Suction pressure is a term used to describe the pressure of a refrigerant gas at the intake point of a refrigerator or air conditioner compressor. Suction or, as it is also known, low side pressure, is one of the critical variants in the operation and diagnostic processes of air conditioning and refrigeration. A correct balance between the suction and discharge pressure ensures that the system is healthy and functioning correctly. This relationship, along with the suction and discharge temperatures and the specific refrigerant used, play an important role in determining the correct refrigerant charges for individual systems.
Suction pressure refers to the pressure of the refrigerant at the point where it enters the compressor after leaving the interior of the building. Measured when the compressor is running, this pressure will typically be in the region of 100 pounds per square inch (PSI). The exact suction pressure an air handling unit is designed to produce is a product of the intended capacity of the system, the specific components employed, and the refrigerant the system is designed to use.
A commonly held misconception is that an air conditioner blows cold air into a room or building. All refrigeration systems work on a basic heat transfer principle in which low pressure, cold refrigerant circulates through a set of coiled tubes inside the area to be cooled. This cold fluid absorbs heat from the atmosphere in the room and is then pumped outside the building via the system compressor where a fan or water spray dissipates the heat. The refrigerant is then returned to the interior of the building to repeat the process until the desired temperature is achieved.
The relationship between suction and compressor outlet pressures and temperatures are known quantities in any given air conditioning or refrigeration system. This finite relationship allows suction pressure values to be used as diagnostic aids when troubleshooting system faults. High suction pressure, for instance, may indicate a clogged condenser coil or faulty condenser fan. Low pressure values, on the other hand, may be an indication of a refrigerant undercharge or restricted indoor airflow.
This general overview of a refrigeration cycle serves to demonstrate the importance of suction pressure. For any air conditioning or refrigeration system to work correctly, the relationship between low and high side pressure needs to be maintained within the systems operating specifications. Anomalous suction pressure values can, however, be a very reliable indication of where faults lie in systems that are not operating correctly.