Working principle and structure of dry, flooded and falling film evaporator
1. Dry type, full liquid type,falling film evaporatorWorking principle and structure:
1.1, dry evaporator
Dry evaporator refrigerant passes through the heat exchange tube and cold water runs outside the high-efficiency heat exchange tube. The heat exchange efficiency of this heat exchanger is relatively low, and its heat exchange coefficient is only about 2 times that of the light tube. However, its advantage is that it is convenient to return oil and its control is relatively simple, while the filling amount of refrigerant is about 1/2-1/3 of that of the full-liquid unit.
1.2, 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, and the refrigerant completely immerses the heat exchange tube and evaporates outside the heat exchange tube after absorbing heat. There are many needle-shaped small holes on the surface of the heat transfer tube of the liquid-filled evaporator, and there are spiral protrusions on the surface of the tube to strengthen the heat transfer on the cold water side. This kind of high-efficiency heat transfer tube, which strengthens the boiling outside the tube and the heat transfer inside the tube at the same time, makes its heat transfer coefficient increase by about 5 times compared with the smooth tube.
1.3, falling film evaporator
Falling film evaporator, also known as spray evaporator, this heat exchanger 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. The refrigerant only forms a thin layer of refrigerant liquid film on the heat exchange tube, so that the refrigerant is reduced when boiling and evaporating. The static liquid level pressure is reduced, thereby improving the heat exchange efficiency, and its heat exchange efficiency is increased by about five times compared with the full liquid unit.
2. Comparison of advantages and disadvantages of dry and flooded evaporators:
The full-liquid shell-and-tube evaporator flows water in the tube, 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 filled in the shell is about 55% ~ 65% 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.
Its advantages are compact structure, convenient operation and management, and high heat transfer coefficient.
Its disadvantages are:
When the evaporation temperature of the refrigeration system is lower than 0 ℃, the water in the tube is easy to freeze and destroy the evaporation tube;
② Large refrigerant charge;
③ Affected by the height of the refrigerant liquid column, the evaporation temperature at the bottom of the cylinder is higher, which will reduce the heat transfer temperature difference;
④ The lower part of the evaporator cylinder will accumulate oil, and there must be reliable oil return measures, otherwise the safe operation of the system will be affected.
The refrigerant of the dry shell-and-tube type evaporator is not flooded with liquid, and the water flows outside the tube cluster. Refrigerant flow usually has several processes, due to the gradual vaporization of refrigerant liquid, usually the more upward, the more the number of process tubes. In order to increase the water side heat exchange, a number of baffles are arranged on the outside of the cylinder heat transfer tube, so that the water flows across the tube cluster many times.
Its advantages are:
① The lubricating oil enters the compressor with the refrigerant, and there is generally no oil accumulation problem.
The filling of the refrigerant is less, generally only about 1/3 of the full liquid;
When t0 is near 0 ℃, water will not freeze.
But the use of this evaporator must pay attention:
① The refrigerant has multiple processes. If the end cover is not handled properly, effusion will be generated, which will cause uneven distribution of the liquid entering the next process and affect the heat transfer effect;
②There is a leakage problem on the water side. Since there is generally a gap of 1~3mm between the outer edge of the baffle and the shell, and a gap of about 2mm between the baffle and the heat transfer tube, it will cause water leakage. Practice has proved that the leakage of water will cause the water side heat transfer coefficient to reduce 20% ~ 30%, the total heat transfer coefficient to reduce 5% ~ 15%.