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UNIT 2 REFRIGERATION CYCLERefrigeration CycleStructure2.1IntroductionObjectives2.2Vapour Compression ple Vapour Compression Refrigeration CycleTheoretical Vapour Compression Cycle with Saturated Vapour after CompressionConditions for Highest COPCarnot Refrigeration CycleTemperature LimitationsDifference between Refrigeration and Heat Pump CyclesVapour Absorption SystemIllustrative ProblemsSummaryAnswers to SAQs2.1 INTRODUCTIONThe term „refrigeration’ may be defined as the process of removing heat from asubstance under controlled conditions. It also includes the process of reducing andmaintaining the temperature of a body below the general temperature of its surroundings.In other words, the refrigeration means a continued extraction of heat from a body whosetemperature is already below temperature of its surroundings. In a refrigerator, heat isvirtually pumped from a lower temperature to a higher temperature. According to SecondLaw of Thermodynamics, this process can only be performed with the aid of someexternal work. It is thus obvious that supply of power is regularly required to drive arefrigerator. Theoretically, a refrigerator is a reversed heat engine or a heat pump whichpumps heat from a cold body and delivers it to a hot body. The substance which works ina pump to extract heat from a cold body and to deliver it to a hot body is known asrefrigerant.ObjectivesAfter studying this unit, you should be able to know what is refrigeration cycle, understand about the vapour compression cycle, describe the vapour compression refrigeration cycle, and solve the problem on refrigeration system.Refrigeration cycle is the basis of all refrigeration systems. So refrigeration cycle shouldbe known to understand the refrigeration system. Some basic refrigeration cycles arediscussed here through different diagrams.2.2 VAPOUR COMPRESSION CYCLEVapour compression cycle is an improved type of air refrigeration cycle in which asuitable working substance, termed as refrigerant, is used. The refrigerants generally usedfor this purpose are ammonia (NH3), carbon dioxide (CO2) and sulphur-dioxide (SO2).The refrigerant used, does not leave the system, but is circulated throughout the systemalternately condensing and evaporating. In evaporating, the refrigerant absorbs its latentheat from the solution which is used for circulating it around the cold chamber and incondensing; it gives out its latent heat to the circulating water of the cooler.17

Refrigeration and AirConditioningThe vapour compression cycle which is used in vapour compression refrigeration systemis now-a-days used for all purpose refrigeration. It is used for all industrial purposes froma small domestic refrigerator to a big air conditioning plant.2.2.1 Simple Vapour Compression Refrigeration SystemIt consists of the following essential parts:CompressorThe low pressure and temperature vapour refrigerant from evaporator is drawn intothe compressor through the inlet or suction valve A, where it is compressed to ahigh pressure and temperature. This high pressure and temperature vapourrefrigerant is discharged into the condenser through the delivery or dischargevalve B.CondenserThe condenser or cooler consists of coils of pipe in which the high pressure andtemperature vapour refrigerant is cooled and condensed.Figure 2.1 : Simple Vapour Compression Refrigeration SystemThe refrigerant, while passing through the condenser, gives up its latent heat to thesurrounding condensing medium which is normally air or water.ReceiverThe condensed liquid refrigerant from the condenser is stored in a vessel known asreceiver from where it is supplied to the evaporator through the expansion valve orrefrigerant control valve.Expansion ValveIt is also called throttle valve or refrigerant control valve. The function of theexpansion valve is to allow the liquid refrigerant under high pressure andtemperature to pass at a controlled rate after reducing its pressure and temperature.Some of the liquid refrigerant evaporates as it passes through the expansion valve,but the greater portion is vaporized in the evaporator at the low pressure andtemperatureEvaporatorAn evaporator consists of coils of pipe in which the liquid-vapour. refrigerant atlow pressure and temperature is evaporated and changed into vapour refrigerant atlow pressure and temperature. In evaporating, the liquid vapour refrigerant absorbsits latent heat of vaporization from the medium (air, water or brine) which is to becooled.18

