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Boiler Operator practice questions

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Question 1Boiler Design, Construction, and Types

In a fire-tube boiler, how are the hot combustion gases and the water arranged?

  1. Water passes through tubes that are surrounded by hot gases.
  2. Water surrounds the furnace only, and gases pass over an open grate.
  3. Hot gases pass through tubes that are surrounded by water. Correct answer.
  4. Hot gases and water are mixed together in a common drum.

Explanation

A fire-tube boiler is defined by the path of the hot gas: the products of combustion travel inside tubes that are submerged in a shell full of water. Heat passes through the tube wall into the water, which is why the tube wall is the main heating surface. The reverse arrangement, water inside tubes with gas outside, is a water-tube boiler.

Question 2Boiler Design, Construction, and Types

How many internal flue tubes does a Lancashire boiler have?

  1. Three
  2. Two Correct answer.
  3. One
  4. Four

Explanation

The Lancashire boiler is a shell boiler with two large internal flue tubes running from the front to the back, each containing a furnace at its front end. The gases return along external side flues before leaving at the chimney. The related Cornish boiler has only a single flue tube.

Question 3Boiler Design, Construction, and Types

What is the purpose of a fusible plug in a boiler?

  1. To melt and warn of overheating caused by low water Correct answer.
  2. To indicate the water level inside the steam drum
  3. To release steam automatically if the pressure exceeds the set point
  4. To drain the boiler shell completely during a shutdown

Explanation

A fusible plug contains a low-melting alloy and is fitted in the firebox crown plate or a tube sheet where it is normally covered by water. If the water level falls so that the plug is exposed, the alloy melts and releases steam and water into the furnace, making a loud warning and quenching the fire. It is a last-line protection against overheating of the metal.

Question 4Boiler Design, Construction, and Types

The water level gauge glass on a boiler is used to:

  1. measure the pressure inside the steam drum
  2. show the water level inside the boiler Correct answer.
  3. show the temperature of the steam leaving the drum
  4. sample boiler water for chemical analysis

Explanation

The gauge glass is connected to the steam space at the top and the water space at the bottom, so the level in the glass equals the level in the boiler. It is the operator's primary indication of water level and must be blown through regularly to prove it is clear. Low water is one of the most dangerous conditions in boiler operation.

Question 5Boiler Design, Construction, and Types

The main function of a boiler safety valve is to:

  1. prevent steam from leaving the drum during warm-up
  2. open and release steam at the set pressure Correct answer.
  3. remove dissolved gases from the feedwater
  4. control the feedwater flow into the drum

Explanation

A safety valve is the final pressure protection of the boiler and must open automatically at or below the set pressure to prevent the shell from being overstressed. It is a direct-loaded, fail-safe device that must be capable of passing the full evaporative capacity of the boiler. It is set and sealed by an authorised person and must never be gagged.

Question 6Boiler Design, Construction, and Types

The Babcock and Wilcox boiler is an example of which type?

  1. horizontal water-tube Correct answer.
  2. vertical water-tube with no drum
  3. shell type with two flues
  4. vertical fire-tube

Explanation

The Babcock and Wilcox unit is a horizontal water-tube boiler with an inclined tube bank connecting upper and lower drums, set in brickwork with baffles to guide the gas across the tubes. Water is inside the tubes and the hot gas passes over the outside. This construction allows high pressure and large capacity compared with shell boilers.

Question 7Boiler Design, Construction, and Types

The economizer in a boiler plant is used to:

  1. heat the combustion air before it enters the furnace
  2. heat the feedwater using the leaving flue gases Correct answer.
  3. superheat the steam before it leaves the boiler
  4. cool the flue gases below the dew point to recover water

Explanation

An economizer is a heat exchanger in the gas path after the evaporator that transfers sensible heat from the leaving flue gas to the incoming feedwater. Raising the feedwater temperature reduces the heat needed in the furnace and lowers the exit gas temperature, improving overall efficiency. It is placed ahead of the air preheater in the direction of gas flow.

Question 8Boiler Design, Construction, and Types

Which boiler is a vertical, multi-tubular fire-tube boiler with a hemispherical crown?

  1. Babcock and Wilcox
  2. Lancashire
  3. Cochran Correct answer.
  4. Stirling

Explanation

The Cochran is a vertical shell boiler with a domed top, a central furnace and a nest of horizontal fire tubes through which the gases pass before turning to the chimney. Its vertical layout needs a small floor area, which suits small plants. It is a fire-tube design, since the water surrounds the tubes.

