Gu, Z.

Li, Y.

Liu, H.

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Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2002Simulation of NH3/CO2 Two-Stage LowTemperature Refrigeration SystemH. LiuXi’an Jiaotong UniversityZ. GuXi’an Jiaotong UniversityY. LiXi’an Jiaotong UniversityFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlLiu, H.; Gu, Z.; and Li, Y., "Simulation of NH3/CO2 Two-Stage Low Temperature Refrigeration System" (2002). InternationalRefrigeration and Air Conditioning Conference. Paper 620.http://docs.lib.purdue.edu/iracc/620                                  R18-5 SIMULATION OF NH3/CO2 TWO- STAGE LOW TEMPERATURE  REFRIGERATION SYSTEM  Hongjuan Liu, M.S. student, Environment and Chemical Engineering School Xi’an Jiaotong University, Xi’an 710049, P. R. China  *Zhaolin Gu, PhD, Professor, Environment and Chemical Engineering School Xi’an Jiaotong University, Xi’an 710049, P. R. China; Tel: 86-29-3268980; Fax: 86-29-3237910; E-mail: guzhaoln@mail.xjtu.edu.cn  Yun Li, PhD, Associate Professor, Environment and Chemical Engineering School Xi’an Jiaotong University, Xi’an 710049, P. R. China  ABSTRACT Studied is a kind of NH3/CO2 two-stage low temperature refrigeration system, in which NH3 is used as the refrigerant in the high-temperature stage and CO2 is used as the refrigerant in the low-temperature stage. The performance of the system is simulated and analyzed by changing the mean temperature of the evaporator-condenser and the temperature of the condenser. The mean temperatures of the evaporator-condenser are calculated and demonstrated to maximize the COP of the system at different condensing temperatures. It is shown that there exists an optimum mean temperature of evaporator-condenser to maximize the COP of the system and the maximal COP of the system is increased lowering the condensing temperature. The relationship between the optimum mean temperature and the condensing temperature is given in this paper.    INTRODUCTION Due to the depletion of the ozonosphere and/or the global warming impact of CFCs, HCFCs and HFCs, natural refrigerants, such as carbon dioxide, ammonia and HCs, are discussed and used for refrigeration systems [1-9]. Carbon dioxide has many advantages as a natural refrigerant. It is non-flammable, inexpensive and abundant. It has great evaporating latent heat and refrigeration capacity per unit. Its kinetic viscidity is low and it’s harmony with common lubricant. Carbon dioxide as a single refrigerant have been studied for automobile air-condition, heat pump and so on, mainly focusing on the transcritical refrigeration cycle [1-3]. The defect of carbon dioxide as a refrigerant is the high system pressure for air-conditioning, more than 100 bar. Ammonia as a refrigerant has advantageous thermodynamic properties. But it is incompatible with ordinary mineral lubricant, so the evaporator must adopt full-liquid evaporator, which increases the filling capacity of ammonia. If there exists some water vapor in the system, ammonia becomes corrosive to copper, so the heat transfer rubes must use steel tube. The capability of heat transfer of steel is not as good as that of the copper, which increases the heat transfer area and the weight of the system. The disadvantages of ammonia as a refrigerant must be overcome in order to expand its market of ammonia refrigeration system [5-7]. Decreasing the filling capacity of ammonia refrigeration system and departing ammonia refrigeration system from the public or half-public area to reduce the fatalness of ammonia refrigeration system, have pressed the technique of ammonia refrigeration to be changed, one of which is using NH3/CO2 double working fluids two-stage refrigeration system instead of the traditional NH3 low temperature refrigeration system. NH3/CO2 two-stage low-temperature refrigeration system takes advantages of the environmental qualities of ammonia and its safety issues [7-9]. This refrigeration system minimizes the charge quantity for NH3 by filling only in the high-temperature stage, using CO2 as a boiling secondary refrigerant/coolant in the low-temperature stage and possible to keep NH3 away from safety requiring areas. With the thermodynamic simulation, the performance of NH3/CO2 two-stage refrigeration system is calculated and analyzed by changing the mean temperature of evaporator-condenser between the two stage system and the temperature of the condenser, which provides a theoretic basis for the optimal designation and practical operation of cascading refrigeration system.  