This set of Thermal Engineering online quiz focuses on “Steam Generators – High Pressure Boilers”.
1. Shell boilers are preferred for high pressure and high output.
Explanation: Water tube boilers are preferred for high pressure and high output while shell boilers are preferred for low pressure and low output. Water tube boilers can be erected easily and its parts are easily transportable.
2. Which of the following statements is FALSE about high pressure boilers?
a) Modern high pressure boilers used for power generation have steam capacities even greater than 650 tonnes/hr
b) Water is circulated through a pump in most of the modern high pressure boilers
c) They are mostly water tube boilers
d) One continuous tube is used for water circulation
Explanation: The pressure drop that takes place due to friction in a single large tube is greater than multiple small tubes. Hence, multiple tubes of small diameter are preferred over one continuous large tube. The Steam capacities for modern high pressure boilers vary from 30 to 650 tonnes/hr and even more with a pressure up to 160 bars.
3. Which of the following is NOT a unique feature of high pressure boiler?
a) Water circulation method
b) Tubing type
c) Firing method
d) Improved method of heating
Explanation: Unique features of high pressure boilers are – method of water circulation, type of tubing, improved method of heating. Pumps are used in most of the high pressure boilers for water circulation, i.e. forced circulation method. Multiple tubes instead of one continuous tube is used since the friction loss in the former one is much smaller than in the latter.
4. Which of the following heating methods is NOT used in high pressure boilers?
a) Evaporation of water above critical pressure of steam
b) Decreasing gas velocity (improves heat transfer coefficient)
c) Increasing water velocity inside tube (improves heat transfer coefficient)
d) Mixing of water with superheated steam
Explanation: The overall heat transfer coefficient is increased when the flow velocity of water inside tube is increased and the gas velocity is increased above sonic velocity. Heat is saved by evirating water above critical pressure of the steam. Higher heat transfer coefficient is obtained by heating water by mixing it with superheated steam.
5. Which of the following is FALSE about high pressure boilers?
a) They are compact
b) Heat of combustion utilized more effectively
c) Heat is saved by evaporating water below critical pressure
d) Pumps are used to circulate water
Explanation: High pressure boilers are compact; hence less floor space is required. Since tubes of small diameter are used heat of combustion is used more effectively. Some amount of heat is saved by evaporation of water above critical pressure. High pressure boilers are forced circulation boilers i.e. pumps are used to circulate water.
6. In LaMont boilers, the circulation of water is maintained by a centrifugal pump which is driven by a steam turbine in which steam is fed from the boiler itself.
Explanation: LaMont boiler is a forced circulation and high pressure boiler. It uses a pump which driven by a steam turbine which uses the steam from the boiler. An electrically driven pump is also installed for emergency.
7. The main drawback of LaMont boilers is _____
a) limited capacity of evaporating drum
b) bubble formation on the inner surface of heating tubes
c) hindrance in the flow of hot gases
d) radiant superheater decreases the efficiency
Explanation: Formation of bubbles on the inner side of heating tube takes place. These bubbles reduce the heat flow due to its high thermal resistance. This reduces the steam generation. This is drawback is rectified in Benson boiler.
8. Which of the following statements is FALSE about Benson boiler?
a) Bubble formation in the heating tubes is prevented by slow heating
b) It is a forced circulation boiler
c) Oil is used as fuel
d) Feed water is compressed to 235 bar
Explanation: Benson boiler is a forced circulation boiler. To avoid bubble formation in the heating tubes the water is compressed to 235 bar. 235 bar is the critical pressure for water. At this temperature water and steam have same density thereby eliminating the chance of bubble formation.
9. What is the maximum working pressure obtained by a commercial Benson boiler?
a) 300 atm
b) 400 atm
c) 500 atm
d) 600 atm
Explanation: 500 atm is the maximum working pressure obtained from a commercial Benson boiler. Benson boiler having steam generation of tonnes/h are also used. It can also generate steam having a temperature of 650°C.
10. Which of the following statements about Benson boiler is correct?
a) It requires large surface area
b) It is prone to explosions more than other boilers
c) It takes a lot of time to start
d) It is lighter than other boilers
Explanation: Benson boiler can be erected on comparatively small floor area. Since small diameter tubes are used explosion hazards are not severe. It takes lesser time to start. It also has a very small storage capacity which makes it even more safe. Since there are no drums in this boiler, it has 20% less weight than other boilers.
11. Which of the following statements about super-critical is TRUE?
a) It only needs preheater and superheater
b) It only needs economizer and preheater
c) It only needs economizer and superheater
d) It only needs preheater
Explanation: A super-critical boiler does not require an economizer. It only requires a preheater and a superheater. Super critical boilers have large heat transfer rate and have higher thermal efficiency.
12. Which of the following statements about Velox boiler is FALSE?
a) It starts quickly
b) It is compact
c) Combustion rates are low
d) Pressurized air is used
Explanation: High combustion rates are possible in Velox boiler. It is compact. Problem of draught is solved by using pressurized air, low excess air is required. In addition to easy operation is also starts quickly.
Sanfoundry Global Education & Learning Series – Thermal Engineering
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