Thermodynamic Study of Diffusion Absorption Refrigeration System with Organic Fluid

Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

THERMODYNAMIC STUDY OF DIFFUSION ABSORPTION REFRIGERATION SYSTEM WITH ORGANIC FLUID

Mukul Kumar1* and R K Das1

*Corresponding Author: Mukul Kumar,  [email protected]

In the recent years, the research interest on Diffusion Absorption Refrigeration (DAR) technology has increased significantly due to its ability to utilize exclusively low-grade heat to produce cooling effect. The operation of diffusion absorption refrigeration system is quiet and reliable therefore often used in hotel rooms and offices. The diffusion-absorption cycle utilizes ammonia-water- hydrogen as working fluid. In this paper, a mathematical model has been established and solved numerically. The model is based on the mass and energy conservation principles applied for every components of the DAR system, through which the working fluids flow. Equations have been developed to estimate mass flow rate, mass concentration and enthalpy of different fluids at various state points of the cycle by considering the mass balance and heat balance equations. Suitable thermodynamic relations have been used for estimating enthalpies at various points corresponding to their state properties of pressure, temperature and dryness fraction. The study showed that the COP of this refrigeration system, although not comparable with the COP of a vapour compression cycle, is encouraging, considering the fact that waste heat can be utilized for running the cycle and COP of the DAR system depends on different parameters like generator temperature, concentration of ammonia in rich solution, evaporator temperature and condenser temperature.

Keywords: Ammonia-water, Bubble pump, COP, Circulation ratio, Diffusion absorption refrigeration

INTRODUCTION There are no moving parts in DAR System, Diffusion Absorption Refrigeration (DAR) therefore this refrigeration system has quiet, system is one of the most fascinating field of reliable and maintenance free operation. DAR research interest during the last few decades, system was pioneer in 1920 by Von Platen and especially in developing countries like India. Munters (Pongsid Srikhirin et al., 2001). The

1 Department of Mechanical Engineering, Indian School of Mines, Dhanbad 826004, India.

473 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 huge advantage of DAR system is that it can of water. This is rarely true, due to the fact that operate without electricity and thus reduces the rectifler is not 100% efficient in separating demand of electricity. Due to lack of moving water from ammonia. parts DAR system has low maintenance cost In this paper, we investigate, by numerical and because of environment-friendly simulation, the performance of a single stage refrigerant, it does not contribute to ozone diffusion-absorption cycle operating with NH / depletion or global-warming. 3 H2O mixture as working fluids and hydrogen DAR cycle utilizes triple fluids namely as auxiliary inert gas. A parametric study of ammonia, water and an auxiliary inert gas the effect of different system parameters on usually hydrogen, where ammonia is used as the COP is also presented by using the refrigerant and water as absorbent. The unique developed model. The performance of this feature of this cycle, as compared to a mixture is simulated with various generator and conventional ammonia-water absorption evaporator temperatures. cycle, is that introduction of the auxiliary inert gas plays its role to reduce the partial pressure SYSTEM DESCRIPTIONS of the refrigerant in the evaporator and allows A simplified schematic diagram of the DAR the refrigerant to evaporate at low temperatures cycle is depicted in Figure 1. The DAR system producing the cooling effect (Zohar and Jelinek, is similar to conventional ammonia water 2007). This refrigeration system is totally heat absorption refrigeration system with an inert operated and there is no need of electrical or gas usually hydrogen, diffused through the mechanical energy. Generally ammonia/water system to maintain a uniform system pressure mixture provides cooling at quite low throughout the cycle. The DAR system temperatures, –10 to –30 °C, depending on includes: (i) a generator which is basically a the diffusion absorption cycle conûguration but combination of a boiler and a bubble pump, requires moderately high driving temperatures (ii) a rectifier, (iii) a condenser, (iv) an which is higher than 150 °C, Although ammonia evaporator containing gas heat exchanger, (v) has excellent thermo-physical properties, it is an absorber, and (vi) a solution heat toxic, explosive, and corrosive to copper and exchanger. Geometry of the bubble pump is other non-ferrous metals. very simple and it contains a cylindrical hollow tube. Application of bubble pump is to raise Zohar et al. (2005) developed a complex the solution from the lower level to the higher thermodynamic model that explored the DAR one. Different operations of a DAR system are cycle performance. They found that the as follows: maximum COP values reachable with a DAR cycle can be obtained when the ammonia • A generator where the rich solution of concentrations in rich and weak solutions are ammonia is heated increasing the 0.25 to 0.30 and 0.10, respectively and that temperature of the solution, the generator temperature is approximately • A bubble pump through which bubbles or 200 °C. They assumed that the vapor exiting vapours formed during heating of solution the rectiûer was pure ammonia with no traces rise to the separator,

