This set of Thermal Engineering Questions and Answers for Entrance exams focuses on “Steam Condensers – Sources and Effects of Air Leakage”.
1. Which of the following statements is not a correct reason for inefficiency in surface condensers?
a) Air leakage
b) High resistance faced by the steam while entering
c) Condensate undercooling
d) Circulating water passing through the condenser almost smoothly
Explanation: Circulating water passes through the condenser with high friction and at a velocity not consistent with high efficiency, this results in increased inefficiency in surface condensers. Since the pressure inside the condenser is less than atmospheric, air leaks into the condenser. This limits the amount of work done by unit mass of steam, thus, results increases inefficiency.
2. Surface condensers require more power for water pumping than jet condensers.
Explanation: For water pumping, surface condensers require more power than jet condensers. Also the floor area required by surface condensers is more than that of jet condensers. Air pumping power required by jet condensers is more than that of surface condensers.
3. Which of the following statements is FALSE regarding air leakage in condensers?
a) The sources of air leakage can be found by putting soap water at infiltration prone points
b) The sources of air leakage can be found out by putting pepprament oil on suspected joints
c) The source of air leakage, if large, can be found out by passing candle flame over possible openings
d) Thermometer and vacuum gauge readings become steady when leakage is present in a condenser
Explanation: In case of presence of air leakage, when the plant is running and temperature and pressure conditions reach a steady state, the thermometer and vacuum gauge readings record a fall. To locate the sources of air leakage, soap water, pepprament oil or a candle flame can be used.
4. Which of the following methods is the correct one for using pepprament oil to detect the source of air leakage?
a) Put the pepprament oil on the suspected joint and wait for the bubbles
b) Put the pepprament oil on the suspected joint and wait for it to get sucked
c) Put the pepprament oil on the suspected joint and check for pepprament odour at the discharge of air ejector
d) Put the pepprament oil on the suspected joint, pepprament oil being coloured will change the colour of condensed water
Explanation: Pepprament oil has a peculiar odour, this property is used to identify the source of air leakage in the condensers. It is put on the suspected joint and if the joint leaks, the oil will be sucked in the condenser and pepprament odour will be observed at the discharge of air ejector.
5. Which of the following statement about condensers is FALSE?
a) Jet condensers are more suitable for high capacity plants owing to their high vacuum efficiency
b) Surface condensers require more cooling water than jet condensers
c) As compared to surface condensers, jet condensers require smaller floor space
d) The manufacturing cost of surface condensers is greater than that of jet condensers
Explanation: vacuum efficiency of surface condensers is higher than that of jet condensers, that’s why surface condensers are more suitable for high capacity plants. In comparison with jet condensers, surface condensers require more cooling water, larger floor space.
6. What is the effect of air leakage in condensers on thermal efficiency of the steam power plant?
a) Thermal efficiency gets lowered
b) Thermal efficiency increases
c) Thermal efficiency remains unchanged
d) It increases or decreases depending upon the degree of leak
Explanation: Air leakage reduces thermal efficiency of a condenser. The leaked air increases back-pressure on the prime mover which results in loss of heat drop and a result the thermal efficiency of the steam power plant is lowered.
7. How does air leakage in a condenser affect the requirement of cooling water?
a) Air leakage reduces the amount of cooling water required
b) Air leakage increases the amount of cooling water required
c) Air leakage doesn’t affect the amount of cooling water required
d) Amount of cooling water required increases or decreases depending upon the rate of air leakage
Explanation: The partial pressure of steam is lowered by the air leakage. It reduces the saturation temperature of steam, therefore, its latent heat increases. Hence, increased amount of cooling water is required.
8. How does air leakage in condensers affect the heat transfer rate?
a) Heat transfer is reduced
b) Heat transfer is increased
c) Heat transfer rate is not affected by air leakage
d) Heat transfer rate increases or decreases depending on the rate of air leakage
Explanation: Air being poor conductor of heat reduces the heat transfer rate in the condenser. The heat transfer from the vapour is reduced. To tackle this problem, the surface area of tubes of a surface condenser is increased.
9. What effect does air leakage in a condenser has on corrosion?
a) Air leakage reduces corrosion
b) Air leakage increases corrosion
c) Air leakage does not affect corrosion
d) Corrosive action increases or decreases depending upon the degree of leak
Explanation: Air leakage introduces air in the condenser. The inside of a condenser is already humid due to presence of water. Hence, the presence of air increases the corrosive action. Corrosion on the tube surface reduces the heat transfer rate.
10. Which of the following is not an effect of air leakage in condensers?
a) Reduced thermal efficiency of steam power plant
b) Increased requirement of cooling water
c) Increase in corrosive action in condenser
d) Increased heat transfer in condenser
Explanation: Air leakage in the condenser reduces the thermal efficiency of the steam power plant. It increases the amount of cooling water required and also increases the corrosive action in the condenser. The heat transfer rate in the condenser is reduced.
Sanfoundry Global Education & Learning Series – Thermal Engineering
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