Suction pressure
Also known as: low-side pressure, low pressure, back pressure
Short definitionThe pressure read on the low side of the circuit, at the compressor inlet. The evaporating temperature in the evaporator is found from it using the refrigerant’s table; a deviation from the expected value is the first clue to a fault.
It is the value shown by the blue needle on the service manifold. It may look like a number on its own, but the room temperature and the evaporator TD set it: an evaporating temperature of −6 °C to −10 °C is expected for a 0 °C room, and −25 °C to −28 °C for a −18 °C freezer room. With R404A the gauge reads about 1 bar g at −30 °C and about 3.3 bar g at −10 °C.
As the pressure falls, the gas entering the compressor gets thinner. When evaporation drops by 1 K, 3–4% of the capacity and 2–3% of the efficiency are lost; that is the price of an undersized evaporator. A low reading suggests undercharge, a blocked filter, a frosted evaporator or a stopped fan. A high reading can come from freshly loaded warm product or from leaking compressor valves. The verdict is reached together with superheat and discharge pressure. The gauge reads gauge pressure; the absolute value is 1.013 bar higher.