Appendix G: Quiz on Boilers and HRSGs with Answers
(The answers to all of these questions can be found in the book. However, e-mail me for clarifications if required. My e-mail is v_ganapathy@yahoo.com.)
1.If boiler efficiency for a typical natural gas fired boiler is 83% on higher heating value basis, what is it approximately on lower heating value basis?
a. 73% b. 83% c. 92%
2.If NOx in a natural gas–fired boiler is 50 ppmv (3% oxygen dry), what is it on lb/MM Btu (HHV) basis?
a. 0.06 b. 0.10 c. 0.20
3.In a 100,000 lb/h packaged boiler, 1 in. WC of additional gas pressure drop is worth about how many kilowatts of fan power consumption?
a. 5 b. 20 c. 50
4.If boiler water concentration in a boiler drum is 1000 ppm and steam purity is 1 ppm, what is the percent steam quality?
a. 99.9 b. 99 c. 99.99
5.Boilers of the same capacity are located at different sites, whose ambient conditions and elevation are as follows. Which case requires the biggest fan?
a. 80°F and sea level b. 100°F and 3000 ft c. 10°F and 7000 ft
6.In a boiler plant, if the conductivity of the condensate, makeup, and feed water is 800, 40, and 150 mmho/cm, respectively, what is the percent condensate returns in the feed water?
a. 5 b. 50 c. 15
7.A 20° F change in exit gas temperature of an oil-fired boiler changes boiler efficiency by approximately what percent?
a. 1 b. 0.5 c. 2.0
8.Approximate airflow (acfm) required in a packaged boiler firing 100 MM Btu/h (HHV) of natural gas is
a. 19,000 b. 30,000 c. 12,000
9.The steam pressure drop in a boiler superheater is 50 psi when generating 600 psig, 650°F steam. What is it likely to be at 400 psig, 600°F with the same flow?
a. 70 b. 30 c. 50
10.Which is the worst-case scenario for an economizer from the viewpoint of sulfuric acid condensation? Assume that the oil-fired boiler flue gas contains 12% water vapor and 0.03% SO2.
a.Flue gas at 680°F and feed water at 200°F
b.Flue gas at 320°F and feed water at 275°F
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Appendix G: Quiz on Boilers and HRSGs with Answers |
11. If vol% of oxygen (dry) in a natural gas–fired boiler is 2.0%, what is the excess air used? a. 15 b. 5 c. 10
12.If boiler casing heat loss is 0.2% at 100% load, what is it at 25% load, assuming that wind velocity and ambient temperature are unchanged?
a. 1.0 b. 2.0 c. 0.8
13.Plant management decides to change the tube inner diameter of an existing superheater from 1.7 in. to 1.5 in. The steam-side pressure drop for the same steam conditions will go up by what percent?
a. 87 b. 65 c. 29
14.The heat transfer coefficient in a finned tube bundle is higher than in a bare tube exchanger for the same gas velocity, temperature, tube size, and geometry.
a.True b. False
15.In a fire tube waste heat boiler, a small diameter tube has a higher tube-side heat transfer coefficient and higher heat flux than a larger tube for the same gas velocity.
a.True b. False
16.Superheated steam temperature from a boiler firing oil will be higher than when firing natural gas at the same steam generation rate (assuming steam temperature is uncontrolled).
a.True b. False
17.More flue gas is generated in a boiler while firing oil than while firing natural gas at the same excess air and steam generation.
a.True b. False
18.The maximum possible fuel (natural gas/distillate oil) that can be fired in an HRSG with exhaust gas flow = 200,000 lb/h and 14% oxygen wet in MM Btu/h (LHV) is
a. 100 b. 150 c. 50
19.Surface area gives a good indication of whether a boiler or HRSG design is adequate or not.
a. True b. False
20.An uncooled soot blower lance is located in a boiler convection bank at 1700°F gas temperature. Its temperature will be
a. >1700°F b. 1700°F c. <1700°F
21.(a) A fire tube waste heat boiler using small-diameter tubes will be longer than the design using larger-diameter tubes for the same duty and gas pressure drop.
