases _1 at higher generation temperatures because a reduction in m
The behaviour of U and f against TGE is shown in Fig. 5 for the five refrigerants chosen. In this case, the condenser and evaporator temperatures are fixed and the system cooling
entails a decrease in dt, while dm remains almost constant. On the other hand, an ejector operating with R290 has lower f values than using R123. This behaviour is caused by the larger _ 1 and m _ 2 required to dm required with R123, due to the higher m operate at the given temperatures and cooling capacity. This means that an ejector using R290 is smaller than another operating with R123. The ejectors using R600a, R152a, R134a and R600 have larger dimensions than the one employing R290 with very similar f values.
Fig. 5 e Entrainment ratio U and ejector area ratio f against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 6 e Entrainment ratio U and ejector area ratio f against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
3.
Analysis of results
3.1.
Ejector Performance
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 7 0 7 e1 7 1 6
Fig. 7 e Entrainment ratio U and ejector area ratio f against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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Fig. 9 e Ejector efficiency hE against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
According to Fig. 5, it is important to point out that an ejector operating with R290 reaches a maximum generator temperature of 95 C, because it operates nearly and under its critical point, which is around 96 C as Table 3 shows. For the other refrigerants, this limitation is not present and it is possible to attain generator temperatures of 100 C. Also, as the TGE of any refrigerant has a corresponding saturation pressure and considering its critical point shown in Table 3, an
ECS working with R290, R152a and R134a will experience the higher pressures and will have the more robust construction. The trend of U and f against TCO is shown in Fig. 6 for a unitary capacity of the cooling system and constant generator and evaporator temperatures. In this plot, U decreases as TCO increases because any increase in this temperature results _ 1 and m _ 2 in order to provide in a pCO growth, causing a rise in m
Fig. 8 e Ejector efficiency hE against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 10 e Ejector efficiency hE against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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Fig. 11 e System coefficient of performance COPs against TGE for a constant TEV of 10 C and TCO of 30 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
Fig. 13 e System coefficient of performance COPs against TEV for a constant TCO of 30 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
a higher secondary fluid recompression and to maintain _ 1 increases a fixed cooling capacity, respectively. Then, as m _ 2 does, U decreases. In relation to the much more than m refrigerant effect on the ejector operation, lower U values are obtained for the ejector using R123 and higher with R290. For the other refrigerants, the ejectors have intermediate U values and follow the tendency above mentioned. Regarding f, the _ 2 as TCO grows, causes an increase in dt and dm _ 1 and m rise in m
in which the growing rate is higher for the first one, giving that f decreases. Once again, the order in the magnitude of f above mentioned in the feTGE plot is repeated and the lowest f values correspond to an ejector operating with R290. For a system unitary cooling capacity with constant generator and condenser temperatures, the change of U and f against TEV is shown in Fig. 7 in which U experiences a growth tendency with TEV increments. As any increase in tempera_ 2 remains almost ture goes with a pressure raise, for which m _ 1 is required as the reduction effect in invariable and a lower m the secondary fluid recompression, U increases. Again, the lower U values are obtained for an ejector employing R123 and the higher using R290. Also, ejectors employing the other refrigerants have intermediate U values and follow the tendency already mentioned. Regarding f, as there is a higher _ 2 as TEV grows, a lower decreasing rate _ 1 than m reduction of m of dm than dt is found and gives rise to a f growth. Again, the tendency already mentioned is repeated with the lower f values belonging to an ejector using R290. From the above U plots, an ejector operating at the same temperatures and using hydrocarbon refrigerants has the highest U values for R290, intermediate for R600a and lower for R600. In regard to the halocarbon refrigerants, ejectors employing R152a and R134a are immediately below that one using R290 with R123 at the end of all. In relation to f, the highest values are given for an ejector using R123 and the lowest for R290. For the other refrigerants, intermediate f values are found. Finally, the last ejector parameter considered is its efficiency, hE, which is shown from Figs. 8 to 10. In the plot of hE against TGE shown in Fig. 8, as this temperature increases hE has a growing tendency with the higher value, for three of the employed refrigerants that according to Table 3 are wet. For the ejectors
Fig. 12 e System coefficient of performance COPs against TCO for a constant TEV of 10 C and TGE of 80 C for refrigerants R290, R600a, R600, R152a, R134a and R123.