474 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

• A rectifier for condensing out the water evaporator through a heat exchanger where vapour, if any, in the ammonia vapour, the liquid ammonia is sub cooled at 9 by the leaving the generator. cold ammonia and hydrogen mixture returning • A condenser for de-superheating and from the evaporator, which is mixed with the condensing ammonia vapour, uncondensed ammonia 8 and the mixture flows into the reservoir. • An evaporator where liquid ammonia and hydrogen gas mix and liquid ammonia The ammonia and hydrogen gas mixture evaporates at partial pressure absorbing enters the absorber coil from the bottom and heat from the region to be cooled, flows upward while the weak solution of ammonia enters the absorber coil from the top • An absorber where ammonia vapour is 16 and flows downward in a counter-current absorbed in the weak solution of ammonia arrangement. The ammonia vapor is readily and water, and absorbed in the weak solution and the resulting • A solution heat exchanger for exchanging rich solution flows to the reservoir. The auxiliary heat between the hot weak solution gas hydrogen is not absorbed and continues returning from the generator and the cold to flow to the evaporator with ammonia rich solution going toward the generator. residuals at 10. The strong solution leaves the reservoir at 14 towards the generator. Rich solution from the reservoir at 14 is Hydrogen leaving the absorber is at higher heated in the solution heat exchanger by the temperature since it absorbs some part of heat return weak solution and enters the generator liberated during absorption of ammonia in at 15. When heat is supplied to the generator weak solution. at 1, bubbles of ammonia gas are produced from the ammonia-water mixture. The Hydrogen gas with traces of ammonia after ammonia bubbles rise through the bubble leaving the absorber at 10 enters the pump lift tube to the separator where evaporator and passes through the heat ammonia vapour is separated and the exchanger. When hydrogen is allowed to mix remaining weak ammonia-water solution with the liquid ammonia at 12, the partial (weak in ammonia) is sent to the absorber at pressure of ammonia is reduced and this 16 through the liquid-liquid solution heat allows the liquid ammonia to evaporate at a exchanger. The ammonia vapor along with lower temperature. The evaporation of some amount of water vapour from the ammonia extracts heat from the evaporator, separator flows through the rectifier, where providing refrigeration to the region to be ammonia and water vapour mixture is cooled cooled. From the evaporator, the mixture of and water vapour is condensed to join the ammonia and hydrogen at 13 returns to the weak solution at 2. Almost pure ammonia reservoir. In the passage this mixture absorbs refrigerant produced by rectification at 6 some amount of heat separately from the liquid enters into the condenser and condenses to ammonia coming from the condenser and liquid at 7 the total pressure of the system. hydrogen gas coming from the absorber and The condensed ammonia flows to the moving towards the evaporator.

475 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

One of the advantages of DAR system is 3. Supply of cooling water in the absorber that it is self circulating system due to the where ammonia vapour is absorbed in the gravity and density differences of the working weak solution liberating latent heat of fluid. liquefaction which is to be carried away by the coolant water. It is necessary to have the following external services for the functioning of the cycle. 4. Supply of coolant in the rectifier. This coolant may be air or water. In the rectifier 1. Supply of the heat at the generator (NH + H O) vapors are cooled and since temperature from the external sources, 3 2 water has higher saturation temperature which may be any source of waste heat than ammonia at any given pressure, available at generator temperature, like water vapour is condensed out and almost exhaust gas from automobiles or boilers, pure ammonia proceeds to the exhaust gas from gas turbines, solar energy, condenser. etc. 2. Supply of cooling water in the condenser, NUMERICAL MODELING OF in which ammonia vapour is de- DAR SYSTEM superheated and condensed to liquid, Numerical modeling of DAR system is liberating latent heat of liquefaction. developed to analyze the performance of the

