Evaporator for screw type water chillers (brief description of difference between full liquid type and dry type)

.Evaporator for screw type water chillers (brief description of difference between full liquid type and dry type)

In screw type water chillers, evaporator types are mainly full-liquid evaporator and dry evaporator two.

In the full-liquid evaporator, the liquid refrigerant enters the evaporator through the throttling device, and the liquid level in the evaporator is kept certain. The heat transfer tube in the evaporator is immersed in the refrigerant liquid. There is still a lot of liquid in the gas-liquid mixture after heat absorption and evaporation, so the moisture escaping from the evaporator is separated by gas-liquid and then returned to the compressor.

The dry evaporator is directly controlled by the thermal expansion valve or electronic expansion valve liquid refrigerant into the evaporator pipe, refrigerant liquid in the pipe into gas, the cooled medium flows in the tube path outside the heat transfer tube.

Comparison of the two:

1.Heat transfer, the surface of the evaporator is liquid wetting, large surface heat transfer coefficient. The evaporating tube surface of the dry evaporator is partially wetted by liquid, and the surface heat transfer coefficient is low. The shell side of the full-liquid evaporator is the liquid refrigerant in the phase change state. The heat transfer form is the heat transfer between liquid and liquid, which belongs to the large-space evaporation heat transfer. The temperature difference of heat transfer is only 2℃. In the full-liquid evaporator, the cold water flows in the tube, can reach the turbulent state, the heat transfer coefficient is high, to the shell side refrigerant, in the boiling state, the heat transfer efficiency is greatly enhanced. The full-liquid evaporator can avoid the limit of superheat degree, and make the heat exchange tube completely immerse in the liquid refrigerant, make full use of the effective evaporating area, and the evaporating temperature can be increased by 2 ~ 3 ° C.

The heat transfer of the dry evaporator is the heat transfer between liquid and gas-liquid mixture. The difference of heat transfer temperature is 5 ° C, and the heat transfer efficiency is low. The dry evaporator is limited by the control condition of the temperature-sensitive thermal expansion valve. The outlet of the evaporator needs to maintain a superheat degree of 5 ~ 8 ° C, resulting in a dry evaporation (superheat) area of 15 ~ 20% of the total area of the heat exchange tubes, the heat transfer of refrigerant in this region is very small and the flow resistance is large, which inhibits the rise of evaporation temperature.

The full-liquid evaporator has higher heat transfer efficiency than the dry evaporator and plate heat exchanger, which increases the energy efficiency ratio (COP) , i. e. the ratio of energy input to energy output. The energy efficiency ratio of full-liquid chillers can generally reach the national first and second class, while that of ordinary dry chillers can only reach the fourth and fifth class. Therefore, the energy efficiency of the full-liquid chillers dry evaporator compared with the use of dry-type chillers save about 15-25% of the operating costs.

2. Oil return performance, it is difficult to return oil of full-liquid evaporator under the condition of oil and refrigerant miscibility, and the condition of oil return directly affects the working condition and oil migration of the unit. The return oil of dry evaporator is stable and convenient.

Therefore, for the full-liquid evaporator, the need for a higher oil return technology to determine the normal oil return unit, so that the unit to achieve long-term stable and efficient operation.

Shanghai KUB Refrigeration Equipment Co., Ltd.

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