The typical vapor compression refrigeration system can be divided into two pressures:
Condensing pressure (also called High-side pressure, head pressure, or discharge pressure) is the pressure at which the refrigerant phase changes from vapor to a liquid.
Evaporating pressure (also called Low-side pressure, suction pressure, or back pressure) is the pressure at which the refrigerant phase changes from liquid to a vapor.
These pressures are divided or separated in the system by the compressor's discharge valve and the metering device. and can be read directly from a pressure gauge connected anywhere between the compressor's discharge valve and the entrance to the metering device, assuming that there is negligible pressure drop.
Superheat (S/H) in HVAC refers to the temperature of a refrigerant vapor above its saturation temperature at a specific pressure. It is measured at the evaporator outlet or the suction line of an HVAC system and is calculated as the difference between the actual temperature of the gas and the saturation temperature of the substance. In HVAC systems, refrigerants often boil at much lower temperatures than water. Having the correct superheat temperature ensures that the refrigerant remains in a gaseous state as it exits the evaporator coil, allowing it to absorb heat effectively during the cooling process and ensures that liquid refrigerant is not coming back to the compressor causing damage.
Subcooling (S/C) is the difference between the saturation temperature and the actual liquid refrigerant temperature. The ideal subcooling number will vary depending on the type of HVAC system you have. However, a good rule of thumb is that the liquid refrigerant should be about 10 degrees colder than the minimum temperature required to keep it from boiling. When the liquid refrigerant is colder than it should be, it can boil and turn into a gas. This can cause problems with the HVAC system, including making it less efficient and causing damage to the system. Subcooling can help prevent these problems by keeping the liquid refrigerant at a safe temperature.
Vapor Saturation Temperature (VSAT) is the temperature of the refrigerant vapor at Saturation. Saturation in HVAC systems is that point where liquid and vapor will exist at the same time and place and generally refers to the refrigerant that is in the process of changing from liquid to vapor, or boiling, in the evaporator or vapor to liquid, condensing, in the condenser. VSAT is a good indication of the temperature of the evaporator coil. A temperature below 32 ºF will cause the condensation across the coil to freeze and may lead to a loss of efficiency or operation in the system.
Suction line temperature (SLT) refers to the temperature of the refrigerant vapor as it enters the compressor’s suction line. An AC suction line is the large copper pipe in charge of transporting refrigerant from the evaporator coil to the compressor. The refrigerant inside is gaseous with low pressure and temperature. It is a critical parameter that directly affects the performance and efficiency of the entire refrigeration system. By monitoring and controlling the suction temperature, we can ensure that the refrigeration system operates optimally. This involves maintaining the suction temperature within a specific range, which is determined by factors such as the type of refrigerant used and the desired cooling requirements. The ideal SLT ranges changes in relation to saturation temperature and system pressure.
Temperature differential (ΔºT) is the difference in temperature between the cool air coming from the supply vents – which has been through the system - and the warmer air going back into the return grille – which needs to be cooled and de-humidified. The air coming out of the AC should be blowing typically at 15-20°F lower than the return air temperature. The evaporator ΔºT represents how much heat the AC removes from the air as it passes through. Both too high and too low a temperature differential is bad for your AC.
