Quantification of energy losses and performance improvement in dx cooling by exergy method
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Quantification of energy losses and performance improvement in dx cooling by exergy method Quantification of energy losses and performance improvement in dx cooling by exergy method Document Transcript

  • INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 3, Issue 3, Septmebr - December (2012), pp. 137-149© IAEME: www.iaeme.com/ijmet.htmlJournal Impact Factor (2012): 3.8071 (Calculated by GISI) ©IAEMEwww.jifactor.com QUANTIFICATION OF ENERGY LOSSES AND PERFORMANCE IMPROVEMENT IN DX COOLING BY EXERGY METHOD Dinkar V. Ghewade*1, Dr S.N.Sapali2 * Department of Mechanical Engineering, Genesis Institute of Technology, Kolhapur 416234 India Professor in Mechanical Engineering, Govt. College of Engineering, Pune 411005 India; E-mail: dvghewade@gmail.com 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 35oC to 4oC in 3 to 3.5 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 0.6 to 0.8 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 0.6 - 0.8 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
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME1. INTRODUCTIONMilk chilling is the primary and one of the important processes in maintaining the goodquality of milk. The temperature of the milk at its harvest is 35oC and the bacteria count is inthe range 10000 to 25000 per ml depending upon hygiene conditions at workplace. If themilk is not chilled within half an hour from the harvest to a temperature of 4oC, the bacteriacount increases at a faster rate. The rise in bacteria count increases the pasteurizationtemperature and decreases the quality of the milk. Hence Bulk Milk Cooler (BMC) is used tochill and store the milk at large dairy farms, chilling centers, and milk collection centers. Thesize of the storage tank depends upon whether the BMC is used for two, four or six milkingconditions. BMC for two milking conditions are the most widely used and are the focus ofthe present study. In the second milking condition, milk is collected by milk processing plantsonce in 24 hours from dairy farms. Milk harvested in the morning (first milking) is poured inthe BMC to half of its capacity and is chilled from 35oC to 4oC. After second milking, thefresh raw milk at 35oC is mixed with the chilled milk and the tank is completely filled to itscapacity. This raises the temperature of milk in the tank to 19oC. The milk is further cooled to4oC and stored till it is transported to milk processing plant.The energy performance of the refrigeration systems is usually evaluated based on first law ofthermodynamics. However compared to energy analysis, exergy analysis can better andaccurately show the location of inefficiencies in the refrigeration system. The results ofexergy analysis can be used to assess and optimize the performance of the system. Exergy isdefined as the maximum useful work that can be obtained from the system at a given statewith respect to a reference environment (i.e. dead state) (Kotas, 1985). The total amount ofexergy 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 asspecified by ISO5708. BMCs are classified based on cooling time as class I, class II, classIII, and class IV. In the present study, class II BMC stipulated to chill the milk in 3.0 hourswhen the tank is half-filled is analyzed for its performance.According to the laboratory test reports of BMC obtained from different manufacturers, thecoefficient of performance (COP) of these bulk milk coolers, over its operational time isfound to be in the range of 1.95 - 2.5. A field survey was conducted by the authors to studythe conditions in which the BMCs are used. From the survey, it is found that the performanceof the BMC further declines when operating in field conditions. Due to low operationalefficiency, the bulk milk coolers are not economic for use. Another finding obtained from thesurvey 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 ofBMC.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 ascientific study to improve its energy efficiency. Exergy analysis tool is the appropriatetechnique to understand the system behavior and to locate inefficiencies. Importantparameters viz. exergy destruction, coefficient of performance, work input, exergy efficiency,second law efficiency are determined to evaluate and compare the system performance. Thework consists of two parts as mentioned below: 138
