Vapour compression cycle is an improved type of air refrigeration cycle in which a suitable working substance, termed as refrigerant, is used. The refrigerants generally used for 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 system alternately condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the solution which is used for circulating it around the cold chamber and in condensing; it gives out its latent heat to the circulating water of the cooler. The vapour compression cycle which is used in vapour compression refrigeration system is now-a-days used for all purpose refrigeration. It is used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant.
Simple Vapour Compression Refrigeration System:
It consists of the following essential parts:
Compressor
The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed to a high pressure and temperature. This high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery or discharge valve B.
Condenser
The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant is cooled and condensed.
The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water.
Receiver
The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is supplied to the evaporator through the expansion valve or refrigerant control valve.
Expansion Valve
It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure and temperature 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 and temperature
Evaporator
An evaporator consists of coils of pipe in which the liquid-vapour. refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which is to be cooled.
Theoretical Vapour Compression Cycle with Dry Saturated Vapour after Compression
A vapour compression cycle with dry saturated vapour after compression is shown on T-s diagrams in Figures 2.2(a) and (b) respectively. At point 1, let T1, p1 and s1 be the temperature, pressure and entropy of the vapour refrigerant respectively. The four processes of the cycle are as follows :
Theoretical vapour Compression Cycle with Dry Saturated Vapour after Compression Compression Process
The vapour refrigerant at low pressure p1 and temperatureT1 is compressed isentropically to dry saturated vapour as shown by the vertical line 1-2 on the T-s diagram and by the curve 1-2 on p-h diagram. The pressure and temperature rise from p1 to p2 and T1 to T2 respectively.
The work done during isentropic compression per kg of refrigerant is given by
w = h2 – h1
where h1 = Enthalpy of vapour refrigerant at temperature T1, i.e. at suction of the compressor, and
h2 = Enthalpy of the vapour refrigerant at temperature T2. i.e. at discharge of the compressor.
Condensing Process
The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure p2 and temperature T2 as shown by the horizontal line 2-3 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing through the condenser, gives its latent heat to the surrounding condensing medium.
Expansion Process
The liquid refrigerant at pressure p3 = p2 and temperature T3 = T2, is expanded by throttling process through the expansion valve to a low pressure p4 = p1 and Temperature T4 = T1 as shown by the curve 3-4 on T-s diagram and by the vertical line 3-4 on p-h diagram. Some of the liquid refrigerant evaporates as it passes through 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 the liquid refrigerant.
Vaporizing Process
The liquid-vapour mixture of the refrigerant at pressure p4 = p1 and temperature T4 = T1 is evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which, is to be cooled, This heat which is absorbed by the refrigerant is called refrigerating effect and it is briefly written as RE. The process of vaporization continues up to point 1 which is the starting point and thus the cycle is completed.
We know that the refrigerating effect or the heat absorbed or extracted by the liquid-vapour refrigerant during evaporation per kg of refrigerant is given by
RE = h1 – h4 = h1 – hf3
where hf3 = Sensible heat at temperature T3, i.e. enthalpy of liquid refrigerant leaving the condenser.
It may be noticed from the cycle that the liquid-vapour refrigerant has extracted heat during evaporation and the work will be done by the compressor for isentropic compression of the high pressure and temperature vapour refrigerant.
Coefficient of performance, C.O.P. = (Refrigerating effect)/( Work done)
The suction pressure (or evaporator pressure) decreases due to the frictional resistance of flow of the refrigerant. Let us consider a theoretical vapour compression cycle 1-2-3-4 when the suction pressure decreases from ps to ps as shown on p-h diagram inFigure
It may be noted that the decrease in suction pressure :
- decreases the refrigerating effect
- Increases the work required for compression
Effect of Suction Pressure
Since 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 the
- of the refrigerating system for the same refrigerant Hence with the decrease in suction pressure the refrigerating capacity of the system decreases and the refrigeration cost increases.
