Pressure drop
Also known as: pressure loss, line loss
Short definitionThe pressure a refrigerant loses to friction along the line. Installers usually talk about it in kelvin rather than bar; the accepted upper limit for the suction pipe is about 1–2 K.
When the outdoor unit sits far from the room, the pipe gets longer, the elbows multiply and the compressor meets a lower pressure than the one in the evaporator. Each kelvin lost on the suction side takes about 3% of the capacity, and the electricity bill grows too. A freezer room is hit harder: with R404A, a loss of 0.1 bar amounts to 0.7 K when evaporating at −10 °C but reaches 1.5 K at −35 °C. Elbows and valves are added to the total as equivalent pipe length.
A larger diameter cuts the loss but slows the gas. If velocity in a vertical suction riser falls below 8 m/s, the oil cannot be carried up and the compressor runs short of oil. In the liquid pipe no more than 0.5–1 K is wanted; otherwise the liquid starts to bubble before it reaches the expansion valve. If the evaporator is 10 m above the condensing unit, height alone takes about 1 bar, so there must be enough subcooling. The allowance for the discharge line is 1 K.