This paper analyzes a CO2/NH3 cascade refrigeration system of two temperature range applied in the cold storage. A mathematical model is presented to determine the optimal middle temperatures of the cascade-condenser for obtaining the maximum coefficient of performance (COP) under different operation conditions. Three main parameters including the evaporation temperature in the cold storage, the evaporation temperature in the refrigerated storage and the condensation temperature in the high temperature stage are used to study the optimal middle temperature of CO2 in the cascade-condenser. The results show that the optimal middle temperature increases with the increment of three main parameters. Moreover, under specific conditions, the optimal temperature is equal to the evaporation temperature of refrigerated storage. The results shown in this paper is helpful to the control strategy of CO2/NH3 cascade refrigeration systems for two temperature ranges

Pan, Xi

Sun, ShiZhong

Wang, Haojie

Wu, Huagen

Xing, Ziwen

English

Purdue E-Pubs

Purdue University Purdue e-Pubs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 2021 Study on Optimal Middle Temperature of Cascade-condenser in CO2/NH3 Cascade Refrigeration Systems with Two Temperature Ranges Xi Pan Xi'an Jiaotong University, panxihouse@163.com ShiZhong Sun Haojie Wang Huagen Wu Ziwen Xing Follow this and additional works at: https://docs.lib.purdue.edu/iracc Pan, Xi; Sun, ShiZhong; Wang, Haojie; Wu, Huagen; and Xing, Ziwen, "Study on Optimal Middle Temperature of Cascade-condenser in CO2/NH3 Cascade Refrigeration Systems with Two Temperature Ranges" (2021). International Refrigeration and Air Conditioning Conference. Paper 2094. https://docs.lib.purdue.edu/iracc/2094 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional 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.html                                                                                                                                                                                                                                                                                                                                                                                   2164, Page 1Study on Optimal Middle Temperature of Cascade-condenser in CO2/NH3 Cascade Refrigeration Systems with Two Temperature RangesXI PAN*, SHIZHONG SUN, HAOJIE WANG, HUAGEN WU, ZIWEN XINGSchool of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, ChinaContact Information (+8615029086807, panxihouse@163.com)* Corresponding AuthorABSTRACTThis paper analyzes a CO2/NH3 cascade refrigeration system of two temperature range applied in the cold storage. Amathematical model is presented to determine the optimal middle temperatures of the cascade-condenser forobtaining the maximum coefficient of performance (COP) under different operation conditions. Three mainparameters including the evaporation temperature in the cold storage, the evaporation temperature in the refrigeratedstorage and the condensation temperature in the high temperature stage are used to study the optimal middletemperature of CO2 in the cascade-condenser. The results show that the optimal middle temperature increases withthe increment of three main parameters. Moreover, under specific conditions, the optimal temperature is equal to the evaporation temperature of refrigerated storage. The results shown in this paper is helpful to the control strategy ofCO2/NH3 cascade refrigeration systems for two temperature ranges.1. INTRODUCTIONIn the field of rapid freezing and the storage of frozen food, the required temperature ranges from -30℃ to -55℃and the ambient temperature is generally from 20℃ to 35℃. A large temperature difference leads to a large pressure difference, which results in the low efficiency of the single- stage refrigeration system. Instead, the cascaderefrigeration system can be a better choice and the CO2/NH3 cascade refrigeration system is generally applied in thecold storage. Compared with other refrigerants, CO2 and NH3 whose global warming potential (GWP) are 1 and 0respectively are environmental-friendly. Additionally, in this system, NH3 operates at high temperature stage, whichis away from refrigeration storage and NH3 charge is low. Due to non-toxic, non-flammable, CO2 is safe enough andoperates at low temperature stage, which can be closed to the foods. Hence, this system has good reliability andsafety.Many studies have been conducted on the CO2/NH3 cascade refrigeration system. Getu and Bansal (2008) presenteda thermodynamic analysis on the CO2/NH3 cascade refrigeration system to determine the operation parameters anddesign. Lee et al. (2006) thermodynamically analyzed a CO2/NH3 cascade refrigeration system to determine the optimal condensing temperature of the cascade-condenser under different design parameters and displayed anequation about the maximum COP. Wang et al. (2009) evaluated the performance of a CO2/NH3 cascaderefrigeration system experimentally and analyzed the effects of different parameters, which showed that theexperimental values were close to those predicted by Lee’s expression. Messineo (2012) showed a comparisonbetween the CO2/NH3 cascade refrigeration system and a hydrofluorocarbon two stage system, which showed that while energy, security and environmental reasons are taken into account, the CO2/NH3 cascade refrigeration systemis an interesting alternative to R404A two-stage refrigeration system for low evaporating temperatures incommercial refrigeration. Aminyavari et al. (2014) analyzed the performances of a CO2/NH3 cascade refrigerationsystem from exergetic, economic and environmental points of view and a genetic algorithm method was used todesign the parameters of this system.