a. True b. False
(b) A fire tube waste heat boiler using small-diameter tubes requires less surface area than the design using larger-diameter tubes for the same duty and gas pressure drop.
a.True b. False
22.For the same mass flow and gas temperature drop, a flue gas containing 16% water vapor will transfer more energy than a gas stream having 5% water vapor.
a.True b. False
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23.Design of tubular air heaters in steam generators can be improved if finned tubes are used instead of plain tubes.
a. True b. False c. Depends on the fuel used
24.In a cross-flow heat transfer situation, an inline arrangement of plain tubes is better than a staggered one.
a.True b. False
25.For the same casing insulation thickness and ambient conditions, aluminum casing will run hotter than carbon steel.
a.True b. False
26.If additional steam is required in a cogeneration plant, supplementary firing the HRSG rather than using a packaged boiler will be more prudent.
a.True b. False
27.Required thickness of a boiler tube subjected to external pressure will be less than when the same tube is subjected to the same internal pressure at the same temperature.
a.True b. False
28.Which is a better choice for fin density for a superheater in an HRSG?
a.5 fins/in. b. 2 fins/in.
29.The exit gas temperature in a single-pressure unfired HRSG generating steam at 600 psig, 700°F can be less than 300°F. (Assume exhaust gas at 950°F and feed water at 230°F.)
a.True b. False
30.A boiler designed for 1000 psig, 800°F steam can be operated at the same steam flow at 300 psig without modifications.
a.True b. False
31.A gas turbine HRSG economizer is likely to steam at which ambient temperature in unfired mode?
a.40°F b. 90°F
32.More energy can be transferred to a boiler evaporator if the circulation ratio is higher.
a.True b. False
33.Heat flux will be higher in a packaged boiler furnace for which fuel? Assume same steam generation.
a. Fuel oil b. Natural gas
34.For the same excess air and exit gas temperature, an oil-fired boiler will have a higher efficiency on HHV basis than a gas-fired boiler.
a.True b. False
35.For the same mass flow per tube and length of tube, superheated steam at 600 psig, 800°F will have a higher pressure drop than 150 psig saturated steam.
a.True b. False
36.Gas-side fouling increases the tube wall temperature in a waste heat boiler.
a. True b. False c. Depends on whether it is a fire tube or water tube boiler
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37.The feed pump requires more power to generate a given amount of steam at a given pressure and temperature in a once-through HRSG than in a natural circulation HRSG.
a.True b. False
38.The volumetric heat release rate is more important in a gas-fired packaged boiler than the area heat release rate.
a.True b. False
39.Large margins on flow and head should not generally be used while selecting the fan for a packaged boiler.
a.True b. False
40.If an economizer with counter-flow arrangement is experiencing low-temperature corrosion problems, then re-piping it with a parallel-flow arrangement can fix the problem.
a.True b. False
41.Exit gas temperature from a single-pressure HRSG having a superheater, evaporator, and economizer increases as steam generation increases.
a.True b. False
42.It is better to preheat condensate or feed water using extraction steam from the steam turbine rather than use the energy in the HRSG exhaust gases.
a.True b. False
43.Steam for deaeration should preferably be taken from the boiler outlet rather than from an extraction point in the steam turbine.
a.True b. False
44.The maldistribution of steam flow through superheater tubes will be the worst at
aboiler load of
a. 20% b. 50% c. 100%
45. Which fuel generates the maximum amount of carbon dioxide per MM Btu fired? a. Oil b. Natural gas c. Coal
46.Is it possible to predict the off-design performance of an HRSG without knowing its mechanical constructional features?
a.Yes b. No
47.Can we have more surface area in an HRSG and yet transfer less duty?
a.Yes b. No
48.Can we use finned tubes for the evaporator or superheater of a gas-fired packaged boiler?
a.Yes b. No
49.What happens to the pinch and approach points of the evaporator in an HRSG as we increase the supplementary firing rate?
a. Both increase b. Both decrease c. Pinch point increases while approach point decreases d. They are unchanged
50.In a packaged boiler, the furnace performance and circulation are more critical in oil firing than in gas firing.
a. True b. False
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51.Can a superheater be located between the evaporator and the economizer in a packaged boiler?
