Aim: – To study the analysis of simple vapour compression cycle and explain
the various types of Vapour compression cycle with T-S and P-H diagram
Apparatus Used: – schematic diagram of Simple vapour compression cycle.
Theory:
A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant, is used. It condenses and evaporates at temperatures and pressures close to the atmospheric conditions. The refrigerants, usually, 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 brine (salt water) which is used for circulating it around the cold chamber. While condensing, it gives out its latent heat to the circulating water of the cooler. The vapour compression refrigeration system is, therefore a latent heat pump, as it pumps its latent heat from the brine and delivers it to the cooler. The vapour compression refrigeration system is now-a-days used for all purpose refrigeration. It is generally used for all industrial purpose from a small domestic refrigerator to a big air conditioning plant.
Mechanism of a Simple Vapour Compression Refrigeration System 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 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.
Types of Vapour Compression Cycles:
We have already discussed that vapour compression cycle essentially consists of compression, condensation, throttling and evaporation. Many scientists have focused their attention to increase the coefficient of performance of the cycle. Though there are many cycles, yet the following are important from the subject point of view:
1. Cycle with dry saturated vapour after compression,
2. Cycle with wet vapour after compression,
3. Cycle with superheated vapour after compression,
4. Cycle with superheated vapour before compression, and
5. Cycle with under-cooling or sub-cooling of refrigerant.
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 and P-H diagrams in (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:
1. Compression Process:
The vapour refrigerant at low pressure p1 and temperature T1 is compressed isentropic ally to dry saturated vapour as shown by the vertical line 1-2 on T-s diagram and by the curve 1-2 p – h diagram. The pressure and temperature rises 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.
2. 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.
3. 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. We have already discussed that 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.
4. 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
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 = (h1 – h4) /( h2 – h1) = (h1 – hf3) / (h2 – h1)
Theoretical Vapour Compression Cycle with Wet Vapour after Compression
In this cycle, the enthalpy at point 2 is found out with the help of dryness fraction at this point. The dryness fraction at points 1 and 2 may be obtained by equating entropies at points 1 and 2.
.’. C.O.P. = Refrigerating effect / Work done = (h1 – hf3) / (h2 – h1)
Theoretical Vapour Compression Cycle with Superheated Vapour after Compression
In this cycle, the enthalpy at point 2 is found out with the help of degree of superheat. The degree of superheat may be found out by equating the entropies at points 1 and 2.
Now the coefficient of performance may be found out as usual from the relation,
.’. C.O.P. = Refrigerating effect / Work done = (h1 – hf3) / (h2 – h1)
A little consideration will show that the superheating increase the refrigerating effect and the amount of work done in the compressor. Since the increase in refrigerating effect is less as compared to the increase in work done, therefore, the net effect of superheating is to have low coefficient of performance.
Theoretical Vapour Compression Cycle with Superheated Vapour before Compression:
In this cycle, the evaporation starts at point 4 and continues up to point 1’, when it is dry saturated. The vapour is now superheated before entering the compressor up to the point 1.
The coefficient of performance may be found out as usual from the relation.
Theoretical Vapour Compression Cycle with under cooling or Sub cooling of Refrigerant:
Sometimes, the refrigerant, after condensation process 2’-3’, is cooled below the saturation temperature (T3’) before expansion by throttling. Such a process is called under cooling or sub cooling of the refrigerant and is generally done along the liquid line as shown in fig. The ultimate effect of the under cooling is to increase the value of coefficient of performance under the same set of conditions. The process of under cooling is generally brought about by circulating more quantity of cooling water through the condenser or by using water colder than the main circulating water. Sometimes, this process is also brought about by employing a heat exchanger. In actual practice, the refrigerant is superheated after compression and under cooled before throttling, as shown. A little consideration will show that the refrigerating effect is increased by adopting both the superheating and under cooling process as compared to a cycle without them, which is shown by dotted lines.
In this case, the refrigerating effect or heat absorbed or extracted,
RE = h1 – h4 = h1- hf3
And work done, w = h2 – h1
.’. C.O.P. = Refrigerating effect / Work done = (h1 – hf3) / (h2 – h1)
Viva Questions
1. What are the advantages of vapour compression refrigeration system?
2. Describe the mechanism of a simple vapour compression refrigeration system?
3. What is sub-cooling and superheating?
4. Why is superheating considered to be good in certain cases?
5. What is tonnage capacity of refrigeration system?
6. What is the effect of superheating and sub cooling on the COP?