Multiple choice question for engineering
1. Proper regulation of Pulverized fuel results in _________
a) better fuel economy
b) high output work
c) efficient regulation of heat
d) proper combustion
Answer: a [Reason:] The rate of pulverized coal feed can be regulated properly resulting in fuel economy. Since coal is in the powdered form, coal of any grade can be used. Wide grade of low grade fuel can be burnt easily. Practically the coal used does not form any slagging or clinkers.
2. Why is combustion rate faster when the usage of pulverized coal is done in pulverized fuel handling?
a) Due to greater surface area
b) For using fuel in powdered form
c) Decrease in excess air
d) The speed at which the coal is fed
Answer: a [Reason:] The combustion rate is faster due to greater surface area of coal per unit mass of coal. It means more coal surface is exposed to heat and oxygen. This decreases excess air required for complete combustion and also decreases fan power. There is an increased rate of evaporation and higher boiler efficiency due to the complete combustion of fuel.
3. What is the advantage of breaking coal into smaller pieces in pulverized fuel handling?
a) Easy coal feeding
b) No clogging coal since they are in powder form
c) Exposes more surface of the coal
d) Increase in combustion rate
Answer: c [Reason:] By breaking, a given mass of coal into smaller pieces exposes more surface area for combustion. The increase of surface area exposed per unit volume with the decreasing diameter of coal particle. As shown in graph below, this increase varies rapidly after 0.01 inch diameter of the coal particle and there is no substantial increase after 0.001 inch diameter.
SURFACE AREA VS COAL PARTICLE DIAMETER
4. What is the main aim of Automatic control combustion in pulverized fuel firing?
a) To regulate the powder coal
b) To draw in air required for combustion
c) To maintain constant steam pressure
d) To draw out the ash formed
Answer: c [Reason:] Automatic control combustion applied to pulverized fuel fired boilers is effective in maintaining an almost constant steam pressure under wide load variations. It also gives fast response to load changes as rate of combustion can be controlled easily and immediately.
5. How is that using the pulverized system causes less trouble?
a) Rate of combustion is high
b) The operation is very quick and easy
c) No moving part in furnace
d) By using pulverized fuel system is totally free from slagging or clinker formation
Answer: c [Reason:] There are no moving parts in the furnace, subjected to high temperature therefore the life of the system is more and the operation is trouble free. The boilers can be started from cold very rapidly and effectively. The external heating surface is free from corrosion and fouling as smokeless combustion is possible.
6. Usage of highly preheated secondary air in pulverized fuel firing helps in ______
a) Heats fuel at pace
b) Takes less time to finish the process
c) Rapid flame propagation
d) Reduces the troubles and problems caused in the system
Answer: c [Reason:] Using highly preheated secondary air helps in raid flame propagation. This also increases the ability of system to use high combustion air temperature. The furnace makes it possible to complete combustion with minimum travel of flame travel since it is pre-heated.
7. In which type of machine is the coal reduced to fine powders?
Answer: c [Reason:] Coal is reduced to fine powder in grinding mill. Coal is pulverized to increase its surface exposure. The pulverization of coal means of exposing a large surface area of the coal to action of oxygen and consequently accelerates combustion.
8. Where are pulverized fuel firers used?
a) Large capacity plants
b) Small capacity plants
c) Medium capacity Plants
d) Domestic plants
Answer: a [Reason:] Pulverized fuel firers are used in large capacity plants. The conventional or stoker firing methods were unable to meet the variable loads on the plants and were unsuitable for large capacity plants. Using pulverized fuel firer gives higher thermal efficiency, better control as per load on the plant and uses low grade coal.
9. With what is pulverized fuel mixed to burn like gas or oil?
b) Crude oil
Answer: b [Reason:] In the pulverized fuel firing system the coal is reduced to fine powder state by using a grinding mill and then projected in the combustion chamber with the help of hot air current. The coal in powder state may be mixed with crude oil and may burn like gas or oil.
