While the resistance of a system can be estimated in terms of static pressure changes, the total pressure method is adopted here. This avoids the use of the somewhat confusing “static regain” concept, and is logical in the total pressure drop is true measure of energy loss. When using total pressure it is important to remember that the loss of total pressure at system outlet must always be included. This appliers whether the air leaves through an outlet grille into a room, leaves through whether louvers to the atmospheres or is directly discharged by the fan itself. In each case there is an item of total pressure drop equal to the velocity pressure corresponding to the average velocity to an outlet.

The loss in each elements of the systems are depend to the average velocity through it, which is taken as:

Volume flow, Q Average Velocity V (m/s) = Gross cross section

From this velocity, and the air density, p (kg/m³) the conventional velocity pressure is determined:

Velocity Pressure = ½ p V2 Pa.

This is then multiplied by a factor, K, and the result is the total pressure drop from the inlet (1) to the outlet (2) of the element. Since we are calling it a pressure drop, we can ignore the negative sign which it would have as a pressure change (from high low in the direction of flow).

Total Pressure drop Pf12 = K ½ p V12                          Or Pf12= K ½ p V22

We to calculate the total pressure drop in an airways system, Pf first on the assumption that it is handling air at 1.20 kg/m³ standard density. This will give the correct volume flow rate irrespective of the actual density (or gas) when used in conjunction with the standard fan characteristic. The reason is that both the fans total pressure and total pressure drop are equally in balance as density changes.

At any density, pkg/m³, the total pressure drop is: