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 = 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
• 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.