2.2.2 Theoretical Vapour Compression Cycle with Dry SaturatedVapour after CompressionRefrigeration CycleA vapour compression cycle with dry saturated vapour after compression is shown on T-sdiagrams in Figures 2.2(a) and (b) respectively. At point 1, let T1, p1 and s1 be thetemperature, pressure and entropy of the vapour refrigerant respectively. The fourprocesses of the cycle are as follows :(a) T-s Diagram(b) p-h DiagramFigure 2.2 : Theoretical vapour Compression Cyclewith Dry Saturated Vapour after CompressionCompression ProcessThe vapour refrigerant at low pressure p1 and temperatureT1 is compressedisentropically to dry saturated vapour as shown by the vertical line 1-2 on the T-sdiagram and by the curve 1-2 on p-h diagram. The pressure and temperature risefrom p1 to p2 and T1 to T2 respectively.The work done during isentropic compression per kg of refrigerant is given byw h2 – h1where h1 Enthalpy of vapour refrigerant at temperature T1, i.e. at suction of thecompressor, andh2 Enthalpy of the vapour refrigerant at temperature T2. i.e. at dischargeof the compressor.Condensing ProcessThe high pressure and temperature vapour refrigerant from the compressor ispassed through the condenser where it is completely condensed at constantpressure p2 and temperature T2 as shown by the horizontal line 2-3 on T-s and p-hdiagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant,while passing through the condenser, gives its latent heat to the surroundingcondensing medium.Expansion ProcessThe liquid refrigerant at pressure p3 p2 and temperature T3 T2, is expanded bythrottling process through the expansion valve to a low pressure p4 p1 andTemperature T4 T1 as shown by the curve 3-4 on T-s diagram and by the verticalline 3-4 on p-h diagram. Some of the liquid refrigerant evaporates as it passesthrough the expansion valve, but the greater portion is vaporized in the evaporator.We know that during the throttling process, no heat is absorbed or rejected by theliquid refrigerant.Vaporizing ProcessThe liquid-vapour mixture of the refrigerant at pressure p4 p1 and temperatureT4 T1 is evaporated and changed into vapour refrigerant at constant pressure andtemperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. Duringevaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporization19

Refrigeration and AirConditioningfrom the medium (air, water or brine) which, is to be cooled, This heat which isabsorbed by the refrigerant is called refrigerating effect and it is briefly written asRE. The process of vaporization continues up to point 1 which is the starting pointand thus the cycle is completed.We know that the refrigerating effect or the heat absorbed or extracted by theliquid-vapour refrigerant during evaporation per kg of refrigerant is given byRE h1 – h4 h1 – hf3where hf3 Sensible heat at temperature T3, i.e. enthalpy of liquid refrigerantleaving the condenser.It may be noticed from the cycle that the liquid-vapour refrigerant has extractedheat during evaporation and the work will be done by the compressor for isentropiccompression of the high pressure and temperature vapour refrigerant.Coefficient of performance, C.O.P. (Refrigerating effect)/( Work done) h1 h4h2 h1h1 h f 3h2 h1Effect of Suction PressureThe suction pressure (or evaporator pressure) decreases due to the frictionalresistance of flow of the refrigerant. Let us consider a theoretical vapourcompression cycle 1-2-3-4 when the suction pressure decreases from ps to ps asshown on p-h diagram in Figure 2.3.It may be noted that the decrease in suction pressure :(a)decreases the refrigerating effect from (h1 – h4) to ( h11 h41 ), and(b)Increases the work required for compression from (h2 – h1) to( h21 h11 ).Figure 2.3 : Effect of Suction PressureSince the C.O.P, of the system is the ratio of refrigerating effect to the work done,therefore with the decrease in suction pressure, the net effect is to decrease theC.O.P. of the refrigerating system for the same refrigerant flow. Hence with thedecrease in suction pressure the refrigerating capacity of the system decreases andthe refrigeration cost increases.Effect of Discharge PressureIn actual practice, the discharge pressure (or condenser pressure) increases due tofrictional resistance of flow of the refrigerant. Let us consider a theoretical vapourcompression cycle l-2-3-4 when the discharge pressure increases from pD to pD‟ asshown on p-h diagram in Figure 2.4 resulting in increased compressor work andreduced refrigeration effect.20

