INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN (Online) Volume 3, Issue 3, Sep- Dec (2012) AND TECHNOLOGY (IJMET) IAEME ISSN (Print) ISSN (Online) Volume 3, Issue 3, Septmebr - December (2012), pp IAEME: wwwiaemecom/ijmethtml Journal Impact Factor (2012): (Calculated by GISI) wwwjifactorcom IJMET I A E M E QUANTIFICATION OF ENERGY LOSSES AND PERFORMANCE IMPROVEMENT IN DX COOLING BY EXERGY METHOD Dinkar V Ghewade* 1, Dr SNSapali 2 * Department of Mechanical Engineering, Genesis Institute of Technology, Kolhapur India Professor in Mechanical Engineering, Govt College of Engineering, Pune India; dvghewade@gmailcom ABSTRACT Direct expansion bulk milk cooling and storage tanks are found commonly in milk chilling centers as well as in large dairy farms These systems are used to pull down the milk temperature from 35 o C to 4 o C in 3 to 35 hours This duration is excess to maintain the quality of the milk at its original Further, the energy consumed by bulk milk cooler is comparatively higher, demanding the performance analysis The refrigeration system used for this purpose consists of standard components available in the market Even though these components are designed for the best individual performance, the performance of a plant as a whole is required to be studied The first law efficiency of the plant is higher, but the second law efficiency is found to be low Exergy analysis is used as a tool to evaluate the performance of the system Exergy flows in the system are experimentally studied to identify and quantify exergy destruction in all components of the system Based on the findings, certain design changes are made in the evaporator of the new system and tested for validation The contributing components to exergy destruction are: (i) compressor, (ii) condenser, (iii) evaporator and (iv) expansion valve, in decreasing order It is found that coefficient of performance (COP) of the new system (model) is improved by 06 to 08 and irreversibilities in compressor, condenser and evaporator are reduced significantly Marginal improvement in second law efficiency of the new model is recorded along with the saving in energy consumption rate of kw The improvement potential in each component is determined and the scheme to achieve the improvement is discussed Keywords Exergy analysis; Exergy efficiency; Bulk Milk Cooler; Improvement potential; Thermodynamic Analysis 137

2 1 INTRODUCTION Milk chilling is the primary and one of the important processes in maintaining the good quality of milk The temperature of the milk at its harvest is 35 o C and the bacteria count is in the range to per ml depending upon hygiene conditions at workplace If the milk is not chilled within half an hour from the harvest to a temperature of 4 o C, the bacteria count increases at a faster rate The rise in bacteria count increases the pasteurization temperature and decreases the quality of the milk Hence Bulk Milk Cooler (BMC) is used to chill and store the milk at large dairy farms, chilling centers, and milk collection centers The size of the storage tank depends upon whether the BMC is used for two, four or six milking conditions BMC for two milking conditions are the most widely used and are the focus of the present study In the second milking condition, milk is collected by milk processing plants once in 24 hours from dairy farms Milk harvested in the morning (first milking) is poured in the BMC to half of its capacity and is chilled from 35 o C to 4 o C After second milking, the fresh raw milk at 35 o C is mixed with the chilled milk and the tank is completely filled to its capacity This raises the temperature of milk in the tank to 19 o C The milk is further cooled to 4 o C and stored till it is transported to milk processing plant The energy performance of the refrigeration systems is usually evaluated based on first law of thermodynamics However compared to energy analysis, exergy analysis can better and accurately show the location of inefficiencies in the refrigeration system The results of exergy analysis can be used to assess and optimize the performance of the system Exergy is defined as the maximum useful work that can be obtained from the system at a given state with respect to a reference environment (ie dead state) (Kotas, 1985) The total amount of exergy is not conserved in a process or a system, but destroyed due to irreversibilities (Kotas, 1985) BMCs are designed and used for chilling the milk in standard duration of three hours as specified by ISO5708 BMCs are classified based on cooling time as class I, class II, class III, and class IV In the present study, class II BMC stipulated to chill the milk in 30 hours when the tank is half-filled is analyzed for its performance According to the laboratory test reports of BMC obtained from different manufacturers, the coefficient of performance (COP) of these bulk milk coolers, over its operational time is found to be in the range of A field survey was conducted by the authors to study the conditions in which the BMCs are used From the survey, it is found that the performance of the BMC further declines when operating in field conditions Due to low operational efficiency, the bulk milk coolers are not economic for use Another finding obtained from the survey is that the factors like size, low operation and maintenance cost, low initial cost, efficient heat transfer, and easy cleaning are very important in optimizing the performance of BMC In an effort to understand and identify the inefficiencies in the equipment and the process, exergy analysis of the BMC is done and exergy efficiency of each component is determined Energy consumption and overall performance of the BMC is major concern and needs a scientific study to improve its energy efficiency Exergy analysis tool is the appropriate technique to understand the system behavior and to locate inefficiencies Important parameters viz exergy destruction, coefficient of performance, work input, exergy efficiency, second law efficiency are determined to evaluate and compare the system performance The work consists of two parts as mentioned below: 138

