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The Difference Between Dry, Flooded And Falling Film Evaporators

Jul 13, 2020

The difference between dry, flooded and falling film evaporators


As we all know, the four major components are, compressor, evaporator, condenser and throttle device. Among them, the type and characteristics of the compressor, which is the heart of the refrigeration system, are better known than the other three components. In addition, the evaporator is another key component. Do you know the types and characteristics of the evaporator? The following is a detailed explanation of the working principle and structural characteristics of dry, flooded and falling film evaporators.


Dry evaporator


The dry evaporator refrigerant passes through the heat exchange tube, and the cold water runs outside the high-efficiency heat exchange tube. The heat exchange efficiency of such a heat exchanger is relatively low, and its heat transfer coefficient is only about twice that of the smooth tube. The advantage is that it is easy to return oil, and the control is relatively simple, and the refrigerant charge is about 1/2 to 1/3 of the charge of the full-fluid unit.


Flooded evaporator


The operation mode of the flooded evaporator and the dry evaporator is just the opposite. Cold water passes through the heat exchange tube, the refrigerant completely immerses the heat exchange tube, and evaporates outside the heat exchange tube after absorbing heat. The surface of the heat transfer tube of the flooded evaporator has many pin-shaped small holes, and there are spiral protrusions on the inner surface of the tube to enhance the heat transfer on the cold water side. This high-efficiency heat transfer tube, which simultaneously strengthens the boiling outside the tube and the heat transfer inside the tube, increases the heat transfer coefficient by about 5 times compared to the smooth tube.


Falling film evaporator


Falling film evaporator, also known as spray evaporator, this heat exchanger is similar to flooded evaporator, but it is different from flooded evaporator. The refrigerant of this evaporator is sprayed onto the heat exchange tube from the upper part of the heat exchanger. The refrigerant only forms a thin layer of refrigerant liquid film on the heat exchange tube, so that the refrigerant is reduced when it boils and evaporates. The static liquid level pressure is increased, thereby improving the heat exchange efficiency, and its heat exchange efficiency is increased by about 5 compared with the full liquid unit.


Falling film evaporation is flow boiling. Due to the small thickness of the liquid film layer on the outer surface of the tube, there is no boiling point increase due to static pressure, and the heat transfer coefficient is high. However, the bubbles generated by flooding evaporation (that is, immersion evaporation) tend to accumulate on the surface of the heat exchange tube, resulting in a decrease in heat exchange efficiency, and its heat exchange effect is not as good as falling film evaporation. In general, falling film evaporation belongs to a small temperature difference, but it is necessary to prevent scaling and affect the heat transfer efficiency.