Performance
Refrigeration capacity from mass flow and enthalpy change
- where
- refrigeration capacity (kW)
- refrigerant mass flow (kg/s)
- enthalpy leaving the evaporator (kJ/kg)
- enthalpy entering the evaporator (kJ/kg)
Explain this equation
Works out the cooling capacity of an evaporator from the refrigerant flow and the enthalpies either side of it. Use it with a P-h chart or refrigerant tables.
Enthalpy is the energy carried by each kilogram of refrigerant. It enters the evaporator as a cold liquid-vapour mix at h₄ (equal to the liquid enthalpy before the expansion valve) and leaves as vapour at h₁. The difference, the refrigerating effect, is the heat each kilogram absorbs. Multiplying by the kilograms per second gives kJ/s, which is kW.
An energy balance on a steady flow: heat in equals mass flow times enthalpy rise.
Sense check: 0.05 kg/s with 150 kJ/kg of refrigerating effect is 7.5 kW. More subcooling lowers h₄ and raises the capacity for the same flow.
Watch out: take h₁ including any useful superheat in the evaporator only; superheat picked up in the suction line outside the space is not useful cooling.
Works out cooling capacity from the refrigerant mass flow and the enthalpy gained across the evaporator.
Enter your values to see the result.
Good to know
Read h1 and h4 from the refrigerant’s pressure-enthalpy chart or tables. h4 equals the liquid enthalpy just before the expansion valve (isenthalpic expansion), so more subcooling lowers h4 and raises capacity. The same form gives condenser heat rejection with h2 − h3.
Common mistake
Using h1 at saturation when the evaporator outlet is superheated, or mixing enthalpy reference states from different tables.
Limits
Steady-state energy balance; ignores heat gains to pipework.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.