# API RP 500, Ventilation

Written by Jerry Ratzlaff on . Posted in Electrical Engineering

## Free Area of Inlet Openings formula

 $$\large{ A = \frac{ V }{ 1200 \; \sqrt{ \frac{ h \; \left( T_i \;-\; T_o \right) }{ T_i } } } }$$

### Where:

$$\large{ A }$$ = free area of inlet (or outlet) opening (s)

$$\large{ V }$$ = volume of building to be ventilated

$$\large{ h }$$ = height from the center of the louver opening to the Neutral Pressure Level (NPL)

$$\large{ T_i }$$ = temperature of indoor air

$$\large{ T_o }$$ = temperature of outdoor air

## Height from the center of the louver opening to the Neutral Pressure Level formula

 $$\large{ h = \frac{ H }{ 1 \;+\; \left[ \left( \frac{ A_1 }{ A_2 } \right)^2 \; \left( \frac{ T_i }{ T_o } \right) \right] } }$$

### Where:

$$\large{ h }$$ = height from the center of the louver opening to the Neutral Pressure Level (NPL).  The NPL is the point on the vertical surface of a building where the interior and exterior pressures are equal.

$$\large{ H }$$ = vertical distance (center to center) between A_1 and A_2

$$\large{ A_1 }$$ = free area of lower opening

$$\large{ A_2 }$$ = free area of upper opening

$$\large{ T_i }$$ = temperature of indoor air

$$\large{ T_o }$$ = temperature of outdoor air