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21. Model proposed by the Friedlander, correlating the air filtration efficiency by impaction can be expressed as (where NSt is the Stokes number)
A. ηimp= 0.075NSt1.2
B. ηimp = 0.75NSt1.2
C. ηimp= 0.25NSt1.2
D. ηimp= 0.025NSt1.2
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 ηimp,ηint,ηdif are the efficiencies by impaction, interception and diffusion respectively)
A. 1 – ηimpηintηdif
B. 1 – (1 – ηimp)(1 – ηint)( 1 – ηdif)
C. 1 – ( ηimpηint/ηdif)
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
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