Question 9Boiler Design, Construction, and Types

Which water-tube boiler has three upper drums and one lower mud drum connected by bent tubes?

  1. Cochran
  2. Lancashire
  3. Cornish
  4. Stirling Correct answer.

Explanation

The Stirling boiler uses a bank of bent tubes joining three upper drums to a single lower mud drum, giving a large heating surface within a brickwork setting. The bent tubes allow free thermal expansion and the three upper drums improve steam and water separation. The other options are shell boilers.

Question 10Boiler Design, Construction, and Types

What does a superheater do?

  1. It increases the steam pressure above the drum pressure.
  2. It preheats the feedwater before it enters the drum.
  3. It raises steam temperature above saturation at constant pressure. Correct answer.
  4. It removes moisture by mechanical separation only.

Explanation

A superheater is a bank of tubes in the hot gas path through which saturated steam passes after leaving the drum. Heat added at constant pressure raises the steam temperature above its saturation value, producing dry superheated steam. This reduces condensation losses in the turbine or process and improves cycle efficiency.

Question 11Boiler Design, Construction, and Types

In the flue gas path of a boiler, the air preheater is normally placed:

  1. after the economizer, at the cold end Correct answer.
  2. ahead of the superheater, nearest the furnace
  3. inside the steam drum, ahead of the superheater
  4. between the furnace and the superheater

Explanation

Heat recovery equipment is arranged so that the hottest gas meets the hottest fluid: superheater first, then economizer, then air preheater at the coldest end of the gas path. This counterflow arrangement extracts the maximum heat and gives the lowest possible stack temperature. Placing the air preheater earlier would waste the high-grade heat it cannot use efficiently.

Question 12Boiler Design, Construction, and Types

Water-wall tubes in a large boiler furnace are mainly provided to:

  1. absorb radiant heat and protect the furnace walls Correct answer.
  2. support the weight of the boiler drum
  3. carry steam from the drum to the superheater
  4. increase the furnace volume for complete combustion

Explanation

Water walls are tubes lining the furnace walls, connected into the circulation circuit, that absorb the intense radiant heat of the flame. They generate a large share of the boiler's steam output and at the same time keep the wall temperature low enough for the enclosing refractory and casing to survive. They also reduce the heat lost through the furnace walls.

Question 13Boiler Design, Construction, and Types

Boiler stays are used to:

  1. support flat plates such as the crown sheet Correct answer.
  2. hold the refractory brickwork in place
  3. brace the boiler shell against wind loads
  4. guide the tubes into the tube plate when they expand

Explanation

Flat surfaces such as the firebox crown sheet, tube plates and flat end plates cannot resist internal pressure by their own shape, so they are tied together by stays. The stays carry the pressure load in tension and keep the plate spans small enough to limit bending stress. Stay spacing is therefore a design parameter that directly governs the plate thickness required.

Question 14Boiler Design, Construction, and Types

A chain grate stoker in a boiler:

  1. blows pulverized coal into the furnace with air
  2. removes ash from the furnace with a screw conveyor
  3. sprays liquid fuel into the furnace
  4. moves a bed of coal through the furnace Correct answer.

Explanation

A chain grate stoker is an endless moving grate that carries a bed of coal slowly through the furnace, so that ignition occurs at the front and burn-out at the rear. Air is supplied from beneath the grate through the bed. It suits solid fuels of reasonably uniform size and is common on shell and small water-tube boilers.

Question 15Boiler Design, Construction, and Types

Which is a characteristic of pulverized coal firing?

  1. it can be used only in fire-tube boilers
  2. the fuel burns in a thick bed on the grate
  3. the fuel burns in suspension, like a gas Correct answer.
  4. it needs no excess air for complete combustion

Explanation

Coal ground to a fine powder is carried into the furnace in an air stream and burns in suspension, giving a very high combustion rate and a steady flame that fills the furnace. Because the particles are small, combustion is rapid and responds quickly to load changes. A large water-cooled furnace and careful control of the air/fuel mixture are needed to complete combustion.

Question 16Boiler Design, Construction, and Types

In a fluidized bed boiler, injecting limestone into the furnace mainly:

  1. raises the furnace temperature to melt the ash
  2. increases the combustion air requirement
  3. prevents the bed material from being fluidized
  4. captures sulphur dioxide formed from the fuel Correct answer.

