Chiller Refrigerant Superheat: A Guide to Understanding and Calculating
When it comes to chiller operation, there’s one important factor you don’t want to overlook: superheat. But what exactly is superheat? Put simply, it’s the difference between the evaporation temperature of the refrigerant gas in the chiller evaporator and the temperature at the compressor inlet.
To calculate superheat, you can use this equation:
Superheat = Evaporator outlet temperature – Compressor inlet temperature
Let’s take a closer look at two different types of chiller evaporators and how to calculate their superheat.
Dry Chiller Evaporator:
If you’re using a chiller with a thermostatic expansion valve for air conditioning, you’ll want to know the evaporation temperature, which is typically around 7°C. The temperature at the evaporator outlet (compressor inlet) will depend on the cooling load and valve setting, but let’s say it’s around 11°C.
Using the superheat equation, we can calculate:
Superheat = Evaporator outlet temperature – Compressor inlet temperature Superheat = 11°C – 7°C = 4°C
Chiller Flooded Evaporator:
In this type of evaporator, the liquid refrigerant from the refrigeration circuit is fed through a float valve to maintain a constant level inside the casing. As the cooling load varies, a refrigerant level control valve ensures that the liquid level is maintained in the shell. The outlet of the refrigerant from the evaporator is located at the top, and is always above the level, so the refrigerant must be in a vapor state to rise and reach the outlet point.
Since the refrigerant leaving the flooded evaporator is a newly formed vapor, the outlet temperature is the same as the evaporation temperature, which we know is around 7°C. Using the superheat equation, we can calculate:
Superheat = Evaporator outlet temperature – Compressor inlet temperature Superheat = 7°C – 7°C = 0°C
Why does superheat matter?
For one, it allows the expansion valve to regulate the amount of refrigerant that enters the chiller’s evaporator. If superheating is greater than zero in dry evaporators, it indicates that all the refrigerant is in the vapor state before entering the compressor. Excessive overheating can cause very high temperatures at the compressor outlet, which can affect the oil.
To improve the efficiency of modern large chillers and refrigeration equipment, superheat should be as close to zero as possible, always above. In chillers with bulb thermostatic expansion valves, the standard superheat value is usually 5°C maximum.
By understanding and calculating chiller refrigerant superheat, you can keep your system running smoothly and efficiently.
