This set of Energy Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Draught System – 1”.
1. Why boiler draught is most essential part of the thermal plants?
a) Serves combustion by supplying ample amount of air
b) Helps in maintaining air ventilation
c) Required to maintain constant air flow
d) Stabilizes the boiler system
Explanation: Boiler draught is the difference of pressure required to maintain constant flow of air into the furnace and to discharge the flue gases to the atmosphere through a chimney. Thus boiler draught is one of the most essential systems of thermal plant. Boiler draught is generated by using chimney, fan, steam jet or air jet or combination of them.
2. Which is the correct formula of total draught loss?
a) Ht = Hv + Hb + Hc + Hcd
b) Ht = Hv + Hb – Hc + Hcd
c) Ht = Hv – Hb + Hc + Hcd
d) Ht = Hv – Hb – Hc – Hcd
Explanation: The total draught required to produce the current of the air and to expel the flue gases is given as follows.
Total draught loss = Velocity head loss at the chimney exit + fuel bed resistance + head loss in
Equipment’s like economizer, air heater etc. + head loss in chimney and ducts
i.e., Ht = Hv + Hb + Hc + Hcd
3. By what is natural draught produced?
a) Air duct
c) Locomotive exhaust
d) Air blower
Explanation: Natural draught is produced by using a chimney. A chimney is a vertical tubular structure made up of brick, steel or reinforced concrete. The draught produced by the chimney is due to the difference in temperature of hot flue gases inside the chimney and the atmospheric air.
4. Which of the following given below as the affect on amount natural draught?
a) Boiler operation
b) Size of the furnace
c) Grade of the fuel
d) Dust content in the fuel is used
Explanation: The draught produced in the chimney is due to the temperature difference of the hot flue gases. In addition to this, height of the above the furnaces grate, whether conditions and boiler operations also have considerable effect on the amount of natural draught.
5. When is artificial draught preferred?
a) When quick evacuation of flue gases is required
b) When we choose large power plant systems
c) When draught produced by chimney is not sufficient
d) When there are many components inside the boiler
Explanation: This type of draught is preferred when draught produced by the chimney is not sufficient or when a certain draught is to be maintained irrespective of atmospheric temperature. Artificial draught as no flexibility, as poor efficiency and requires tall chimney.
6. Artificial draught is also called as _______
a) Force draught
b) Induced draught
c) Balanced draught
d) Mechanical draught
Explanation: In modern high pressure boilers, use of economizers, super heaters, air pre heaters reduces the exit flue gas temperature considerably. This results in increased chimney height, whose cost is not justified. By using mechanical draught chimney height is considerably reduced.
7. What is installed to transfer air into furnace in forced draught system?
a) Air blower
c) Draft gauges
d) Cyclone separator
Explanation: In force draught system an air blower is installed near the boiler which blows into the furnace. The pressure of air throughout its path is above atmospheric pressure. This system also uses chimney to discharge hot flue gases into the atmosphere.
8. Where is blower installed in induced draught system?
a) Near the boiler
b) Base of the chimney
c) Above the boiler
d) Between economizer and air preheater
Explanation: In induced draught system the blower is installed near the base of the chimney so as to facilitate sucking of flue gas from the furnace. Thus pressure inside the furnace is reduced below atmospheric pressure inducing outside air into the furnace.
9. How can exhaust gas be recovered in the induced draught?
a) By installing the forced draught fan at the starting point
b) By extending the chimney’s height
c) By inletting high amount of air from multiple air ducts
d) By installing air preheater and economizer along the gas path
Explanation: The heat in exhaust gases can be recovered as much as possible by installing an air preheater and economizer along the gas flow path. As the draught produced in this system is independent of flue gas temperature care should be taken such that the fan handles gas at its lowest possible temperature.
10. Which is most preferred type of draught system?
a) Chimney draught
b) Fan draught
c) Force draught
d) Balanced draught
Explanation: A balanced draught system combines the features of forced draught and induced draught. In case of forced draught, when the furnace doors are opened high pressure air rushes outside and even blow out the fire entirely. In case of induced draught, when the furnace doors are opened to atmosphere air enters the furnace and causes imbalance in the draught. This difficulty is overcome by using balanced draught.
11. How pressure in the furnace is maintained below atmospheric pressure in balanced draught system?
a) By forced draught fan
b) By induced fan
c) Depends on the height of chimney
d) By installing Natural draught
Explanation: In balanced draught system, a forced draught fan is installed near the boiler which helps in overcoming the resistance of fuel bed by supplying sufficient air for combustion. An induced draught fan is installed near the chimney base removes the flue gases coming from the furnace and maintains pressure in the furnace slightly below the atmospheric pressure.
12. Which type of draught system is used in the locomotives?
a) Natural draught system
b) Balanced draught system
c) Forced draught system
d) Induced Steam draught system
Explanation: Induced steam draught system is used in a boiler for a locomotive. When the locomotive is stationary steam generated in the boiler is fed to the smoke box through the nozzle to create the draught. When locomotive is moving air enters through the dampers and makes its way through grate and smoke box.
13. Which draught system is also known as turbine draught?
a) Forced steam draught
b) Induced steam draught
c) Natural draught
d) Fan draught
Explanation: Forced steam draught system is also is known as turbine draught. In this case, steam from the boiler is throttled to a pressure of 1.5 to 2 bar and passed through a series of nozzles fitted to a diffuser pipe.
14. Which draught system is used as conjunction with chimney draught?
a) Forced steam draught
b) Forced draught
c) Induced draught
d) Balanced draught
Explanation: The high velocity steam coming out from the nozzle drags the air column along with it creating suction. The kinetic energy of air and steam mixture is converted into pressure energy forcing air through the coal bed to the furnace. So, Forced steam draught is used as a booster in conjunction with chimney draught.
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