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 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 designchanges based on exergy analysis lead to improvement in performance of the system. Thispaper analyzes the performance of BMC with respect to important parameters as mentionedabove in point no. (1). An attempt is made to explain the nature of irreversibilities andpractical limits to their reduction.2. LITERATURE REVIEWThe theory of exergy analysis is discussed at length by Kotas (1985) and is applied in thermaland chemical plant analysis by many researchers. The refrigeration systems used as heatpumps with R22 as a refrigerant are analyzed for exergy loss by Hepbasali (2005). Entropygeneration in thermodynamic process causes exergy destruction, which is a cause of low COPand consequently high energy consumption. It is necessary to identify, locate and quantify theirreversibilities to improve energy efficiency of refrigeration systems. The compressorperformance is analysed using exergy method by McGovern(1995) and the refrigerant flow inthe evaporator coils and air cooled condenser coils is analysed for various mass flowdensities, inlet temperatures and tube lengths by Liang (2001). The heat transfer coefficient isfound to be low in low vapour quality two phase flow region and high in high vapour qualitytwo phase flow region. Ratio of irreversibility rate with augmented heat transfer in a tube toirreversibility 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-dimensionalnumber of irreversibility and non-dimensional irreversibility balance is defined by Pons(2004), and the entropy generation numbers are defined by A. Bejan (1982). The above-mentioned studies provide an adequate framework for setting the research experiment andanalysing the refrigeration system in the present study. 3. METHODOLOGYThe methodology adopted in this study consists of two parts: (i) the experimentation scheme, and (ii) theanalysis scheme. This methodology is explained in brief in the following paragraphs.3.1 Experimentation SchemeThe experiment is designed on two types of BMC: the old model and the new model. The oldmodel 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 fourrow air cooled condensers are used which consist of grooved copper tube of an outerdiameter of 9.525 mm, and fin density of 14 fins per inch. Evaporator, divided into two equalparts, 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. Thesetwo units operate simultaneously to chill the milk to required temperature. Thermostaticexpansion valve is used to regulate the mass flow rate with respect to evaporator exittemperature of refrigerant. 139
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 3 Condenser Compressor 2 Pressure measurement points-1,2,7& 8 4 5 Temperature measurement points- 1 to 8 E1 1 Expansion Valve 8 6 Evaporator 7 condensing unit 1 I1 I2 condensing unit 2 Evaporator Expansion Valve E2 Compressor 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 E1and 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 milkare very similar to that of water [12]. Physically milk is a rather dilute emulsion combinedwith colloidal dispersion in which the continuous phase is the solution. Table no 1 gives theproperties of water and milk at 20oC. Table No. 1: Properties of water and milk at 20oc. Sr. No. Name of Property Water Milk 1. Specific Gravity 1.0 1.0321 2. Specific Heat 4.183 kJ/kg 3.9315 kJ/kg 3 Thermal Conductivity 0.599 W/m-K 0.550-0.580 W/m-K 4 Viscosity 1.004x10-3 N-s/m2 2 x10-3 N-s/m2 5 Refractive index 1.3329 1.3440The findings for the water will equally hold good for the milk without much variations.Pressures are measured by piezoelectric transducers across the compressor and evaporatorwith an accuracy of ±0.01 MPa. The pressure losses in the condenser are insignificant, andtherefore neglected. Temperatures are measured across compressor, condenser, expansionvalve and evaporator by RTD with an accuracy ±0.1oC. Data acquisition system is used torecord the temperatures and pressures at specific intervals at salient points of the cycle overits operation. Mass flow rate is measured by Coriolis effect mass flow rate meter (in kg/min)within an accuracy of ±0.2%. Water is used as the chilling medium instead of milk, as bothhave the similar properties. For the first milking condition test the tank is half filled (i.e. 500liter) by water and it is heated to 35oC with the auxiliary heater provided. For the secondmilking condition tank is filled to its capacity (1000 liter.) and heated to 19oC. In both thecases the water is chilled to 4oC. Data of Pressures (4 no.), Temperatures (12 no.), Mass flowrates 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 areconfirmed. 140
