Enthalpy (h) is the heat content of one kilogram of refrigerant, in kJ/kg. On a P-h diagram, enthalpy runs along the bottom (x-axis). Pressure runs up the side (y-axis), usually on a log scale.

A dome-shaped curve sits in the middle. Left of the dome is liquid. Inside it is a liquid–vapour mix. Right of it is vapour. The left edge is the saturated liquid line; the right edge is the saturated vapour line.

The cycle draws as a rough box:

• 4 → 1, evaporation: horizontal line at low pressure, moving right. The length is heat absorbed.
• 1 → 2, compression: a line going up and to the right. The horizontal shift is compressor work.
• 2 → 3, condensation: horizontal line at high pressure, moving left. The length is heat delivered.
• 3 → 4, expansion: a vertical line straight down. Enthalpy does not change.

Worked example — COP from a P-h diagram (illustrative enthalpies).

• Read from the diagram: h1 = 420 kJ/kg, h2 = 470 kJ/kg, h3 = h4 = 260 kJ/kg.
• Heat delivered q_cond = 470 − 260 = 210 kJ/kg.
• Compressor work w = 470 − 420 = 50 kJ/kg.
• COP = 210 ÷ 50 = 4.2.
• Check: q_evap = 420 − 260 = 160 kJ/kg, and 160 + 50 = 210. The balance holds.

Mass flow for a 12 kW unit: m = 12 ÷ 210 ≈ 0.057 kg/s.

Remember

• Horizontal = pressure constant; vertical = enthalpy constant.
• Evaporation and condensation are horizontal lines.
• Expansion is vertical: no energy added or removed.
• COP = condenser enthalpy change ÷ compressor enthalpy change.