3 1 The testing of existing BMC system (termed as the old model) is done and its performance in terms of COP, exergy destruction, work input, second law efficiency is evaluated 2 New model is developed, based on findings from the analysis of the old model, with some design changes in evaporator and the performance is measured as above Results of both the models are presented in this paper, wherein it is observed that design changes based on exergy analysis lead to improvement in performance of the system This paper analyzes the performance of BMC with respect to important parameters as mentioned above in point no (1) An attempt is made to explain the nature of irreversibilities and practical limits to their reduction 2 LITERATURE REVIEW The theory of exergy analysis is discussed at length by Kotas (1985) and is applied in thermal and chemical plant analysis by many researchers The refrigeration systems used as heat pumps with R22 as a refrigerant are analyzed for exergy loss by Hepbasali (2005) Entropy generation in thermodynamic process causes exergy destruction, which is a cause of low COP and consequently high energy consumption It is necessary to identify, locate and quantify the irreversibilities to improve energy efficiency of refrigeration systems The compressor performance is analysed using exergy method by McGovern(1995) and the refrigerant flow in the evaporator coils and air cooled condenser coils is analysed for various mass flow densities, inlet temperatures and tube lengths by Liang (2001) The heat transfer coefficient is found to be low in low vapour quality two phase flow region and high in high vapour quality two phase flow region Ratio of irreversibility rate with augmented heat transfer in a tube to irreversibility rate in heat transfer in smooth tube for turbulent flow is studied by Bali (2008) and Wang (2003) Irreversibility rate depends on Reynolds number (Re) and increases with it Rate of increase in ratio of irreversibilities deceases towards higher Re Non-dimensional number of irreversibility and non-dimensional irreversibility balance is defined by Pons (2004), and the entropy generation numbers are defined by A Bejan (1982) The abovementioned studies provide an adequate framework for setting the research experiment and analysing the refrigeration system in the present study 3 METHODOLOGY The methodology adopted in this study consists of two parts: (i) the experimentation scheme, and (ii) the analysis scheme This methodology is explained in brief in the following paragraphs 31 Experimentation Scheme The experiment is designed on two types of BMC: the old model and the new model The old model refers to the existing system, while the new model refers to the modified BMC design Refrigeration system of Bulk milk cooler consists of compressor, air cooled condenser, receiver, thermostatic expansion valve and dimple type (jacketed) evaporator Standard four row air cooled condensers are used which consist of grooved copper tube of an outer diameter of 9525 mm, and fin density of 14 fins per inch Evaporator, divided into two equal parts, is at the base of the tank, which cools the milk by direct expansion of the refrigerant R22 is used as the refrigerant Block diagram of 1000 liter BMC plant is shown in Figure-1 The BMC plant consists of two refrigeration units each for half part of the evaporator These two units operate simultaneously to chill the milk to required temperature Thermostatic expansion valve is used to regulate the mass flow rate with respect to evaporator exit temperature of refrigerant 139

