Superheat Refrigerant in Suction Line of Evaporative Condensed Chillers


As you know the refrigerant fluid is evaporated and unheated in the evaporator coils.

The fluid refrigerant starts moving through the chiller’s evaporator coils as soon as it is evaporated. The increased evaporation of refrigerant will result in the reduced volume of the fluid refrigerant, so that at the end of the evaporator coil the refrigerant is fully evaporated despite being cooler than the objects surrounding the coil. Therefore, the sensible heat entering the evaporator will be absorbed by the evaporated refrigerant. However, since the refrigerant has been completely evaporated, its temperature will exceed the saturation temperature by absorbing more heat. Under such conditions, the refrigerant is called the evaporator superheat.

Superheat temperature is indeed the temperature added on the suction gas of the condensed cooling cycle, which is above the saturation temperature of the refrigerant gas.

Aim of superheating the refrigerant is not to let the fluid enter the chiller compressor, because due to incompressibility of fluids, permeation of fluid to the compressor shall break the compressor valves. On the other hand, when the evaporator is in shortage of refrigerant fluid, there is no enough refrigerant in the evaporator, and the existing refrigerant is evaporated quicker than it should, as a result of which the refrigerant will not absorb the full thermal load of the building.

In direct expansion (DX) chillers and system, where the refrigerant fluid is injected directly to cooling coils of the chiller, shortage of the refrigerant can be measured by measuring accurate quantity of the superheat refrigerant gas after exiting the evaporator, especially at the full load.

If thermostatic valves are used in the cooling systems, considering the design type, the superheat quantity is almost fixed.

In normal working conditions this refrigerant temperature difference is 10 to 20°F or 5-10°C, and if this temperature remains the same under all load variations, the gas is charged properly.