10. Which air is supplied separately to ensure complete combustion in pulverized fuel firing system?
a) Primary air
b) Turbulence air
c) Secondary air
d) Blower air
Answer: c [Reason:] The amount of secondary air required to complete the combustion is blown separately and resulting turbulence in the high pressure combustion chamber helps in through mixing of fuel and air. For better combustion result this type of mixing is done.
11. What decides the efficiency of pulverized fuel firing system?
a) Turbulence of air
b) Size of coal powder
c) Primary air
Answer: b [Reason:] The size coal powder decides the efficiency of the pulverized fuel firing system. The fineness of the coal should be such as 70%. Of it would pass through a 200 mesh sieve and 90% through 50 mesh sieve.
1. Why do large power plants use pulverized fuel firing system?
a) Higher thermal efficiency
b) Uses high grade coal
c) Less smoke formation
d) Troubles created in this system are less
Answer: a [Reason:] Most of the large power plants use pulverized fuel firing system since it is higher in its thermal efficiency. And selection of it mainly depends upon the size of boiler unit, type of the coal available, nature of load on the plant, load factor, cost of the fuel and availability of skilled labors.
2. How is the fuel selected in the pulverized fuel system?
a) Depending on its size
b) Depending on its grade
c) Depending on volatile matter content
d) Depending the chemical composition
Answer: c [Reason:] In pulverized fuel firing system, coal with 20% volatile matter is considered as the best fuel for the plant, as high volatile coals are harder to grind when compared to low volatile coals. In stoker firing, the characteristics of the coal plays very important role in selection of coal.
3. What is the advantage of using pulverized fuel firing system?
a) Decreases maintenance of plant
b) Less wastage of the products
c) Less ash formation
d) No formation of clinkers
Answer: a [Reason:] The use of pulverized system increases the capital of and operating costs, but decreases maintenance of the plant and increases thermal efficiency. The pulverizer used to grind the coal should consume less power and should deliver the rated tonnage of coal.
4. What is unit pulverizer system also called?
a) Burner and pulverizer firing system
b) Central system firing system
c) Bin system firing system
d) Direct firing system
Answer: d [Reason:] The unit system is also called as the direct firing system. Most of the power plants with pulverized coal as the fuel are being installed with unit pulverizer. In this system each burner or a group of burner and pulverizer constitute a unit. The over head bunker supplies raw coal by gravity into a feeder where it is dried with the help of hot air.
5. On which factor is the coal fed to the pulverizing mill in pulverized fuel firing system?
a) Steam generating rate
b) Capacity of intake of coal
c) On the rate of speed of the feeder
d) Depending on the temperature that is governed inside the combustion chamber.
Answer: a [Reason:] The feeder supplies coal too the pulverizing mill after drying coal in hot air. It is supplied at a variable rate governed by the combustion requirements of furnace and steam generating rate. In the pulverizing mill the coal is crushed to the required size (fine powder).
6. Which type of draught system is used in the pulverized fuel firing system?
a) Forced draught
b) Balanced draught
c) Induced draught
d) Natural draught
Answer: c [Reason:] Induced draught system is used in pulverized fuel firing system. The primary air from the Induced draught fan carries pulverized coal from the mill to the burner through short delivery pipe. In the separator, the big coal particles are separated from the fine dust these again fall down into the mill.
7. Which system was used before direct firing system in pulverized fuel firing?
a) Unit system
b) Burner and pulverizer system
c) Bin system
Answer: c [Reason:] Bin system was widely used before pulverizing equipment became reliable enough for continuous steady operation. This system employs a limited number of large capacity pulverizes at a central point to prepare coal for all the burners.
8. Why is direct firing system considered better than bin system in pulverizer fuel firing?
a) It has long operating procedure
b) The auxiliaries used in system consume large amount of power
c) The initial cost of system is very high
d) The transportation of coal is much more complex
Answer: a [Reason:] The bin system involves many stages of drying, storing and transportation and hence the process takes extra amount of time & chances of fire hazards are more. Never the less, it is still in use at many older plants.