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021                                                                                                                                                                                                                                                                                                                                          High-temperature stage ( NH 3) Low-temperature stage ( CO 2) T hrottling valve B 11 Separator B Condensation te mperature:25-40°C 2 3 Condenser Throttlmg V valve A ,. . . · 6 9 I O Throttling valve L} ..... ' Com,== ' : 15 ·· ·O ··· Evapo rator A (/\\ Separator A 7 ._ ____ _, 8 Y Compressor B Pump 1 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• I ]4 j The evaporation temperature in the j j v--_ -  ~:=:=}~'.~~==~=~=;~::.:~  ,,~.;.~ ,; '- 0 Evapo rator B ., L _ the cold storage:_-30~-SO"C _ j 12 ~--~ 13 Pump 2164, Page 2In order to meet the temperature demands of different foods, the low temperature stage is generally divided intoseveral circuits for different temperature ranges in the practical applications and these relatively complicated systemhave received little attention. In this paper, the low temperature stage of the CO2/NH3 cascade refrigeration system isdivided into two circuits, one for refrigerated storage with the evaporation temperature from -5℃ to -15℃ and theother for cold storage with the evaporation temperature from -30℃ to -50℃. A mathematical model is applied toobtain the optimal middle temperatures of the cascade-condenser under different operation conditions which includethe evaporation temperature in the cold storage, the evaporation temperature in the refrigerated storage and thecondensation temperature at the high temperature stage. A discussion about the effect of different parameters on theCOP is presented in detail.2. CYCLE SYSTEM DESCRIPTIONA schematic diagram of the CO2/NH3 cascade refrigeration system which has been considered in the present study isdisplayed in Figure 1. This CO2/NH3 cascade refrigeration system is constituted by two single stages, hightemperature stage using NH3 as working fluid and low temperature stage using CO2 as working fluid, connected by acascade-condenser. The high temperature stage is aimed at cooling CO2. The low temperature stage is used tocooling different foods.It can be clearly seen from Figure 1 that the low temperature stage is constituted by two circuits, one for refrigeratedstorage (dotted line) and the other for cold storage (chain dotted line). CO2 at the outlet of cascade-condenser isdivided in to two fluids. Fluid A passes though the throttling valve A and changes into two-phase fluid which flowsinto separator A. The liquid of fluid A in the separator is driven by the pump and flows into evaporator A where the liquid of fluid A absorbs heat for cooling the refrigerated storage. The gas of fluid A is compressed by the compressor A and then back to the cascade-condenser where the gas of fluid A releases heat. The evaporationtemperature in the refrigerated storage is assumed to be between -5 ℃ and -15 ℃ . The cooling capacity ofrefrigerated storage is assumed as 100kW. The flow process of fluid B is similar to the flow process of fluid A andthe evaporation temperature in the cold storage is assumed to be between -30℃ and -50℃. Besides, the coolingcapacity of cold storage is also assumed as 100kW.The main components of the high temperature stage are condenser, NH3 compressor, throttling valve and cascade-condenser. The condensation temperature is assumed to be between 25℃ and 40℃ and the temperature difference inthe cascade-condenser is assumed as 5℃.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021                                                                                                                                                                                                                                                                                                                                                        Wcom,NH3 = rnNH3 (h2s - h1)/rJcom,NH3 h2 = h2s - h1 + hi 17com,NH3 Qcc,NH3 = mNH3 (h1 - h4)Qcc,C02 = (mA +ms) * (h16 - hs)WcomA = mA(h10s - h9)/rJcom,C02 WcomB = ms(h1ss - h14)/rJcom,C02 QeA = mA(h6 - h9) Qes = ms(hu - h14)'Y/com,NH3 = - 0.00097 (::r -0.01026 (;:) + 0.83955 'Y/com,C02 = 0.00476 (::r -0.09238 (;:) + 0.898102164, Page 3Figure 1: Schematic diagram of the CO2/NH3 cascade refrigeration systems for two temperature ranges3. CYCLE MATHEMATICAL MODELING3.1 Assumptions for Mathematical ModelTo obtain the maximum COP under different operation conditions including the evaporation temperature in the coldstorage, the evaporation temperature in the refrigerated storage and the condensation temperature in the hightemperature stage, a mathematical model based on mass conservation and energy conservation is proposed. Some assumptions for simplifying analysis are considered as follows:(1) All components of this system operate under steady conditions.