What is Heat duty?
The heat added to (or removed from) a gas stream per second, in watts.
It sizes heaters and heat exchangers and sets fuel or power consumption.
Heating 100 kg/h of air from 20 °C to 300 °C takes about 8 kW.
The Heater either takes an outlet temperature (and reports the duty) or a duty (and reports the outlet temperature). Composition is kept unchanged.
\dot{Q} = \dot{m}\,\left(h_{\mathrm{out}} - h_{\mathrm{in}}\right)
Positive values heat the stream, negative values cool it.