Figure 1: Schematic Diagram of Diffusion Absorption Refrigeration System

476 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 diffusion-absorption refrigeration cycle. The Rectifier Energy balance: In the rectifier, modeling proceeds by the following steps: gaseous ammonia-water solution (at 3G) is • To develop mass and energy balance cooled to produce almost pure ammonia equation at various component of DAR refrigerant (6) and condensed solution (4), system. which returns into separator to form weak solution at 5. Thus energy balance equation • To specify the fundamental operating for rectifier gives the heat rejection from the conditions like, driving heat source rectifier as:

temperature (Tgen), temperature of the cold     Qrec  m3Gh3G  m6h6  m4h4 ...(3) place (T evp) the temperature of the surrounding (T ). a Also, heat balance for the mixing of • To characterize the heat and mass transfer condensate from the rectifier (4) and weak

processes in various heat and mass solution (3L) gives: exchanging devices in DAR system.    m5h5  m3Lh3L  m4h4 ...(4) Combined Generator, Bubble Pump, Separator and Rectifier Figure 2: Combined Block Diagram of Generator Bubble Pump Separator, In the generator, the rich solution (15) is heated Rectifier and Heat Exchanger by external heat source. Vapours (1) produced due to heating rises through the bubble lift pump to the separator. Vapour (1) contain both gasses and liquid portion of the solution. Energy Equations: Generator energy balance: In the generator, strong solution (15) is heated by external heat source (Qgen) and saturated mixture of ammonia water (1) passes through the bubble pump. Therefore energy balance equation for generator can be written as:

    Qgen  m1Lh1L  m1G h1G  m15h15  ...(1) Bubble Pump energy balance: The bubble pump acts as a heat exchanger, in which the returning weak solution (5) absorbs some amount of heat from the rising vapours of NH 3 Condenser and H2O. Therefore, Refrigerant ammonia vapour after leaving the Heat balance in the bubble pump: rectifier (6) condenses in the condenser by       m1Lh1L  m1Gh1G  m3Lh3L  m3Gh3G   m2h2  m5h5 rejecting heat into the atmosphere thus mass ...(2) balance equation in the condenser.

477 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

   Energy balance equation: Ammonia m12L  m12G  mR12 ...(7) refrigerant is condensed at constant pressure     m12L  m12G  m9  mv ...(8) in the condenser. Thus Energy balance equation: Energy balance Heat rejected in the condenser can be equation for sub cooled ammonia refrigerant written as: hydrogen and ammonia residual gas leaves     at expansion at 12. Qcon  m7 X 7h7G  m7 1 X7 h7L  m6h6 ...(5) m h  m h  m h The liquid portion of the refrigerant (7) from v 11 H H,11 9 9    the condenser flows through the gas heat  mH hH,12  m12L h12f  m12Gh12G ...(9) exchanger where it is sub cooled while the Evaporator Ammonia Sub Cooling vapour portion of the refrigerant by passes the and Gas Heat Exchangers heat exchanger-evaporator assembly and Condensed ammonia refrigerant enters into flows to the reservoir after mixing with the (NH 3 the heat exchanger portion of the coupled + H O) gas coming out of the gas heat 2 evaporator and gas heat exchanger where it exchanger. is sub cooled up to 9, by the cold gas mixture

Expansion Chamber (NH3 + H2) coming out from the evaporator, Sub cooled liquid ammonia 9 meets with Hydrogen gas and residual ammonia mixture hydrogen gas with some residual ammonia released from the absorber (10) is also cooled gas in the expansion chamber (11). This by the cold gas mixture from the evaporator causes pressures of both ammonia and upto 11 before mixing with the sub cooled ammonia at 9. hydrogen to drop and mixture of NH3 and H2 with their respective partial pressures enters After mixing pressure of ammonia reduces the evaporation 12. Mass balance and energy to partial pressure in a mixture of saturated balance for expansion chamber are presented liquid ammonia, saturated vapour ammonia below. and hydrogen gas at 12. This mixture then Mass flow rate of ammonia vapor after enters the evaporation and absorbs heat from expansion the evaporation, While the liquid ammonia is first evaporated to saturated vapour and then m  m  m X 12G 9 v 12 the whole mixture is heated beyond   m9  m7 1 X 7  temperature at 12. After leaving the evaporator the ammonia X12 is the dryness fraction of ammonia vapour after mixing and expansion (12) and hydrogen gas mixture enters the gas heat exchanger where it absorbs some amount of Mass flow rate of liquid ammonia after heat from the liquid ammonia (7L) and also expansion from mixture of hydrogen and residual    m12L  m9  mv  1 X12  ...(6) ammonia (10). General mass balance of ammonia at exit Mass flow rate of ammonia vapor leaving of expansion 12. the evaporator at 13 is given below.

