Refrigeration Systems
The vapor-compression cycle
Four processes: isentropic-ish compression (1→2), constant-pressure desuperheating, condensation, and subcooling (2→3), isenthalpic expansion through a metering device (3→4), and constant-pressure evaporation with superheat (4→1). Capacity is the mass flow times the enthalpy rise across the evaporator: Q = ṁ(h1 − h4). Compressor work is ṁ(h2 − h1), so COP = (h1 − h4)/(h2 − h1).
Ratings and hardware
EER = Btu/h per watt, related to COP by COP = EER/3.412; SEER and IEER are seasonal metrics, not the same as COP at a single point. Compressor types: reciprocating, scroll, screw, and centrifugal (large chillers). Metering devices are thermostatic expansion valves, electronic expansion valves, or fixed orifices/capillary tubes. Air-cooled condensers reject heat at roughly 10–15°F approach above ambient; evaporative condensers use wet-bulb.
Traps
Confusing saturated suction temperature with evaporator-air leaving temperature; assuming subcooling is free (it increases capacity but consumes condenser area); mishandling zeotropic blends with temperature glide; forgetting that lower condensing temperature raises capacity and EER; and ignoring that oil return depends on adequate refrigerant velocity.
Worked example
A unit is rated EER 11.5. COP = 11.5/3.412 = 3.37. If the compressor draws 10 kW, delivered cooling = 33.7 kW ≈ 9.6 tons. Alternatively, from a P-h diagram with h1 = 108, h2 = 122, h4 = 45 Btu/lb: COP = (108 − 45)/(122 − 108) = 4.5.