The difference between the three types of evaporators dry, flooded, and falling film evaporators
1. Dry evaporator
The refrigerant in the dry evaporator passes through the heat exchange tube, and the cold water runs outside the high-efficiency heat exchange tube. Such a heat exchanger has relatively low heat exchange efficiency, and its heat transfer coefficient is only about twice that of a bare tube, but Its 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-liquid unit.
2. Flooded evaporator
The operation mode of the full-liquid evaporator is exactly the opposite of that of the dry evaporator. The 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. There are many needle-shaped holes on the surface of the heat transfer tube of the flooded evaporator, 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 heat transfer inside the tube, increases the heat transfer coefficient by about 5 times compared with the bare tube.
3. 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 from the upper part of the heat exchanger onto the heat exchange tube. The refrigerant only forms a thin film of refrigerant on the heat exchange tube, so that the refrigerant will be reduced when it boils and evaporates. The static liquid level pressure is improved, thereby improving the heat exchange efficiency, which is about 5 higher than that of the flooded unit.
Falling film evaporation is flow boiling. Because the thickness of the liquid film layer on the outer surface of the tube is small, there is no increase in boiling point caused by static pressure, and the heat transfer coefficient is high. However, the bubbles generated by flooded evaporation (that is, immersed 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 is a case of small temperature difference, but it is necessary to prevent fouling and affect heat transfer efficiency.
"Chiller" is the customary nomenclature for a kind of refrigeration unit. This "chiller" is generally used for the cold source of central air-conditioning, or the refrigeration of air-conditioning conditions, and the output is low-temperature cold water, usually called "chilled water" ", hence the name. Generally, those that can only refrigerate are called chillers, while those that can produce heat at the same time are called "heat pump" units.
The "full-liquid type" means that the "shell and tube evaporator" used by the unit adopts the form of "full-liquid evaporator", which is a shell and tube type that is different from the "dry" and "falling film" types. Evaporator. It has a refrigerant cycle in the "shell side" and a chilled water cycle in the "tube side". From the cross-sectional view, it seems that there is more than half of the cylinder in the cylinder, and the water-carrying tube bundle is immersed in the refrigerant. It is just the opposite of the "dry evaporator". The dry type uses refrigerant in the "tube side" and water in the "shell side", just like the refrigerant tube bundle is immersed in water.
The flooded evaporator and the flooded unit have higher heat transfer efficiency than dry evaporator/dry unit. The temperature difference between the outlet water temperature and the evaporation temperature is small, and the resistance along the way is small, which is suitable for large circulation. The unit (such as a centrifuge) has a good cooling effect. However, the refrigerant charge requirement is large, and a special oil return system is needed to help the compressor return oil.
The full liquid type means that the refrigerant is between the copper tube and the shell tube, while the chilled water flows in the copper tube. The dry type is the opposite of the other two. The refrigerant evaporates in the copper tube, and the water flows between the copper tube and the shell tube. They are mainly used for heat pump air conditioning. In industrial low-temperature chillers, ordinary dry evaporators are generally used.
Fourth, the advantages and disadvantages of dry and flooded evaporators
The full-liquid shell and tube evaporator takes 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. The refrigerant liquid absorbs heat and vaporizes through the liquid separator on the top of the cylinder and returns to the compressor. Convenient operation and management, high heat transfer coefficient.
Its disadvantages are:
①When the evaporating temperature of the refrigeration system is lower than 0℃, the water in the tube is easy to freeze and damage the evaporating tube;
②The refrigerant charge is large;
③Affected by the height of the refrigerant liquid column, the evaporation temperature at the bottom of the cylinder is too high, which will reduce the heat transfer temperature difference;
④ Oil will accumulate in the lower part of the evaporator cylinder, so reliable oil return measures must be taken, otherwise the safe operation of the system will be affected.
Dry shell and tube type, that is, non-filled evaporator, the refrigerant flows in the tube, and the water flows outside the tube cluster. The refrigerant flow usually has several processes. Due to the gradual vaporization of the refrigerant liquid, the more upward, the more the number of process tubes. In order to increase the heat exchange on the water side, a number of baffles are arranged on the outer side of the heat transfer tube of the cylinder to make the water flow across the tube clusters many times.
Its advantages are:
①Lubricating oil enters the compressor with the refrigerant, generally there is no problem of oil accumulation
②The refrigerant charged is less, generally only about 1/3 of the full-liquid type;
③When t0 is around 0℃, the water will not freeze.
But you must pay attention to the use of this evaporator:
① The refrigerant has multiple processes. If the end cover is not properly handled, it will produce liquid accumulation, which will make the liquid distribution into the next process uneven and affect the heat transfer effect;
② There 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 water leakage will reduce the heat transfer coefficient on the water side by 20% to 30%, and the overall heat transfer coefficient by 5% to 15%.
