Refrigeration can be defined as the process of transferring heat from low temperature region to high temperature region. In other words, it is the process of cooling the substance. This can be achieved only if heat is removed from that substance.
Principle of refrigeration: Principle of refrigeration is based on second law of thermodynamics. It states that heat doesn’t flow from low temperature body to high temperature body without the help of external work. In refrigeration process, since heat has to be transferred from low temperature body to high temperature body some external work has to be done according to second law of thermodynamics as shown below (fig 2.) Here, the external work is done by compressor, condenser etc. The machine which works under this principle and serves the purpose of refrigeration is called refrigerator.
Types of Refrigeration System
i) Vapor compression refrigeration system (VCRS)
ii) Vapor absorption refrigeration system (VARS).
Vapor compressor refrigeration system (VCRS): This type of refrigeration system is most commonly used system in domestic refrigerators. In VCRS, the vapor alternately undergoes a change of phase from vapor to liquid and vice versa during cycle. The compression system operates on the mechanical energy produced by motor driven compressor. This system consists of following major components
4. Expansion valveCondenser: Condenser changes gas into liquid form. Its main purpose is to liquefy the refrigerants sucked by the compressor from the evaporator. High vapor pressure in condenser changes vapor refrigerant into liquid refrigerant again with little heat. The liquid refrigerant will then flow from the condenser to the liquid line. As condensation begins, heat will flow from the condenser into the air, only if the condensation temperature is higher than that of the atmosphere.
Compressor: The compressor’s use is to pull the low-temperature and low pressure vapor from the evaporator through suction line. Once the vapor is drawn, it will be compressed. This will cause the vapor’s temperature to rise. Its main function is to transform low temperature vapor into high temperature vapor to increase pressure. Vapor is released from compressor into discharged line.
Evaporator: An evaporator is used to turn any liquid material into gas. In this process, heat is absorbed. The evaporator transfers hat from refrigerated space into heat pump through liquid refrigerant, which boils in evaporator at low pressure. In achieving heat transfer, liquid refrigerant should be lower than the good being cooled in refrigerator. After the transfer, liquid refrigerant is drawn by compressor from evaporator through suction line. Liquid refrigerant will be in vapor form upon leaving the evaporator coil.
Expansion valve: This is also called throttling devise. it is commonly placed before the evaporator and at the end of liquid line. As liquid refrigerant reaches expansion valve, its temperature will decrease to a level below its atmosphere. This liquid will then be pumped into the evaporator.
How Vapor Compression Refrigeration System (VCRS) Works?
The refrigerant which is at low pressure and temperature flows into the compressor. In the compressor, the refrigerant is compressed into high pressure and high temperature refrigerant. This high pressure and high temperature refrigerant in vapor form is converted to high pressure liquid refrigerant by condenser. This high pressure liquid refrigerant is converted to low pressure liquid as it passes through expansion valve. This partly liquid and vapor refrigerant is mostly evaporated and converted to low pressure vapor while it absorbs latent heat of vaporization from material that is to be cooled in the refrigerator.
Course of refrigerant through the system is activated by the compressor. It is divided into high and low pressure side. The high pressure side is that portion of the system through which refrigerant travels under high pressure. This side starts at discharge valve of compressor and include condenser, receiver or storage tank and liquid line.
The low pressure side is that part of system through which refrigerant moves under low pressure. This side starts at the expansion valve which divides the high and low pressure side and includes cooling unit or evaporator, suction line and suction valve of the compressor. The compressor is essentially a cylinder in which piston works. It consists of two valves; i) suction valve and ii) discharge valve.
The refrigerant in the form of gas under low pressure is drawn from the suction line into the compressor. Gas enters the compressor cylinder through suction valve as the piston moves in its downward stroke. When the piston moves upward, pressure and temperature of gas rise. The compressed gas is finally pushed through discharge valve into the condenser.
The refrigerant enter the condenser enter the condenser as a high pressure gas and its temperature is higher than that of the air or water cooling the condenser. As heat flows from the gas to the cooling agent, the gas condenses, changes to high pressure liquid and passes into liquid receiver or storage tank. From the receiver, the high pressure liquid refrigerant passes through liquid line on its way to cooling unit.
The high pressure refrigerant must now be reduced to low pressure liquid so that it can evaporate at lower temperature. For this, the refrigerant passes through an expansion valve which divide high and low pressure side and controls the flow of refrigerant into the cooling unit.
The refrigerant enters the cooling unit under reduced pressure caused by suction stroke of the compressor. The refrigerant in the tube evaporates absorbing heat from the food in the refrigerator and providing what we know as refrigeration. The refrigerant now at low pressure gas is drawn through suction line back to compressor completing the cycle.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.