111 CHAPTER 7 PERFORMANCE ANALYSIS OF VAPOUR COMPRESSION REFRIGERATION SYSTEM IN HYBRID REFRIGERATION SYSTEM 7.1 INTRODUCTION Energy is the primary component to run any system in the world. According to the second law of thermodynamics, there is an external work required to run the refrigerator which transfers the heat from the sink to the source. In order to maximize the performance of the refrigeration system, the input energy supplied to the refrigeration system needs to be utilized effectively. Most of the industries let out their process heat and flue gases to the atmosphere which are responsible for air and thermal pollutions in the environment. These heat sources can be employed as input energy to VAR system and also leads to reduction in thermal pollution. HRS is having the potential to empower the performance of simple VCR system and it is most suitable for industrial applications where low grade and high grade energy sources are available. Low grade energy sources like exhaust flue gas, hot stream, geothermal sources, etc., can be used as a heating source for VAR system. Integration of these two refrigeration systems having the salient features like higher COP, improvement in cooling performance, momentous energy savings and healthier environmental condition due to the usage of eco friendly refrigerants (HCM1 as refrigerant).
112 7.2 VAPOUR ABSORPTION REFRIGERATION SYSTEM VAR system consists of a condenser, an expansion device, and an evaporator which are similar to VCR system. In addition to the above, an absorber, circulating pump and a generator or desorber are used in the VAR system. A solution heat exchanger is additionally added in between the generator and an absorber to maximize the heat recovery from weak refrigerant solution. The experimental arrangement of VAR system is shown in Figure 7.1 and the main components of VAR system are indicated. The evaporator and the insulated evaporator cabin are indicated in the interior view of the VAR system and shown in Figure 7.2. Thermocouples are used to measure the temperature at various locations of the VAR system and these are connected to the digital temperature indicator. Figure 7.1 Experimental arrangement of VAR system
113 Figure 7.2 Interior view of VAR system 7.3 EXPERIMENTAL ARRANGEMENT OF HYBRID REFRIGERATION SYSTEM A simple VCR system can be cascaded with VAR system which provides the arrangement of Hybrid Refrigeration System. Figure 7.3 represents the schematic arrangement of HRS. Hermetically sealed compressor is employed to elevate the pressure of refrigerant from P evap to P cond in the VCR system. Air cooled condenser is assembled with VCR system and further enhancement in heat rejection rate is achieved through subcooling arrangement. Evaporator of the VAR system is acting as Evaporator cum Condenser (ECC) in HRS. The cooling effect produced by ECC unit is utilized for subcooling of HCM1 refrigerant coming from air cooled condenser (3). Expansion valve (EV) is used to expand the refrigerant from P cond to P evap and refrigerating effect (4a-1) is obtained through this expansion
114 valve. While the refrigeration system is working through the state points (1-2-3-4), indicates that the refrigeration system is coming under the configuration of simple VCR system. Valves V 1, V 2 and V 3 are used to operate the experimental arrangement in simple VCR system and HRS through the state points (1-2-3-3a-4a) respectively. High pressure and high temperature saturated vapour refrigerant leaving from the generator (13) is condensed into saturated liquid refrigerant through air cooled condenser (14) by constant pressure heat rejection process in VAR system. Then it is expanded into low pressure and low temperature refrigerant by the expansion device (EV). Expanded refrigerant (5) is allowed to flow inside of the ECC unit where the evaporator cabin heat will be absorbed. After absorption of the evaporator cabin heat, the vapour refrigerant (6) will be absorbed by the absorber and converted into a strong refrigerant solution (mixture of refrigerant and absorbent). Circulation pump is used to elevate the low pressure (7) into high pressure strong solution (8) and supplied to the generator (9). Figure 7.3 Schematic arrangement of Hybrid Refrigeration System
115 Pumped strong refrigerant solution is heated by means of applied heat source in the generator and is segregated into high pressure and high temperature vapour refrigerant (13) and weak refrigerant solution (major portion of absorbent). The weak refrigerant solution at high pressure and high temperature (10) is supplied to the absorber through the solution heat exchanger (11). High pressure and high temperature saturated vapour refrigerant (13) is condensed by air cooled condenser and the refrigeration cycle is repeated to produce the required cooling effect in the evaporator cabin of VAR system. Photographic view of the experimental arrangement of HRS is shown in Figure 7.4. The arrangement of HRS is quite complex but the momentous features of VCR and VAR systems are gained through HRS. Normally, condensation process of VCR system is carried out by air cooled condenser. After air cooled condensation, the HCM1 refrigerant is subcooled through VAR system. VAR system is assembled with VCR system through ECC unit. The cooling effect offered by the evaporator of VAR system has been employed to subcool the saturated liquid refrigerant (HCM1). The arrangement of subcooling coil inside the VAR system is also shown in Figure 7.4. The p-h diagram of the refrigeration cycle with VAR system subcooling is plotted in Figure 7.5. This shows that the subcooling (Process 3-3a) has been admitted after air cooled condensation. By this experimental arrangement, degree of subcooling is obtained about 6 C through VAR system. Due to extension of condensation process from state 3 to state 3a, the refrigerating effect produced is also prolonged from state 4a to state 1 (Instead of 4-1).