Explanation

Limestone or dolomite is fed into the bed to act as a sorbent: it calcines and reacts with the sulphur dioxide released from the burning fuel, forming a dry solid sulphate that leaves with the ash. This allows high-sulphur fuels to be burnt without a separate flue gas desulphurisation plant. The low, uniform bed temperature of about 850 °C also favours the capture reaction and suppresses thermal NOx.

Question 17Boiler Design, Construction, and Types

A once-through boiler is characterized by:

  1. no drum; water evaporates in a single pass Correct answer.
  2. circulation of water many times through the evaporator
  3. operation only at subcritical pressure
  4. a large drum that stores steam for peak loads

Explanation

In a once-through boiler the feedwater passes through the evaporator tubes once and leaves as steam, so the whole unit acts as a single tube circuit with no steam drum. Without a drum the design is not limited by drum thickness and can be operated above the critical pressure, where there is no distinct evaporation stage. Water chemistry and control must be stricter because there is no blowdown from a drum.

Question 18Boiler Design, Construction, and Types

The mud drum of a water-tube boiler:

  1. distributes feedwater to the superheater
  2. contains the burner and the fuel nozzle assembly
  3. stores steam for the superheater
  4. collects sludge at the bottom of circulation Correct answer.

Explanation

The mud drum is the lowest drum of the circulation circuit, where water leaving the generating tubes turns and returns up the risers. Solids carried in the water settle there and are removed by regular bottom blowdown. Keeping sludge out of the heated tubes prevents overheating and tube failure.

Question 19Boiler Design, Construction, and Types

Baffles in the shell of a fire-tube boiler are fitted to:

  1. direct the flue gases over the tubes in passes Correct answer.
  2. reduce the pressure drop of the flue gas through the boiler
  3. support the tubes against vibration
  4. separate the steam from the water

Explanation

Baffles are plates that force the hot gases to travel across the tube bank in a series of passes rather than straight through. This increases gas velocity across the tubes, improves the heat transfer coefficient and raises efficiency, at the cost of a higher gas-side pressure drop. They are arranged so that the gas path is lengthened without short-circuiting.

Question 20Boiler Design, Construction, and Types

A boiler circulates 30 kg/s of water through its evaporator and produces 5 kg/s of steam. What is the circulation ratio?

  1. 6 Correct answer.
  2. 0.17
  3. 30
  4. 5

Explanation

The circulation ratio is the mass flow of water circulating through the evaporator divided by the mass flow of steam produced. Here 30 kg/s divided by 5 kg/s gives a ratio of 6, meaning each kilogram of steam is formed from six kilograms of circulating water. A high ratio keeps tube walls wetted and prevents departure from nucleate boiling.

Question 21Boiler Design, Construction, and Types

Superheated steam temperature is controlled by attemperation. This means:

  1. bypassing part of the flue gas around the superheater to raise steam temperature
  2. increasing the firing rate to hold the temperature constant
  3. adding a second superheater in parallel to increase the flow
  4. spraying feedwater into the steam to reduce its temperature Correct answer.

Explanation

An attemperator is a spray desuperheater that injects high-purity feedwater into the steam between superheater stages to bring the outlet temperature down to the required value. It responds quickly to load changes, protecting the superheater and turbine from overtemperature. The water used must be of boiler-water quality so that dissolved solids are not carried into the steam.

Question 22Boiler Design, Construction, and Types

For superheater tubes operating with metal temperatures around 580 °C, the material is selected primarily on its:

  1. density
  2. electrical resistance
  3. creep rupture strength Correct answer.
  4. thermal conductivity

Explanation

At high metal temperature a tube under internal pressure deforms slowly and continuously by creep, so the design is based on the stress that gives an acceptable rupture life over the design hours. Alloy content, such as chromium and molybdenum, is chosen to maintain that creep strength in the operating range. Thermal conductivity and density affect temperature distribution and weight but are not the governing design property.

Question 23Boiler Design, Construction, and Types

A boiler shell has a longitudinal seam with joint efficiency 0.80 instead of 1.00. Compared with a seamless shell of the same diameter and pressure, the shell must be:

  1. made of a lower grade material to compensate
  2. the same thickness because efficiency applies only to circumferential seams
  3. thicker, because the allowable stress is reduced by the joint efficiency Correct answer.
  4. thinner, because less material is used in the seam

Explanation

In the shell thickness formula the allowable stress is multiplied by the joint efficiency, so a lower efficiency means a lower permissible stress and therefore a thicker plate for the same pressure and diameter. The longitudinal seam governs because the hoop stress, which is twice the longitudinal stress, acts across it. Seam radiography and weld quality are controlled precisely to keep this efficiency high.

