Frequently used formula in HVAC engineering

Shaft power (break horse power)

\(\dot{W}\) [hp] = \(\frac{CFM \times DP [in.w.g.]}{6356 \times \eta_{M}}\)

Temperature increase due to fan heat

1. Motor in located outside of air stream

\(\frac{\Delta p}{\eta_{fan} c_{air} \rho_{air} }\) = 0.363 \(\frac{\Delta p [in.w.g.]}{\eta_{fan}}\)

2. Motor in located inside of air stream

\(\frac{\Delta p}{\eta_{fan} \eta_{motor} c_{air} \rho_{air} }\) = 0.363 \(\frac{\Delta p [in.w.g.]}{\eta_{fan} \eta_{motor}}\)

Simplified formula of heat transfer through cooling/heating coil

1. working fluid: air

\(q_{sen}\) = 1.08 x cfm x \(\Delta T\)

\(q_{lat}\) = 0.68 x cfm x \(\Delta W [gr/lb]\) = 4840 x cfm x \(\Delta W [gr/lb]\)

\(q_{total}\) = 4.5 x cfm x \(\Delta H [btu/lb]\)

2. working fluid: water

\(q_{sen}\) = 500 x gpm x \(\Delta T\)

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