Refrigerant superheat in a chiller is the difference between the evaporation temperature of the refrigerant gas in the chiller evaporator and the temperature at the inlet of its compressor.
Superheat = Evaporator outlet temperature – Compressor inlet temperature
Calculation of refrigerant superheat in dry chiller evaporator:
A chiller operating with a thermostatic expansion valve, intended for an air conditioning system, has an evaporation temperature of 7°C.
Depending on the cooling load and the expansion valve setting, the temperature at the evaporator outlet (compressor inlet) is 11°C.
- Superheat = Evaporator outlet temperature – Compressor inlet temperature
- Superheat = 11°C – 7°C = 4°C
Superheat calculation in flooded chiller evaporator:
This type of evaporator is mainly characterized by:
- The liquid coolant in the cooling circuit is fed through a float valve, which is responsible for maintaining a constant level of coolant inside the housing.
- As the cooling load varies, a coolant level control valve acts to maintain the liquid level in the housing.
- The housing will be the evaporator of the cooling circuit.
- The outlet of the refrigerant from the evaporator or casing is located at the top, and is always above the level, so to exit, the refrigerant must be in a vapor state, in order to rise and reach the exit point.
Since the refrigerant leaving the flooded evaporator is newly formed vapor, the temperature at the outlet is the same as the evaporation temperature.
- Superheat = Evaporator outlet temperature – Compressor inlet temperature
- Superheat = 7°C – 7°C = 0°C
The Superheat in a flooded evaporator will always have a value of zero.
Importance of chiller refrigerant superheat:
- In dry evaporators, superheat greater than zero indicates that all the refrigerant is in the vapor state before entering the compressor.
- Superheating allows the expansion valve to regulate the amount of refrigerant that must enter the chiller evaporator.
- Excessive superheat indicates a system with a lack of liquid refrigerant in the evaporator.
- Excessive overheating can cause very high temperatures at the compressor outlet, which can affect the oil.
- In modern large chillers and refrigeration equipment, to improve the efficiency of the equipment, the superheat should be as close to zero as possible, always above.
- In chillers with a bulb thermostatic expansion valve, the standard superheat value is usually 5°C maximum.