9. Which separator is used to separate the pulverized coal in pulverized fuel firing?
a) Central inlet separator
b) Magnetics separator
c) Cyclone separator
d) Inertial separator
Answer: c [Reason:] Cyclones are mostly used for removing industrial dust from air or process gases. They are the principal type of gas-solid separator. The particulate is made to spin rapidly, heavier particle settle down and all the dust is removed out from the ducts. To also remove dust from the pulverized coal cyclone separator are used.
10. What is used to remove moisture air after cyclone separation in the pulverized fuel system?
a) Sponge pad
b) Dry filters
c) Fabric bag
d) Hot air
Answer: c [Reason:] A fabric bag filter is used to separate and exhaust the moisture air to the atmosphere and discharge the pulverized coal to storage bins, through the conveyor. This system uses all the equipments as used in unit system with higher capacity of each part. In addition to that the system also uses storage bins.
11. In what manner is the coal is fed to all the burners in pulverized fuel system?
a) Through a common feeder
b) Parallel feeding
c) From separate feeder
d) Series mesh feeding
Answer: c [Reason:] The pulverized coal is fed to the various burners through separate feeders. The bin may contain coal required for 12 to 24 hours of supply. The quantities of fuel and air can be regulated accurately and separately. This leads greater degree of flexibility.
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
Answer: a [Reason:] 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 130m/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
Answer: c [Reason:] 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
Answer: a [Reason:] 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
Answer: c [Reason:] 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
Answer: a [Reason:] 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
Answer: a [Reason:] 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.
Answer: c [Reason:] 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
Answer: d [Reason:] 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
Answer: a [Reason:] 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
Answer: a [Reason:] 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
Answer: c [Reason:] 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
Answer: a [Reason:] 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
Answer: a [Reason:] 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
Answer: d [Reason:] 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.
1. Which is the net pressure equation used to find chimney height?
Answer: a [Reason:] The pressure acting on the grate from chimney side,
Pressure acting on the grate from atmospheric side
= Atm pressure
= weight density of air
= Weight density of hot gases
The net pressure acting on the combustion chamber due to the pressure exerted by gas column and air column is given by
P = P2 – P1 (as Wa > Wg )
P = (pa + Wa H) – (Pa + Wa H)
P = H (Wa – Wg )
This pressure difference is known as static draught and is responsible for causing the flow of air through the chimney.
2. Which is the equation used to find chimney diameter?
Answer: a [Reason:] The mass of gases flowing through any cross section of the chimney is given by
3. The portion of flue gases carried away to produce draught could be utilized to _______
a) Heat the air entering furnace
b) Blow out the combustion products such as soot and ash
c) Heat the fuel in ash chamber
d) Support the combustion
Answer: a [Reason:] It is evident that the draught is created at the cost of thermal efficiency of boiler plant installation since a portion of flue gases carried away by the flue gases to produce the required draught could have been used either in heating the air entering the furnace or in heating the feed water, which would increase the thermal efficiency.
4. Determine the height of chimney above grate level. Where diameter of chimney is 1.75m and produces a draught of 1.8cms of water. Temperature of flue gases is 290oC. The flue gases formed per kg of fuel burnt are 23kg. Neglect the losses and assume atmospheric temperature as 20oC?
a) H = 23.28m
b) H = 18.56m
c) H = 32.77m
d) H = 41.92m
Answer: c [Reason:] Given: D = 1.75m,
Draught in mm of water = 1.8cm = 18mm
Flue gas temperature = Tg = 290o
C = 563K
Ambient temperature = Ta = 20o
C = 293k
Flue gases formed per kg of fuel burnt: (ma
+ 1) = 23kg
5. A draught of 15mm of water is produced by using a chimney of height 30m. The ambient air and flue gases are at 27oC and 300oC respectively. The coal burned on the grate contains 81% carbon, 5% moisture and remaining ash. Neglect the losses and assume values of burnt products equivalent to the volume of air supplied and for the complete combustion of fuel, find the percentage of excess of air supplied?