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEIn 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 GD D 2 n= Gd d 2 3 Condenser Compressor 2 4 5 E3 E2 E1 1 Expansion Valve 8 6 Evaporator I3 I2 I1 condensing unit 1 7 I6 I5 I4 condensing unit 2 Evaporator Expansion Valve E6 E5 E4 Compressor 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 byadopting the same experimental procedure as employed for the old model.3.2 Analysis schemeFor analyzing the data obtained from the experiments, a technique of “exergy analysis” isused. Exergy analysis combines the first and the second laws of thermodynamics, and is apowerful tool for analyzing both the quantity and quality of energy utilization. The maximumwork obtainable from system using environmental parameters as reference state is calledexergy and is expressible in terms of four components: physical exergy, kinetic exergy,potential exergy and chemical exergy. However the kinetic exergy and potential exergies areusually neglected and chemical exergy is zero as there is no departure of chemical substancesto environment. Therefore in this analysis physical exergy is only considered and iscalculated. Physical exergy of the material stream can be defined as the maximum work thatcan be obtained from when it is taken to physical equilibrium state with the environment. 141
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME . Ex = ( h − ho ) + To ( s − s o ) (1)where h and s are enthalpy and entropy respectively and To is the dead state temperature. Theenthalpy and entropy of the substance have to be evaluated at its pressure and temperatureconditions (P, T) and the pressure and temperature at dead state (P0, T0). For a process 1-2 thechange in exergy is given by: . ∆ E x = ( h2 − h1 ) + To ( s 2 − s1 ) (2)This change in exergy represents the minimum amount of work to be added or removed tochange from state 1 to state 2 when there is an increase and decrease in internal energy orenthalpy resulting from change.General exergy balance can be expressed in rate form as . . .E xin − E xout = E x dest (3)Considering control volume at steady state (Fig. 2) the exergy balance can be expressed as . . . . . Ex in + Ex Qin + Win = Ex out + Ex Qout + I (4)The exergy analysis is mainly concerned for the calculation of exergy efficiency and lostwork for each unit operation.The total exergy destruction in a cycle is simply the sum of the exergy destruction incondenser, compressor, evaporator and expansion valve. The overall exergetic efficiency isdefined as . . . E xout − E xin I total η ex = . = 1− . (5) W actual W actualThe energy efficiency is simply the ratio of useful output energy to input energy and isreferred as coefficient of performance (COP) for refrigeration system. In this context theenergy efficiency of BMC unit (COPactual) can be defined as follows: QeCOPactual = (6) WinThe ideal COP obtained from the carnot cycle is given as, TeCOPideal = (7) Tc − TeIdeal COP is calculated on basis of effective condenser temperature (Tc) and the effectiveevaporator temperature (Te).Effective condenser temperature is defined as T c = T cin − T cexit and effective evaporator ln(T cin T cexit )temperature is defined as, T eexit − T ein . T e = ln( T eexit T ein )One of the form of representing rational efficiency (Second law efficiency) is: 142
