# Pressure drop

The transportation of the entire air volume of a ventilation system requires corresponding differences in pressure. Air volume and pressure losses of connected components (including ventilated luminaires) which meet the air flow with a certain resistance (pressure loss) are material to the dimensioning of ventilator pressures (power). Pressure losses of components are determined in suitable measurement facilities. For many typical ventilation components (air ducts, bends, transition points in cross-section), these pressure losses are compiled in tables. Pressure losses of ventilated luminaires are specified by their manufacturers based on measurements.

In terms of pressure loss, a distinction is made between static Δpstat and dynamic pressure loss Δpdyn. Both add up to overall pressure loss Δptotal], which is lower than the static pressure loss. The numerical value equation is as follows:

.

 ζ Resistance coefficient ρ Air density w Air velocity

The static pressure loss corresponds to the pressure difference between absolute pressure before and behind the resistance. It increases with the square of the velocity w. The resistance coefficient ζ (zeta) depends on the flow properties of the air resistance. For common air resistances such as cross-sectional constriction, bends, slides etc., resistance values are standardised in technical tables. For ventilated luminaires, pressure losses are determined in dedicated measuring facilities.

For ventilated luminaires without direct duct connection, meaning for low-pressure ceilings, the static pressure loss is decisive for the dimensioning of ventilator pressure. The dynamic pressure loss does not apply in this case due to the air velocity inside the suspended ceiling; w = 0. For luminaires with duct connection (air outlet hood), overall pressure loss is the corresponding dimensioning value.

Dynamic pressure loss corresponds to the acceleration energy (kinetic energy) of flowing air and increases at the square of the velocity. It is regained at the end of the ventilation system (at the exhaust ventilator) – which is why it appears as a negative amount in the numerical value equation.

Ventilation design data for ventilated luminaires are provided by the ventilation luminaires’ manufacturers.  Figure depicts an example of this. It shows the overall pressure loss Δpges in relation to air volume flow.

Traditional units still occasionally appearing in air conditioning documentation can be converted to SI units as follows:

The parameter values specify the A-weighted sound power level in dBA measured in a reverberation room according to EN 23741 which does not factor in any room absorption. Regarding acoustic luminaire properties see chapter .