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using the other refrigerants, which are dry, hE reaches a maximum value and starts decreasing. The behaviour above mentioned is given by the growth rate of U and Ui with TGE, regarding the wet refrigerants is lower for Ui than U and higher for the dry refrigerants. Also, the change in ejector efficiency growth tendency for the wet refrigerants is caused by the ejector ideal exit, which falls inside the saturation curve. So, the higher efficiency is obtained for the ejector operating with R290 and the lower when R123 is used. In a descending order come the ejectors using R152a and R134a, with very similar values, then comes an ejector using R600a and finally that employing R600. The plot of hE against TCO is shown in Fig. 9. As this temperature grows, a maximum efficiency is reached by the ejector operating with R290 and then starts decreasing. For the other refrigerants, the ejectors have efficiencies with a decreasing tendency. Again, the better performance is for an ejector using R290 and the poorer when employing R123, while the ejectors operating with the rest of refrigerants repeat the above mentioned order in the magnitude of hE. The variation of hE against TEV is shown in Fig. 10. As this temperature increases, a maximum efficiency is reached once more for the ejector employing R290 to start decreasing. Ejectors operating with the rest of refrigerants have an increasing tendency. The higher and lower hE values are given by the ejector using R290 and R123, respectively. For ejectors employing the other refrigerants, the above indicated order in the magnitude of hE is followed. From above hE plots, the highest values are obtained for an ejector operating with R290, which also has higher U and lowest f values. In regard to the ejector using refrigerants R134a and R152a, they have similar hE values, which are lower than those of R290, followed by ejectors employing R600a and R600 with R123 at the end.
3.2.
Ejector cooling system performance
Figs. 11e13 show the tendency of the system coefficient of performance, COPs. Its variation with TGE is shown in Fig. 11 and follows a similar U growing path, due to its dependence with this parameter as well as the refrigerant properties, according to Eq. (7a). The best performance corresponds to the system using R290 and R152a. So, at a TGE of 95 C an ECS employing the first refrigerant has a COP around 0.72 and of 0.69 for the second, while the system operating with R134a comes immediately below them. As well, COPs values for an ECS operating with R600a and R600 are lower. At a temperature of 100 C, the system operating with R152a reaches the higher COPs and if the tendencies for the ECS using refrigerants R134a, R600, R600a and R123 are continued, they can operate at higher temperatures and efficiencies, having only as limit their critical temperatures. Therefore, the best performance using a hydrocarbon refrigerant is obtained for an ECS employing R290 at TGE not higher than 95 C. The change of COPs with TCO is shown in Fig. 12. As this temperature grows the COPs follows the U decreasing tendency, repeating the trend shown in Fig. 6. The variation of COPs against TEV is shown in Fig. 13. As this temperature is varied, a similar pattern between COPs and U is observed. The highest COPs values belong to the system using R290 and R152a with the lowest for R123. In between, higher
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values belong to the system employing R134a and then follow those for R600a and R600, which are similar. As observed from Figs. 11e13, the upper COPs is achieved at higher TGE and TEV with lower TCO. For an ECS using hydrocarbon refrigerants, the highest COPs are obtained with a system employing R290, the intermediate values for that with R600a and the lower for the one with R600. In regard to the halocarbon refrigerants, the system with R152a is below and very close to that with R290, the system with R134a is under them and the one with R123 is at the end of all.
4.
Conclusions
The theoretical behaviour of an ECS is obtained with a validated ejector one-dimensional mathematical model with secondary fluid choking. The refrigerants considers are hydrocarbons R290, R600a and R600 as well as halocarbons R152a and R134a, among which only the last one is partially environmentally friendly. For a system constant unitary capacity, a parametric study is carried out varying the temperatures of the generator, condenser and evaporator. The ejector parameters found are: the entrainment ratio; area ratio and efficiency. The system parameter found is the coefficient of performance. In regard to the ejector, the best performance is obtained when the hydrocarbon refrigerant R290 is used because has the maximum U values, which mean lowest primary fluid mass flow rates; least f values, meaning smallest sizes; upper hE values denoting proficiency. Below this outstanding behaviour, come those for ejectors employing halocarbon refrigerants R152a and R134a with hydrocarbon refrigerants R600a and R600 at the end. For the ECS, upper COPs values are achieved at higher TGE and TEV with lower TCO. An ECS operating with hydrocarbon refrigerants has the highest COPs using R290, the intermediate values when R600a is employed and the smallest ones when operating with R600. In regard to the halocarbon refrigerants, the COPs for a system using R152a is below but very close to that of R290 with R134a under them and R123 at the end of all. Therefore, the systems operating with R290 or R152a are good options. Also, an ECS working with R290, R152a and R134a will experience the higher pressures and will have the more robust construction. From the above mentioned data, it is important to consider the system and ejector operation parameters to have a complete sight of the system performance. So, the best system is that using hydrocarbon R290 because it has the highest COPs, meaning the most efficient system; and its ejector has the highest U, connotation of the minimum primary mass flow rate; lower f, denotation of the smallest ejector; and the highest hE, implying the most efficient device. Therefore, the use of R290 in an ECS gives the best performance for TGE ranging from 70 C to 95 C, which are appropriate temperatures for thermal cooling systems. As to each combination of the generation, condensation and evaporator temperatures corresponds one and only one ejector geometry, the obtained data represent possible design points of the efficient operation of an ECS.
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Acknowledgements We thank to the Consejo Nacional de Ciencia y Tecnologı´a, CONACYT, the financial support given to this research through Project U-44764-Y.
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