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME COPactual η th = (8) COPidealExergy efficiency can be written as follows: . . E xout E x dest η ex = . = 1− . (9) E xin E xinMaximum 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 toemploy the concept of improvement potential when analyzing the different processes. Thisimprovement 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 calculatedusing the exergy balance as follows: I. Compressor 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 = Win (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) . Since the thermal exergy associated with heat transfer is zero ( EQ = 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 IV = Ex7 − Ex8 + E Q (16) .The performance of the condenser and evaporator is analyzed by defining the parameter I / Qi.e., the ratio of irreversibility rate to heat transfer rate. The ratio indicates the relative changein irreversibility rate with respect to heat transfer rate.4. RESULTSThe observations were collected by conducting the experiments according to theexperimentation scheme, and the analysis was carried out based on the analysis scheme. Theresults obtained from the analysis of the collected data are presented in graphical form in thissection.Data was separately collected for both the models with same instruments, ensuring equalaccuracy. The exergy rate is calculated at inlet and exit of each component of therefrigeration system. The reference state for R22 is taken as normal atmospheric conditions of 143
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEtemperature and pressure as 298.16 K and 101.325 kPa respectively. The two condensingunits operate simultaneously and exhibit nearly similar performance with insignificantvariations at the end of cooling period. The refrigerant properties are calculated by usingCoolPack software. Figure-3: Refrigeration cycle for old model at Figure-4: Refrigeration cycle for new model at t=72 min t=72 min Table-2: Results of exergy calculations at t=72 min (for old model) Sr. Salient point Pressure Temperature Sp. Sp. Exergy No. (kPa) (K) Enthalpy Entropy Rate (kJ/kg) (kJ/kg) (kW) Compressor 1 482.58 284.06 413.52 1.78 2.03 inlet Compressor 1840.70 368.76 461.88 1.82 3.82 2 exit Condenser 1840.70 368.76 461.88 1.82 3.82 3 inlet Condenser 1840.70 323.46 263.68 1.21 3.04 4 exit Expansion 1840.70 323.46 263.68 1.19 3.36 5 Valve inlet Expansion 620.46 287.76 263.68 1.20 3.17 6 Valve exit Evaporator 620.46 287.76 256.46 1.20 2.83 7 inlet Evaporator 551.52 281.76 410.49 1.76 2.21 8 exit Qcond= 9.28 kW ExQcond= 1.27 kW Teff,cond= 345.62 K Qevap= 7.21 kW ExQevap= 0.34 kW Teff,evap= 284.75 K Win= 2.79 kW COPactual=2.37 COPideal=4.67Exergy loss in the system over cooling period of 180 minutes is tabulated below in Table 3. 144
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Table-3: Exergy Loss in components of BMC system (old model) Expansion Total Compressor Condenser Valve Evaporator Ex Time Ex Loss Ex loss Ex loss Ex loss Loss (min) (kW) (kW) (kW) (kW) (kW) 10 1.24 1.20 0.36 0.40 3.20 30 1.04 1.07 0.14 0.65 2.90 60 1.00 0.87 0.12 0.88 2.88 72 1.00 0.78 0.20 0.96 2.93 90 0.96 0.71 0.21 0.97 2.85 120 0.99 0.58 0.17 1.01 2.75 150 1.09 0.43 0.19 0.99 2.70 180 1.06 0.40 0.18 1.03 2.67 Similarly the exergy loss calculations for new model are calculated and tabulated intable 4. Table-4: Exergy Loss in components of BMC system (new model) Total Compressor Condenser Expansion Evaporator Time Ex Ex Loss Ex loss Valve Ex Ex Loss (min) Loss (kW) (kW) Loss (kW) (kW) (kW) 3 1.04 0.88 0.25 0.58 2.75 18 0.92 0.79 0.25 0.72 2.68 33 0.99 0.94 0.24 0.72 2.89 48 0.96 0.81 0.22 0.81 2.81 63 0.87 0.67 0.25 0.86 2.65 78 0.88 0.61 0.24 0.90 2.62 93 0.85 0.58 0.21 0.96 2.60 108 0.85 0.49 0.23 1.00 2.56 123 0.80 0.42 0.21 1.02 2.45 138 0.80 0.36 0.19 1.02 2.37 153 0.81 0.33 0.20 1.03 2.35Exergy loss rate in each component of BMC refrigeration system for old and new model isdetermined and presented in Figure-6 and Figure-7. Exergy loss in compressor for the oldmodel varies over the range 1.24 to 0.96 and that for the new model from 1.16 to 0.9. Exergyefficiency 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 between0.32-0.88 kW as against 0.4-1.2 kW for old model. In addition, the exergy destruction rate inthermostatic expansion valve is found to be the lowest amongst all (about 10%) of the totalwork input to the plant, which is considered as insignificant. The exergy loss in evaporator isfound to vary from 0.58- 1.03 kW for new model as against 0.4-1.03 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