NH3/CO2 CASCADE REFRIGERATION SYSTEM AND SIMULATION Figure 1 shows the schematic NH3/CO2 two-sage low temperature refrigeration system, including the high temperature loop with ammonia as the working fluid and the low temperature loop with carbon dioxide as the working fluid. The mean heat exchanger (Evaporator-Condenser) is an important heat exchanger, which operating parameters influence the COP of the system. Carbon dioxide as the refrigerant or coolant in the low temperature loop works as a common cascade refrigeration system. The filling capacity of ammonia in the high temperature loop is also decreased compared to ammonia low temperature refrigeration system and the problem of safety will be solved departing NH3 loop from the public area. In addition, the volumetric refrigeration capacity of carbon dioxide is 8 times that of ammonia, which decreased the volumetric flow of coolant greatly. Because of the great phase-change latent of carbon dioxide, heat transfer is improved and the effective area of heat exchanger is reduced. Figure 2 shows the T-S diagram of NH3/CO2 low temperature refrigeration system. The power input consists of two parts, the power of CO2 compressor in the low temperature loop (N1) and the power of NH3 compressor in the high temperature loop (N2). In the mean heat exchanger (Evaporator-Condenser), α kg NH3 is needed to cool and condense 1 kg CO2 assuming no heat loss and heat transfer only by the phase change of CO2. α = qc / qg                                                       (1) Where, qc is the CO2 refrigeration capacity per unit mass, kJ/kg, qg is the NH3 refrigeration capacity per unit mass, kJ/kg.      The mass flow of CO2 in the low temperature loop for the capacity of the system, Q0 kW Gd = Q0 / qd = Q0 / (h1R-h5)                                             (2) Therefore, the mass flow of ammonia in the high temperature loop is Gg = α Gd                                                    (3) The coefficient of performance (COP) of this system is ε = Q0 /[Gd ( h2R - h1R )+Gg ( h7R - h6R )]                                    (4) The evaporating temperature of high temperature loop and the condensing temperature of low temperature loop must match to keep the good COP of the system. Therefore, the mean temperature of evaporator-condenser should be designed carefully. Generally speaking, there are two methods to choose the mean temperature of evaporator-condenser. One aims at maximizing the COP of the system, and the other aims at equalizing the pressure ratio of two compressors to save power input. The optimal mean temperature is determined to maximize the COP of the system in this paper. Assuming pressure losses and isentropic efficiencies of two compressors in the system and given the temperature difference for heat transfer of heat exchanger (Condenser and Evaporator), the evaporating temperature of the low temperature loop and the condensing temperature of the high temperature loop, the performance of the system can be simulated using Eq.(1)-Eq.(4) and the state equations of CO2 and NH3.  RESULTS AND ANALYSIS Calculation conditions: Evaporating temperature of low temperature loop, 233K; Condensing temperature, 308K; Refrigeration capacity, 1.5KW. The temperature difference for heat transfer of heat exchanger is given 5K. The isentropic efficiency of compressors and the mechanical efficiency of compressor shaft are given, 0.75 and 0.98, respectively. Figure 3 shows the COP of the system changing of the mean temperature of the evaporator-condenser in the range of 250.65K ~265.65K. The mean temperature of the evaporator-condenser influences the COP and there is an optimum mean temperature to get the maximum COP. In Figure 3 the COP gets the maximum value, 1.486 at the mean temperature, 257.65K. However, the COP changes slightly within ±5K of the optimal mean temperature, which provides the flexibility for the design and operation of the NH3/CO2 low temperature refrigeration system. After determining the optimal mean temperature, the state points of the cycle and heat capacities of heat exchangers can be calculated. Table 1and Table 2 demonstrate the results of every state point at the optical mean temperature, the heat load of heat exchangers and the power input of two compressors. Changing the condensing temperatures of the high temperature loop, the COP is shown in Figure 4. The COP varies with the condensing temperatures. For example, when the condensing temperature is 298.15K, the COP reaches 1.75 at the condensing temperature 298.15K while the COP is only 1.19 at the condensing temperature, 323.15K. Figure 5 shows the relation of the optimal mean temperatures of the evaporator-condenser with different condensing temperature. The higher the condensing temperature, the higher the optimal mean temperature is. The optimal mean temperature changes from the range of 255.65K to 260.65K while the condensing temperature changes from the range of the 298.15K to 323.15K. This curve is nearly linear and can be formulated as TM = 196.02+0.2×TK                                              (5) Where, TK changing from 