116 Figure 7.4 Photographic view of experimental arrangement of Hybrid Refrigeration System Figure 7.5 p-h diagram of refrigeration cycle with VAR system subcooling
117 7.4 THERMODYNAMIC ANALYSIS OF HYBRID REFRIGERATION SYSTEM Thermodynamic analysis of any refrigeration system has been represented by energy analysis as per the statement of first law of thermodynamics. Thermodynamic behavior of HRS has been considered with the following assumptions. i) All the components of HRS are in steady state condition. ii) iii) iv) Pressure losses in various components of HRS are neglected. No heat losses from the system or to the system are assumed. Kinetic and potential energy are neglected. v) Isentropic compression takes place in the compressor. vi) vii) viii) ix) In VCR system, the refrigerant entered to the compressor is in dry saturated condition (Dryness fraction is unity). Iso-enthalpic process is considered in expansion devices. The power requirement of circulation pump in VAR system has been neglected. The refrigerant leaving from the generator of VAR system is in saturated condition. The work input supplied to VAR system has been neglected.
118 7.4.1 Mass and Energy Balance of VCR System Components VCR system consists of four major components namely compressor, condenser, expansion device and evaporator. Heat balance and energy balance of these components will make perfect energy assessment of the refrigeration system and obtained by the following relations shown in Table 7.1. Table 7.1 Mass and energy balance equations of components in VCR system Components Mass balance Energy Balance Heat capacity (kw) Compressor m 1 = m 2 m 1 h 1 =m 2 h 2 W comp = m VCR-ref (h 2 h 1 ) Condenser m 2 = m 3 m 2 h 2 = m 3 h 3 Q VCR-cond = m VCR-ref (h 2 -h 3 ) Expansion device m 3 = m 4 m 3 h 3 = m 4 h 4 - Evaporator m 4 = m 1 m 4 h 4 = m 1 h 1 Q VCR-evp = m VCR-ref (h 1 -h 4 ) COP of the VCR system is calculated by using the following relation COP =Q VCR VCR-evp /W comp (7.1) 7.4.2 Mass and Energy Balance of VAR System Components The performance analysis of VAR system is obtained by applying the mass and energy conservation equations to each component of the system. Heat capacities of each component in VAR system can be calculated by considering the heat and energy balance of each component. The following equations (7.2) and (7.3) represents the total mass and energy conservation of any thermal system and Table 7.2 represents the mass and energy balance equations of various components in VAR system.