Question 24Boiler Design, Construction, and Types

One end of a long boiler is mounted on supports that allow it to slide. The main reason is to:

  1. reduce the load on the foundation bolts during hydrotest
  2. allow the drum to rotate for inspection
  3. permit thermal expansion without overstressing the shell Correct answer.
  4. allow the boiler to be moved during operation

Explanation

A boiler shell expands when it is heated and contracts when it cools, and a long shell may grow by several centimetres between cold and operating conditions. If both ends were fully restrained, the resulting forces would overstress the shell and the attached pipework. Sliding supports and fixed points are arranged so that expansion is guided to a known point, and expansion joints or bends are used in connections.

Question 25Combustion, Fuels, and Draft

Which of the following best defines theoretical (stoichiometric) air for a fuel?

  1. The minimum airflow that keeps the flame detector from tripping
  2. Air that leaks into the furnace through the casing
  3. Exact air needed for complete combustion with no excess Correct answer.
  4. The air supplied when the burner is at maximum firing rate

Explanation

Theoretical air is the exact quantity of air that supplies just enough oxygen to burn every molecule of fuel, leaving neither unburned fuel nor free oxygen in the products. Real burners run above this value because furnace mixing is never perfect.

Question 26Combustion, Fuels, and Draft

Why is more air than the theoretical amount normally supplied to a burner?

  1. To lower the water dew point of the flue gas leaving the unit
  2. To cool the furnace walls and the superheater tubes
  3. To ensure the fuel burns completely in a real furnace Correct answer.
  4. To increase the heating value of the fuel gas before it burns

Explanation

Theoretical air assumes perfect mixing, which never occurs in a real furnace. A small amount of excess air supplies the oxygen needed to burn out carbon monoxide and unburned fuel so that combustion goes to completion.

Question 27Combustion, Fuels, and Draft

Which group of gases is normally found in the products of complete combustion of a hydrocarbon fuel burned with excess air?

  1. Nitrogen, sulfur trioxide, and methane
  2. Carbon dioxide, water vapor, nitrogen, and oxygen Correct answer.
  3. Carbon dioxide, hydrogen sulfide, and carbon
  4. Carbon monoxide, hydrogen gas, and unburned carbon particles

Explanation

Burning a hydrocarbon with more than theoretical air produces carbon dioxide and water vapor from the fuel, plus the nitrogen that came in with the air and the oxygen that was not consumed. Carbon monoxide and unburned carbon indicate incomplete combustion.

Question 28Combustion, Fuels, and Draft

In what unit is boiler draft normally measured?

  1. Inches of water column Correct answer.
  2. Inches of mercury
  3. Pounds per square inch gauge
  4. Feet per second

Explanation

Draft is a very small pressure difference, usually only a few inches of water or less. The water column unit resolves these small values conveniently, while psi and inches of mercury are far too coarse for furnace draft work.

Question 29Combustion, Fuels, and Draft

A natural-draft boiler depends on which principle to move combustion air through the furnace?

  1. The centrifugal force produced by the rotating burner impeller
  2. The density difference between hot stack gas and outside air Correct answer.
  3. The suction created by the steam jet in the injector
  4. The pressure rise created by the forced draft fan

Explanation

Hot flue gas in the stack is lighter than the outside air, so the column of gas in the stack is balanced against a taller equivalent column of cold air. That imbalance, called stack effect, produces the small negative pressure that pulls air through the furnace.

Question 30Combustion, Fuels, and Draft

What is the main purpose of atomizing fuel oil before it burns?

  1. To raise the flash point of the oil before firing
  2. To remove water from the oil before firing
  3. To break the oil into fine droplets for rapid vaporization Correct answer.
  4. To increase the sulfur content of the fuel oil so it burns faster

Explanation

Oil burns as a vapor, not as a liquid, so it must be broken into very fine droplets that heat up and vaporize quickly. Fine atomization creates the large surface area needed to mix the vapor with combustion air in the short time available in the furnace.

Question 31Combustion, Fuels, and Draft

Which statement about the flash point of fuel oil is correct?

  1. It is the temperature at which the oil burns with a steady flame
  2. It is the temperature at which the oil stops flowing through the line
  3. It is the temperature at which the fuel oil pump begins to cavitate
  4. It is the temperature at which the oil gives off flammable vapor Correct answer.

