Heat Recovered by Cold Air Formula

The heat recovered is determined by the limiting heat capacity (minimum of air or flue gas), the heat exchanger efficiency, and the temperature difference between the flue gas and inlet air. The limiting capacity ensures the calculation respects thermodynamic constraints.

\begin{equation}\label{eq:air-heating-exhaust-heat-recovered} Q_{recovered} = \eta_{hx} \cdot \min(C_{air}, C_{flue}) \cdot (T_{flue} - T_{inlet})\end{equation}

Symbols

\(Q_{recovered}\)	Heat recovered by cold air \([\unit{ \btu\per\hour}]\)
\(\eta_{hx}\)	Heat exchanger efficiency \([\unit{ \unitless}]\)
\(C_{air}\)	Heat capacity of air \([\unit{ \btu\per\degreeFahrenheit}]\)
\(C_{flue}\)	Heat capacity of flue gas \([\unit{ \btu\per\degreeFahrenheit}]\)
\(T_{flue}\)	Flue gas temperature \([\unit{ \degreeFahrenheit}]\)
\(T_{inlet}\)	Inlet air temperature \([\unit{ \degreeFahrenheit}]\)