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Section summary |
---|

1. General
calculation formula |

2. Calculation
without change of state |

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*(H _{2}-H_{1})**

Q = heat flow rate in kW

m = mass flowrate in kg/s

H_{1} = mass enthalpy at condition 1 in kJ/kg

H_{2} = 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 H_{1} and H_{2} is the same.

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*(T _{2}-T_{1})**

Q = heat flow rate in kW

m = mass flowrate in kg/s

c = specific heat in kJ/kg/K

T_{1} = temperature at condition 1 in K

T_{2} = 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.