Comparison of dry, full liquid and falling film evaporators and comparison of advantages and disadvantages
It is well known that the four major components of a refrigeration system are a compressor, an evaporator, a
Condenser and a throttling device. Among them, the type and characteristics of the compressor as the heart of the refrigeration system are more well known than the other three components. In addition, the evaporator is another key component. Do you know about the type and characteristics of the evaporator? The working principle and structural characteristics of the dry, full liquid and falling film evaporators are explained below.
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 the heat transfer coefficient is only about twice the heat transfer coefficient of the light pipe, but The advantage is that it is easy to return oil, and the control is relatively simple, and the charging amount of the refrigerant is about 1/2 to 1/3 of the filling amount of the full liquid type unit.
Comparison of the difference, advantages and disadvantages of dry, full liquid and falling film evaporators for full liquid evaporators
The full-liquid evaporator and the dry evaporator operate in the opposite way. The cold water passes through the heat exchange tubes, the refrigerant completely immerses the heat exchange tubes, and absorbs heat and evaporates outside the heat exchange tubes. The surface of the heat transfer tube of the full liquid evaporator has a plurality of needle-shaped small holes, and a spiral protrusion on the inner surface of the tube enhances the heat exchange on the cold water side. This high-efficiency heat transfer tube, which simultaneously enhances the boiling outside the tube and the heat transfer inside the tube, has a heat transfer coefficient that is about 5 times higher than that of the light tube.
Falling film evaporator
Falling film evaporators, also known as spray evaporators, are similar to full liquid evaporators, but they differ from full liquid evaporators. The refrigerant of the evaporator is sprayed from the upper part of the heat exchanger to the heat exchange tube, and the refrigerant only forms a thin film of the cold liquid on the heat exchange tube, so that the refrigerant is reduced in boiling evaporation. The hydrostatic pressure is increased, which improves the heat exchange efficiency, and the heat exchange efficiency is increased by about 5 compared with the full liquid unit. Comparison of dry, full liquid and falling film evaporators and comparison of advantages and disadvantages
Falling film evaporation is flow boiling. Since the thickness of the liquid film on the outer surface of the tube is small, the boiling point without static pressure increases, and the heat transfer coefficient is high. The bubbles generated by full-liquid evaporation (that is, immersed evaporation) tend to accumulate on the surface of the heat exchange tubes, resulting in a decrease in heat exchange efficiency, and the heat transfer effect is not as good as that of falling film evaporation. In general, falling film evaporation is a small temperature difference, but it is necessary to prevent fouling and affect heat transfer efficiency.
November 25, 2022
