Exergy study of a refrigeration cycle for vapor compression Augusto Cuevas Snchez Sebastin Puentes GarridoEPC criterion is defined as the ratio of production rate exergy irreversibility full (or loss rate of availability). This analysis was carried out for a system of vapor compression refrigeration for different refrigerants where, according to the results, the refrigerant R134a shows a poor performance in terms of EPC among other refrigerants (R22 and R125). The effects of temperature and pressure drop in the evaporator and the condenser and the ambient temperature on the destruction of exergy and EPC for the components of the refrigeration system of a vapor compression have been investigated in detail for the refrigerant R134a.Keywords: Exergy, Vapour compression refrigerators, Performance analysis, R134a, Efficiency, cycle. Nomenclature Exergy rate (kW)h Specific enthalpy (kJ/kg)s Specific entropy (kJ/kgK) Entropy rate (kW)in Inputout Outpout Mass flow rate of the refrigerant (kg/s) P Pressure drop (kPa)T Temperature (C) Exergy efficiency EfficiencyPower (kW) Rate of heat transfer (KW)comp Compressorcond Condensercs Cooled spacedest Destroyede Electricalevap Evaporatorgen Generatedm Mechanicalvalv Valve0 Dead stateIntroductionA carrier gas refrigerant is not more than a substance having the ability to transport and heat exchange with the environment, releasing heat to high temperature and absorbing to low temperature. A good coolant must meet multiple qualities, unfortunately not all can be satisfied at once.It becomes apparent that to the extent that nature of the refrigerant is such that the P - T condensation approximate those environment, will need less energy to compress and to cool it, and with it the consumption indicator will be smaller refrigeration unit . In turn, if it coincides its latent heat difference (relative to atmosphere) was high enough to transfer heat would require less amount of refrigerant to execute the job and thus less compression. Both qualities are essential in the energy consumption. In addition to other characteristic of the chemical nature of the coolant, which can provide cooling perform the work more or less efficiency.[1]In this study, the principal objective is to analize the theoretical performance of a system of refrigeration based on exergetic performance coefficient (EPC). Bringing will be possible to find the best exergetic performance conditions to a minimum loss rate of availability.Order to achieve carry out the foregoing will be made a model based on the mass balance, energy and exergy. after different refrigerants (R22, R134A, and R125) were analyzed to find the best coolant that gives the highest EPC and finally, for each refrigerant selected, measured the effects of important parameters such as condenser temperature, evaporator temperature, pressure drop and heat exchange with the surroundings.Refrigerant gases informationBe detailed in a brief way the characteristics of the refrigerants used for this study:The R-134a (HFC-134a) has been developed to become one of the substitute CFC and HCFC refrigerants. R-134a is a substitute long-term, environmentally safe and harmless to the ozone layer. It can be used in domestic and commercial refrigeration and air conditioning in commercial and industrial.It belongs to the group of HFCs, having no chloro are not miscible with mineral oils, base oil only used ESTER. Evaporates at -26 C at atmospheric pressure and is the permanent replacement for R-12. HFCs are very hygroscopic and absorb a lot of moisture.[2] The R-22 is an HCFC refrigerant at high pressure works but with a minimum compressor displacement. R-22 is used especially in applications domestic, commercial and industrial applications and also is used as a means to produce fluoropolymers and as a blowing agent in foam applications rigid.[3]This refrigerant is HCFC group, was originally designed for air conditioning but until recently used to everything. Evaporated at -40.8 C at atmospheric pressure, is miscible with the mineral and synthetic oil, but in low temperatures is recommended to use the oil separator. Accepts little overheating because otherwise too increase the discharge temperature. Product R 125 is an HFC refrigerant gas, used as a component of refrigerant blends for applications to high, medium and low temperature.[4]to differentiate these refrigerants by its type one considers that CFCs are refrigerants whose molecules contain chlorine atoms, fluorine and carbon while HCFC refrigerants are gases whose molecules contain atomshydrogen, chlorine, fluorine and carbon. The HFC refrigerants are gases whose moleculeshydrogen atoms, fluorine and carbon.[5]Development
The principles and thermodynamic analysis methodologies are well established, mainly for the case of thermal power plants.[6,7,8,9,10,11,12].An additional difficulty in applying the method to exergetic cooling cycles lies in the definition of the reference temperature, data necessary for calculating the irreversibility. Some authors [13,10,14,15] take different references for primary and secondary coolant, for example for the primary temperature and water inlet temperature to the evaporator for the secondary.As these systems operate interchangeably with temperatures below and above the environmental problem arises exergies obtaining negative at some points, apparently contradicts thermodynamic principle. Szargut [16] solves this problem by adding to each of the system streams exergy most negative exergy obtained by changing the scale of the reference drop and converting all exergy negative currents in positive terms, a procedure used in the present research.Thermodynamic analysis for systemThe cycle to analyze is the vapor-compression refrigeration (VCR) and is defined as a closed system in which the process of absorption and release of heat is performed by means of a coolant flowing in a vapor compression cycle. To perform the compression work performed by the cooling system requires power consumption that will be provided by an electric motor. In its simplest form, a refrigeration system comprises five components: Compressor, Condenser, Evaporator, Expansion Device and Pipeline.The cycle operates between a heat source of temperature Tcs (cooled space temperature) and a heat sink of temperature T0 (warm environment). The fluid enters the compressors at state 1, where the temperature is elevated by mechanical compression (state 2). The vapour condenses at this pressure, and the resultant heat is dissipated to the environment. The high pressure liquid (state 3) then passes through an expansion valve through which the fluid pressure is lowered. The low pressure fluid enters the evaporator at state 4, where it evaporates by absorbing heat from the cooled space and re-enters the compressor. The whole cycle is repeated.
