Super Heat and Subcooling
What is Super-Heat?
Superheat refers to heating a gas above its liquid boiling point. E.g; water boils at 100 °C at sea level. Imagine placing a pot of water on a heater. Water heats up to 100 °C or its boiling point and does not go beyond this temperature in liquid form. If you use a thermometer to check the temperature of the water boiling in the pot, the water will never exceed this point of 100 °C. Then, after all the water in the container has evaporated into gas, the gas can get hotter. Superheat is when the temperature of the gas rises above the boiling point of the liquid. For example, after all the water has evaporated and the gas reaches 101 °C, 1 °C is said to be superheated.
To prevent the liquid refrigerant from entering the compressor by gasifying the refrigerant, which cannot be completely gasified in the evaporator for any reason, is called super-heat in cooling systems. As mentioned above, superheat is a measurable value.
Why is Super-Heat Important?
Calculating supereheat is critical in refrigeration systems because this value can tell the refrigeration system user how far the liquid has traveled through the pipes of the evaporator. Higher superheat means less liquid and more gas in the pipes, which can result in lower system capacity.
Lower superheat means more liquid and less gas in the pipes, which can cause compressor overflow and damage. Correct calculation of superheat requires an HVAC technician to check both temperature and pressure in the same place and at the same time.
How is Super-Heat Calculated?
- Record the actual temperature at the bulb part of the expansion valve (TXV).
- Record the evaporation pressure at the bulb portion of the expansion valve (TXV). (Low Side Gauge Pressure)
- By looking at the gas characteristics, convert the evaporation pressure you have measured into temperature. Example: 117 kPa = -7.8 °C (R-134a)
- The difference between the actual temperature you have measured and the evaporation temperature you have calculated gives the true ‘Super-Heat’ value.
What is Sub-Cooling?
Similarly, when a substance drops below its saturation temperature, it is supercooled. Just as only gas can be superheated, only liquids and solids can be supercooled. So, if our boiling water source drops below 100 °C to 99 °C, we can say 1 °C subcooling.
Subcooling provides an increase in cooling capacity without any change in energy consumption in the compressor. For any reason, the refrigerant, which cannot be fully liquefied in the condenser, is turned into liquid by passing through systems or circuits designed for subcooling, and is sent to the expansion valve as liquid.
Why is Sub-Cooling important?
The most basic meaning of subcooling is any temperature below the saturation temperature. The colder the refrigerant is as it flows into the evaporator coil, the more heat it can absorb. In this context, subcooling acts as a useful predictor of overall efficiency. To provide useful information, the exact measurement of subcooling is required. The difference between the temperature at the condenser outlet and the saturation temperature gives you the exact amount of subcooling.
One of the most common sources of air conditioning inefficiency is insufficient subcooling. This limits the amount of heat the coolant can absorb. In extreme cases, insufficient subcooling can even cause the refrigerant to return to a gaseous form before reaching the evaporator piping. The most common cause of insufficient subcooling is low refrigerant charge. However, too much subcooling can damage your system. An excessive amount of coolant will raise subcooling to a problematic level. At this level, the refrigerant cools so much that it does not turn into gas inside the evaporator coil. As this liquid refrigerant flows into the compressor, it causes dangerously high pressure levels. Liquids, unlike gases, are incompressible. Therefore, liquid refrigerant should never enter your compressor.
How is Sub-Cooling Calculated?
- Record the actual temperature at the condenser outlet.
- Record the high pressure value at the condenser outlet. (High Side Gauge Pressure)
- By looking at the gas characteristics, convert the high pressure value you have measured into temperature. Example: 930 kPa = 40.4 °C (R-134a)
- The difference between the actual temperature you have measured and the high pressure temperature you have calculated gives the true ‘Super-Heat’ value.
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