4 2 3 Condenser Pressure measurement points-1,2,7& 8 Temperature measurement points- 1 to 8 Evaporator I1 E Expansion Valve condensing unit 1 Evaporator I2 condensing unit 2 E2 Expansion Valve Condenser 1-compressor inlet; 2- compressor exit; 3- condenser inlet; 4- condenser exit 5-expansion valve inlet; 6-expansion valve exit; 7-evaporator inlet; 8-evaporator exit, I1,I2- inlets to evaporator E1,E2-exits from evaporator Figure-1: Schematic diagram of Bulk Milk Cooler 1000 liter capacity (the old model) The refrigerant after leaving the expansion valve enters evaporator through inlet I1 and I2, and flows through the passage formed by a seam weld in the evaporator towards the exit E1 and E2 In existing system (the old model) there is one inlet and one exit for the refrigerant Experiments are done on chilling of water instead of milk as the physical properties of milk are very similar to that of water [12] Physically milk is a rather dilute emulsion combined with colloidal dispersion in which the continuous phase is the solution Table no 1 gives the properties of water and milk at 20 o C Table No 1: Properties of water and milk at 20 o c Sr No Name of Property Water Milk 1 Specific Gravity Specific Heat 4183 kj/kg kj/kg 3 Thermal Conductivity 0599 W/m-K W/m-K 4 Viscosity 1004x10-3 N-s/m 2 2 x10-3 N-s/m 2 5 Refractive index The findings for the water will equally hold good for the milk without much variations Pressures are measured by piezoelectric transducers across the compressor and evaporator with an accuracy of ±001 MPa The pressure losses in the condenser are insignificant, and therefore neglected Temperatures are measured across compressor, condenser, expansion valve and evaporator by RTD with an accuracy ±01 o C Data acquisition system is used to record the temperatures and pressures at specific intervals at salient points of the cycle over its operation Mass flow rate is measured by Coriolis effect mass flow rate meter (in kg/min) within an accuracy of ±02% Water is used as the chilling medium instead of milk, as both have the similar properties For the first milking condition test the tank is half filled (ie 500 liter) by water and it is heated to 35 o C with the auxiliary heater provided For the second milking condition tank is filled to its capacity (1000 liter) and heated to 19 o C In both the cases the water is chilled to 4 o C Data of Pressures (4 no), Temperatures (12 no), Mass flow rates and work inputs to compressor are recorded over the cooling period at specific intervals Tests are repeated minimum once for each set of parameters and the observations are confirmed 140

5 In the new model, two major design changes are made as shown in Figure-2 1 Seam weld provided on the evaporator to guide the fluid flow causes large pressure drop (138 kpa to 35 kpa) in the evaporator resulting in higher irreversibility rate Secondly the liquid refrigerant entering in evaporator jacket evaporates immediately and flows as gas in further portion of evaporator Since the heat transfer coefficient for the gas-surface interface is low, evaporator performance is low Hence in the new design flow restrictions in evaporator are removed 2 Instead of one inlet and one exit for the refrigerant at the evaporator, three inlets and three exits are provided to ensure liquid refrigerant is distributed equally throughout the jackets at the lower side of the evaporator Three parallel channels are employed in the new evaporator (for the each tube in original design) The mass flux (G) and heat flux for the new evaporator inlet of small diameter (d) tubes would be different than those of original evaporator inlet of large diameter (D) tubes For the same total mass flow rate the number of small diameter tubes (n) replacing large diameter tube is given by G n = G D d D d Condenser 4 5 E3 I3 Evaporator E2 I2 E1 I Expansion Valve 6 condensing unit 1 I6 Evaporator I5 I4 condensing unit 2 E6 E5 E4 Expansion Valve Condenser 1-compressor inlet; 2- compressor exit; 3- condenser inlet; 4- condenser exit 5-expansion valve inlet; 6-expansion valve exit; 7-evaporator inlet; 8-evaporator exit, I1-I6- inlets to evaporator E1-E6-exits from evaporator Figure-2: Schematic diagram of Bulk Milk Cooler 1000 liter capacity (the new model) After making the required design changes, the new model is tested for performance by adopting the same experimental procedure as employed for the old model 32 Analysis scheme For analyzing the data obtained from the experiments, a technique of exergy analysis is used Exergy analysis combines the first and the second laws of thermodynamics, and is a powerful tool for analyzing both the quantity and quality of energy utilization The maximum work obtainable from system using environmental parameters as reference state is called exergy and is expressible in terms of four components: physical exergy, kinetic exergy, potential exergy and chemical exergy However the kinetic exergy and potential exergies are usually neglected and chemical exergy is zero as there is no departure of chemical substances to environment Therefore in this analysis physical exergy is only considered and is calculated Physical exergy of the material stream can be defined as the maximum work that can be obtained from when it is taken to physical equilibrium state with the environment 141

