Air Ducts - Major Friction Loss vs. Temperature and Pressure

Air ducts friction loss and pressure drop diagrams are normally made for air at Normal Temperature and Pressure (NTP) condition - with a temperature of 20 ^oC and a atmospheric pressure of 1 bar.

Influence of Temperature

The friction coefficient in the D'Arcy-Weisbach Equation increases with higher temperature but the coefficient will also be outweighed by the reduced pressure loss due to reduced density of the air.

• the pressure loss in an air duct is reduced with higher temperature

With air temperature other than 20 ^oC - a "Temperature Compensating Coefficient" can be used to modify the pressure loss:

Δp = k_t Δp_NTP         (1)

where

Δp = actual friction loss (pressure or head)

k_t = Temperature Compensating Coefficient

Δp_NTP = friction loss at NTP conditions (pressure or head)

"Temperature Compensating Coefficient" for various air temperatures:

Example - Air with Temperature 100^oC

For air with temperature 100^oC the "Temperature Compensating Coefficient" is approximately 0.83. So,

• the pressure loss in air with temperature 100^oC is approximately 83 % of the pressure loss in air at standard NTP conditions 20 ^oC

Influence of Pressure

If the actual pressure differ from atmospheric pressure (1 bar abs) - friction loss in air ducts should be compensated with a "Pressure Compensating Coefficient":

Δp = k_p Δp_NTP         (2)

where

Δp = actual friction loss (pressure or head)

k_p = Pressure Compensating Coefficient

Δp_NTP = friction loss at NTP conditions (pressure or head)

The "Pressure Compensating Coefficient" can be estimated for various pressures from the diagram below:

• 1000 mbar = 1 bar = 10⁵ Pa (N/m²) = 0.1 N/mm² = 10,197 kp/m² = 10.20 m H₂O = 0.9869 atm = 14.50 psi (lb_f/in²) = 10⁶ dyn/cm² = 750 mmHg