Turbine Auxilaries Q & A

1. Where is an evaporative condenser used in practice?
Answer: In those cases where the shortage of cooling water is acute.
2. What should be the basic criteria for an efficient steam condenser?
Answers:
1. Maximum amount of steam condensed per unit area of available heat transfer surface.
2. Minimum quantity of circulating coolant required.
3. Minimum heat transfer surface required per kW capacity
4. Minimum power drawn by the auxiliaries.
3. Why must a vacuum be maintained in the steam condenser?
Answers:
1. By maintaining a vacuum in the steam condenser, the efficiency of the steam-power plant can be increased as greater the vacuum in the system, greater will be the enthalpy drop of steam. Therefore, more work will be available per kg of steam condensing.
2. Secondly, the non-condensate (air) can be removed from the condensate-steam circuit by pulling and maintaining a vacuum in the steam side. Therefore, the condensate can be used as boiler feed.
4. What are the limitations of a surface condenser?
Answers:
1. It is very bulky and as such requires more floor space.
2. Its manufacturing, running and maintenance costs are high.
5. What should be the requirements of an ideal surface condenser used for steam power plants?
Answers:
1. Uniform distribution of exhaust steam throughout the heat transfer surface of the condenser.
2. Absence of condensate sub cooling.
3. There should not be any leakage of air into the condenser.
4. There should not be any tube leakage.
5. The heat transfer surface in contact with cooling water must be free from any deposit as scaling reduces the efficiency of heat exchangers.
6. What do you mean by vacuum?
Answer: Vacuum means any pressure below atmospheric pressure.
7. How is vacuum in a condenser usually measured?
Answer: It is measured by means of a Bourdon pressure gauge, which is calibrated to read the pressure in mm of mercury below atmospheric pressure.
8. On what factors does the degree of vacuum in a condenser depend?
Answers: It depends on the partial pressure of steam and the partial pressure of air in the condenser.
9. What is the vacuum efficiency of a condenser?
Answers: It is the ratio of the actual vacuum at the steam inlet to the maximum obtainable vacuum in a perfect condensing plant, i.e., it is the ratio of actual vacuum to ideal vacuum.
10. What are the effects of air leakage in the condenser?
Answers:
1. It increases the back pressure on the turbine with the effect that there is less heat drop and low thermal efficiency of the plant
2. The pressure of air in the condenser lowers the partial pressure of steam, which means steam, will condense at a lower temperature and that will require greater amount of cooling water.
3. It reduces the rate of condensation of steam, because air having poor thermal conductivity impairs the overall heat transfer from the steam-air mixture.
11. What is a steam condenser?
Answers:
1. It is a heat exchanger wherein steam is condensed either in direct contact with cooling water or indirect contact with cooling water through a heat transfer medium separating them.
2. That is, a steam condenser is either a direct contact or indirect contact heat exchanger.
12. How many types of steam condensers are known?
Answers:
1. Jet Condensers
- direct contact heat exchanger.
2. Surface Condensers - indirect contact heat exchanger.
13. What is a surface condenser?
Answer: It is a shell-and-tube heat exchanger in which steam is condensed on the shell-side while cooling water flows through the tubes. The condensate and cooling water leave the system separately.
14. How does the down-flow type surface condenser act?
Answer: Exhaust steam is admitted to the top of the condenser, which is a tube-and-shell type crossflow heat exchanger. Cooling water flows through the tubes and extracts heat from the steam, which is on the shell-side. Mter having been condensed on the surface of the water tubes, steam is converted into condensate which is discharged from the condenser bottom.
15. How does the central flow type surface condenser work?
Answer: It is also a shell-and-tube type crossflow heat exchanger at the center of which is located the suction of an air extraction pump, so that the entire steam moves radially inward and comes in better contact with the outer surface of the nest of tubes through which the cooling water flows. The steam condensate is extracted from the bottom by the condensate-extraction pump.
16. How does the inverted type surface condenser work?
Answer: In this type of condenser, steam is admitted at the bottom and flows upwards in cross-flow with the cooling water flowing in the tubes. The air extraction pump draws its suction from the top of the condenser, maintaining a steady upward current of steam, which after having been condensed on the outer surface of water tubes is removed by the condensate extraction pump.
17. How does the evaporative condenser function?
Answer: Exhaust steam from the turbine is condensed inside the finned tubes as cooling water rains down from the top through the nozzles. A part of the cooling water in contact with the tube surface evaporates by drawing enthalpy from the steam, which upon losing its latent heat condenses and discharges out as condensate.
18. What are the primary functions of a condenser?
Answers: There are two important functions of a condenser:
1. It reduces the back pressure upon the turbine by a considerable degree and therefore, the work done per lb of steam during expansion is increased
2. The exhaust steam condensate can be recycled as boiler feedwater
19. Why else is steam from an HRSG used?
Answer: For steam injection into the gas turbine for NO_x control.
20. What gas is used in the SCR method of controlling NO_x?
Answer: Air-diluted ammonia vapor is injected into the flue gas stream before it enters the catalyst units consisting of honeycomb-shaped ceramic material. These cells, with the ammonia vapor, convert nitrogen oxides to nitrogen and water vapor for discharge into the atmosphere.