Heat Transfer Rate, Q
Heat exchanger calculations with the heat exchanger design equation require a value for the heat transfer rate, Q, which can be calculated from the known flow rate of one of the fluids, its heat capacity, and the required temperature change. Following is the equation to be used:
Q = mH CpH (THin - THout) = mC CpC (TCout - TCin), where
mH = mass flow rate of hot fluid, slugs/hr,
CpH = heat capacity of the hot fluid, Btu/slug-oF
mC = mass flow rate of cold fluid, slugs/hr,
CpC = heat capacity of the cold fluid, Btu/slug-oF,
and the temperatures are as defined in the previous section.
The required heat transfer rate can be determined from known flow rate, heat capacity and temperature change for either the hot fluid or the cold fluid. Then either the flow rate of the other fluid for a specified temperature change, or the outlet temperature for known flow rate and inlet temperature can be calculated.