Refrigeration CycleFigure 2.4 : Effect of Discharge Pressure2.2.3 Conditions for Highest COPEffect of Evaporator PressureConsider a simple saturation cycle 1-2-3-4 with Freon 12 as the refrigerant asshown in Figure 2.5 for operating conditions of tk 40 C and t – 5 C.Now consider a change in the evaporator pressure corresponding to a decrease inthe evaporator temperature to – 10 C. The changed cycle is shown as 1 -2 -3-4 inFigure 2.5.It is therefore, seen that a drop in evaporator pressure corresponding to a drop of5 C in saturated suction temperature increases the volume of suction vapour andhence decreases the capacity of a reciprocating compressor and increases thepower consumption per unit refrigeration.Figure 2.5 : Effect of Evaporator PressureIt is observed that a decrease in evaporator temperature results in :(a)Decrease in refrigerating effect from (h1 – h4) to (h1 ‟ – h4 )(b)Increase in the specific volume of suction vapour from v1 to v1 ‟(c)Decrease in volumetric efficiency, due to increase in the pressureratio, from v to v '(d)Increase in compressor work from (h2 – h1) to (h2 ‟ – h1 ) due toincrease in the pressure ratio as well as change from steeper isentropic1-2 to flatter isentropic 1 -2.SinceQ0 mq0 andW * m* w vV pv1q0. . . (2.1). . . (2.2)21

Refrigeration and AirConditioningExpressions for the dependence of capacity and unit power consumption may nowbe written as follows :Q0 q0 h1 h2 . . . (2.3)1v1 vandW * m* 11 q0 h1 h4. . . (2.4) w h2 h1Effect of Condenser PressureAn increase in condenser pressure, similarly results in a decrease in therefrigerating capacity and an increase in power consumption, as seen from thechanged cycle 1 -2 -3 -4 for tk 45 C in Figure 2.6. The decrease in refrigeratingcapacity is due to a decrease in the refrigerating effect and volumetric efficiency.The increase in power consumption is due to increased mass flow (due todecreased refrigerating effect) and an increase in specific work (due to increasedpressure ratio), although the isentropic line remains unchanged. Accordingly, onecan write for the ratiosQ0' h1 h4' v' Q0 h1 h4 v. . . (2.5)W *' c' h1 h4 h2' h1 W * c h1' h4 h2 h1. . . (2.6)'Figure 2.6 : Effect of Condenser PressureIt is obvious that COP decreases both with decreasing evaporator and increasingcondenser pressures.It may, however, be noted that the effect of increase in condenser pressure is not asserver, on the refrigerating capacity and power consumption per ton ofrefrigeration, as that of the decrease in evaporator pressure.Effect of Suction Vapour SuperheatSuperheating of the suction vapour is advisable in practice because it ensurescomplete vaporization of the liquid in the evaporator before it enters thecompressor. Also, in most refrigeration and air-conditioning systems, the degree ofsuperheat serves as a means of actuating and modulating the capacity of theexpansion valve. It has also been seen that for some refrigerants such as Freon 12,maximum COP is obtained with superheating of the suction vapour.22

Refrigeration CycleFigure 2.7: Effect of Suction Vapour SuperheatIt can be seen from Figure 2.7, that the effect of superheating of the vapour fromt1 t0 to t1 ‟ is as follows :(a)Increase in specific volume of suction vapour from v1 to v1‟(b)Increase in refrigerating effect from (h1 – h4) to (h1 – h4)(c)Increase in specific work from (h2 – h1) to (h2 – h1 )It is to be noted that (h2 – h1 ) is greater than (h2 – h1). This is because, althoughthe pressure ratio is the same for both lines, the initial temperature t1,‟ is greaterthan t1 and the work given by the expression 1 RT1 1 p/p21 1 Increases with the initial temperature. That is why isentropic lines on thep-h diagram become flatter in higher temperatures. An increase in specific volumedecreases the capacity. On the contrary, an increase in refrigerating effect willincrease the capacity effect of super- heating is to theoretically reduce the capacityin ammonia systems and to increase it in Freon 12 systems.Effect of Liquid SubcoolingIt is possible to reduce the temperature of the liquid refrigerant to within a fewdegrees of the temperature of the water entering the condenser. In some condenserdesigns it is achieved by installing a sub-cooler between the condenser and theexpansion valve.The effect of sub-cooling of the liquid from t3 tk to t3 is shown inFigure 2.8. It will be seen that sub-cooling reduces flashing of the liquid duringexpansion and increases the refrigerating effect. Consequently, the pistondisplacement and horsepower per ton are reduced for all refrigerants. The percentgain is less pronounced in the case of ammonia because of its larger latent heat ofvaporization as compared to liquid specific heat.Figure 2.8 : Effect of Liquid Sub