6 Ex = ( h h ) + T ( s s ) (1) o o o where h and s are enthalpy and entropy respectively and T o is the dead state temperature The enthalpy and entropy of the substance have to be evaluated at its pressure and temperature conditions (P, T) and the pressure and temperature at dead state (P 0, T 0 ) For a process 1-2 the change in exergy is given by: E x = h h ) + To ( s s ) (2) ( This change in exergy represents the minimum amount of work to be added or removed to change from state 1 to state 2 when there is an increase and decrease in internal energy or enthalpy resulting from change General exergy balance can be expressed in rate form as E x in E x = E x (3) out dest Considering control volume at steady state (Fig 2) the exergy balance can be expressed as in Ex Qin + Win = Ex out + Ex Qout + I Ex + (4) The exergy analysis is mainly concerned for the calculation of exergy efficiency and lost work for each unit operation The total exergy destruction in a cycle is simply the sum of the exergy destruction in condenser, compressor, evaporator and expansion valve The overall exergetic efficiency is defined as E x out in total η ex = = 1 (5) W E x actual I W actual The energy efficiency is simply the ratio of useful output energy to input energy and is referred as coefficient of performance (COP) for refrigeration system In this context the energy efficiency of BMC unit (COP actual ) can be defined as follows: Q e COP actual = (6) Win The ideal COP obtained from the carnot cycle is given as, COP T e ideal = (7) Tc Te Ideal COP is calculated on basis of effective condenser temperature (T c ) and the effective evaporator temperature (T e ) T cin T cexit Effective condenser temperature is defined as T = and effective evaporator c ln( T T ) cin cexit temperature is defined as, T eexit T ein T = e ln ( T T ) eexit ein One of the form of representing rational efficiency (Second law efficiency) is: 142

7 COP actual η th = (8) COPideal Exergy efficiency can be written as follows: E x E x out dest η ex = = 1 (9) E xin E xin Maximum improvement in the exergy efficiency for a process or system is obviously achieved when the exergy loss or irreversibility ( E xin E xout ) is minimized It is useful to employ the concept of improvement potential when analyzing the different processes This improvement potential on rate basis is given by Hammond and Stapleton as: IP = (1 η ex )( E xin E xout ) (10) The irreversibility rates corresponding to various components of the system are calculated using the exergy balance as follows: I and motor (process 1-2) The exergy balance for this component control region is, I I = Win + Ex1 Ex2 (11) Mechanical electrical losses can be obtained from the following relation: I me = W in (1 η mechη motor ) (12) Internal irreversibility due to fluid friction is given by, I int = I I I me (13) II Condenser (process 3-4) 143 Since the thermal exergy associated with heat transfer is zero ( E = 0), the exergy balance in this case is written as, I II = Ex3 Ex4 (14) III Exergy balance for the Expansion Valve (process 5-6) I III = Ex5 Ex6 (15) IV Exergy balance for the Evaporator (process 7-8) I = Ex Ex + (16) IV 7 8 E Q The performance of the condenser and evaporator is analyzed by defining the parameter I/ Q ie, the ratio of irreversibility rate to heat transfer rate The ratio indicates the relative change in irreversibility rate with respect to heat transfer rate 4 RESULTS The observations were collected by conducting the experiments according to the experimentation scheme, and the analysis was carried out based on the analysis scheme The results obtained from the analysis of the collected data are presented in graphical form in this section Data was separately collected for both the models with same instruments, ensuring equal accuracy The exergy rate is calculated at inlet and exit of each component of the refrigeration system The reference state for R22 is taken as normal atmospheric conditions of Q

8 temperature and pressure as K and kpa respectively The two condensing units operate simultaneously and exhibit nearly similar performance with insignificant variations at the end of cooling period The refrigerant properties are calculated by using CoolPack software Figure-3: Refrigeration cycle for old model at t=72 min Figure-4: Refrigeration cycle for new model at t=72 min Table-2: Results of exergy calculations at t=72 min (for old model) Sr No Salient point Pressure (kpa) Temperature (K) Sp Enthalpy (kj/kg) Sp Entropy (kj/kg) Exergy Rate (kw) inlet exit Condenser inlet Condenser exit Expansion Valve inlet Expansion Valve exit Evaporator inlet Evaporator exit Q cond = 928 kw Ex Qcond = 127 kw T eff,cond = K Q evap = 721 kw Ex Qevap = 034 kw T eff,evap = K W in = 279 kw COP actual =237 COP ideal =467 Exergy loss in the system over cooling period of 180 minutes is tabulated below in Table 3 144

