Comparison of advantages and disadvantages of dry evaporator and flooded evaporator

Comparison of advantages and disadvantages of dry evaporator and flooded evaporator

The refrigerant of the dry expansion evaporator evaporates in the tube. The superheat at the outlet end of the evaporator is usually about 5 to 10K. The expansion valve uses the superheat detected by the temperature sensor to adjust the refrigerant flow required by the evaporator.

　　The advantages of dry expansion evaporator:

　　 Most of them use temperature-sensitive expansion valve (TXV) as the throttling device, which can adapt to load changes, has good controllability, and is easy to construct and low in cost.

　　The refrigerant flows through the pipe. Because of the high flow rate, the refrigerant oil can be directly brought back to the compressor by the speed of the gaseous refrigerant.

　　 Disadvantages:

　　 Overheated refrigerant vapor accounts for about 20% of the heat transfer area of the evaporator, but it can only provide a small amount of refrigeration enthalpy, so the total heat transfer efficiency of the evaporator is low.

　　 The refrigerant flowing through TXV must maintain a certain pressure difference to ensure the refrigerant circulation and cooling capacity of the system. Usually, a high pressure control (HeadPressureControl) that sets the cooling water inlet temperature to 30°C is used to maintain a certain pressure difference. However, this control mode must maintain a certain system high pressure. When the operating conditions on the high pressure side are lower than the design conditions or partly Under load, the system efficiency cannot be improved accordingly.

　　In a multi-compressor system, the refrigerant circulation is independent. After unloading, part of the idle heat exchange area cannot be used, so the partial load performance of the system is limited.

　　 Flooded evaporators are often used in combination with centrifugal compressors or screw compressors in medium and large ice water machines. The system has high efficiency, high manufacturing costs and high technical difficulties. The refrigerant evaporates outside the tube. The flow rate of the refrigerant required by the evaporator can be adjusted by a liquid level controller (such as a float control valve) on the high-pressure side or the low-pressure side. In principle, the boiling refrigerant liquid can completely wet the heat transfer surface, so it can have better heat transfer efficiency.

　　 The advantages of liquid-filled evaporator:

　　 The fully wetted heat transfer surface can increase the efficiency of the evaporator and increase the pressure on the low pressure side of the system.

　　The liquid refrigerant boils on the shell side of the evaporator, the pressure loss is small, and the temperature is relatively uniform; and because the vapor at the suction end enters the compressor in a nearly saturated gas state, the compression efficiency and mass flow rate of the compressor can be increased

　　Because multiple compressor units can share an evaporator and condenser, the heat transfer area can still be used effectively under partial load, so it can have higher partial load efficiency.

have to be aware of is:

　　The liquid-gas separation space should be able to separate most of the gaseous state from the liquid refrigerant, so as not to cause a large amount of liquid refrigerant to enter the compressor and cause liquid compression.

　　 Since the refrigerant oil entering the evaporator cannot directly return to the compressor with the gaseous refrigerant, the problem of oil return must be handled with special care.