Energy Engineering Multiple Choice Questions on “Function of Boilers”.
1. What is the function of boiler?
A. To burn the fuel in a confined closed system with the supply of air
B. To generate steam at varying pressure
C. To generate steam at constant pressure
D. To produce flue gases by burning fuel at a given pressure
clarification: Function of boiler is to generate steam at constant pressure as per the process requirement. The steam generated may be nearly dry saturated or superheated steam. Constant pressure in the boiler is maintained by generating the steam equal to the steam flowing out of the boiler.
2. Why single boiler unit per turbine is equipped commonly?
A. For better turbine control
B. To deduce the costs
C. For overcoming losses of power
D. To improve the efficiency
clarification: In modern power plants it is very common to use a single boiler unit per turbine. This leads to simpler piping systems and relatively easier boiler and better boiler control. And also operating is easier when the connection is made in this type.
3. What is the critical average pressure at which the single boiler unit per turbines is designed to handle?
A. 150 bar
B. 220 bar
C. 740 bar
D. 575 bar
clarification: Single boiler per unit turbines is designed to operate at average critical temperature of 220bar. If the boilers are designed to operate above or below the critical pressure, than they are known as super critical boilers are once through boilers. If the boilers are designed to operate below the critical pressure than they are known as sub critical or drum boilers.
4. How can be the plant efficiency of steam power plant is increased?
A. By using high pressure and high temperature steam
B. By equipping ash and flue gas clearance system
C. By maintaining the boiler on daily basis by cleaning the soot and combustion products
D. By using better and efficient type of fuel for combustion
clarification: The plant efficiency can be increased by using high pressure and high temperature steam. This also reduces the cost of electricity production. The development of newer materials permitted the use of higher operating pressure and temperature of the boiler.
5. Which type of boilers is preferred cost wise when the boilers are required to raise less than 30 tonnes of steam?
A. Flued boiler
B. Cornish boiler
C. Butterley boiler
D. Shell boiler
clarification: The use of shell boilers are considered to be cheaper when the boilers are required to raise less than 30 tonnes of steam per hour and at pressures 30bar. Above these limits, i.e. for high pressure and high output, water tube boilers are preferred.
6. What is the temperature at which the steam boilers are capable to withstand?
clarification: The modern high pressure boilers have steam capacities of 30 to 650 tonnes/hour and with pressure of upto 160 bar & maximum steam temperature of about 540oC. Also the furnace shape is not limited to cylindrical form; hence furnace shape can be changed to suit wide range of situations.
7. The _________ may have water circulation either by natural means due to difference in density or by external means.
clarification: The boiler may have a water circulation either by natural means due to difference in density or by external means (forced circulation). All modern high pressure boiler plants use a pump to force the water to circulate through the boiler plant. The use of natural circulation is limited to the subcritical boilers.
8. What increases as steam pressure increases inside a boiler?
C. Rate of steam conversion
clarification: As the steam pressure increases, its density also increases while the corresponding increase in temperature causes density of water to decrease. This makes the pressure difference causing water flow to decrease.
9. What is needed to be increased to increase the heat transfer rate in the boiler?
A. Gas velocities
B. Fuel input
C. Water velocities
D. Air supply
clarification: Water velocity is required to be increased rather than gas velocities to increase the rate of heat transfer rate in the boiler, because a considerable increase in pressure is more easily produced with water than with gas. Hence, for a given output in boiler, smaller diameter tube may be used.
10. In what is water in high pressure boiler circulated through?
clarification: In high pressure boilers, water is circulated through the tubes. If the flow of water takes place through one continuous tube, there is a large resistance due to friction and also pressure drop will be more. If the flow is arranged through a parallel system of tubing i.e., through several sets of tubes which are arranged parallel pressure loss will be reduced. This also provides better control over the steam quality.