We know that in the refrigeration cycle the refrigerant is always changing phase in the evaporator (from liquid to vapor) and in the condenser (from vapor to liquid).
If the refrigerant is pure, while it is changing phase its temperature remains constant, but after the complete change, if we continue to provide heat, the temperature increases.
At this temperature rise is what we call Overheating. Analyze the figure below:
After the expansion device to the point "X" marked in the figure, the refrigerant is changing phase at a constant temperature that we can determine through the table Pressure x Saturation Temperature. After this point, the fluid that is already fully in the vapor phase continues to receive heat and its temperature increases, ie the steam will be overheated.
The difference in the temperature of the suction line and the saturation temperature determines the value of the superheat.
Therefore, to verify the overheating that has occurred in the Evaporator, we need to install a contact thermometer on the suction line and check the suction pressure.
With the pressure value in place, we determine the saturation (evaporation) temperature of the refrigerant and subtract it from the first.
In short:
SA = Temp. L. Suction - Temp. Of Saturation.
Ex .: Suction pressure measured = 70 psig;
Coolant - R - 22;
Saturation temperature = 4ºC (according to table)
Suction line temperature = 10ºC
SA = 10-4 = 6K.
OBS:
Overheating of the refrigerant serves to ensure that 100% steam arrives at the compressor. The SA should not be too high as it would reduce the performance of the equipment and would cause damage to the compressor and not too low as it could get in a liquid form to the compressor, damaging it.
The range of SA recommended by manufacturers of Split System is 5 to 8K.
If in the check you find value below 5, it means that the system is overflowing and if it is above 8, a deficiency.
