# Heat flow rate calculation

## How to calculate the heat exchange required ?

Section summary
1. General calculation formula
2. Calculation without change of state

## 1. General calculation formula

### How to calculate the heat that must be exchanged to bring a substance from condition 1 to condition 2 ?

The heat required to bring a substance from a condition 1 to a condition 2 can be calculated thanks to enthalpies. The substance at condition 1 has a given enthalpy H1 (which you can assimilate to a quantity of energy) and the same substance in condition 2 has another enthalpy H2. The difference in between condition 1 and condition 2 can be only a matter of temperature but can also include a change of phase (vaporization for example).

The heat required to bring a mass m from condition 1 to condition 2 is then, with enthalpies referring to mass :

Q = m*(H2-H1)

Q = heat flow rate in kW
m = mass flowrate in kg/s
H1 = mass enthalpy at condition 1 in kJ/kg
H2 = mass enthalpy at condition 2 in kJ/kg

Note that Q can be positive or negative.

This method allows to cover change of states if it happens in between conditions 1 and 2. It however requires that the engineer has a table or a graph allowing to determine the enthalpies. It is also very important that the reference state for both enthalpies H1 and H2 is the same.

## 2. Calculation without change of state

Enthalpy values are not always available, the specific heat c is more often known. It can be at constant pressure, Cp, or constant volume Cv. It can be used the following way when measuring the heat required to go from conditions 1 to conditions 2, again, without change of state.

Q = m*c*(T2-T1)

Q = heat flow rate in kW
m = mass flowrate in kg/s
c = specific heat in kJ/kg/K
T1 = temperature at condition 1 in K
T2 = temperature at condition 2 in K

Note that Q can be positive or negative.

c is the general expression for the specific heat of a substance, in practice, values at constant pressure Cp and sometimes at constant volume Cv are available.