119 m m 0 (7.2) in out m h m h 0 (7.3) in in out out Table 7.2 Mass and energy balance equations of components in VAR system Components Mass balance Energy balance Heat capacity (kw) Generator m 9 = m 10 + m 13 m 9h 9 = (m 10 h 10 + m 13 h 13) Q gen = (m 10 h 10 + m 13 h 13 - m 9 h 9 ) Condenser m 13 = m 14 m 13 h 13 = m 14 h 14 Q VAR-cond = m VAR-ref (h 13 -h 14 ) Evaporator m 5 = m 6 m 5 h 5 = m 6 h 6 Q VAR-evp = m VAR-ref (h 6 -h 5 ) Absorber m 7 = m 6 +m 12 m 7 h 7 =m 6 h 6 +m 12 h 12 Q VAR-abs = (m 6 h 6 +m 12 h 12 - m 7 h 7 ) Circulation ratio of VAR system is defined as the ratio (f) between the concentration of strong refrigerant solution (X 9 ) and the difference in concentration of strong refrigerant solution (X 9 ) and weak refrigerant solution (X 10 ). Circulation ratio is given by f = m /m 10 13 = X / X -X (7.4) 9 10 9 COP of the VAR system is calculated by using the following relation COP =Q VAR VAR-evp / Q gen (7.5) 7.4.3 Mass and Energy Balance of Hybrid Refrigeration System Cooling effect (Q VAR-evp ) produced by VAR system will be equal to the subcooling effect (Q HRS-sub ) on the refrigerant of VCR system. Energy balance of ECC is given by
120 Q = Q (7.6) HRS-sub VAR-evp h 3a = h3- m VAR-ref /mvcr-ref h6-h 5 (7.7) given by From (7.6), the refrigerating effect (RE) produced in the HRS is RE = m h h - m /m h -h HRS - (7.8) VCR-ref 1 3 VAR-ref VCR-ref 6 5 From (7.7), COP of the HRS is obtained by COP = RE / m h - h (7.9) HRS HRS VCR-ref 2 1 The above equation emphasis that refrigerating effect of the HRS is directly proportional to the refrigerating effect produced in the VAR and VCR systems respectively, and it is inversely proportional to the work input to the compressor. 7.5 METHODOLOGY Experimental analysis has been carried out on VCR system in HRS to identify the economic loading under various evaporator loading conditions. The experimental steps followed in chapter 4 have been adopted in part load performance analysis of VCR system in HRS at 25, 50, 75 and 100% loading conditions in the evaporator. Water has been taken as a substance which is to be cooled in the evaporator and is filled according to the loading conditions. During the experimentation in HRS, VCR and VAR systems are in on condition. VAR system adds the subcooling effect to the HCM1 refrigerant after air cooled condensation of VCR system.
121 Cycling test (on/off) is performed for all loading conditions in the evaporator. Experimental study has been started with 25% of loading condition and experimental observations are noted down for the water temperature in the evaporator from 26 to 2 C. The observations are recorded for every 2 C drop in water temperature in the evaporator. During the experimentation the measurements like pressure and temperature at various state points (1, 2, 3, 3a and 4a) are noticed. Based on the measured pressure and temperature, the enthalpy value of each state points of the refrigeration system is calculated using the thermodynamic properties of HCM1. Various thermodynamic properties of HCM1 are obtained from the REFPROP software. The performance indexes like work input to compressor, condenser heat rejection rate, refrigerating effect and COP are evaluated based on the unit mass flow rate of refrigerant in the refrigeration system. The same procedure is repeated for 50, 75 and 100% loading conditions respectively. The results of various performance indexes are compared at different loading conditions and discussed in the following titles. 7.6 RESULTS AND DISCUSSION The part load performance results of VCR system in HRS engender a significant enhancement in all loading conditions compared to simple VCR system. The effect of VAR system subcooling on various loading conditions has been compared with each other and also with simple VCR system. The main requirement of any refrigeration system is to get the minimum possible temperature compared to the environmental condition. Since, water is used as the substance in the evaporator and the various performance indexes of VCR system in HRS are compared at lowest water temperature (i.e. 2 C) in this analysis.
