Friction Loss - Temperature and Pressure

Friction loss in air ducts will depend on the actual temperature and pressure of the air

Most of the friction loss diagrams and tables available for air ducts are estimated for air at Normal Temperature and Pressure - NTP - conditions (20^oC and 1 bar).

Temperature

If the actual temperature condition is otherwise the  friction loss can be compensated like

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)

The temperature compensating coefficient can be estimated with the diagram below:

Pressure

If the actual pressure is otherwise the  friction loss can be compensated like

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 with 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

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