Air Ducts - Major Friction Loss vs. Temperature and Pressure
The influence of temperature and air pressure on major friction loss.
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)
where
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)
where
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
Related Topics
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Ventilation Systems
Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Related Documents
-
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.