478 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

m  m 1 X   m  X ...(10) n R,13G R 7 v 13 Ppart NH3  n n ...(15) P NH  H Mass flow rate of ammonia liquid refrigerant total 3 2 leaving the evaporator at 13 is given below. Absorber and Reservoir    mR,13L  mR 1 X 7  mv  1 X13  ...(11) In the Reservoir ammonia and hydrogen gas mixture leaving the evaporator at 13 and Mass balance equation for ammonia uncondensed ammonia gas from the refrigerant in evaporator gas heat exchanger. condenser mix with the weak solution from Since refrigerant at 13 may consider both the solution heat exchanger. Ammonia saturated liquid and saturated vapour. vapour is readily absorbed in the weak    mR,13L  mR,13G  mR,13 ...(12) ammonia solution to produce strong ammonia solution (14). During the m  m  m  m  1 X  R,13L R,13G R 9, v 13 absorption processes, hydrogen gas is   liberated which leaves the reservoir along  mR 9,  mv  X13 ...(13) with some amount of unabsorbed ammonia Combined energy balance equation for gas. Thus residual gas mixture (10) condense ammonia refrigerant enters at 7, (hydrogen and unabsorbed ammonia) leaves residual gas mixture (ammonia and hydrogen) the reservoir and moves towards gas heat enters at 10 and passing through the tube exchanger. meets with sub cooled ammonia at 12, ammonia and hydrogen gas mixture leaves the Energy balance equation: Heat rejected in evaporator at 13. the absorber is given by        Qabs  m14h14  mH hH,10  mv hR,10  Qevap  mR 1 X7  h9  hR 7, f   mHCp,H T11 T10         m7 X 7 hR ,8, G  mR,13,GhR,13,G  mR,13,L hR,13,f  mv hh,11  hR,10  mR,13,G hR,13,G  mR,13,LhR,13,f        mH hH,13  m16h16  ...(16)  mR,12,G hR,12,G  mR,12,L hR,12,f  mHCp,H T13 T12  ...(14) Heat of absorption is liberated when More the liquid evaporates while traversing ammonia is absorbed in weak solution solution. Due to this temperature of leaving the evaporator more the NH3 refrigerant partial pressure in the vapor phase increases. strong solution (14) and inside gas (10) Therefore temperature through the evaporator- increases. Also heat is rejected from the gas heat exchanger increases. The evaporator- absorber to the atmosphere. gas heat exchanger outlet temperature (T ) is 13 Solution Heat Exchanger a model input parameter of the simulation. In the solution heat exchanger, the strong Many factors affect the evaporation process solution is heated from state 14 to state 15 by such as refrigerant mass flow, NH partial 3 extracting heat from the weak solution returning pressure, temperature, absorber from the bubble pump. effectiveness, etc. The partial pressure of NH3     in the gas mixture is defined as: m15h15  m16h16  m2h2  m14h14   0 ...(17)