Fig1 Vapour compression refrigeration system and its temperature specific entropy diagram
Table 1 Base case model parameters for simulation of VCR system
ParametersUnitValue
Mass flowrate of refrigerant fluidKg/s1
Condenser operating temperatureK300
Ambient temperatureK298
Ambient pressurekPa101.325
Pressure loss in condenser and evaporatorkPa15
Isentropic compressor efficiency%75
Mechanical efficiency%85
Electrical engine efficiency%90
Evaporator operating temperatureK223
Temperature difference between cooled space and evaporatorK10
To find the heat exchange of the condenser and the evaporator will be used the following formulates: (1) (2)The electrical power input for the compressor is given as
Where the compressor isentropic efficiency is defined as the ratio of an isentropic power and the actual power
(4)
Coefficient of operationIn the classical performance analysis of refrigeration systems, the COP is used as a major performance criterion. The COP gives information about the necessary electrical power input in order to produce a certain amount of cooling load. From the first law of thermodynamics, the COP is defined as the ratio of cooling load to the electrical power input for VCR cycle and given as (5)Using equations (1) and (3) into equation (4), COP becomes: (6)To find Exergy flow (or current), we use the following equation:
(7)The general exergy balance can be expressed in rate form as: (8)With this be graph the COP of the refrigerant with respect to the condensation and evaporation temperatures within a range of :
(a)
(b)Fig.2 variation in COP of VCR system for different refrigerants with (a) condenser temperature and (b) evaporator temperature
Exergy destruction rate in system componentsThe exergy destruction rate in each component of the VCR system is obtained as given below (9)
(10)Where the thermal exergy rate related with: cond is zero. (11) (12)Where is the thermal exergy rate related with:and can be defined as: (13)Where the total exergy destruction of the Vapour compression refrigeration system is obtained as: (14)
With this be graph the exergy percentage for each device of the refrigerant with respect to the condensation and evaporation temperatures within a range of :
Fig.3 variation in exergy destruction rate of VCR system and its components for R134a with respect to a condenser temperature (a),(b),(c) and (d).
Fig.4 variation in exergy destruction rate of VCR system and its components for R134a with respect to a evaporator temperature
Fig.5 variation in exergy destruction rate of VCR system and its components for R22 with respect to a condenser temperature
Fig.6 variation in exergy destruction rate of VCR system and its components for R22 with respect to evaporator temperature
Fig.7 variation in exergy destruction rate of VCR system and its components for R125 with respect to a condenser temperature Fig.8 variation in exergy destruction rate of VCR system and its components for R125 with respect to evaporator temperature
The exergetic efficiency The exergetic efficiency of the VCR system is defined as (15)Where the exergy input is equal to the electrical power input for the compressor: (16)And the exergy output is the exergy rate of heat transferred to the evaporator from the cooled space at temperature Tcs,
(17)Using equations (16) and (17) in equation (15), the exergetic efficiency becomes(18)With this be graph the exergy efficiency for each device of the refrigerant with respect to the condensation and evaporation temperatures within a range of :
(a)
(b)
Fig.9 variation in exergetic efficiency of VCR system for different refrigerants with respect to (a) condenser temperature and (b) evaporation temperature
Exergetic performance coefficientThe EPC objective function for a VCR system is defined as the ratio of exergy output to the total exergy destruction (or loss rate of availability): (19)Exergetic performance coefficient is related to the COP and the exergetic efficiency :
Reducing it is like this: (20)
With this be graph the EPC for each device of the refrigerant with respect to the condensation and evaporation temperatures within a range of :
(a)
(b)Fig.10 Variation in EPC of system of VCR system for R134a with respect to (a) condensation temperature and (b) evaporation temperature.
