Refrigerant Mass Flow Rate

A refrigeration circuit's capacity is mass flow times refrigerating effect: how many pounds of refrigerant circulate each hour, and how much heat each pound picks up between the metering device and the compressor. The refrigerating effect is read off a pressure–enthalpy diagram as the horizontal span of the evaporator process, typically 60–80 BTU/lb for R-410A at air-conditioning conditions and around 70 BTU/lb for R-134a in a chiller.

This is the number that sizes compressors, line diameters and metering orifices. Worked example: a 10-ton coil (120,000 BTU/hr) with a 70 BTU/lb refrigerating effect circulates 120,000 ÷ 70 ≈ 1,714 lb/hr, or 778 kg/hr on this page. Turn it around to audit a machine: 5 tons of capacity moving 400 lb/hr implies 150 BTU/lb of effect, which for a halocarbon is impossible and tells you a measurement is wrong. That number belongs to ammonia, whose 470 BTU/lb effect is exactly why industrial plants tolerate its toxicity and run tiny pipes. The trap is forgetting that subcooling changes Δh: every extra degree of liquid subcooling adds roughly 0.3–0.5 BTU/lb of refrigerating effect for free, which is why liquid-suction heat exchangers exist and why a hot, undersized liquid line quietly steals capacity.