Answer: c [Reason:] Given: Draught in mm of water= 15mm
Height of chimney =30m
Ambient temperature, Ta = 27 + 273 = 300K
Flue gas temperature, Tg = 300 + 273 = 573K
% of carbon = 81%
6. Determine the height of the chimney to produce a static draught of 22mm of water if the mean flue gas temperature in chimney is 290oC and ambient temperature in boiler house is 20oC. The gas constant for air is 29.26Kgm/Kgk and for chimney flue gas is 26.2 Kgfm/Kgk. Assume reading as 760mm of mercury?
Answer: b [Reason:] Density of air at 290K ρg = P/RT= [1.033 * 104 / 29.26 * 290]
= 1.217Kg/m3 Density of fuel gas at 563K, ρg = [1.033 * 104 / 26.2 * 563]
= 0.7 Kg/m3 Static Draught, p = H (ρa– ρg)
22 = H (1.217 – 0.7)
H = 42.55m
7. The flue gases of natural draught are at higher temperature when compared to flue gases in artificial gas?
Answer: a [Reason:] The flue gases of natural draught are at higher temperature when compared to flue gases of artificial gas because to maintain certain minimum temperature required to produce a given draught for the given height of the chimney. Due to higher flue gas temperature, the heat lost with flue gases is more in natural draught.
8. Determine the height of the chimney to get net draught of 12mm if the total losses are 4mm. the temperature of air is 25oC and the temperature of chimney gases is 300oC. The mass of air used per kg of fuel used is 18kg. One kg of air occupies a volume of 0.7734m3 at normal temperature?
Answer: c [Reason:] Density of air at normal temperature = 1/0.7734
= 1.293Kg/m3 Density of air at 298K, ρa = 1.293 ×273/298
= 1.1845 Kg/m3 Density of gases at 573K, ρg = 1.293 × ((18+1)/18) × 273/573
= 0.65 Kg/m3 But, P = 12 +4
= 16 mm of water
P = H (ρa- ρg)
16 = H (1.1845 – 0.65)
H = 29.93m
9. A 15kg of air is supplied per kg of fuel burnt to the combustion chamber of a boiler using fuel 600kg/hr. the temperature of flue gases and ambient air are 273oC and 32oC. If the minimum draught required to start the flue is 9.5mm of water, find out the minimum height of the chimney?
Answer: a [Reason:] Temperature of fuel gases Tg
= 237 + 273 = 510K
Temperature of ambient air Ta
= 32 + 273 = 305K
Drought in mm of water Hw
Mass of air mg = 15kg
10. Using which of the given formula the chimney height is calculated to get the answer of 67.4m when the coal burnt is 18.8TPH and considering 0.5% of sulfur content in coal?
a) H = 12(Q)13
b) H = 17(Q)13
c) H = 14(Q)13
d) H = 22(Q)13
Answer: c [Reason:] Given: Coal = 18.8TPH (ton per hour) = 18800kg/hr
Sulfur = 5%
1. Rainfall is also known as ___________
d) Down pour
Answer: a [Reason:] The rainfall is also known as precipitation, is a natural process of atmospheric vapor into water. Atmospheric vapor are nothing but clouds which are contamination of air and water. When the temperature goes below dew point then precipitation takes place.
2. In which of the following season is evaporation loss from free water is large?
Answer: d [Reason:] During summer, the evaporation loss from free water surface is considerably large and this evaporated water finds room in the air mass. The water holding capacity of air in the form of vapor is also considerably large in hot weather.
3. Rate of rainfall is expressed in _____________
Answer: a [Reason:] The rate of rainfall is expressed in centimeters of water during a given period of time. One centimeter rainfall is the quantity of water collected on a certain area due to rainfall which becomes one centimeter in height.
