The air flow rate is determined from either nameplate data or field measurements. When using nameplate data, the flow rate is taken directly from the equipment nameplate. When using measured data, the flow rate is computed as the product of the duct cross-sectional area and the measured air velocity. The heat flow rate is then computed from the air flow rate and the temperature rise across the heat exchanger using an empirical air heat capacity coefficient. Annual energy use is scaled by operating hours, units, system efficiency, and boiler efficiency. The constant \(k_{air} = 0.072381\) \([\unit{\kJ\minute\per\cubic\meter\per\hour\per\K}]\) is derived from the standard air heat capacity \(1.08\) \([\unit{\btu\hour\per\scf\per\hour\per\degreeR}]\).

Flow Rate: \begin{equation}\label{eq:steam-reduction-air-flow-measured} \dot{V}_{air} = v_{air} \cdot A_{duct}\end{equation}

Energy Use

\begin{equation}\label{eq:steam-reduction-air-heat-flow} \dot{Q}_{air} = k_{air} \cdot \dot{V}_{air} \cdot (T_{out} - T_{in})\end{equation}

\begin{equation}\label{eq:steam-reduction-air-energy} E_{use} = \frac{k_{units} \cdot \dot{Q}_{air} \cdot t_{op}}{\eta_{sys} \cdot \eta_{boiler}}\end{equation}

Symbols

\(\dot{V}_{air}\)	Air flow rate \([\unit{ \cubic\meter\per\minute}]\)
\(v_{air}\)	Measured air velocity in the duct \([\unit{ \m\per\minute}]\)
\(A_{duct}\)	Cross-sectional area of the duct \([\unit{ \squareMeter}]\)
\(\dot{Q}_{air}\)	Air heat flow rate \([\unit{ \kJ\per\hour}]\)
\(k_{air}\)	Air heat capacity coefficient (0.072381) \([\unit{ \kJ\minute\per\cubic\meter\per\hour\per\K}]\)
\(T_{out}\)	Outlet temperature \([\unit{ \degreeCelsius}]\)
\(T_{in}\)	Inlet temperature \([\unit{ \degreeCelsius}]\)
\(E_{use}\)	Annual energy use \([\unit{ \kJ\per\year}]\)
\(k_{units}\)	Quantity multiplier (number of identical units) \([\unit{ \unitless}]\)
\(t_{op}\)	Annual operating hours \([\unit{ \hour\per\year}]\)
\(\eta_{sys}\)	System efficiency \([\unit{ \unitless}]\)
\(\eta_{boiler}\)	Boiler efficiency \([\unit{ \unitless}]\)