Mechanical Operation: Practical Questions & Answers

a)      The power and power number for a helical ribbon and propeller in turbulent region.
Ans: the power number for a helical ribbon is lesser but the power consumption will be more due to high rotational speed required in turbulent region.

b)      Do the particles move along with the balls in the ball mill?
Ans: Particles move along with the ball depending on their size shape and friction with wall and
        crushed by attrition, rubbing and impact.

c)      Define mesh number and also explain the difference between BSS 5 and Tyler 5 mesh screens.
Ans: Mesh number is the number of square openings in 1 inch. the BSS5 and Tyler 5 mesh have
        different openings due to different wire diameters.

d)     Relation between slurry feed concentration and concentration per unit volume of filtrate.
Ans: The volume of filtrate (liquid) is less than that in slurry feed because some water is present in
        the cake. 

e)     Why the pressure drop doesn't change with velocity of fluid in fluidized bed?
Ans: after fluidization the bed start expanding and porosity increases which leads to constant pressure.

f) In scale up of a baffled vessel (D1:D2 = 1:3), in order to maintain an equal rate of mass transfer 
(equal power/volume), under turbulent condition, the ratio of agitator speed (N2:N1) should be:
Ans: equal power/volume= n^3D^2 is constant.

g)   How the settling velocity of a particle can be increased to achieve faster separation?
Ans: To increase the settling velocity, the density and viscosity can be decreased by increasing the
        temperature.

h) The significance of Froude number in power calculation of mixer/agitator.
Ans: Froude number signifies the effect of vortex formation. in a baffled tank or in the process 
       where Reynolds number is less than 300, Froude number is insignificant. 

i) Efficiency of crushing equipment is defined as: 
Theoretical power required to crush the material/actual power consumption

j) What kind of equipment would you suggest for reducing size of hard material: impact/cutting

k) In order to produce solid particles between 5 and 10 µm, the appropriate size reducing equipment 
     is fluid energy mill.

l) The clarifying filter could be used to filter a slurry containing 0.05 % CaCO3.

m) Sticky and pasty materials are transported by: screw (ribbon) conveyor

n) The porosity of a compressible cake is minimum the filter medium.

o) What are the methods available for determination of particle size?
Ans: Screening, SEM, TEM, particle size analyser. 

p)  Why are different sizes of ball used in Ball mill? What will happen if you use same sizes balls?
Ans: different size balls will have different critical speed, leads to different impact time.

q) Can the screening prior to classification be beneficial? Explain.
Ans: for a close size range, classification will give sharp separation according to their settling velocity.

r) To handle large gas flows what can be better: a large diameter cyclone separator or a no of 
small diameter cyclones separators and why.
Ans: large diameter cyclones will have less efficiency.