298.15 to 323.15K    CONCLUSIONS NH3/CO2 two-stage low-temperature refrigeration system takes advantages of the environmental qualities of ammonia and its safety issues. This refrigeration system minimizes the charge quantity for NH3 by filling only in the high-temperature stage, using CO2 as a boiling secondary refrigerant / coolant in the low-temperature stage and possible to keep NH3 away from safety requiring areas. With the thermodynamic simulation, the performance of NH3/CO2 two-stage refrigeration system is calculated and analyzed by changing the mean temperature of evaporator-condenser between the two stage system and the temperature of the condenser, which provides a theoretic basis for the optimal designation and practical operation of cascading refrigeration system. The mean temperature of the evaporator-condenser influences the COP and there is an optimum mean temperature to get the maximum COP. However, the COP changes slightly within ±5K of the optimal mean temperature, which provides the flexibility for the design and operation of the NH3/CO2 low temperature refrigeration system. The COP varies with the condensing temperatures. For example, when the condensing temperature is 298.15K, the COP reaches 1.75 at the condensing temperature 298.15K while the COP is only 1.19 at the condensing temperature, 323.15K.  REFERENCES [1] Lorentzen G, Pettersen J, A new, efficient and environmentally benign system for car air conditioning, Int J Refrig. 1993,16(1):4~12. [2] Lorenten G, Revival of carbon dioxide as a refrigerant, Int J Refrig., 1994,17(4):292~301. [3] Lin Gaopin, Gu Zhaolin, Performance of CO2 transcritical cycle, Journal of Xi’an Jiaotong University, 1998,32 (8): 35-38 [4] Klocker K, Schmidt E L, Steimle F, Carbon dioxide as a working fluid in a drying heat pumps, Int. J. Refrig., 2001, 24(1): 15-24 [5] Paul De Larminat, Expanding the use of ammonia, ASHRAE Journal, 2000, No. 3: 35-40 [6] Forbes S, Pearson, Ammonia refrigeration systems [J], ASHRAE Journal, 1999, No. 3: 24-29 [7] Miyasaki A, Ammonia, carbon dioxide and ice slurry--the commercial refrigerants of the future, Refrigeration of JSRAE, 1999,74(2): 120~125 [8] Enkemann T, Arnemann M, Investigation of CO2 as a secondary refrigerant [A], Proc. Int. Conf. IIR, New Applications of Natural Working Fluids in Refrigeration and Air Conditioning [C], Hannover, Germany, 1994 [9] Infante Ferreira C A, Boukens R A, Carbon dioxide-secondary coolant or refrigerant for cascade systems? , Proc. Int. Conf. IIR, Applications for Natural Refrigerants, Aarhus, Denmark, 1996        Table 1 Results of the state points of the refrigeration system Point    T/K    P/MPa  H/kJ kg-1   S/kJ(kg.·k)-1  1      233.15    1.004   629.66      5.0288 1R     232.89    0.984   629.66      5.0322 2AR      280.86    2.474   668.39      5.0322 2      -------     2.426   681.29      -------- 3      260.15    2.426   -------       -------- 4      260.15    2.426   364.70      3.8718 5      -------     -------   364.70      -------- 6      255.15    0.208   1639.86     8.8717 6R     254.96    0.204   1639.86     8.8811 7AR    395.82    1.375   1925.97     8.8811 7R      -------    1.375   2021.34     -------- 7      -------     1.3488  2021.34     -------- 8      308.15    1.348    --------     -------- 9      308.15    -------    565.16     4.5638 10     --------    -------    565.16     --------   Table 2 Loads of heat exchangers and compressors (COP=1.486)          Load          Qe/KW  Qc/kw   q/kg s-1  Ne/KW  Low-temperature loop    1.50     1.792   5.66      0.318 High-temperature loop   1.792     2.428   1.67      0.692   CO2  1        2      NH3      3                                     4                                          Figure 1 Schematic NH3/CO2 two-sage refrigeration system 5                                               1-CO2 compressor; 2-Evaporator –Condenser; 3-NH3 compressor;                           8                   4-Condenser; 5-Evaporator; 6 and 8-Throttle valve; 7-Accumlator         6        7        250 252 254 256 258 260 262 264 2661.4701.4721.4741.4761.4781.4801.4821.4841.4861.488TK=308.15KCopTM/K Figure 3 COP at different mean temperatures  Figure 2 T-S diagram of the cycle 25 7AR 26R 2R 347R 10 98 7 1 1R 6 TS 0 250 252 254 256 258 260 262 264 2661.11.21.31.41.51.61.71.8TK=323.15KTK=318.15KTK=313.15KTK=308.15KTK=303.15KTK=298.15KCopTM/K                       Figure 4 COP at different condensing temperatures   295 300 305 310 315 320 325255256257258259260261T m opt/KTK/K Figure 5 Relation of the optimal mean temperature and the condensing temperature  

Simulation of NH3/CO2 Two-Stage Low Temperature Refrigeration System

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Simulation of NH3/CO2 Two-Stage Low Temperature Refrigeration System

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