Almost all processes occurring in refrigeration plants are of the flow type, like in Figure 2.7b, so the change in enthalpy applies, and this is why we almost exclusively use enthalpy in making energy calculations. To summarize, the rate of heat transfer in kW (Btu/min) in a heat exchanger, such as the evaporator in Figure 2.8a, is the product of the mass rate of flow kg/s (lb/min) and the difference of enthalpies in kJ/kg (Btu/lb) leaving and entering the evaporator.