heat exchanger efiiciency?? I have done some design work calculations for a double pipe heat exchanger. I have the inlet and outlet temperatures and mass flow rates etc. I have calculated Q the heat transfer rate. However im confused with the idea of the thermal efficiency of the heat exchanger. In order to select the required capacity of heat exchanger i need to factor in an efficiency somehow. Do I work out the effectiveness and then when i get this value I choose my capacity of heat exchanger (e.g if the effectiveness is 0.5 then I double the initial Q value to select the heat exchanger capacity)??