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – Technology6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Sepfor the condenser in new model is found to be low in the range 0.05-0.08 as compared to that 0.05 0.08of old model range 0.06-1.0. Similarly the ratio is determined for evaporator in new and old 1.0.model varying from 0.04-0.08 and 0.04 0.08 0.04-0.12 respectively.The exergy destruction rate in the condenser is primarily due to heat exchang with the exchangeenvironment. Exergy destruction in condenser takes place due to pressure and temperatureloss. The exergy destruction rate due to pressure loss in condenser is very small and is about0.01-0.02 kW and rest is due to temperature loss. Large pressure losses in the evaporator of 0.02 pressureold model are reduced to greater extent in new model. It is observed that the amount ofpressure losses in old model were in the range 138 kPa to 35 kPa which are reducedsignificantly to 35 kPa to 14 kPa, Figure Figure-10 indicates the Second law efficiency for newmodel, which is higher than that of old model by 2 4%. The exergy input and its 2-4%.consumption in components of the system is shown in Figure Figure-11. At particular instant, out ofthe total exergy input to the system, it is found that about 38% of exergy is consumed in thecompressor, 26% in condenser, 24% in evaporator, 10% in expansion valve and 2% isunaccounted loss.COP of new model is found to be higher than old model by about 0.6 0.8 as shown in Figure- 0.6-0.8 Figure12. Carnot COP is determined on the basis of effective condenser and effective evaporatortemperatures. COP actual varies from 2.49 to 1.95 for old model as against the new modelfrom 3.5 to 2.75. Comp Ex Loss 1.40 Compressor Ex Loss 2 Condenser Ex. Loss 1.20 Condenser Ex Loss 1.00 Ex. Loss in KW Exergy Loss in kW 0.80 0.60 0.40 0 0.20 10 30 60Time in90 120 150 180 72 min 0.00 0 50 Time in min 150 100 200 Figure- 5: Exergy destruction in Figure-6: Exergy destruction in components components of the BMC (the old model) of the BMC (the new model) 0.12 Old 0.15 0.10 model 0.10 0.08 0.06 0.05 0.04 Old… 0.00 New… 0.02 0 50 Time in min 150 100 200 0.00 0 50 100 Time in min 150 200Figure- 7: Variation in ratio of irreversibility Figure-8: Variation in ratio of irreversibility rate to heat transfer rate in condenser rate to heat transfer rate in evaporator ev 146
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – Technology6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Sep 60.00 Second Law Efficiency 50.00 Exergy Consumption 40.00 (New Model) 30.00 2% 20.00 Compressor old 10.00 mod… 24% 38% 0.00 10% Condenser 26% 0 50 100 Time in min 150 200Figure-9: Variation in second law efficiency Figure-10: Pattern of exergy consumption in system components. 4.00 3.00 COP 2.00 1.00 old 0.00 model 0 50 100 Time in min 150 200Figure-11: COP comparison between old and 11: Figure-12: Comparison of Improvement new model potential in both models. odels.It is observed that the improvement potential in the new model has been reduced whichshows that there is overall improvement in the performance of the system. The systemperformance has been improved resulting in power saving of 0.2 kW.5 DISCUSSIONSAs observed from Figure-5 and Figure , exergy destruction rate is the highest in compressor Figure-6,as compared to other components of the system. Initially, the exergy loss rate is higher,1.24kW, due to high discharge temperature because of high pressure ratio and decreasesslowly to 0.9kW towards the end of cooling period for old model. Similar trend is observedfor new model with lower values of exergy destruction The most significant exergydestruction rate is due to throttling, followed by internal convection. Friction, mixing of fluid, convection.conduction and external convection and radiation also contribute to the exergy destruction incompressor. Exergy destruction could be reduced by reducing the internal convective heattransfer coefficient, swirl and turbulence in the cylinder and by increasing the areas of suction turbulenceand discharge valve ports.Improvement in the design of evaporator leads to reduction in exergy destruction incompressor and condenser. Isentropic efficiency of the compressor is found to be in the range63% to 65%. Manufacturer’s compressor performance data and actual work input to thecompressor 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 oldmodel. Due to high discharge pressure and high discharge temperature in old model, exergy 147
