The direct method of finding boiler efficiency compares
Answer: A
It needs only the steam and fuel quantities and their enthalpies, which makes it quick, but it says nothing about where the missing energy went. The indirect method accounts for each loss separately and so points at what to fix.
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In the loss method of assessing a boiler, the efficiency is arrived at as
Answer: B
Each loss is measured or estimated in turn and subtracted, so the result carries a breakdown with it. It is also less sensitive to an error in the steam flow, which is often the least reliable measurement on a boiler house.
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The indirect method is preferred in an audit because it
Answer: C
Knowing that a boiler is eighty per cent efficient is of little use; knowing that seven points are leaving as dry flue gas and two as blowdown tells the auditor what to attack. It needs more measurements, not fewer, and the calorific value is essential to both.
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Three of the following are losses accounted for in the indirect method. The exception is
Answer: D
Correct answer: the loss in the feed pump's motor
The indirect method accounts for the losses in the path from the fuel’s energy to the steam - dry flue gas, hydrogen and moisture, unburnt fuel, radiation and convection, and blowdown. The feed pump’s motor consumes electricity, not the fuel being burned, so its loss belongs to the plant’s auxiliary power account and never appears in the boiler’s heat balance.
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The largest single loss in a typical industrial boiler is usually
Answer: A
Hot gas leaving the stack commonly carries six to twelve per cent of the input, and it is the loss most amenable to improvement through excess air control and better heat transfer. Radiation and air moisture are usually under two per cent between them.
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The dry flue gas loss depends on the mass of flue gas and on
Answer: B
The loss is the mass multiplied by the specific heat and by the rise above ambient, so both the quantity of gas, which follows from the excess air, and its exit temperature enter. Hydrogen and fuel moisture are separate loss heads.
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The loss due to hydrogen in the fuel arises because the hydrogen
Answer: C
Nine kilograms of water are formed for each kilogram of hydrogen, and its latent heat is carried away as vapour unless the flue gas is condensed. It is unavoidable in an ordinary boiler and is the difference between the gross and net calorific bases.
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Excess air above the optimum raises the flue gas loss because it
Answer: D
Correct answer: adds mass that is heated and then discarded
Every kilogram of excess air is drawn in cold, heated to flue gas temperature by the fuel, and then thrown up the stack still hot. It contributes nothing to combustion and carries away energy in proportion to its mass, which is why trimming the excess air to the least that still burns the fuel cleanly is the first move in any boiler audit.
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The optimum excess air for a boiler is the level at which
Answer: A
Reducing air cuts the dry flue gas loss and eventually raises the unburnt loss, so the two curves cross at a minimum a few per cent of oxygen above stoichiometric. Chasing zero oxygen produces carbon monoxide and soot and costs more than it saves.
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A high flue gas temperature with correct excess air points at
Answer: B
If the gas quantity is right and it is still leaving hot, the heat has not been transferred, and soot on the fire side or scale on the water side is the usual reason. Cleaning restores it, and monitoring the temperature is how the need is detected.
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