Understand dry evaporator and flooded evaporator in one article

February 21, 2024

Understand dry evaporator and flooded evaporator in one article


The evaporator is an important component of the air conditioning system, mainly composed of a heating chamber and an evaporation chamber, responsible for boiling and vaporizing liquids to achieve refrigeration effects. According to the state of refrigerant in the evaporator, the evaporator is mainly divided into two types: dry evaporator and full liquid evaporator.

Dry evaporator:

The refrigerant flows inside the pipe, while water or air flows outside the pipe.

The refrigerant flows through a thermal expansion device (such as a thermal expansion valve), atomizing into small droplets and entering the evaporator tube. The surface area of small droplets is large and they absorb heat from frozen water or air outside the tube. Non azeotropic refrigerants with temperature slip are fractionated inside the tube, and the components with low boiling points evaporate first, followed by other components. The high superheat of a dry evaporator ensures that all refrigerant components evaporate into gas, keeping the proportion of refrigerant components constant.


The refrigerant in the dry evaporator runs inside the tube, while water (refrigeration unit) or air (direct expansion coil) runs outside the tube.

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​Full liquid evaporator:

The refrigerant of the full liquid evaporator goes through the shell side, and the water goes through the pipe.

All heat exchange tubes are immersed in the refrigerant liquid of the shell side. The heat of the frozen water inside the pipe causes the refrigerant to evaporate. The heat transfer tube surface of a full liquid evaporator has many pinholes, and the inner surface of the tube has spiral protrusions to enhance heat exchange on the cold water side. This efficient heat transfer tube enhances both external boiling and internal heat transfer, resulting in a heat transfer coefficient about 5 times higher than that of a bare tube.

The shape of the full liquid evaporator and the dry evaporator are similar, but the refrigerant process is significantly different. The refrigerant in the full liquid evaporator enters from the lower part of the shell, flows and boils outside the heat exchange tube, and the boiling steam is discharged from the upper part of the shell.



dry evaporator

Flooded evaporator

fluid arrangement

Refrigerant tube process, chilled water shell process

The refrigerant goes through the shell and the chilled water goes through the pipe.

Filling capacity

The refrigerant filling amount is small. The liquid filling amount is only about 40% of the inner volume of the tube , which is one-third of that of a full-liquid evaporator with the same cooling capacity.


The refrigerant filling amount is large. Generally, the liquid level height is 55%~65% of the cylinder diameter , and 1~2 rows of heat exchange tubes are left on the upper part to expose the liquid surface. (If the refrigerant filling level is too high, liquid droplets are easily included in the steam. If the separation is incomplete, it may easily cause liquid shock in the compressor; if the liquid level is too low, the heat transfer area cannot be fully utilized.)

Chilled water volume

Chilled water demand is relatively large

Under the premise of maintaining the same efficiency, the temperature difference of flooded heat transfer is smaller than that of dry heat transfer, and the water demand is greatly reduced.

Superheat evaporation temperature

There is a certain degree of superheat and the evaporation temperature is relatively low

No need for superheating, the evaporation temperature can be greatly increased

Oil return performance

Since the flow rate of the refrigerant in the refrigerant pipe is relatively large, the lubricating oil can be brought back to the compressor without an oil return device.

Oil return is difficult and unstable, so reliable oil return measures must be taken. (Special oil separation measures and oil return pipelines are key technologies for flooded units)

Gas-liquid separator

Due to a certain degree of superheat, a gas-liquid separator is generally not needed

Most of them are equipped with gas-liquid separators to separate gaseous and liquid refrigerants to avoid liquid compression.

Separation phenomenon

It is easy to cause uneven refrigerant distribution flow in each tube, especially in multiple processes.

There is no uneven gas-liquid phase separation.

Freezing hazard

The liquid to be cooled is outside the tube, so there is less cooling loss, which can alleviate the risk of freezing.

When the evaporation temperature is too low or the flow rate of the refrigerant is too slow, the refrigerant may freeze and freeze the pipes.

Heat exchange performance

Part of the surface of the heat exchange tube is wetted by liquid, and the surface heat transfer coefficient is slightly lower. The baffle and shell leak, reducing the water side heat exchange effect.

The surface of the heat exchange tube is wetted by liquid, and the surface heat transfer coefficient is high.

When the diameter of the shell is larger, the evaporation temperature of the bottom liquid increases due to the influence of the hydrostatic pressure, which reduces the heat transfer temperature difference. Especially since Freon has a high density, the impact is more significant.

Refrigerant side resistance

relatively bigger

Relatively small

Fouling performance

The shell-side frozen water scale easily adheres to the outer surface of the heat exchange tube, making it difficult to clean.

Chilled water scales on the inner surface of heat exchange tubes, which is relatively easy to clean.

Expansion valve

Most of them use temperature-sensitive expansion valves (electromagnetic or thermal expansion valves). The thermal expansion valve adjusts the opening through the compressor suction superheat, and has good control performance.  

Electronic expansion valve, the opening of the valve is controlled by the liquid level sensor and the compressor exhaust superheat (the cost is too high) ; or the evaporator heat exchange temperature difference and the exhaust superheat control the opening


COP is relatively low and performance is average

Higher COP 







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