Every pipe, elbow, valve and filter resists the flow; the fluid overcomes this resistance by spending some of its pressure. Because pressure and saturation temperature are tied together in refrigeration, the loss is mostly stated as "so many kelvin of saturation temperature". A loss of 1 K in the suction line means that the compressor draws at a pressure 1 K lower than the evaporator; this reduces capacity by about 3% and increases consumption. A loss of the same value in bar is more expensive at low temperature: with R404A, 0.1 bar amounts to 0.7 K at −10 °C and 1.5 K at −35 °C.
Accepted design limits: the equivalent of 1–2 K in the suction line, 1 K in the discharge line and 0.5–1 K in the liquid line. The pipe diameter is chosen to these limits, with the refrigerant velocity also taken into account: in the suction line at least 4 m/s in horizontal runs and at least 8 m/s in vertical risers, so that the oil is carried back to the compressor; the upper limit, because of noise and loss, is 15–20 m/s. In the liquid line the velocity is kept at 0.5–1.5 m/s. Elbows and valves are added to the total length as "equivalent pipe length"; a short line with many elbows can give more loss than a long straight one.
In the liquid line the danger is different: if the pressure falls, the liquid starts to boil before it reaches the expansion valve (flash gas) and the valve capacity falls. In addition to friction, height also takes away pressure: as liquid rises, R404A and similar refrigerants lose about 0.1 bar per metre. If the evaporator is 10 m above the condensing unit, 1 bar, that is, about 2–2.5 K of subcooling, is used up by height alone. The solution is sufficient subcooling, the correct diameter and, if necessary, a liquid–suction heat exchanger.
On site pressure drop is found from the pressure or temperature difference measured at two points. A temperature difference that can be felt by hand between the inlet and outlet of the filter drier, or sweating at the outlet, shows that the filter is blocked. If the evaporator’s own internal loss is large (multi-circuit coils with a distributor) an externally equalised expansion valve is used; otherwise the valve measures the superheat wrongly. In split installations with long lines the pipe diameter may be chosen larger than the connection diameter in our catalog. You can read how many kelvin the pressure difference you measure amounts to from our refrigerant pressure–temperature chart.