479 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

Relation between mole fraction and mass the DAR system is the amount of cooling fraction is presented below: achieved by a refrigerating machine per unit heat supplied. However COP is also defined 17.03x   ...(18) as the refrigeration rate over the rate of heat 17.03x  1 x18.01 addition at the generator. Numbers of equations developed for finding Q the enthalpy such that the results are obtained COP  evp  ...(24) in terms of refrigeration capacity, coefficient Qgen of performance, heat addition in the generator, heat rejection in the condenser and absorber. RESULTS AND DISCUSSION Governing equation for enthalpy of ammonia- In this section, the effects of variation of water solution in liquid phase as well as gas generator temperature (T_GEN), ammonia phase is expressed below: mass concentration (ZETA) in rich solution, evaporator temperature and condenser mi  T  ni temperature on performance parameters like hL T, x  h0 i ai  1 x ...(19)  T   0  COP, refrigeration capacity and circulation ratio (ƒ) are presented. mi  T  h T, y  h a 1  1 y ni 4/ 6   0  i i     ...(20) Figures 3 and 4 present the variation of  T0  COP as a function of generator temperature Temperature-pressure and mole fraction and ammonia concentrations in rich solution (liquid phase and gas phase) relation is (ZETA), respectively. It can be seen that for all presented below. concentrations, COP of the cycle is very low at temperatures which are closer to the bubble ni   p  T p, x  T a 1 x mi ln  o  point for a particular concentration. COP   0  i i      ...(21)   p  increase quickly with increase in generator temperature, reaches the peak and then   p  T p, y  T a 1 y mi 4/ ln  o    0  i i      ...(22)   p  Figure 3: Effect of Generator Temp. on COP Circulation Ratio Circulation Ratio (CR) is defined as the ratio of the mass flow rate of the strong solution entering the generator to the mass flow rate of refrigerant.

m f  15  ...(23) m6

Cycle Performance The performance of the DAR cycle can be expressed in terms of COP. Performance of

480 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

maximum COP occurs at generator Figure 4: Effect of Ammonia Concentration on COP temperature of 100 °C, while for ZETA = 0.3, the same occurs at 160 °C of generator temperature. • The maximum value of COP obtained at a particular ZETA is higher at higher concentrations, which is justified by lower circulation ratio at higher concentrations. The maximum COP obtained is 0.56 for ZETA of 0.9 and while the same for ZETA = 0.3, is only 0.18. • For ZETA = 0.9, the maximum COP occurs at generator temperature of 100 °C, while decreases slowly with further increase in for ZETA = 0.3, the same occurs at 160 °C generator temperature. of generator temperature. At temperature closer to the bubble point, • Variation of COP with temperature (i.e., amount of ammonia evaporated in the difference if the maximum and the minimum generator is very low causing to lower COP at a particular ZETA) is higher at higher refrigeration capacity (Figure 5). After reaching concentrations. the peak, when generator temperature is increased, both ammonia and water are • At temperatures higher than 220 °C, COPs heated to higher temperatures and amount of for all concentrations are very low and heat input for the mass flow rate increases, approach towards zero. most part of which is rejected through the Figure 4 exhibits the variation of COP with rectifier and condenser without any significant ammonia concentration in rich solution (ZETA) increase in refrigeration capacity (Figure 5). at different generator temperature. The figure Since COP is defined as the ratio of reveals that COP depends significantly ZETA refrigeration capacity and the rate heat input only at lower temperatures, while at higher in the generator, COP decreases with temperatures, there is very little dependence increase in generator temperature. The other of COP on the concentration. revelations from these figures are as follows: Figures 5 and 6 show the variation of • At a particular generator temperature, COP refrigeration capacity as a function of increases with increase in ammonia generator temperature and ZETA, respectively. concentration, but at higher temperatures, Fig. 5 reveals that the refrigeration capacity, COP is less sensitive to the concentration. at a particular concentration, increases initially, • The generator temperature at which the reaches the peak and then starts decreasing peak COP occurs is lower for higher sharply with increase in generator concentration (ZETA). For ZETA = 0.9, the temperature. On the other hand, increase of

481 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

of the same is 743.8 kW for ZETA = 0.3. Figure 5: Effect of Generator Temperature on Refrigeration Capacity Here, it may be mentioned that the above performances are obtained for a mass flow rate of 5 kg/s through the generator. • Temperature, at which the maximum refrigeration capacity occurs at a particular ZETA, is lower for higher concentration. For ZETA = 0.9, the maximum refrigeration capacity occurs at generator temperature of 150 °C, while for ZETA = 0.3, the same occurs at 200 °C of generator temperature. • The generator temperature, at which the maximum COP occurs, is, in general, not equal to the generator temperature, at which Figure 6: Effect of Ammonia Concentration on Refrigeration Capacity the maximum refrigeration capacity, is obtained.