(2) The pressure drop and heat loss of the refrigerant flowing inside the heat exchangers and pipelines are neglected.(3) All throttling processes are isenthalpic.(4) The compression process is irreversible and the isentropic efficiencies of the compressors related to the pressure ratio are taken into account (Stoecker, 1998; Petter et al., 2004).(5) The superheat degree and the subcooling degree are assumed as 0℃.(6) The power consumption of pump can be neglected.3.2 Energy AnalysisFor the high temperature stage:For the low temperature stage:(1)(2)(3)(4)(5)(6)(7)(8)In this system, the COP can be obtained as follows:(9)The isentropic efficiencies of the compressors are strongly related with the pressure ratio, which are shown asfollows (Stoecker, 1998; Petter et al., 2004):(10)(11)Additionally, the thermodynamic properties of CO2 and NH3 are calculated by using REFPROP and the calculationprocedure is written by using MATLAB.4. RESULTS AND DISCUSSION4.1 Effect of the Condensation Temperature in the High Temperature StageFigure 2 displays the results of COP versus middle temperature of CO2 in the cascade-condenser at the six different condensation temperatures of high temperature stage and the optimal middle temperatures of six curves are markedby arrows. The evaporation temperature in the cold storage and the evaporation temperature in the refrigeratedstorage are -30℃ and -10℃, respectively. the condensation temperature is from 25℃ to 40℃. It is clearly seen thatthe condensation temperature has a significant effect on the COP which decreases dramatically with thecondensation temperature. The major reason is that an increment of temperature difference in the high temperaturestage leads to a pressure difference lift. Hence, the power consumption of high temperature stage increases greatly.In addition, with the increase of middle temperature, six curves of COP increase firstly and then drop. The range ofgreat COP is obtained at the corresponding range of middle temperature. Moreover, the optimal middle temperature increases with the increase of condensation temperature, which is helpful to the control of this refrigeration system.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021                                                                                                                             3.1 2.9 P-, 0 2.7 u P-, 0 u 2.5 2.3 2.1 3.0 2.9 2.8 2.7 2.6 2.5 3.1 842 • 2.9638 -• 2.7659 • 2.5874 • -o---□--0-er-o--0--c:r-o--0-0-0--0-o-o--o-o-D--o-o-D--o--o-a -,.-0•0 2.4253 • e--0--0--e--0--o-e--0-~-e--e--0--e--0--0--e--e-~-e .. ..., . .,..0-•e·-<>·<>-· 2.2775 • .. -l,--tr-6.--A--l!r--ir -A--l!r--tr--A--b---tr-A--.!r--l:r-A--A-~-t!.--A--b--A--A--6---6 ·8 ·6 .4 ·2 0 Middle temperature of CO2 ( 0 C) 2.9674 • .20 ·16 ·12 ·8 .4 0 4 Middle temperature of CO2 (0 C) 8 Condensation temperature ---2s0 c -+-28°C -+-31°C ·-0·· 34°c ....... 37°c TEA= -10°C Tm= -30°C 2 Evaporation temperature in the refrigerated storage - -5°C -+--7°C ---9°C ··~·· -11°C ··8·· -13°C TcN = 30°C Tm= - 30°C 12 2164, Page 4Figure 2: COP versus middle temperature of CO2 at different condensation temperature4.2 Effect of the Evaporation Temperature in the Refrigerated StorageFigure 3 shows the results of COP versus middle temperature of CO2 in the cascade-condenser at the six differentevaporation temperature of refrigerated storage. The optimal middle temperatures of six curves are marked byarrows. The condensation temperature of high temperature stage is 30℃ and the evaporation temperature of coldstorage is -30℃. Besides, the evaporation temperature of refrigerated storage is from -5℃ to -15℃ and the middle temperature should be equal or larger than the evaporation temperature of the refrigerated storage. It is clearly seenthat an increment of evaporation temperature in the refrigerated storage results in a COP lift. And the optimal middle temperature increases with the evaporation temperature of the refrigerated storage. In addition, with the increase ofmiddle temperature, six curves of COP increase firstly and then drop. The range of great COP is obtained at the corresponding range of middle temperature.Figure 3: COP versus middle temperature of CO2 at different evaporation temperature of refrigerated storage18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021                                                                                                                                                                                                                                 2.8 2.6 p... 2.4 0 u 2.2 2.0 1.8 1.6 -12 2.8296 • ..... ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 2.6392 ■ • += t I I I I I f I I I I I I I 2.4550 • I l t I t I t * t 2.2765 I I I I I I I • -10 -8 -6 -4 -2 0 Middle temperature of CO2 (°C) Evaporation temperature in the cold storage ---30°C -+- -34°C ---38°C --13-- -42°C --e-- -46°C __ ., __ -50°C TcN = 30°C TEA = -10°C 2 2164, Page 54.3 Effect of the Evaporation Temperature in the Cold StorageFig. 4 displays the variations of COP with middle temperature of CO2 in the cascade-condenser at the six different evaporation temperature of cold storage and the optimal middle temperatures of six curves are marked by arrowsand dots. The condensation temperature and the evaporation temperature of refrigerated storage are 30℃ and -10℃, respectively. The evaporation temperature of cold storage is from -30 ℃ to -50℃, which is important to the COP.With the increase of evaporation temperature in the cold storage, the pressure difference of compressor B decreases and the power consumption of low temperature stage drops, which leads to the increase of COP. Middle temperature has a relatively slight effect on COP and the fluctuations of six curves are small. When the evaporation temperatureof cold storage is from -30℃ to -42℃, the curves of COP increase firstly and then decrease. However, when theevaporation temperature of cold storage is from -46℃ to -50℃, the optimal middle temperature for COP is equal tothe evaporation temperature of refrigerated storage. The reason is that the middle temperature should be equal orlarger than the evaporation temperature of the refrigerated storage. Besides, the optimal middle temperature growswith the growth of evaporation temperature in the cold storage.Figure 4: COP versus middle temperature of CO2 at different evaporation temperature of cold storage5. CONCLUSIONSThis paper analyzes the performance of a CO2/NH3 cascade refrigeration system with two temperature ranges. Three main parameters including the evaporation temperature in the cold storage, the evaporation temperature in the refrigerated storage and the condensation temperature in the high temperature stage are used to study the optimal middle temperature of CO2 in the cascade-condenser. The conclusions from analytical results forobtaining the optimal performance are shown in detail as follows: COP increases with the decrease of condensation temperature in the high temperature stage and the optimalmiddle temperature increases with the increase of condensation temperature Both COP and optimal middle temperature increase with the increment of evaporation temperature in the refrigerated storage. Both COP and optimal middle temperature increase with the increment of evaporation temperature in the cold storage.NOMENCLATUREW work or power (kW)18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021                                                                                                                                                                                                                                                                                       2164, Page 6m mass flow rate -1)(kg∙sh specific enthalpy (kJ∙kg-1)η efficiency (–)Q heating capacity (kW)COP coefficient of performance (–)p pressure (MPa)Subscriptcom compressor1,2,3··· points shown in Figure 1s isentropiccc cascade-condensercomA compressor AcomB compressor BA Fluid AB Fluid BeA evaporator AeB evaporator BREFERENCESAminyavari, M., Najafi, B., Shirazi, A., Rinaldi, F. 2014. Exergetic, economic and environmental (3E) analyses, andmulti-objective optimization of a CO2/NH3 cascade refrigeration system. Applied Thermal Engineering, 65, 42-50.Getu, H.M., Bansal, P.K. 2008. Thermodynamic analysis of an R744–R717 cascade refrigeration system.International Journal of Refrigeration, 31 (1), 45–54.Lee, T., Liu, C., Chen, T. 2006. Thermodynamic analysis of optimal condensing temperature of cascade-condenserin CO2/NH3 cascade refrigeration systems. International Journal of Refrigeration, (29), 1100-1108.Messineo, A. 2012. R744-R717 Cascade refrigeration system: performance evaluation compared with a HFC two-stage system. Energy Procedia, 14, 56-65.Petter, N., Filippo, D., Havard, R., Arne, B. 2004. Measurements and experience on semi-hermetic CO2 compressors.Fifth International Conference on Compressors and Coolants, IIR, Slovak Republic.Stoecker, W.F. 1998. Industrial Refrigeration Handbook, McGraw Hill.Wang, B., Wu, H., Li, J., Xing, Z. 2009. Experimental investigation on the performance of NH3 /CO2 cascade refrigeration system with twin-screw compressor. International Journal of Refrigeration, 32, 1358-1365.18th International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021

Study on Optimal Middle Temperature of Cascade-condenser in CO2/NH3 Cascade Refrigeration Systems with Two Temperature Ranges

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Study on Optimal Middle Temperature of Cascade-condenser in CO2/NH3 Cascade Refrigeration Systems with Two Temperature Ranges

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