Fig.11 sankey diagram energy
Fig.12 sankey diagram exergy
Analysis and resultsThis analysis was performed using R134a, R22 and R125. The VCR system thermodynamic analysis was performed based on the following assumptions: steady state operation in all components chemical energy, kinetic and potential energy of the omitted components pressure drops in pipe networks are neglected heat transfer to from the compressor and the expansion valve are neglected isenthalpic expansion of refrigerant expansion valves.Model parameters for the base case VCR system simulation are chosen as shown in Table 1.The variation in the objective function of the VCR system EPC different refrigerants with respect to temperature and temperature evaporator condenser.In fig.5 shows that R22 represents the maximum among other refrigerants EPC for all values and .Also seen that the objective function EPC first increases and reaches a maximum and then decreases with increasing Tevap. Therefore, there an optimum value for a selected set of operating parameters.In Figure2 shows that the COP decreases as the condensation temperature increases and in turn increased when the evaporating temperature is increased.Therefore, in order to analyze the effects of , , temperature and pressure drops in the evaporator and condenser ( = ) in exergy destruction and EPCs for VCR system components, R134a refrigerant was chosen as the model for manual analysisData thermodynamic properties calculated for each node of the refrigeration cycle, where the performance results exergy VCR system base and its components are given in Tables 2 and 3 respectively.In Figures 3 and 4, it can be seen that the rate of higher irreversibility in the system occurs in the compressor section.This is a consequence of the efficiencies electrical, mechanical and isentropic compressor. As irreversibility rate is higher in the compressor, which is more than half of the total rate of exergy destruction, much interest should be given in the selection of the compressor for a refrigeration system.Also seen from Table 3 higher than the EPC components occur in the condenser. This is followed by evaporator, the compressor and the expansion valve for the base case condition.
Table 2 base case simulation results at each node of VCR system for refrigerant R134a
stateTemp CP kPah(kJ/kg)s(kJ/kgk)quality
1-59.1522.78219.46550.98297151
268.317718.46306.381.04729
326.85703.4689.04550.33330
4-43.916937.7889.04550.388580.41163
Table3Base case exergetic performance results of VCR system and its components for refrigerant R134a
components or system Exergy destruction rate (kJ/s) Exergy destruction ratio %
compressor4759.76
condenser4.265.42
valve15.8920.20
evaporator11.514.62
system78.65100
In figure 2 VCR system performance decreases with increasing temperature and increases condensation when the evaporation temperature increasesIn Figures 3 and 4 observed:1- the destruction of exergy in the compressor decreases as the condensation temperature increases and more destruction of exergy when the evaporation temperature rises. In the compressor the refrigerant r125 is the most exergy is destroyed, the r22 is the one that presents less destruction and r134a is somewhere in between.
2- Observed that exergy destruction capacitor is proportional to the condensation temperature and in turn increased inversely proportional to the evaporation temperature, the change of the evaporation temperature does not have significant variations r125.
3- The expansion valve exergy destruction rate increases as temperature increases and condensation is reduced by lowering the temperature of the evaporator.
4- Observed that in the evaporator the exergy destruction rate is inversely proportional to the increase in condensing temperature in which R134a and R22 would be in similar conditions.The exergy destruction rate increases as the evaporation temperature.In Figure 9 can be seen that the exergetic efficiency like the COP decreases with increasing the condensation temperature, and rises with increasing evaporation temperatureIn Figure 10 can be seen that the EPC like the COP and exergetic efficiency decreases with increasing the condensation temperature, and rises with increasing evaporation temperature
With Figures 11 and 12 represent the flow of energy and exergy destruction of the system respectively. With this method, which improved the explanation of benefit and harm flows our VCR
Conclusions
The irreversibility analysis by sample cycle components most sensitive components are the valve and the compressor, which reported an incidence about 80% of the total irreversibilitys of the system.The best performing component exergy (EPC) compared to the condensation temperature and evaporation is the capacitor due to the minimal exergy destroyed posing in the process.The COP improvement obtained by increasing the evaporation temperature and the condensation temperature decreases while maintaining the constant pressures by optimizing the energy consumption by the compressor.
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