Safety Protection in Refrigeration Devices

Safety protection in refrigeration devices mainly includes two aspects: automatic protection of compressors and overpressure protection of pressure vessels.
I. Automatic protection of compressors mainly includes suction and discharge pressure protection, pressure difference protection, and compressor temperature protection.
(1) Suction and discharge pressure protection
The purpose of suction and discharge pressure protection for compressors is to control the suction pressure and discharge pressure when the compressor is running. If the discharge pressure is too high, not only will the power consumption of the refrigeration device increase, but it may also lead to malfunctions of the refrigeration device; if the suction pressure is too low, outside air and moisture may enter the refrigeration system, affecting the normal operation of the refrigeration device or affecting the oil supply of lubricating oil, causing wear and damage to the moving parts in the compressor due to lack of oil. The protection element used for suction and discharge pressure protection is called a pressure controller.

Pressure controllers come in adjustable and fixed forms, automatic reset and manual reset types.
A low-pressure controller is used for suction pressure protection, and a high-pressure controller is used for discharge pressure protection.

The combination of the two is called a high-low pressure controller. The high-low pressure controller is an electrical switch controlled by pressure signals. Its electrical circuit is connected to the control circuit of the compressor motor. According to the received pressure signal, it directly controls the start and stop of the compressor. The pressure relay installed on the refrigeration device should be tested at least once a year, especially the high-pressure control part, to avoid major malfunctions caused by relay failure.
(2) Pressure difference protection
For compressors using pressure lubrication, when the pressure difference between the lubricating oil pressure and the suction pressure is too small, the moving parts will be damaged due to lack of oil.

Therefore, at this time, the compressor needs to be able to automatically shut down. For refrigeration devices using pump liquid supply, in order to prevent cavitation of the pump, when the pressure difference of the refrigerant entering and leaving the pump is too small, the pump also needs to be automatically stopped. The pressure difference controller is a control element used for pressure difference protection. When the pressure difference at both ends is less than the specified value, it can automatically cut off the power supply. The control setting value of the pressure difference controller at the factory can be appropriately adjusted within its use range if it does not meet the requirements of actual application. During adjustment, according to the oil pressure control range of the compressor, move the pressure difference adjustment gear and set the pointer to the actually required value. When the compressor is shut down due to pressure difference protection, after troubleshooting, manual reset is required to make the time-delay switch contact of the controller work normally.
(3) Compressor temperature protection
Mainly includes exhaust temperature protection and oil temperature control of lubricating oil. If the exhaust temperature is too high, on the one hand, the power consumption of the compressor will increase. In addition, too high exhaust temperature will cause carbonization and deterioration of lubricating oil. In some small refrigeration devices with relatively high exhaust temperatures, a liquid injection valve is used to limit the increase in exhaust temperature. In large refrigeration devices, a temperature controller is used to adjust the cooling water volume to control the oil temperature.
II. Overpressure protection of pressure vessels in refrigeration devices is generally achieved by using safety valves, fusible plugs or safety membranes to automatically release the pressure inside the container when high pressure occurs.
(1) Safety valve
The common safety valve structure in refrigeration devices is spring-type. When the difference between the inlet pressure and outlet pressure of the valve exceeds the set value, the valve disc will be lifted, opening the safety valve completely, and the refrigerant will be discharged from the container to relieve the internal pressure of the container. The safety valve must open within 10% of the container overpressure. Therefore, when selecting a safety valve, the discharge capacity of the safety valve is calculated as 10% higher than the pressure design value of the container. The discharge capacity of a safety valve refers to its minimum aperture. The material of the safety valve is selected according to the characteristics of the system medium. For example, a safety valve containing ammonia medium cannot be made of copper or contain copper. When the safety valve leaves the factory, the opening pressure has been adjusted and sealed with lead. Users must not unseal or adjust it arbitrarily when using it.
(2) Fusible plug
The fusible plug is generally installed on pressure vessels. The fusible plug is made of a low-melting-point alloy. When the pressure and temperature of the container rise to the limit value, the fusible plug will melt, allowing the refrigerant in the container to be discharged to achieve the purpose of relieving pressure on the container.