CONCLUSION A mathematical model has been developed to predict the performance of the diffusion absorption refrigeration system for various generator temperatures and concentrations of the refrigerant (ammonia) in the rich solution of ammonia-water solution. It is found that the performance of the diffusion absorption refrigeration system, in general, is poor due to large amount of heat ZETA at any temperature causes increase in lost during cooling process in rectifier, refrigeration capacity. The other observations condenser, absorber and gas heat exchanger. from these figures are as follows: Although NH3/H2O diffusion absorption • The maximum refrigeration capacity refrigeration system can be used for obtained at a particular ZETA is higher for maintaining temperatures below 0 °C, NH3/ higher concentrations, which is justified by H2O mixture may not be a good potential pair higher mass of refrigerant flow rate through for absorption refrigeration cycles operating the evaporator for given mass flow rate of with lower generation temperatures. The ammonia-water solution through the additional advantage of this system is that the generator at higher concentrations. The system can utilize heat sources like solar, maximum refrigeration capacity obtained is geothermal, industrial waste or others. 3160.7 kW for ZETA of 0.9 while the value

482 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015

REFERENCES and Supachart Chungpaibulpatana 1. Ben Ezzine N et al. (2010), “A Numerical (2001), Renewable and Sustainable Investigation of a Diffusion-Absorption Energy Reviews, Vol. 5, pp. 343-372. Refrigeration Cycle Based on R124- 7. Wang Q et al. (2011), “A Numerical DMAC Mixture for Solar Cooling”, Investigation of a Diffusion Absorption International Journal of Energy, Vol. 35, Refrigerator Operating with the Binary pp. 1874-1883. Refrigerant for Low Temperature 2. Chen J, Kin K J and Herold K E (1996), Applications”, Applied Thermal “Performance Enhancement of a Engineering, Vol. 31, pp. 1763-1769. Diffusion-Absorption Refrigerator”, 8. Zohar A et al. (2007), “The Influence of International Journal of Refrigeration, Diffusion Absorption Refrigeration Cycle Vol. 19, No. 3, pp. 208-218. Configuration on the Performance”, 3. Giuseppe Starace and Lorenzo De Applied Thermal Engineering, Vol. 27, Pascalis (2011), “An Advanced Analytical pp. 2213-2219. Model of the Diffusion Absorption 9. Zohar A, Jelinek M et al. (2005), Refrigerator Cycle”, International Journal “Numerical Investigation of a Diffusion of Refrigeration, pp. 1-8. Absorption Refrigeration Cycle”, 4. Koyfman A et al. (2003), “An Experimental International Journal of Refrigeration, Investigation of Bubble Pump Vol. 28, pp. 515-525. Performance for Diffusion Absorption 10. Zohar A, Jelinek M et al. (2007), “The Refrigeration System with Organic Influence of Diffusion Absorption Working Fluids”, Applied Thermal Refrigeration Cycle Configuration on the Engineering, Vol. 23, pp. 1881-1894. Performance”, Applied Thermal 5. Patek J and Klomfar J (1995), “Simple Engineering, Vol. 27, pp. 2213-2219. Functions for Fast Calculations of 11. Zohar A, Jelinek M et al. (2009), Selected Thermo Dynamic Properties of “Performance of Diffusion Absorption Ammonia-Water System”, Int. J. Refrig., Refrigeration Cycle with Organic Working Vol. 18, pp. 228-234. Fluids”, International Journal of 6. Pongsid Srikhirin, Satha Aphornratana Refrigeration, Vol. 32, pp. 1241-1246.

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APPENDIX

Nomenclature

Q Heat transfer rate (W) x Mole fraction of ammonia in liquid phase y Mole fraction of ammonia in vapor phase T Temperature (°C) L Liquid h Enthalpy (kJ Kg-1)  Ammonia mass fraction in solution G Gas COP Coefficient of performance ƒ Circulation ratio (CR)

m Mass flow rate (kg s-1) X Quality of gas C Specific heat (kJ kg-1 K-1) P Pressure (bar) p R Refrigerant Tg Generator temperature C Concentration H Hydrogen

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