a.Yes b. No
52.Good steam-separating devices cannot prevent carryover of silica from boiler water into steam at high pressures.
a.True b. False
53.Superheated steam for use in turbines should have better steam purity than saturated steam.
a.True b. False
54.Feed water used for attemperation in a desuperheater for steam temperature control should preferably have low to zero solids.
a.True b. False
55.Tube-side heat flux will be higher in a plain tube evaporator than in a finned tube evaporator for the same gasand steam-side conditions.
a.True b. False
56.In a waste heat boiler containing hydrogen chloride gas, a low steam temperature (say, 700°F versus 850°F) is preferred.
a.True b. False
57.A higher steam pressure requires a higher steam temperature to minimize wetness in steam after expansion in a steam turbine.
a.True b. False
58.An ammonia–water mixture has a varying boiling point and hence is a better fluid for energy recovery from waste flue gases than steam.
a.True b. False
59.The cross section of a 100,000 lb/h packaged boiler will be much smaller than that of an unfired gas turbine HRSG generating the same amount of steam.
a.True b. False
60.Gas conditions being the same, as steam pressure increases, the steam generation in an unfired HRSG
a. Increases b. Decreases c. Is unchanged
61.The cross section of a forced circulation HRSG and its surface area will be much different from a natural circulation HRSG for the same duty and pressure drop.
a. True b. False c. Can’t say
62.A fire tube waste heat boiler generally responds faster to load changes than an equivalent water tube design.
a.True b. False
63.The amount of deaeration steam is impacted by the conductivity of boiler feed water.
a.True b. False
64.In a boiler or HRSG evaporator, the allowable steam quality to avoid DNB conditions decreases as the heat flux increases.
a.True b. False
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65.A natural circulation HRSG using vertical evaporator tubes can handle higher heat flux than a forced circulation or once-through unit using horizontal tubes.
a.True b. False
66.A gas turbine plant has two options: a supplementary-fired HRSG and an unfired HRSG. The cross section of the supplementary-fired HRSG generating twice the amount of steam as the unfired HRSG should be much larger.
a.True b. False
Think about It!
1.Why is multiple-pressure steam generation often required in HRSGs but not in a packaged boiler?
2.Explain how surface areas can be different in steam generators (or HRSGs) and yet the duty transferred is the same.
3.Why is supplementary firing very efficient in HRSGs?
4.Why is an economizer preferred to an air heater in oiland gas-fired packaged boilers? Give at least two reasons.
5.Why is steaming in the economizer often a concern in HRSGs and not in packaged boilers?
6.Why can we achieve a low exit gas temperature in a packaged boiler at any steam pressure, whereas it is difficult in a single-pressure unfired HRSG?
7.Why is the superheated steam temperature generally lower with oil firing than with gas firing in a packaged boiler?
8.Why is a low fin density, say, 2 fins/in., preferred in an HRSG superheater over, say, 5 fins/in.?
9.Why does raising the gas temperature at the economizer alone not help minimize low-temperature corrosion problems?
10.Compute typical operating costs of fuel and electricity for various boilers and HRSGs in your plant and suggest how to lower these costs.
11.Is a supplementary-fired HRSG a better choice than an unfired HRSG in a combined cycle plant?
12.Why do we not worry about pinch and approach points in a packaged boiler, whereas they are very important in an HRSG?
13.What are the advantages of a convective superheater in a packaged boiler over a radiant design?
14.What are the various factors to be considered while modifying an existing packaged boiler to meet lower emissions of NOx and CO?
15.In a packaged boiler, why is interstage attemperation for steam temperature control generally preferred to attemperation at the superheater exit?
Appendix G: Quiz on Boilers and HRSGs with Answers |
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16.A single-pressure unfired HRSG generates 600 psig steam at 750°F using 230°F feed water with an exit gas temperature of 380°F. To lower the exit gas temperature, is it more prudent to add a condensate heater rather than increase the surface area of the evaporator significantly?
17.Explain why rules of thumb relating surface areas with steam generation can be misleading.
18.An economizer has been removed from a packaged boiler for maintenance. Can the plant generate the same amount of steam as before? What are the concerns?
Answers to Quiz
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