9 Table-3: Exergy Loss in components of BMC system (old model) Condenser Expansion Valve Evaporator Total Ex Time Ex Loss Ex loss Ex loss Ex loss Loss (min) (kw) (kw) (kw) (kw) (kw) Similarly the exergy loss calculations for new model are calculated and tabulated in table 4 Table-4: Exergy Loss in components of BMC system (new model) Time (min) Ex Loss (kw) Condenser Ex loss (kw) Expansion Valve Ex Loss (kw) Evaporator Ex Loss (kw) Total Ex Loss (kw) Exergy loss rate in each component of BMC refrigeration system for old and new model is determined and presented in Figure-6 and Figure-7 Exergy loss in compressor for the old model varies over the range 124 to 096 and that for the new model from 116 to 09 Exergy efficiency of compressor for the new model is in the range 76% - 83% as against 70% - 81% for the old model The amount of exergy loss in condenser for new model lies in between kw as against kw for old model In addition, the exergy destruction rate in thermostatic expansion valve is found to be the lowest amongst all (about 10%) of the total work input to the plant, which is considered as insignificant The exergy loss in evaporator is found to vary from kw for new model as against kw for the old model The improvement in the performance is indicated by the ratio of irreversibility rate to the heat transfer rate in condenser and evaporator as shown in Figure-8 and Figure-9 The ratio I/ Q 145

10 for the condenser in new model is found to be low in the range as compared to that of old model range Similarly the ratio is determined for evaporator in new and old model varying from and respectively The exergy destruction rate in the condenser is primarily due to heat exchange with the environment Exergy destruction in condenser takes place due to pressure and temperature loss The exergy destruction rate due to pressure loss in condenser is very smalll and is about kw and rest is due to temperature loss Large pressure losses in the evaporator of old model are reduced to greater extent in new model It is observed that the amount of pressure losses in old model were in the range 138 kpa to 35 kpa whichh are reduced significantly to 35 kpa to 14 kpa, Figure-10 indicates the Second law efficiency for new model, which is higher than that of old model by 2-4% The exergy input and its consumption in components of the system is shown in Figure-11 At particular instant, out of the total exergy input to the system, it is found that about 38% of exergy is consumed in the compressor, 26% in condenser, 24% in evaporator, 10% in expansion valve and 2% is unaccounted loss COP of new model is found to be higher than old model by about as shown in Figure- evaporator 12 Carnot COP is determined on the basis of effective condenser and effective temperatures COP actual varies from 249 to 195 for old model as against the new model from 35 to 275 Ex Loss in KW Comp Ex Loss 2 Condenser Ex Loss Time 72 in 90 min Figure- 5: Exergy destruction in components of the BMC (the old model) Exergy Loss in kw Ex Loss Condenser Ex Loss 0 50 Time 100 in min Figure-6: Exergy destruction in components of the BMC (the new model) Old model 0 50 Time 100 in min Figure- 7: Variation in ratio of irreversibility rate to heat transfer rate in condenser Old New 0 50 Time 100 in min Figure-8: Variation in ratio of irreversibility rate to heat transfer rate in evaporator 146

11 Second Law Efficiency Time 100 in min old mod Figure-9: Variation in second law efficiency Exergy Consumption (New Model) 2% 24% 38% 10% 26% Condenser Figure-10: Pattern of exergy consumption in system components COP Time 100 in min old model Figure-11: COP comparison between old and new model Figure-12: Comparison of Improvement potential in both models It is observed that the improvement potential in the new model has been reduced which shows that there is overall improvement in the performance of the system The system performance has been improved resulting in power saving of 02 kw 5 DISCUSSIONS As observed from Figure-5 and Figure-6, exergy destruction rate is the highest in compressor as compared to other components of the system Initially, the exergy loss rate is higher, 124kW, due to high discharge temperature because of high pressure ratio and decreases slowly to 09kW towards the end of cooling period for old model Similar trend is observed for new model with lower values of exergy destruction The most significant exergy destruction rate is due to throttling, followed by internal convection Friction, mixing of fluid, conduction and external convection and radiation also contribute to the exergy destruction in compressor Exergy destruction could be reduced by reducing the internal convective heat transfer coefficient, swirl and turbulence in the cylinder and by increasing the areas of suction and discharge valve ports Improvement in the design of evaporator leads to reduction in exergy destruction in compressor and condenser Isentropic efficiency of the compressor is found to be in the range 63% to 65% Manufacturer s compressor performance data and actual work input to the compressor closely match with the theoretical work input and varies within 10% % The exergy destruction in the condenser for new model is less as compared to that of the old model Due to high discharge pressure and high discharge temperature in old model, exergy 147

