How to calculate the pressure loss in smoke exhaust ventilation system?

Because the gas is viscous, it will produce friction resistance when flowing. The process of airflow flowing in the smoke exhaust and ventilation system is the process of airflow power loss. Except that the fan inputs mechanical energy to the airflow, all components are consuming mechanical energy, resulting in system pressure loss. Whether the selected fan can overcome the friction resistance of the system and ensure the air flow, it is necessary to calculate the pressure loss of the system.

(1) Frictional drag and local pressure loss

The energy consumed by the flow of gas per unit weight in a straight pipe is called thefrictional drag. When the magnitude and direction of flow velocity change dramatically in the pipeline of the system, turbulence is formed. The pressure loss caused by the consumed energy is called local pressure loss. The pressure loss caused by elbow, reducer, air valve, diffuser, smoke exhaust hood and oil fume purifier are all local pressure losses.

(2) Absolute roughness of pipe material

Absolute roughness: the roughness of the wall surface is expressed by the protrusion height K of the coarse particle (sand particle diameter).

Relative roughness: the ratio of roughness protrusion height to pipe diameter, which can reflect the influence of wall roughness on resistance in pipes with different diameters. The expression of relative roughness is K / d.

Absolute roughness is often used in the resistance calculation of kitchen smoke exhaust pipe. The following table shows the absolute roughness of pipes of different materials.

pipe material Absolute roughness/ mm


Galvanized iron sheet 0.05-0.18


Plastic plate 0.01-0.05


Aluminum plate 0.03


Plywood 1.0


Board 0.2-1.0


Concrete slab 1.0-3.0