Ⅰ. Dry evaporator
The refrigerant of the dry evaporator passes through the heat exchange tube, and the cold water runs outside the high-efficiency heat exchange tube. The advantage is that oil can be returned and the expansion can be controlled, and the refrigerant charging amount is about 1/2~1/3 of the flooded unit charging amount.
Ⅱ, flooded evaporator
The flooded evaporator operates in the opposite way to the dry evaporator. Cold water passes through the heat exchange tubes, and the refrigerant completely submerges the heat exchange tubes. After absorbing heat, it evaporates outside the heat exchange tubes. There are many needle-shaped small holes on the surface of the tube, and there are spiral protrusions on the inner surface of the tube to strengthen the heat exchange on the cold water side. This high-efficiency heat transfer tube, which simultaneously enhances boiling outside the tube and heat transfer inside the tube, improves the heat transfer coefficient by about 5 times compared with the bare tube.
Ⅲ, falling film evaporator
The falling film evaporator, also known as the spray evaporator, is similar to the flooded evaporator, but it is different from the flooded evaporator. The refrigerant of this evaporator is sprayed from the upper part of the heat exchanger to the heat exchange tube, and the refrigerant only forms a thin layer of refrigerant liquid film on the heat exchange tube, so that the refrigerant decreases when it boils and evaporates. The static liquid level pressure is improved, thereby improving the heat exchange efficiency, and its heat exchange efficiency is about 5 times higher than that of the flooded unit.
Falling film evaporation is flow boiling. Since the thickness of the liquid film layer on the outer surface of the tube is small, there is no increase in the boiling point caused by static pressure, and the heat transfer coefficient is high. However, the bubbles generated by flooded evaporation (that is, submerged evaporation) are easy 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 that of falling film evaporation. Generally speaking, falling film evaporation belongs to the case of small temperature difference, but it is necessary to prevent scaling and affect heat transfer efficiency.
Ⅳ, the advantages and disadvantages of dry and flooded evaporators
In the flooded shell and tube evaporator, water flows in the tubes, and the refrigerant evaporates outside the tube cluster, so the heat transfer surface is basically in contact with the liquid refrigerant. Generally, the amount of refrigerant charged in the shell is about 55% \\ x7e65% of the effective volume of the cylinder. After the refrigerant liquid absorbs heat and vaporizes, it returns to the compressor through the liquid separator at the top of the cylinder. Easy operation and management, high heat transfer coefficient.
Its disadvantages are:
① When the evaporation temperature of the refrigeration system is lower than 0°C, the water in the tube is easy to freeze and damage the evaporation tube;
② Large amount of refrigerant charge;
③Affected by the height of the refrigerant liquid column, the evaporation temperature at the bottom of the cylinder is too high, which will decompose the heat transfer temperature difference;
④ Oil will accumulate in the lower part of the evaporator cylinder, and reliable oil return measures must be taken, otherwise the safe operation of the system will be affected.
In dry shell-and-tube evaporators, that is, non-flooded evaporators, the refrigerant flows in the tubes, and the water flows outside the tube clusters. The flow of refrigerant usually has several processes. Due to the gradual vaporization of refrigerant liquid, generally the higher the direction, the more the number of process tubes. In order to increase the heat transfer on the water side, several baffles are arranged in the transverse direction of the heat transfer tubes of the cylinder, so that the water can move across the tubes multiple times.
Its advantages are:
① Lubricating oil enters the compressor with the refrigerant, and generally there is no problem of oil accumulation
② The amount of refrigerant charged is less, generally only about 1/3 of that of the flooded type;
③When the temperature is near 0℃, the water will not freeze.
But use this evaporator must pay attention to:
① Refrigerant has multiple processes. If the corner of the end cover is not handled properly, liquid accumulation will occur, which will make the liquid distribution into the next process uneven and affect the heat transfer effect;
②There is a leakage problem on the water side, because there is generally a 1\x7e3mm gap between the outer edge of the baffle and the shell, and a gap of about 2mm between the baffle and the heat transfer tube, which will cause water leakage. Practice has proved that water leakage will cause the water side heat transfer coefficient to decrease by 20% to 30%, and the total heat transfer coefficient to decrease by 5% to 15%.