12 loss is higher ie12kw at the starting period of cooling time, which later decreases sharply to 04kW as the temperature of the water in the tank approaches 4 o C at the end of the cooling time The factors contributing to the exergy destruction in condenser are fluid friction and turbulence caused due to change in direction of flow Higher the refrigerant mass velocity, higher is the exergy destruction rate An ideal condenser coil should have a high heat transfer coefficient with a low pressure drop Both the heat transfer coefficient and pressure drop are closely related to refrigerant mass velocity Varying the refrigerant mass velocity in different regions would balance the refrigerant side heat transfer coefficient and pressure drop Appropriate suitable complex refrigerant circuitry can improve the coil performance Significant reduction in pressure loss in the evaporator of the new model leads to decrease in exergy losses Exergy destruction in evaporator in the old model is comparatively high due to the turbulent flow and mixing of the refrigerant fluid The refrigerant side heat transfer coefficient is low due to a vapour film with low thermal conductivity between the liquid and the evaporator plate In the new model, path constraint for the fluid flow is removed Therefore, the refrigerant vapor moves to exit after heat absorption As a result, more amount of liquid refrigerant comes in contact with the heat transfer surface This design change in the evaporator leads innovation leads to higher overall performance of the evaporator and noticeable improvement in the second law efficiency The study reveals that further improvement in second law efficiency is possible by changing other design parameters, which may be explored in future research The mixing of the incoming fluid with the fluid already present in the system when they have different temperatures is one of the causes of irreversibility Further, when the fluid is throttled it will be at a temperature different from the fluid within the equilibrium system Unless the entering fluid has the same temperature as the fluid in the equilibrium system, exergy destruction will occur Finally, the COP of the entire system can further be increased by reducing unuseful heat gain in the suction line and in the condensing unit 6 CONCLUSIONS Significant amount of energy is lost in irreversibilities caused by improper process design and poor design of components The energy lost can be recovered by improving design and process parameters The compressor is the major contributor to the exergy destruction Evaporator and condenser, if redesigned to reduce the irreversibilities in flow, result in sizeable energy savings Proper sizing of evaporator inlets and exits for refrigerant will further reduce the pressure loss and hence the exergy loss The net savings in work input for the new model is recorded as 8-10% as that of work input to old model and reduction in cooling time is around 10% 148

13 Abbreviations and symbols Subscripts COP : Coefficient of performance 0 : Reference state Ė x : Exergy rate kw 1 : state 1 in process 1-2 G : Mass flux kg/m 2 s 2 : state 2 in process 1-2 h : Specific enthalpy kj/kg K c : condenser I : Irreversibility rate kw D : large diameter IP : Improvement potential kw d : small diameter Q : Heat Transfer Rate kw dest : destruction s : Specific entropy kj/kg K e : evaporator T : Temperature K ex : exergetic W : Mechanical or electrical energy in : Inlet kw η : efficiency int : internal th : thermodynamic exit : Outlet me : mechanical electrical mech : mechanical motor : electric motor REFERENCES [1] Bali,Tulin, Sarac, Betul Ayhan, Exergy Analysis of heat transfer in a turbulent pipe flow by a decaying Swirl generator,international journal of exergy, vol 5, No1, 2008 [2] Bansal, PK, Rupsinghe, AS, An empirical model for sizing of capillary tubes, International journal of Refrigeration, vol 19 No 8, pp ,1996 [3] Bejan A, Entropy generation through heat and fluid flow, New York, John Wiley and sons, 1982 [4] Bejan A, Entropy generation minimization, New York, CRC press, 1996 [5] Hepbasali,Arif, Thermodynamic analysis of a ground source heat pump system for district heating, International journal of energy research, 2005, 29: [6] Kotas,TJ, The Exergy Method of Thermal Plant Analysis, Butterworths,1985 [7] Liang,SY, Woong,TN, Nathan,GK, Numerical and experimental studies of refrigerant circuitry of evaporator coils, International journal of refrigeration,24 (2001) [8] McGovern, JA, Harte,S, An exergy method for compressor performance analysis, International Journal of Refrigeration, vol 18, No 6, pp , 1995 [9] Pons, Michel, Irreversibility in energy processes: Non-dimensional quantification and balance, Journal of non equilibrium thermodynamics, 2004,vol 29, pp [10] Tirandazi,B, Mehrpooya, M, Vatani, A, Effect of valve pressure drop in Exergy + Analysis of C 2 Recovery Plants Refrigeration Cycles, Int Journal of Electrical and Electronics Engineering, 4:4, 2009 [11] Wang, SP, Chen, QL, Yin, QH, Hua, B, Exergy destruction due to mean flow and fluctuating motion in compressible turbulent flows through a tube, Energy, 28 (2003), [12] Robert Jenness, Noble P Wong, Marth, Keeney, Fundamentals of Dairy Chemistry, Springer 1998, PP