Insulating that surface so it sits at 35 \( ^\circ \)C reduces the loss to about
Answer: A
The difference is now five degrees, so the loss is fifty watts a square metre, a saving of five sixths. The surface temperature is what an infrared survey measures, which is why it is the figure quoted in such calculations.
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Condensate at 10 bar and 180 \( ^\circ \)C is flashed to atmospheric pressure. With liquid enthalpies of 763 and 419 kJ/kg and a latent heat of 2257 kJ/kg, the flash fraction is about
Answer: B
Correct answer: 15 per cent
\[ x = \frac{h_{f1}-h_{f2}}{h_{fg2}} = \frac{763-419}{2257} = \frac{344}{2257} = 0.152 \approx 15\%. \] Fifteen per cent of the condensate leaves as steam, and it carries a disproportionate share of the energy because of the latent heat. Recovering it in a flash vessel and using it at low pressure is standard practice.
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Returning condensate at 90 \( ^\circ \)C instead of using make-up at 30 \( ^\circ \)C saves, per kilogram of feed, about
Answer: C
Correct answer: 251 kJ
\[ c_p \Delta T = 4.19 \times (90-30) = 251\ \mathrm{kJ/kg}. \] Only the sensible heat between the two feed temperatures is saved - the latent heat of 2257 kJ/kg is a distractor, since the boiler must supply that in either case. Against a total of roughly 2700 kJ/kg this is about nine per cent of the fuel.
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A furnace exhausts 2 kg/s of gas at 600 \( ^\circ \)C which could be cooled to 200 \( ^\circ \)C, with a specific heat of 1.1 kJ/kgK. The recoverable heat is about
Answer: D
The mass flow times the specific heat times the four hundred degree drop is \( 2 \times 1.1 \times 400 = 880 \) kilowatts. Whether it is worth recovering depends on whether a use exists for heat at that grade and on how continuously the furnace runs.
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A heat exchanger's approach temperature has risen from 10 to 25 \( ^\circ \)C over a year at constant duty. The indication is
Answer: A
More resistance for the same duty means the cold stream leaves further from the hot stream, and a trend of that kind is exactly what a cleaning schedule should be based on. A change in flow or inlet temperature would show up in the duty as well.
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Match the recovery device with the stream it heats.
1. Economiser 2. Air preheater 3. Waste heat boiler
P. Water, raising steam Q. Feedwater R. Combustion air
Answer: B
All three take heat from a flue or exhaust and differ in what they give it to, and all three are limited at the cold end by the acid dew point. Which suits a site depends on whether it needs more feedwater heat, hotter air or more steam.
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Match the loss with what reduces it.
1. Dry flue gas loss 2. Blowdown loss 3. Surface loss
P. Better feedwater treatment Q. Trimming the excess air R. Insulating the shell and pipework
Answer: C
Correct answer: 1-Q, 2-P, 3-R
Each loss is attacked at its own source. The dry flue gas loss is driven by the mass of gas leaving, so trimming the excess air reduces it. Blowdown is driven by the solids entering, so better feedwater treatment reduces it. And the surface loss is driven by exposed hot area, so insulation reduces it.
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Put the steps of an indirect boiler efficiency calculation in order.
1. Compute the flue gas mass per unit of fuel 2. Measure the flue gas composition and temperature 3. Subtract the sum of the losses from a hundred per cent 4. Compute each loss in turn
Answer: D
Correct answer: 2, 1, 4, 3
Measurement comes first, because the flue gas composition is what the whole calculation rests on. The composition then gives the gas mass per unit of fuel, the mass and temperature give each loss in turn, and the losses subtracted from a hundred per cent give the efficiency. The order is forced: nothing later can be computed before the step above it.
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Consider the following statements about boiler efficiency.
1. The direct method needs the steam and fuel quantities 2. The indirect method identifies the individual losses 3. The two methods must give identical answers
Answer: A
The first two are correct and describe why both methods are used. In practice they differ by a point or two because of measurement error in the steam flow and estimation in the smaller loss heads, and a large discrepancy is itself informative.
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Which of the following would NOT reduce a boiler's fuel consumption for the same steam output?
Answer: B
Correct answer: raising the boiler's operating pressure above the process requirement
Raising the pressure beyond what the process needs raises the saturation temperature, so the shell and the distribution mains run hotter and lose more, and the flash losses at every trap grow. Nothing is gained in return. Returning condensate, fitting an economiser and trimming the excess air all reduce fuel for the same output.
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