Energy balance calculation is often done for designing a heat exchanger to determine operating parameters for hot and cold fluids such as - inlet / outlet temperatures and flow rates.

For hot fluid side of a heat exchanger let,

m_{H} : mass flow rate of the hot fluid in kg/hr

Cp_{H} : mass heat capacity of the hot fluid in Joules/kg^{0}C

Ti_{H} and To_{H} : Respectively inlet and outlet temperatures on exchanger hot side in ^{0}C

m_{C} : mass flow rate of the cold fluid in kg/hr

Cp_{C} : mass heat capacity of the cold fluid in Joules/kg^{0}C

Ti_{C} and To_{C} : Respectively inlet and outlet temperatures on exchanger cold side in ^{0}C

Heat lost by the hot fluid = -Q = m_{H} × Cp_{H} × (To_{H} - Ti_{H}) … (1)

Heat gained by the cold side = Q = m_{C} × Cp_{C} × (To_{C} - Ti_{C}) … (2)

Comparing equations (1) and (2),

m_{H} × Cp_{H} × (Ti_{H} - To_{H}) = m_{C} × Cp_{C} × (To_{C} - Ti_{C}) … (3) (Heat balance equation)

This energy (heat) balance equation can be solved for one variable for any given case. Out of total six variables in the equation (3), five should be fixed to determine the unknown variable.

It should also be noted that the mass balance equation is already applied in this case to develop equation (3).

The fact that m_{H}in = m_{H}out = m_{H} and m_{C}in = m_{C}out = m_{C} is already considered while writing equations (1) and (2). Hence application of mass balance equation for heat exchanger does not present any new information here.