The Refrigeration Cycle

A basic refrigeration cycle works in a continuous loop and requires four main components. At the first stage the compressor increases the temperature and pressure of the refrigerant, turning it into a gas. The condenser then cools and condenses the hot gas turning it from vapour to a high-pressure liquid. The expansion valve in the system lowers the pressure, causing the refrigerant to expand and reduces in temperature before entering the evaporator coils. The evaporator turns the hot liquid into cold gas again, cooling the coils, which reduces the temperature of the cold room. Heat from inside the cold room can at this point be absorbed by the refrigerant, in vapour form and carried through the evaporator outlet. The ongoing cycle of compression, condensation, expansion, and evaporation of the refrigerant maintains the cold temperatures inside the cold room.

The Refrigeration Cycle - Axair Refrigeration
The Refrigeration Cycle - Axair Refrigeration

Various types of refrigeration systems are available depending on the temperatures required. Single stage systems can achieve temperatures no lower than -40 ℃, whereas two stage and cascade refrigeration systems are suitable for much lower temperatures.

 

Single Stage Refrigeration System

In a single stage system, the compressor circulates the refrigerant around the system, it draws in the refrigerant as a low-pressure vapour from the evaporator, where it has collected all the unwanted heat from the facility. The low-pressure vapour is compressed into a smaller volume & the temperature is increased. It leaves the compressor as a hot, high-pressure vapour as it flows to the condenser where it is cooled down. The heat is discharged to the outside via the help of fans which forces the hot air outside. Water pumps can also be applied at this stage to help remove excess heat. As the heat is removed the refrigerant passes through as a high-pressure liquid to the receiver, the storage vessel. From here the refrigerant flows to the expansion valve which regulates the refrigerant pressure into the evaporator. The liquid refrigerant then flows into the liquid separator, where it is sucked into refrigerant pumps to manage the circulation rate with cooling load variants. The refrigerant is then forced into the expansion valve where the flow is regulated. The cold refrigerant enters the evaporator passing through pipes. A fan is positioned in front of the pipes and blows the warm air inside the facility across the pipes. The cold refrigerant absorbs the heat leaving cooler air flowing into the space. The refrigerant boils across the evaporator pipes turning into part vapour part liquid, and as it evaporates the heat is carried away. The refrigerant then flows back into the separator, where the liquid part drops to the bottom of the separator and flows back round through the evaporator, the vapour rises and is drawn back into the compressor to repeat the whole cycle again.

 

Two Stage System

A two-stage system is a similar set up as a single stage system but with a few extra components and cycles. An intermediate cooler tank sits between the receiver and the expansion valve. The refrigerant passes through the coil inside the tank and into the main expansion valve, then flows to the separator, evaporator, and back to the separator. A second stream of refrigerant flows off the main line and is sprayed into the cooler tank via an expansion valve to produce a cooling effect. As it evaporates in the tank the submerged coil is cooled which cools the main flow of refrigerant inside the coil before it flows to the main expansion valve. There are two compressors in this system, therefore from the separator tank, the vapour refrigerant is drawn out to the low stage compressor to increase pressure, and is released into the inter mediate cooler tank, which helps to condense the refrigerant. The vapour refrigerant is then drawn out of the tank to the high stage compressor where it then flows back into the condenser and the cycle is repeated.

 

Cascade System

A cascade system is more a complicated system, suitable where different temperature ranges are required for cooling loads. There can be two or more separate refrigeration cycles in a cascade system that use different refrigerants that have different boiling points. There is a high temperature circuit and a low temperature circuit with a heat exchanger, known as a cascade condenser, connecting the circuits. The condenser acts as a condenser for the high temperature circuit and an evaporator for the low temperature circuit.