This process is named as vapour compression refrigeration system because the vapour is compressed in this system to produce refrigeration.
Vapour Compression Refrigeration Systems are the most commonly used refrigeration system among all the refrigeration system.
In this system, the working fluid undergoes phase change twice from liquid to vapour and again from vapour to liquid.
In this type of system refrigeration is obtained as refrigerant evaporates at low temperature providing the cooling effect.
The actual vapour compression cycle is based on Evans-Perkins Cycle which is also known as Reverse Rankine Cycle.
Components used in Vapour Compression Refrigeration System:
All the components of Vapour Compression Refrigeration System are connected through a pipe.
The four components of this system are:
The evaporator is used to heat the working fluid and turn it into vapour.
After the evaporator, the working fluid is passed to the compressor. The compressor is used to compress the working fluid.
After the compressor, the vapour is transferred to the condenser. This condenser is used to condense the vapour back to the liquid.
4 Expansion Valve or Throttling device:
From the condenser, the saturated liquid formed is sent to the expansion valve. In the expansion valve, the working fluid will be expanded and its pressure will decrease.
A fan is used to supply warm air to the evaporator.
Working Of Vapour Compression Refrigeration System:
There are four steps involved in the working of the vapour compression refrigeration system:
Step 1: Compression (Reversible adiabatic compression. )
In this compressor, high pressure and high temperature superheated vapour will be created by the compressor and this superheated vapour will be transferred to the condenser.
Step 2: Condensation (Constant pressure heat rejection)
In this condenser, the superheated vapour will give its latent heat to the cooling fluid in the condenser. This cooling fluid can be air or water.
After entering the condenser, the superheated vapour will be converted into a liquid as heat is extracted from the vapour. This is called saturated liquid. After that, this liquid goes to the expansion valve.
Step 3: Throttling And Expansion ( Reversible adiabatic expansion )
In the expansion valve, the pressure and temperature of the saturated liquid will decrease.
After that this low temperature and pressure liquid will be transferred to the evaporator.
Step 4: Evaporation ( Constant pressure heat addition )
In the evaporator, the low pressure and temperature liquid will be converted into vapour. This liquid will take all the heat from the evaporator and will get converted to vapour using the heat taken from the evaporator. Hence, this liquid will decrease the temperature inside the evaporator.
The fan present near the evaporator will supply warm air and on another side, we will get cold air.
After that, these vapours will go to the compressor again and this cyclic process will be repeated again and again.
Performance Of Vapour Compression Refrigeration System:
Applying Steady Flow Energy Equation (SFEE) at Evaporator, we get
h4 + Qe = h1 + 0
Qe = h1 – h4
Applying Steady Flow Energy Equation (SFEE) at Compressor, we get
h1 + 0 = h2 – W
W = h2 – h1
Applying Steady Flow Energy Equation (SFEE) at Condenser, we get
h2 – Qc = h3
Qc = h3 – h2
Applying Steady Flow Energy Equation (SFEE) at Expansion Valve, we get
h3 + Q = h4 + W
But we have Q=0 and W=0
So, h3 = h4
Co-efficient Of Performance:
COP = Desired Effect / Work Input
= h1 – h4 / h2 – h1
Problems in the vapour compression refrigeration system :
Some problems in the vapour compression refrigeration system that may effect the efficiency of the system are:
i) Compressor leakage/failure:
Compressor leakage or failure is a major in the vapour compression refrigeration system.
Compressor failure can cost the company a lot if the compressor fails inside the warranty period. It also affects the reputation of the company a lot.
Some problems that may cause compressor failure are lubrication problems, slugging, overheating, and contamination.
Fouling is defined as an accumulation of unwanted deposits pipes of the water and refrigerant transfer surface.
The foulant layer imposes an additional resistance to the flow of water or the refrigerant.
Steps to reduce fouling:
1) Cleaning the evaporator surface frequently.
2) Keeping the condenser tubes clean.
3) Effective water treatment of the water used in the system should be done.
iii) Liquid Line Restriction:
The restriction in the thermostatic expansion valve is called liquid line restriction. This problem can reduce the cooling capacity up to 50%.
Causes of a liquid line restriction:
1) Wax buildup in the valve because of the use of the wrong oil in the system.
2) Too much oil in the system.
3) Sludge formation due to compressor burnout.
Symptoms of liquid line restrictions: (How to know that problem is due to liquid line restriction):
1) Very high discharge temperature.
2) Low condensing pressures.
3) Bubbles in the sight glass.
4) Low current draw.
Test to conduct to know that the problem is liquid line restriction:
1) Temperature drop test.
2) Performing a freeze test.
3) Using thermal imaging can easily pinpoint the problem.
iv) Cooling Problem:
It is one of the most common problems in vapour compression refrigeration system. Due to this problem, the efficiency of the system decreases significantly.
Possible causes of the cooling problem:
a) Dirty air passages.
b) Blocked air filters due to irregular cleaning.
Steps to prevent cooling problems:
a) Regular cleaning of air filters and air passages.
b) Regular check of chiller logs especially the comparison between amperage and voltage.