Explanation

The flash point is the temperature at which the oil releases enough vapor to form a momentary flash when an ignition source is passed over it. It is a safety indicator of how readily the fuel gives off ignitable vapor during storage and handling.

Question 32Combustion, Fuels, and Draft

The higher heating value (HHV) of a fuel differs from the lower heating value (LHV) because HHV includes:

  1. The sensible heat carried into the furnace by the combustion air
  2. The heat lost up the stack in the flue gas
  3. The latent heat of the water vapor formed during combustion Correct answer.
  4. The heat needed to vaporize the steam used in the atomizer

Explanation

Burning a fuel that contains hydrogen produces water vapor. The higher heating value counts the latent heat that would be released if that vapor were condensed, while the lower heating value does not.

Question 33Combustion, Fuels, and Draft

As excess air is increased well past the optimum level, what happens to boiler efficiency?

  1. It falls, because more heat leaves with the flue gas Correct answer.
  2. It stays the same, since fuel input is unchanged
  3. It falls, because the flame temperature rises sharply
  4. It rises, because combustion becomes more complete

Explanation

Every pound of excess air must be heated from ambient up to stack temperature and then thrown away. Beyond the point where combustion is already complete, adding air simply increases the dry gas loss and lowers efficiency.

Question 34Combustion, Fuels, and Draft

A flue gas reading shows high carbon monoxide with very low oxygen. What is the most likely cause?

  1. Excessively high steam pressure at the atomizer
  2. A leaking air preheater between the gas paths
  3. Far too much excess air is being supplied to the burner
  4. Insufficient combustion air or poor fuel-air mixing Correct answer.

Explanation

Low oxygen means there is little oxygen left after combustion, and the CO present shows that some carbon never found enough oxygen to finish burning. In practice this points to air that is either short in quantity or poorly mixed with the fuel at the burner.

Question 35Combustion, Fuels, and Draft

In a balanced-draft unit, how do the forced draft and induced draft fans normally work together?

  1. Both fans discharge together into the base of the stack
  2. The ID fan is located upstream of the furnace and the FD fan downstream of it
  3. The FD fan pushes air in while the ID fan holds furnace pressure near zero Correct answer.
  4. The FD fan runs continuously and the ID fan is used only during light-off

Explanation

In balanced draft, the forced draft fan supplies combustion air to the windbox and the induced draft fan removes the flue gas downstream of the furnace. Their speeds are trimmed against each other so the furnace stays slightly negative, which keeps hot gas and smoke from escaping the casing.

Question 36Combustion, Fuels, and Draft

Which condition most directly increases the stack heat loss from a boiler?

  1. A higher flue gas temperature leaving the unit Correct answer.
  2. Reduced firing rate on the burner
  3. Cleaner heat transfer surfaces in the economizer
  4. A lower excess-air level at the burner

Explanation

Dry gas loss is proportional to the mass of gas leaving and to how much hotter it is than the incoming air. Raising the exit gas temperature therefore sends more usable heat up the stack for the same amount of steam produced.

Question 37Combustion, Fuels, and Draft

Why is the acid dew point of flue gas from a high-sulfur fuel oil higher than that from natural gas?

  1. Sulfur raises the flame temperature and the velocity of gas leaving the unit
  2. Sulfur in the fuel increases the water vapor content of the flue gas noticeably
  3. Sulfur acts as a catalyst that increases the nitrogen content of the gas
  4. Sulfur burns to SO3, forming acid that condenses at a higher temperature Correct answer.

Explanation

A small fraction of the sulfur dioxide formed during combustion oxidizes further to sulfur trioxide, which combines with water vapor to make sulfuric acid. Sulfuric acid condenses at a much higher temperature than pure water vapor, so the cold end of the unit must be kept warmer when burning high-sulfur oil.

Question 38Combustion, Fuels, and Draft

Using the common rule of thumb, about how many cubic feet of air are needed to burn 1 cubic foot of natural gas?

  1. About 2 cubic feet
  2. About 25 cubic feet
  3. About 50 cubic feet
  4. About 10 cubic feet Correct answer.

Explanation

Natural gas is mostly methane and releases roughly 1,000 Btu per cubic foot. A widely used approximation is about 10 cubic feet of air per 1,000 Btu of fuel fired, which matches the stoichiometry of methane closely.