Effect of Discharge Pressure
In actual practice, the discharge pressure (or condenser pressure) increases due to frictional resistance of flow of the refrigerant. Let us consider a theoretical vapour compression cycle l- when the discharge pressure increases from pD to pD‟ as shown on p-h diagram in Figure resulting in increased compressor work and reduced refrigeration effect.
Effect of Discharge Pressure
Conditions for Highest COP Effect of Evaporator Pressure
Consider a simple saturation cycle 1-2-3-4 with Freon 12 as the refrigerant as shown in Figure for operating conditions of tk = 40°C and t = – 5°C.
Now consider a change in the evaporator pressure corresponding to a decrease in the evaporator temperature to – 10°C.
It is therefore, seen that a drop in evaporator pressure corresponding to a drop of 5°C in saturated suction temperature increases the volume of suction vapour and hence decreases the capacity of a reciprocating compressor and increases the power consumption per unit refrigeration.
Effect of Evaporator Pressure
It is observed that a decrease in evaporator temperature results in :
- Decrease in refrigerating effect from
- Increase in the specific volume of suction vapour from
- Decrease in volumetric efficiency, due to increase in the pressure ratio,
- Increase in compressor work due to increase in the pressure ratio as well as change from steeper isentropic to flatter isentropic
Effect of Condenser Pressure
An increase in condenser pressure, similarly results in a decrease in the refrigerating capacity and an increase in power consumption, as seen from the changed cycle 1 – – – tk
= 45°C in Figure 2.6. The decrease in refrigerating capacity is due to a decrease in the refrigerating effect and volumetric efficiency. The increase in power consumption is due to increased mass flow (due to decreased refrigerating effect) and an increase in specific work (due to increased pressure ratio), although the isentropic line remains unchanged. Accordingly, one can write for the ratios
Effect of Condenser Pressure
It is obvious that COP decreases both with decreasing evaporator and increasing condenser pressures.
It may, however, be noted that the effect of increase in condenser pressure is not as server, on the refrigerating capacity and power consumption per ton of refrigeration, as that of the decrease in evaporator pressure.
Effect of Suction Vapour Superheat
Superheating of the suction vapour is advisable in practice because it ensures complete vaporization of the liquid in the evaporator before it enters the compressor. Also, in most refrigeration and air- conditioning systems, the degree of superheat serves as a means of actuating and modulating the capacity of the expansion valve. It has also been seen that for some refrigerants such as Freon 12, maximum COP is obtained with superheating of the suction vapour.
Effect of Suction Vapour Superheat
It can be seen from Figure 2.7, that the effect of superheating of the vapour from is as follows Increase in specific volume of suction vapour
- Increase in refrigerating effect
- Increase in specific work
This is because, although the pressure ratio is the same for both lines, the initial temperature t1,‟ is greater than t1 and the work given by the expression increases with the initial temperature.
That is why isentropic lines on the diagram become flatter in higher temperatures. An increase in specific volume decreases the capacity. On the contrary, an increase in refrigerating effect will increase the capacity effect of super- heating is to theoretically reduce the capacity in ammonia systems and to increase it in Freon 12 systems.
Effect of Liquid Subcooling
It is possible to reduce the temperature of the liquid refrigerant to within a few degrees of the temperature of the water entering the condenser. In some condenser designs it is achieved by installing a sub-cooler between the condenser and the expansion valve.
It will be seen that sub-cooling reduces flashing of the liquid during expansion and increases the refrigerating effect. Consequently, the piston displacement and horsepower per ton are reduced for all refrigerants. The percent gain is less pronounced in the case of ammonia because of its larger latent heat of vaporization as compared to liquid specific heat.
Effect of Liquid Subcooling
Normally, cooling water first passes through the subcooler and then through the condenser. Thus, the coolest water comes in contact with the liquid being subcooled. But this results in a warmer water entering the condenser and hence a higher condensing temperature and pressure. Thus, the advantage of subcooling is offset by the increased work of compression.
