21. Model proposed by the Friedlander, correlating the air filtration efficiency by impaction can be expressed as (where N_St is the Stokes number)

A. η_imp= 0.075N_St^1.2
B. η_imp = 0.75N_St^1.2
C. η_imp= 0.25N_St^1.2
D. η_imp= 0.025N_St^1.2

22. The collection efficiency by impaction increases with

A. increasing particle diameter
B. increasing airflow velocity
C. both (a) and (b)
D. increasing particle size

23. The efficiency of the fibrous filter by combined mechanism is given by(where η_imp,η_int,η_dif are the efficiencies by impaction, interception and diffusion respectively)

A. 1 – η_impη_intη_dif
B. 1 – (1 – η_imp)(1 – η_int)( 1 – η_dif)
C. 1 – ( η_impη_int/η_dif)
D. None of the above

24. The heat conduction in dry air is

A. less rapid than in steam
B. more rapid than in steam
C. similar to steam
D. none of these

25. For the laminar flow of Newtonian fluid through a smooth round pipe, the ratio of average fluid velocity to the maximum velocity is

A. 0.5
B. 0.75
C. 0.87
D. 0.37