Mechanical Operations Multiple Choice Questions on “Screens”.
1. ______ are used on the bottom of the blades.
a) Spikes
b) Serrations
c) Doctor knife
d) Both spikes and serration
Answer: d
Clarification: Spikes or serrations are used on the bottom of the blades, when the material handled tends to compact and become hard.
2. _____ is defined as peripheral speed or arm tip speed.
a) Critical speed
b) Rake speed
c) Elevation speed
d) Enhanced speed
Answer: b
Clarification: The rake speed defined as the peripheral speed or arm tip speed varies with the type of material handled, it usually ranges from 20 to 25 ft./min.
3. For a cyclone separator, the separating force is ____
a) 5 times gravity
b) 10 times gravity
c) 50 times gravity
d) 2 times gravity
Answer: a
Clarification: The centrifugal separating speed or force may range from five times the gravity in every large diameter, to 2500 times gravity in very small cyclones.
4. Cyclones can be employed for _______
a) Solid, liquid from gas
b) Solids from liquid
c) Liquid from solid
d) Interphase separation
Answer: d
Clarification: Cyclones have been employed to remove solid and liquid from gases and solid from gases, these units offers a least expensive separation.
5. Cyclones can be operated at temperature as high as ____
a) 1000oC
b) 50o C
c) 2000o C
d) 1500o C
Answer: a
Clarification: They can be operated at a temperature as high as 1000o C and pressure up to 5 atmospheres.
6. Cyclone can separate particles as small as ___
a) 100 microns
b) 5 microns
c) 50 microns
d) 20 microns
Answer: b
Clarification: Cyclones removing solids from gases are generally applicable when particles of over 5 microns (0.0002) inches diameter are involved.
7. When the gas enters the cyclone, velocity increases with decrease in ____
a) Radius
b) Flow rate
c) Force
d) Mass
Answer: a
Clarification: As gas enters the velocity goes redistribution so that the tangential component of the velocity increases and radius decreases.
8. What does the following called?
Vct ~ r-n
a) Decreasing radius
b) Increasing radius
c) Decreasing vector
d) Increasing vector
Answer: a
Clarification: Decreasing radius is expressed by the above equation, where n=1 in the absence of wall friction.
9. For the decreasing radius, the value of n ranges from ____
a) 0.2 to 0.3
b) 0.5 to 0.7
c) 0.6 to 1.0
d) 1.0 to 2.0
Answer: b
Clarification: However, n will take the value in the range of 0.5 to 0.7 over a large portion of the cylinder radius, n is usually 0.52.
10. For a good cyclone design, velocity should not be ____
a) > 600 ft./min
b) < 600 ft./min
c) < 1000 ft./min
d) > 1000 ft./min
Answer: a
Clarification: Select an outlet diameter to give a gas velocity not exceeding 600 ft. /min, only special design can have higher velocities.