The refrigeration cycle and COP

Vapor-compression refrigeration is a reliable source of exam points on both HVAC & Refrigeration and Thermal & Fluid Systems — and it's very learnable, because the whole cycle lives on one diagram.

The cycle, as four moves on a P-h diagram

• Evaporator — refrigerant absorbs heat at low pressure and boils (the useful cooling effect).
• Compressor — raises pressure and temperature; this is the work input.
• Condenser — rejects heat at high pressure and condenses.
• Expansion device — drops pressure back down (roughly constant enthalpy). Plot those on a pressure-enthalpy (P-h) chart for the refrigerant and the energy balances become differences in enthalpy you can read off.

COP — get the definition right

Coefficient of performance is useful effect over work in:

• Cooling: evaporator heat absorbed ÷ compressor work.
• Heating (heat pump): condenser heat rejected ÷ compressor work — always one more than the cooling COP for the same cycle. Watch the superheat at the compressor inlet and subcooling at the condenser outlet, and the isentropic compressor efficiency that separates ideal from actual work.

Where people lose points

• Mixing up cooling vs. heating COP — or which heat transfer goes in the numerator.
• Reading the P-h chart / property tables slowly or off the wrong line.
• Tonnage conversions — forgetting that a ton of refrigeration is 12,000 Btu/h when you back out refrigerant mass flow.
• Ideal vs. actual — applying isentropic work when the problem gives a compressor efficiency.

How to study it

Practice drawing the cycle on a P-h diagram from memory, then solve for each enthalpy from property data and balance the energy. Do enough that locating states and reading properties is fast — that reading speed is half the battle on a closed-book, timed exam.

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