Turbine Auxilaries Q & A-ii

20. 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.
21. How many types of steam condenser are known?
Answers:
1. Jet Condensers
- direct contact heat exchanger.
2. Surface Condensers - indirect contact heat exchanger using water.
3. Air Condensers - direct contact heat exchangers using air.
22. What is a jet condenser?
Answers:
1. It is a direct contact heat exchanger in which steam to be condensed comes into direct contact with the cooling water (cold condensate) which is usually introduced in the form of a spray from a jet. (Fig. 30.1)
2. Upon contact with the cooling water, the steam gives up its enthalpy and gets cooled and ultimately settles as condensate.
23. 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.
24. How many types of jet condensers are known?
Answers:
1. Parallel flow jet type condenser
- It is a kind of jet condenser in which both exhaust steam and cooling water enter the condenser at the top, both flow downward and the steam condensate discharges out from the bottom of the condenser. (Fig. 30.2)
2. Contra flow type jet condenser - The cooling fluid (cold condensate) and exhaust steam flow in a counter-current direction - steam goes up and cold condensate rains down.
3. Ejector type jet condenser - It is one kind of jet condenser in which the mixing of cooling water and steam takes place in a series of combining cones and the kinetic energy of the steam is expended to drain off the condensate and cooling water from the condenser. Cooling water is forced through a series of cones and gets mixed with steam coming through ports. As the cooling water flows through the series of nozzles, it suffers more and more pressure drop and at the same time its velocity gradually increases. Due to this pressure drop, more and more steam is drawn through the ports, gets intimately mixed with the cooling water jet and condenses thereafter.
25. What is the principle of operation of a high-level-parallel-flow jet condenser?
Answer: This condenser, also called barometric condenser, works as follows - The condenser is mounted on a long pipe (at least 10.34 m) called barometric leg which acts in a way identical to a barometer. Now if water is used in a barometer then the barometric height would be 10.34 m. If some vacuum exists in the condenser, the height of water column (h) will be less than 10.34 in. Now it is possible, by using this condenser leg, to drain away the condensate from the condenser.
26. How many types of surface condensers are known?
Answers:
1. Down flow type
- Exhaust steam is admitted to the top of the condenser, which is a tube-and-shell type cross flow heat exchanger. Cooling water flows through the tubes and extracts heat from the steam which is on the shell-side. After having been condensed on the surface of the water tubes, steam is converted into condensate, which is discharged from the condenser bottom. (Fig. 30.7)
2. Central flow type - It is also a shell-and-tube type cross flow 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.
3. Inverted flow type - 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.
4. Evaporative condenser type - 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.
27. Where is the evaporative condenser used in practice?
Answers: In those cases where the shortage of cooling water is acute.
28. What are the two prime functions of a condenser?
Answers:
1. It reduces the backpressure upon the turbine by a considerable degree and therefore, the work done per kg of steam during expansion is increased.
2. The exhaust steam condensate can be recycled as boiler feedwater.
29. What are the auxiliary equipment required for operating a steam condenser?
Answers:
1. Cooling water (which may be cold condensate) circulation pump. Generally, it is a centrifugal one.
2. Arrangement for cooling the condensate (i.e., a heat exchanger) in case the condensate is recycled to extract heat from the exhaust steam.
3. An air pump or steam ejector to remove air and other non-condensing gases from the condenser.
4. An extraction pump (usually centrifugal) to remove the condensate from the condenser.
30. What should be the basic criteria for an efficient steam condenser?
Answer:
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.
31. Why is vacuum 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.
32. What are the advantages of a jet condenser over a surface condenser?
Answers:
1. Simplicity in design.
2. Lower in manufacturing cost.
3. Lower maintenance cost.
4. Occupies lesser floor space.
5. Requires lesser amount of cooling water.
33. What are the advantages of a surface condenser over a jet condenser?
Answers:
1. It imparts to power generation plant a higher thermal efficiency.
2. The condensate can be reused as boiler feedwater.
3. Auxiliary power requirement is less than that of a jet condenser.
4. Less amount of air is carried to the boiler.
34. 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.
35. 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 subcooling.
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.
36. What do you mean by vacuum?
Answer: Vacuum means any pressure below atmospheric pressure.
37. How is vacuum in a condenser usually measured?
Answers: It is measured by means of a Bourdon pressure gauge, which is calibrated to read the pressure in mm of mercury below atmospheric pressure.
38. If the gauge pressure of a condenser is 630 mm of Hg, what will be the absolute pressure in the condenser?
Answer: It means the pressure in the condenser is 630 mm below atmospheric pressure. The atmospheric pressure is 760 mm of Hg, the absolute pressure in the condenser.
39. On what factors does the degree of vacuum in a condenser depend?
Answer: It depends on the partial pressure of steam and the partial pressure of air in the condenser.
40. How could air enter the condenser?
Answers:
1. With the boiler feedwater as dissolved gases.
2. Flange leakage.
3. Cooling water (for jet condenser) containing a certain amount of dissolved air in it.