122 7.6.1 Effect of Water Temperature in the Evaporator on Work input to the Compressor at Various Loading Conditions The quantity of work requirement to run the compressor is evaluated at different loading conditions and shown in Figure 7.6. This shows that maximum and minimum work input given to the compressor is obtained in 100% and 75% loading conditions respectively. In 100% loading condition, the amount of heat to be extracted from the water is more and consequently the more work is required for 100% loading condition. The work requirement is getting on increased while the water temperature is decreased. The work requirement for 75% of loading condition is found lower than that of 25, 50 and 100% by 9, 7.2 and 23.6% respectively. The influence of water temperature on different loading conditions of the evaporator specifies that VCR system in HRS works efficiently at 75% of loading condition. Figure 7.6 Effect of water temperature in the evaporator on work input to the compressor at various loading conditions in HRS
123 7.6.2 Effect of Water Temperature in the Evaporator on Condenser Heat Rejection Rate at Various Loading Conditions The influence of water temperature on condenser heat rejection rate is high for 100% loading condition. The comparison of condenser heat rejection rate at various loading conditions is shown in Figure 7.7. Due to more withdrawal of heat from the water at 100% of loading condition, higher heat rejection rate has been accomplished. Highest and lowest heat rejection rate is attained in 100% and 25% loads respectively in VCR system in HRS. Figure 7.7 Effect of water temperature in the evaporator on condenser heat rejection rate at various loading conditions in HRS
124 7.6.3 Effect of Water Temperature in the Evaporator on Refrigerating Effect at Various Loading Conditions Enhanced refrigerating effect is obtained in VCR system in HRS and variation in refrigerating effect is shown in Figure 7.8. Better refrigerating effect is achieved in HRS mode. Highest and lowest refrigerating effect is produced in 75% and 100% loads respectively. In HRS mode, the highest refrigerating effect is attained in 75% of load in the evaporator over the entire span of temperature. Ultimately, the enhancement in refrigerating effect and reduction in work input to the compressor leads to increase in COP of the refrigeration system. Figure 7.8 Effect of water temperature in the evaporator on refrigerating effect at various loading conditions in HRS
125 7.6.4 Effect of Water Temperature in the Evaporator on COP at Various Loading Conditions The variation of COP with respect to water temperature is compared and shown in Figure 7.9, which shows that 75% and 100% loading conditions yields highest and lowest COP values respectively at lowest water temperature in the evaporator. Among the various loadings, 75% loading condition holds the maximum COP value and is about 11.8, 9.1 and 25.1% greater than that of 25, 50 and 100% loading conditions respectively. Experimental results show that better performance is accomplished in 75% of loading condition in the evaporator. Figure 7.9 Effect of water temperature in the evaporator on COP at various loading conditions in HRS
126 7.7 SUMMARY Cascading of VAR system with simple VCR system provides improvements in cooling effect and COP for the HRS. With respect to various loading conditions in the evaporator, 75% loading condition accomplish superior performance and the gained outcomes are summarized below: Cooling effect offered by the VAR system has been supplied to HCM1 refrigerant after air cooled condensation process of the simple VCR system and 6 C of subcooling has been reached, which holds the improvement in condensation effect and refrigerating effect. The work input to run the compressor for 75% of loading condition is got lower than that of 25, 50 and 100% loading conditions by 9, 7.2 and 23.6% respectively. Among the various loadings, 75% loading condition holds the maximum COP value and is about 11.8, 9.1 and 25.1% greater than 25, 50 and 100% loading conditions respectively. Compared to simple VCR system, better condenser heat rejection rate and refrigerating effect has arrived for 75% loading condition. HRS is having the provision to use low grade energy for VAR system and high grade energy for VCR system. Hence, the work requirement for HRS is less than that of simple VCR system. Implementation of HRS holds the higher cooling performance and substantial energy saving potential.
127 Hence the above empowered results specify that the simple VCR system performance could be advanced by cascading it with VAR system where the availability of low grade energy is more. HRS is having massive potential in industrial applications and will have eco friendly accessibility due to the usage of HCM1 as refrigerant in the VCR system. The results of the three subcooling techniques discussed in chapter 5, 6 and 7 entails that optimized performance is achieved in 75% of loading condition through the performance indexes like higher COP, improved refrigerating effect, reduced work input to the compressor and better condenser heat rejection rate. Hence for cooling of any liquid substance in VCR system, 75% loading is the energy efficient loading condition among the others. COP is one of the key parameter to represent the performance of any refrigeration system. Hence COP values of VCR system with the subcooling techniques discussed in chapter 5, 6 and 7 are compared under 75% loading condition and is given in Figure 7.10. The above comparison is done for the lowest water temperature (2 C) in the evaporator. Among these three techniques, VCR system in HRS provides better COP value. Compared to the air cooled condensation, Thermoelectric cooling and HAC, the COP of HRS is move ahead about 13.76, 9.49 and 5.3% respectively at 75% of loading condition.
Figure 7.10 Comparison of COP values for different subcooling techniques 128