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

$\frac{\mathrm{\Delta }p}{{\eta }_{fan}{c}_{air}{\rho }_{air}}$ = 0.363 $\frac{\mathrm{\Delta }p[in.w.g.]}{{\eta }_{fan}}$

2. Motor in located inside of air stream

$\frac{\mathrm{\Delta }p}{{\eta }_{fan}{\eta }_{motor}{c}_{air}{\rho }_{air}}$ = 0.363 $\frac{\mathrm{\Delta }p[in.w.g.]}{{\eta }_{fan}{\eta }_{motor}}$

Simplified formula of heat transfer through cooling/heating coil

1. working fluid: air

$q_{sen}$ = 1.08 x cfm x $\mathrm{\Delta }T$

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

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

2. working fluid: water

$q_{sen}$ = 500 x gpm x $\mathrm{\Delta }T$

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