# Agitation and Aeration – Section 2

31. Larger propellers turn at the speed of

A. 20 to 150 rpm
B. 150 to 300 rpm
C. 400 to 800 rpm
D. 1150 rpm

Correct Answer: C. 400 to 800 rpm

32. A microbial population in a suspension culture will only be limited by oxygen availability if the dissolved oxygen

A. concentration is less than the critical concentration
B. concentration is greater than the critical concentration
C. levels are greater than the saturation concentration of oxygen
D. levels are equal to saturation oxygen level

Correct Answer: A. concentration is less than the critical concentration

33. Which of the following model holds correct for the two-film theory? (where KL is the mass transfer coefficient, DAB diffusivity and Zf film thickness)

A. KL= DAB/Zf
B. KL= DAB
C. KL = Zf/DAB
D. KL= DAB.Zf

34. Froude number is avoided in the calculation of power requirements for low viscosity liquid mixing systems when

A. baffled systems are used
B. Reynolds number is less than 300
C. both (a)and(b)
D. none of these