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.

Fluid Flow: Type of Flow

The flow can be classified as uniform flow and non uniform flow or steady flow and unsteady flow. Uniform Flow: when the velocity is same throughout the system at any time, it is called uniform flow.  the velocity can be different wrt time.

Steady Flow: when the velocity is constant with respect to time at any point. the velocity can vary with position. 

In a water tank, water comes out of the tank under gravity, the velocity will change with time due to decrease in height of water in tank. i.e. unsteady flow but according to the continuity equation velocity will be same i.e uniform flow.

 In a venturimeter, the area changes, so does the velocity i.e. non-uniform flow. if the flow rate is constant, then the velocity will be constant at any point with time i.e. steady flow.  

Fluid Flow: Flow Measurement Devices

To measure fluid flow rate, there are different flow meters like venturimeter, orifice meter, wier, notches and rotameter.

Venturimeter, orifice meter and rotameter are used to measure flow in closed pipe/channel while wier, notches are used for open flow like river. 

These flow meters are classified as variable head meter and variable area meter. venturimeter, orifice meter are variable head meters where velocity head is converted into pressure head and in rotameter the area of flow in rotameter increases along the flow. when the fluid flows in rotameter, the float starts rising due to buoyancy and drag force. As the flow rate increases, the drag force ( at earlier point) increases that takes the float up in the meter, where the area is larger, leads to decrease in velocity to the earlier value and the the float attains equilibrium.