This set of Energy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Pulverised Mills – 1”.
1. How is coal pulverized in the ball mill?
a) By rotating the cylinder
b) By rolling the cylinder
c) By hammering the cylinder
d) By jolting the cylinder
Explanation: The pulverization in ball mill takes place as a result of action between the balls and coal, as the cylinder they are put in is made to rotate. The cylinder is partly filled with various sized steel balls. The cylinder is rotated at a velocity of 130 m/min periphery.
2. Why pre-heater air and the tempered air are introduced in the pulverizer of ball mill?
a) To carry the ash out
b) To increase the combustion rate
c) To carry pulverized coal
d) To dry the coal by removing the moisture
Explanation: A mixture of tempering air and hot air from pre-heater is introduced in the pulverizer. These streams of air carry the pulverized coal and pass through the classifier. The over-sized particles are thrown out of the air stream in the classifier and fine material is passed to the burner through exhaust fan.
3. The output of the mill can be controlled by the _________ located in the exhaust fan inlet duct.
c) Air duct
Explanation: The output of the mill can be controlled by the dampers located in the exhaust fan inlet duct. These dampers vary the flow of air through the mill, thus controlling the rate of fuel removed from the mill. The dampers are operated by the boilers automatic combustion control.
4. Feeder output in cylinder of a ball mill is regulated on the basis of?
a) Cylinder capacity
b) No of balls
c) Level of coal
d) Rotation speed of cylinder
Explanation: The feeder output is regulated by the coal level in the cylinder. When coal level in the cylinder attains sufficient height to seal off the lower channel than the differential control operates to stop the coal feed.
5. What is the pulverization capacity of the ball mill?
a) 10 tons of coal per hour
b) 20 tons of coal per hour
c) 50 tons of coal per hour
d) 80 tons of coal per hour
Explanation: A ball has the capacity of pulverizing 10 tons of coal per hour containing 4% of moisture requires 28 tons of steel balls and consumes 20 to 25 KW hr energy per ton of coal. A wide range of fuels including anthracite and bituminous coal which are difficult pulverize, can still be used.
6. Why ball and race mill is also called as contact mill?
a) Because of rolling action between the balls and races
b) Because of ramming action between balls and races
c) Because of swirling action of balls and races
d) Because of mixture of balls and races
Explanation: The pulverization or crushing action in ball and race mill takes between the two moving surfaces called balls and races. The rolling action happens between these two. Then the coal passes between the rotating elements time and again until it has been pulverized to the desired degree of fineness.
7. The pulverization is completed by a combination of crushing impact and _______ between grinding surfaces.
Explanation: The pulverization is completed by a combination of crushing impact and attrition between grinding surfaces. The coal is crushed between two moving surfaces: balls and races. The upper stationary race and lower rotating race is driven by a worm and gear, hold the balls between them.
8. What type of feeder is used to supply the coal in ball and race mill?
a) Ring main feeder
b) Parallel feeder
c) Radial feeder
d) Rotating table feeder
Explanation: The coal is supplied through the rotating table feeder at the upper right to fall on the inner side of the races. The moving balls and races trap coal between them to crush it to powder. Spring holds down the upper stationary race and adjusts the force needed for crushing.
9. Which type of fan is used in the ball and race mill?
a) Forced draught fan
b) Balanced draught fan
c) Artificial draught fan
d) Natural draught fan
Explanation: A forced draught fan is used to supply hot air to the mill through the annular space surrounding the races. The air picks up the coal dust as it flows between the balls and races and then enters into the classifier above it.
10. What is the purpose of fixed vanes in the ball and race mill?
a) Forms a cyclonic flow
b) For easy movement of the coal
c) For directing the airflow
d) For barring the exhaust air to enter back into mill
Explanation: The fixed vanes make the entering air to form a cyclonic flow throwing the oversized particles to the walls of classifier. The oversized particles slide down for further grinding in the mill. The coal particles of required size are taken to the burners with air from the top of the cylinder.
11. Why are heavy impurities provided in the process of ball and race mill?
a) To resist upward thrust caused by secondary air stream
b) To resist downward thrust caused by primary air stream
c) To resist upward thrust caused by primary air stream
d) To resist downward thrust caused by secondary air stream
Explanation: The mill is provided with a means of separating heavy impurities from the coal and thus reducing wear and possible damage to the grinding element. These heavy particles resist the upward thrust caused by the primary air stream and collect in a compartment in the base of mill, and then they are removed periodically.
12. What is done when there is requirement for more coal in the ball and race mill?
a) The primary air flow is increased
b) The combustion intensity is reduced
c) The secondary air flow sucks out the light coal particles
d) The hopper is fully opened
Explanation: When there is requirement for more coal, the primary air flow is increased automatically and its higher velocity in the mill carries additional coal in the furnace. This action reduces the amount of coal in the pulverizer and decreases the pressure drop, causing the feeder controller to supply more coal.
13. What happens due to the pressure in the pulverizer?
a) Pulverized fuel is blown into boiler room
b) Increases the rate of combustion
c) Makes the process is vigorous
d) The large particles in the pulverized fuels are removed out
Explanation: As the casing of the pulverizer is under pressure, the leakage of fine coal through the mill casing causes the pulverized fuel to be blown out into the boiler room. The fan used with this mill handles only air therefore the blades erosion by coal particle is eliminated.
14. What is the main disadvantage of race and ball mill pulverizer?
a) Mill requires extra floor area
b) It is directly connected to the motor
c) High power consumption
d) Greater wear
Explanation: Ball and race mill have greater wear compared to other pulverizers. The advantages of lower space occupied, lower power consumption in kW hr/ton of coal pulverized, lower weight and lower capital cost have outweighed the wear problem and these pulverizers found general acceptance.
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