This can be avoided by installing parallel cooling water inlets to the subcooler and condenser. In that case, however, the degree of subcooling will be small and the added cost of the subcooler and pump work may not be worthwhile. It may be more desirable to use the cooling water effectively in the condenser itself to keep the condensing temperature as near to the temperature of the cooling water inlet as possible.
Carnot Refrigeration Cycle
In refrigeration system, the Carnot cycle considered is reversed Carnot cycle. We know that a heat engine working on Carnot engine has the highest efficiency. Similarly, a refrigeration system working on the reversed cycle, has the maximum coefficient of performance.
Reversed Carnot Cycle
A reversed Carnot cycle, using air as the working medium is shown on p-v and T-s diagrams in Figures 2.9(a) and (b) respectively. At point 1, let p1, v1, T1 be the pressure, specific volume and temperature of air respectively.
The four processes of the cycle are as follows:
Isentropic Compression Process
The air is compressed isentropically as shown by the curve 1-2 on p-v and T-s diagrams. During this process, the pressure of air increases from p1 to p2, specific volume decreases from v1 to v2 and temperature increases from T1 to T2. We know that during isentropic compression, no heat is absorbed or rejected by the air.
Isothermal Compression Process
The air is now compressed isothermally (i.e. at constant temperature, T2 = T3) as shown by the curve 2-3 on p-v and T-s diagrams. During this process, the pressure of air increases from p2 to p3 and specific volume decreases from v2 to v3. We know that the heat rejected by the air during isothermal compression per kg of air,
= T3 (s2 –s3)
= T2 (s2 –s3)
Isentropic Expansion Process
The air is now expanded isentropically as shown by the curve 3-4 on p-v and T-s diagrams. The pressure of air decreases from p3 to p4, specific volume increases from v3 to v4 and temperature decreases from T3 to T4. We know that during isentropic expansion, no heat is absorbed or rejected by the air.
Isothermal Expansion Process
The air is now expanded isothermally (i.e. at constant temperature, T4 = T1) as shown by the curve 4- 1 on p-v and T-s diagrams. During this process, the pressure of air decreases from p4 to p1 and specific volume increases from v4 to v1. We know that the heat absorbed by the air during isothermal compression per kg of air,
= T4 (s1 –s4)
= T4 (s2 –s3)
= T1 (s2 –s3)
We know that work done during the cycle per kg of air
= Heat rejected – Heat absorbed
= q2-3 – q4-1
= T2 (s2 – s3) – T1 (s2 – s3)
Therefore, coefficient of performance of the refrigeration system working on reversed Carnot cycle,
C.O.P. = Ratio of Heat Absorbed to Work Done Temperature Limitations for Reversed Carnot Cycle The C.O.P. of the reversed Carnot cycle can be improved by
- Decreasingthe higher temperature (i.e. temperature of hot body, T2) or
- Increasingthe lower temperature (i.e. temperature of cold body, T1).
It may be noted that temperature T1 and T2 cannot be varied at will, due to certain functional limitations. It should be kept in mind that the higher temperature (T2) is the temperature of cooling water or air available for rejection of heat and the lower temperature (T1) is the temperature to be maintained in the refrigerator. The heat transfer will take place in the right direction only when the higher temperature is more than the temperature of cooling water or air to which heat is to be rejected, while the lower temperature must be less than the temperature of substance to be cooled.
Thus if the temperature of cooling water or air (i.e. T2) available for heat rejection is low, the will be high. Since T2 in winter is less than T2 in summer, therefore, C.O.P. in winter will be higher than C.O.P. in summer. In other words, the Carnot refrigerator works more efficiently in winter than in summer. Similarly, if the lower temperature (T1) is high, the C.O.P. of the Carnot refrigerator will be high.
Difference between Refrigeration and Heat Pump System
The major difference between the refrigeration and heat pump system is that refrigerator delivers heat from lower temperature to a higher temperature, whereas heat pump delivers heat from higher temperature to lower temperature body.