The volumetric flow rate is calculated from the opening area, furnace draft pressure, specific gravity of gas, coefficient of discharge, and correction factor using standard orifice flow equations.

\begin{equation}\label{eq:leakage-cfh} \text{CFH} = 1655 \cdot C_{d} \cdot f_{\text{corr}} \cdot (A \cdot 144) \cdot \sqrt{\frac{P}{SG}}\end{equation}

Symbols

\(\text{CFH}\)	Volumetric flow rate of leakage \([\unit{ \cubicFoot\per\hour}]\)
\(1655\)	Flow constant for orifice equation \([\unit{ \unitless}]\)
\(C_{d}\)	Coefficient of discharge or flow coefficient \([\unit{ \unitless}]\)
\(f_{\text{corr}}\)	Correction factor \([\unit{ \unitless}]\)
\(A\)	Opening area \([\unit{ \foot\squared}]\)
\(144\)	Conversion factor (square feet to square inches) \([\unit{ \inch\squared\per\foot\squared}]\)
\(P\)	Furnace draft pressure \([\unit{ \inchWaterColumn}]\)
\(SG\)	Specific gravity of gas \([\unit{ \unitless}]\)

Note

The coefficient of discharge depends on the angle of convergence in degrees. (Source: Eclipse Combustion Engineering Guide, 1986, Chapter 1, Page #6) PDF link