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEloss is higher i.e.1.2kW at the starting period of cooling time, which later decreases sharply to0.4kW as the temperature of the water in the tank approaches 4oC at the end of the coolingtime. The factors contributing to the exergy destruction in condenser are fluid friction andturbulence 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 transfercoefficient with a low pressure drop. Both the heat transfer coefficient and pressure drop areclosely related to refrigerant mass velocity. Varying the refrigerant mass velocity in differentregions 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 inexergy losses. Exergy destruction in evaporator in the old model is comparatively high due tothe turbulent flow and mixing of the refrigerant fluid. The refrigerant side heat transfercoefficient is low due to a vapour film with low thermal conductivity between the liquid andthe 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 amountof liquid refrigerant comes in contact with the heat transfer surface. This design change in theevaporator leads innovation leads to higher overall performance of the evaporator andnoticeable improvement in the second law efficiency. The study reveals that furtherimprovement 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 havedifferent temperatures is one of the causes of irreversibility. Further, when the fluid isthrottled 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 gainin the suction line and in the condensing unit.6. CONCLUSIONSSignificant amount of energy is lost in irreversibilities caused by improper process design andpoor design of components. The energy lost can be recovered by improving design andprocess parameters. The compressor is the major contributor to the exergy destruction.Evaporator and condenser, if redesigned to reduce the irreversibilities in flow, result insizeable energy savings. Proper sizing of evaporator inlets and exits for refrigerant willfurther reduce the pressure loss and hence the exergy loss. The net savings in work input forthe new model is recorded as 8-10% as that of work input to old model and reduction incooling time is around 10%. 148
  • International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Abbreviations and symbols SubscriptsCOP : Coefficient of performance 0 : Reference state . : Exergy rate kW 1 : state 1 in process 1-2ExG : Mass flux kg/m2 s 2 : state 2 in process 1-2h : Specific enthalpy kJ/kg K c : condenser. : Irreversibility rate kW D : large diameter IIP : Improvement potential kW d : small diameterQ : Heat Transfer Rate kW dest : destructions : Specific entropy kJ/kg K e : evaporatorT : Temperature K ex : exergeticW : Mechanical or electrical energy in : Inlet kWη : efficiency int : internal th : thermodynamic exit : Outlet me : mechanical electrical mech : mechanical motor : electric motorREFERENCES[1] Bali,Tulin, Sarac, Betul Ayhan, Exergy Analysis of heat transfer in a turbulent pipe flowby a decaying Swirl generator,International journal of exergy, vol. 5, No.1, 2008[2] Bansal, P.K., Rupsinghe, A.S., An empirical model for sizing of capillary tubes, International journal of Refrigeration, vol. 19 No. 8, pp 497-508,1996[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, 29:671-687.[6] Kotas,T.J., The Exergy Method of Thermal Plant Analysis, Butterworths,1985 [7] Liang,S.Y., Woong,T.N., Nathan,G.K., Numerical and experimental studies of refrigerant circuitry of evaporator coils, International journal of refrigeration,24 (2001) 823-833[8] McGovern, J.A., Harte,S., An exergy method for compressor performance analysis, International Journal of Refrigeration, vol 18, No 6, pp 421-433, 1995[9] Pons, Michel, Irreversibility in energy processes: Non-dimensional quantification and balance, Journal of non equilibrium thermodynamics, 2004,vol 29, pp 157-175.[10] Tirandazi,B., Mehrpooya, M., Vatani, A., Effect of valve pressure drop in ExergyAnalysis of C2+ Recovery Plants Refrigeration Cycles, Int. Journal of Electrical andElectronics Engineering, 4:4, 2009[11] Wang, S.P., Chen, Q.L., Yin, Q.H., Hua, B., Exergy destruction due to mean flow and fluctuating motion in compressible turbulent flows through a tube, Energy, 28 (2003), 809-823.[12] Robert Jenness, Noble P. Wong, Marth, Keeney, Fundamentals of Dairy Chemistry,Springer 1998, PP 409-445. 149