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Air Ducts - Major Friction Loss vs. Temperature and Pressure

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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 = kt ΔpNTP         (1)


Δp = actual friction loss (pressure or head)

kt = Temperature Compensating Coefficient

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

"Temperature Compensating Coefficient" for various air temperatures:

Example - Air with Temperature 100oC

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

  • the pressure loss in air with temperature 100oC 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 = kp ΔpNTP         (2)


Δp = actual friction loss (pressure or head)

kp = Pressure Compensating Coefficient

ΔpNTP = 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 (lbf/in2) = 106 dyn/cm2 = 750 mmHg
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Air Ducts - Friction Loss Diagram

A major friction loss diagram for air ducts - SI units.

Darcy-Weisbach Equation - Major Pressure and Head Loss due to Friction

The Darcy-Weisbach equation can be used to calculate the major pressure and head loss due to friction in ducts, pipes or tubes.

STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure

The definition of STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure.

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