4. Average annual rainfall ranging to dessert to hilly regions would be ________________
Answer: a [Reason:] The rainfall varies widely from one part of the world to another, ranging from desert regions to the hilly regions where the average annual rainfall may be over 1100cm. in some regions, the seasonal variation is very slight and monthly rainfalls are relatively uniform.
5. The annual rainfall at any given station varies from _________
a) month to month
b) weekly basis
c) year to Year
Answer: c [Reason:] The annual rainfall at any given station varies irregularly from year to year. The range of this variation marks the reliability of the rainfall and has great importance in the design of storage reservoirs.
6. Intensity of rainfall is measured by___________
a) Continuously recording gauge
Answer: a [Reason:] The intensity of rainfall is expressed as an amount of precipitation in a stead period. During heavy rainstorm the intensity of rainfall varies widely from minute to minute and can be measured only with a continuously recording gauge.
7. The relation between the area of rainstorm and its average intensity is used in assessing _________
a) Amount of water
b) Amount of rain
c) Amount of wind
d) Amount of duration of rain
Answer: b [Reason:] The relation between the area of rainstorm and its average intensity is used in assessing the amount of rain which may be expected to fall upon a catchment area within a given period. The intensity of rainfall is equally important in the design of spillways during heavy rain periods.
8. The intensity of rainfall is given by ______
a) I = R/(T+C)
b) I = R/(T×C)
c) I = (T×C)/R
d) I = (T+R)/C
Answer: a [Reason:] The intensity of rainfall is given by,
I = R/(T+C)
Where, I = intensity in inches/hr.
T = Duration of rain storm in hours
R & C = these are constants quoted by different authorities for different areas of world.
The intensity also depends on the area selected for the measurement.
9. In what way are all the precipitations measured?
a) Vertical depth
b) Horizontal area
c) Width of area
d) At slope
Answer: a [Reason:] All forms of precipitation are measured on the basis of vertical depth of water which would accumulate on the level surface if all the precipitation remained where it fell. The rainfall is usually measured with rain gauges.
10. How many types of rain gauges are there?
Answer: a [Reason:] There are two types of rain gauges. They are Non-recording type and recording type. Non-recording type do not record the depth of rainfall but only collect the amount of rainfall and recording type records the depth of rainfall in mm or cm.
11. Movement and filtering of fluid from porous material is called as __________
Answer: a [Reason:] Percolation refers to movement and filtering of fluid from porous material. Usually the water which falls on the ground is filtered into the earth surface and is stored under the surface of earth. And this water also improves the ground fertility.
12. Where are funnel and receiver in Non-recording type placed?
a) Inside a Metal case
b) On top of the equipment
c) Below the base of equipment
d) In between the metal case
Answer: a [Reason:] The non-recording gauge usually consists of a standard funnel discharging into a receiver large enough to hold the maximum possible day’s rainfall. The funnel and receiver are placed in a metal casing with suitable packing.
13. The base of the non-recording type rainfall is permanently fixed in the concrete block.
Answer: a [Reason:] The base of the gauge is permanently fixed into a concrete block at a site where rainfall is to be measured. The precaution is taken during fixing, to level perfectly. The gauge is fixed in the block in such a way that the top of the gauge will be 30 cm above the natural surface level.
14. Which gauge gives the permanent record of rainfall?
a) Recording gauge
b) Non-recoding gauge
c) Copper daily gauge
d) Plastic gauge
Answer: a [Reason:] These are rain gauges which can give a permanent, automatic rainfall record in the form of a pen mounted on a clock driven chart. From the chart intensity or rate of rainfall in cm per hour. The recording is done by an attached siphon.
15. The drum situated in recording type makes one rotation for how many hours?
a) 4 hours
b) 24 hours
c) 6 hours
d) 12 hours
Answer: b [Reason:] The rotating drum is kept rotating continuously with the help of electric motor. The drum makes generally one rotation during 24 hours. On the rotating drum the graph paper is placed throughout its rotation the recording is made.