Cold Storage Refrigeration Principles and Process

This article explains the refrigeration principles of cold storage systems with the help of diagrams and process analysis. The reason refrigeration principles are repeatedly emphasized is that only by understanding the underlying logic can operational problems be accurately connected with their causes in real situations, allowing faults to be analyzed more thoroughly and matched with precise solutions. This article uses simple language as much as possible to explain the process.

Whether it is a cold storage system or an air conditioner, neither truly “produces cold.” Instead, refrigeration is essentially a transfer of heat. It is achieved through the continuous conversion and circulation of refrigerant between liquid and gaseous states, thereby accomplishing heat exchange. The entire cycle consists of four stages: compression, condensation, throttling, and evaporation, forming a continuous closed loop. During circulation, the refrigerant continuously absorbs and releases heat while working together with the four major core components and auxiliary devices to transfer heat.

However, during this process, when is the refrigerant in liquid form? When does it become gas? When is it under high pressure, and when is it under low pressure? What are the principles behind these changes?

Compression Process

The Compressor Compresses Low-Temperature, Low-Pressure Refrigerant Gas into High-Temperature, High-Pressure Vapor

Next, let us begin with the “heart” of the refrigeration system - the compressor. As the name suggests, the compressor’s core function is compression. However, the object being compressed must always be gas. This is because liquids are nearly incompressible. If liquid refrigerant enters the compressor, it can cause liquid slugging.

Imagine liquid being forcibly compressed inside the compressor chamber - it acts like a hammer striking the internal components. If this condition continues, the compressor will quickly fail and become permanently damaged. Therefore, a gas-liquid separator is usually installed before the compressor. It acts like a goalkeeper, separating any refrigerant that has not fully evaporated and allowing only gas to enter the compressor.

After the gas enters the compressor, its volume decreases rapidly while pressure and temperature rise sharply, eventually becoming high-temperature, high-pressure gas that is discharged from the compressor. This process is commonly referred to as discharge.

Discharge temperature is a critical parameter because it directly reflects the operating condition of the compressor and the health of the refrigeration system. Therefore, refrigeration units are usually equipped with discharge temperature display devices.

At the same time, the high-pressure gauge is directly connected to the discharge pipeline to monitor pressure values in real time. The high-pressure side of the high-low pressure controller is also connected here. Once discharge pressure exceeds the safe range, the high-pressure protection system is triggered, automatically shutting down the compressor to prevent equipment damage.

In addition, when high-temperature, high-pressure gas leaves the compressor, it also carries part of the compressor oil. At this point, the oil separator plays an important role. It separates the oil and returns it to the compressor, maintaining lubrication and ensuring normal operation.

Compressor Example Diagram

Condensation Process

High-Temperature Vapor Flows Through the Condenser, Releases Heat to the Outside, and Condenses into High-Pressure Liquid. 

The condenser acts as the cooling station for high-temperature gas. After being compressed by the compressor, the high-temperature, high-pressure refrigerant gas enters the condenser as its next destination. Here, it undergoes an important transformation - changing from gas into liquid.

This process is similar to how water vapor condenses into water when cooled. To help the refrigerant cool and liquefy quickly, the condenser is equipped with condenser fans. However, the fans do not run continuously. Their operation is precisely controlled according to system pressure.

This is because if the fans start too early, the refrigerant may liquefy before sufficient heat is released, reducing refrigeration efficiency. On the other hand, if the fans start too late, system pressure may become excessively high.

Therefore, the system uses a controller connected to the discharge pipeline or liquid receiver to detect pressure and intelligently control condenser fan operation. Once the refrigerant successfully completes liquefaction, it flows into the liquid receiver and waits for the next stage of its journey.

After leaving the liquid receiver, the refrigerant first passes through the filter drier, which acts as a purification stage by removing impurities, moisture, and acidic substances from the refrigerant.

Some filter driers containing activated carbon are specifically designed to absorb paraffin wax because refrigeration oil may precipitate wax when temperatures fall below -35°C. Medium- and high-temperature refrigeration systems generally do not require this type of wax removal.

After purification, the refrigerant enters the sight glass.

The sight glass is a key component for judging refrigeration system conditions. Technicians can observe the sight glass to determine the dryness of the pipeline and also identify whether refrigerant charge is sufficient or whether air has entered the system by checking for bubbles inside the refrigerant.

At this stage, the refrigerant is still in a high-pressure liquid state. To prevent refrigerant from continuing to flow into the evaporator after shutdown, a solenoid valve is installed after the sight glass to achieve precise flow control.

Condenser Example Diagram

Throttling Process

High-Pressure Liquid Passes Through the Expansion Valve, Instantly Drops in Pressure, and Becomes a Low-Temperature, Low-Pressure Gas-Liquid Mixture

After passing through the solenoid valve, the refrigerant reaches the expansion valve. The expansion valve acts as the pressure-reduction hub of the refrigeration system. When high-pressure liquid refrigerant passes through the expansion valve, the throttling orifice rapidly reduces pressure, causing both refrigerant pressure and temperature to drop significantly. The refrigerant then becomes a low-temperature, low-pressure gas-liquid mixture.

The throttling process itself does not change the total heat content of the refrigerant. However, the sudden pressure drop lowers the temperature and creates a gas-liquid mixture capable of absorbing heat from inside the cold storage room.

At the same time, the expansion valve can automatically adjust refrigerant flow according to the cooling load of the cold storage room, ensuring refrigeration efficiency and temperature stability.

 Expansion Valve Example Diagram

Evaporation Process

Refrigerant Enters the Evaporator, Absorbs Heat from the Cold Storage Room, Vaporizes Again, Cools the Air, and Returns to the Compressor to Complete the Cycle

Afterward, the low-temperature gas-liquid mixture enters the evaporator, which is the air cooler located inside the cold storage room.

The evaporator is the component that truly produces the refrigeration effect. Here, the refrigerant absorbs heat from inside the cold storage room and gradually changes from liquid into low-temperature, low-pressure gas.

As heat is removed from the storage air, the air temperature decreases, allowing the cold storage room to achieve its required low-temperature condition.

When the refrigerant leaves the evaporator, it has completely transformed into saturated gas and returns to the compressor to begin the next refrigeration cycle.

Evaporator Example Diagram