Compression Refrigeration Cycle – What is it? How it Works?



August 2, 2021

Regardless of the refrigerator type and its use, their cooling system usually follows a general rule. Portable refrigerators and truck refrigerators cooling system is no exception to this rule. Each refrigerator and freezer generally consists of two parts; a cooling unit that operates on the basis of a compression refrigeration cycle. Its role is to transfer heat from inside the refrigerator compartment to the outside and the refrigerator compartment, which is made of insulating materials, its role is to maintain the temperature inside the chamber and prevent it from rising and falling. This article focuses on the compression refrigeration cycle and how it works, with a special focus on the refrigerators of trucks and other refrigerated vehicles.

Compression refrigeration cycle

Turn your head around, you will encounter a variety of cooling devices, all of which use a kind of refrigeration cycle. From gas coolers to home and industrial refrigerators and freezers.

The compression refrigeration cycle is actually a thermodynamic cycle during which heat is transferred from a lower temperature point to a higher temperature point. This is possible with the help of a refrigerant and a mechanical circuit consisting of an evaporator, condenser and compressor, and of course energy consumption. The compression refrigeration cycle has four main stages:

1. Compression stage

The first step in compression refrigeration cycles is compression of the refrigerant. At this stage, they use a compressor. During this process, fluid temperature and pressure increase and its volume decreases. The driving force required to start the compressor can also be provided by electricity or fossil fuels.

2. Cross the condenser

The condenser is actually a tube shell converter. When the fluid comes out of the compressor, it has a high temperature and pressure. The purpose is to reduce the temperature inside the condenser. The thermodynamic process at this stage is designed as a constant pressure, so that the temperature of the fluid is greatly reduced; As a result, the fluid becomes a mixture of steam and saturated liquid. In compression cycles, airflow is usually used to cool the fluid.

3. Cross pressure-reducing valve

The fluid passes through pressure-reducing valve is the third step in a refrigeration cycle. At this point, the fluid pressure drops dramatically; The result will be fluid preparation to reach the final stage of the compression cycle.

4. Evaporator

Heat transfer from the environment to the refrigerant occurs at this stage. The evaporator is designed in the form of a shell-tube heat exchanger. The fluid entering the evaporator has a low vapor pressure and is a mixture of liquid and saturated vapor. That is why it has a great desire to evaporate.

This vapor pressure causes the refrigerant to absorb the surrounding heat, thereby cooling the refrigerator compartment or its surroundings. The fluid leaving the evaporator becomes gaseous and is sent back to the compressor.

Compression refrigeration cycle in refrigerated vehicles

Compression refrigeration cycles are used to cooling in refrigerated vehicles (vans, pickups and refrigerated trucks). Of course, Refrigeration cycle design of refrigerated vehicles is done in two units, outdoor and interior, and has a separate structure. The outdoor unit is usually mounted on the roof of the vehicle or in the front of the refrigerator container; And includes refrigeration cycle compressor and condenser. The condenser is located in the outdoor unit in order to use the air flow to cool the refrigerant.

The indoor unit will be connected to the outdoor unit by a pipeline. The indoor unit is installed in the refrigerator compartment; Which plays a major role in cooling the cabin compartment. It is clear that the evaporator is the most important component in the indoor unit.

Another point that is important about refrigerated vehicles and their compression refrigeration cycle is the type of refrigeration cycle; Based on operating temperature and cooling rate, they are divided into two types of refrigerators: Below zero and above-zero.

Above zero vehicle refrigerator

In above-zero vehicles, the operating temperature of the refrigerator is usually defined as higher than the freezing point. This operating temperature will be suitable for carrying materials that are less sensitive to temperature changes. For example, proteins, fruits and vegetables, and most medicines can be included in this category. Compression refrigeration cycles in these vehicles undergo less pressure and naturally consume less energy.

Below zero vehicle refrigerator

Below zero vehicle is the opposite point of the first category. The operating temperature in these refrigerators should always be less than zero degrees Celsius. (This temperature varies according to need.) Below-zero refrigerators are used to transport temperature-sensitive food and medicine. For example, a below-zero compression refrigeration cycle must be used to move certain dairy products, frozen meats, and certain drugs. Otherwise, there is no guarantee for maintaining the quality and health of these products.


Vehicle refrigerators are used to move food, pharmaceutical, chemical, etc. products in urban and interurban distances. These refrigerators also use a compression refrigeration cycle, just like other cooling systems. Refrigeration cycle of transport refrigerators is designed in two outdoor and indoor units. The outdoor unit will usually be mounted on the roof of the vehicle and the indoor unit will be installed inside the cab. Transport refrigeration cycles are made in two types, below-zero and above zero; Above zero refrigerators are used for intra-city transportation and transportation of materials that do not require complete freezing. Below-zero refrigerators are also used to move materials that need to be completely frozen and lose the quality and freshness of the car if the temperature rises.

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