Question 39Combustion, Fuels, and Draft

As excess air increases, the measured carbon dioxide concentration in the flue gas:

  1. increases in direct proportion to the excess air
  2. remains constant regardless of excess air
  3. decreases because the excess air dilutes the flue gas Correct answer.
  4. rises until carbon monoxide appears and then levels off

Explanation

The CO2 peak occurs at theoretical air, when no oxygen is left over to dilute the products. Any additional air adds nitrogen and oxygen that carry no CO2, so the percentage of CO2 drops as excess air rises.

Question 40Combustion, Fuels, and Draft

Before lighting off a burner in a cold furnace, why must the furnace be purged?

  1. To check the response of the flame scanner
  2. To warm the refractory lining evenly before the burner is fired
  3. To clear combustible gases that could explode at light-off Correct answer.
  4. To bring the furnace draft up to its normal design value

Explanation

Unburned fuel can accumulate in a furnace after a failed light-off, a trip, or from leaking fuel valves. A purge with air at adequate flow for a set period sweeps those combustibles out so the ignition source cannot set off a furnace explosion.

Question 41Combustion, Fuels, and Draft

What is the main benefit of an air preheater on a boiler?

  1. It uses flue gas heat to warm the combustion air Correct answer.
  2. It raises the burner draft by lowering the density of the gas
  3. It cools the flue gas so that no induced draft fan is required
  4. It removes moisture from the combustion air before it is fired

Explanation

An air preheater transfers heat from the leaving flue gas to the incoming combustion air. This lowers the stack temperature and returns otherwise wasted heat to the furnace, raising overall boiler efficiency.

Question 42Combustion, Fuels, and Draft

Fuel oil must be heated before atomization mainly because:

  1. Heating raises the flash point so the oil burns more safely
  2. Heating removes all sulfur from the oil
  3. Heating raises the heating value of the oil so less fuel is needed
  4. The viscosity must be lowered so the oil atomizes properly Correct answer.

Explanation

Heavy fuel oils are too viscous to break into fine droplets at ambient temperature. Heating to the correct atomizing viscosity lets the burner produce a fine, uniform spray that mixes well with air.

Question 43Combustion, Fuels, and Draft

An increase in the smoke spot number on an oil-fired boiler most likely indicates:

  1. A large amount of excess air being supplied at the burner
  2. Free water at the bottom of the fuel oil storage tank
  3. Incomplete combustion from poor atomization or low air Correct answer.
  4. A failed flame scanner

Explanation

Smoke spot testing measures how much unburned carbon is collected from a sample of flue gas. A rising spot number means fuel is not burning out, typically because of too little air, cold or coarse atomization, or poor mixing.

Question 44Combustion, Fuels, and Draft

Natural gas has a specific gravity of about 0.6 relative to air. What does this mean for a leak inside a boiler room?

  1. The gas will dissolve into the boiler feedwater
  2. The gas will rise and collect near the roof or vents Correct answer.
  3. The gas will stay evenly mixed throughout the room
  4. The gas will settle and collect at floor level

Explanation

A specific gravity below 1.0 means the gas is lighter than air, so a leak tends to rise. Gas detectors and ventilation openings are therefore placed high in the boiler room where the gas accumulates.

Question 45Combustion, Fuels, and Draft

A burner shows 3 percent oxygen and 400 ppm carbon monoxide in the flue gas. Which change would most likely improve efficiency without creating smoke?

  1. Reduce excess air further to cut the stack loss
  2. Lower the fuel oil temperature at the atomizer
  3. Improve burner fuel-air mixing so the CO burns with the same air Correct answer.
  4. Raise the excess air to about 15 percent to guarantee that all CO is burned

Explanation

The CO with low oxygen points to locally fuel-rich zones rather than a shortage of total air. Correcting the mixing, through register or burner adjustment, burns that CO using oxygen already in the furnace and lets the unit run at low excess air cleanly.

Question 46Combustion, Fuels, and Draft

At zero excess air, the maximum theoretical carbon dioxide content of flue gas from natural gas firing is about:

  1. 12 percent Correct answer.
  2. 6 percent
  3. 30 percent
  4. 20 percent

Explanation

Natural gas is mostly methane, and burning it completely leaves about two volumes of water vapor and one of CO2 for every volume of methane, diluted by the nitrogen from the air. The resulting maximum CO2 concentration is roughly 12 percent, and any excess air lowers it from there.

Question 47Combustion, Fuels, and Draft

Which condition will reduce the natural draft produced by a given stack?

  1. A colder ambient air temperature outside the boiler house
  2. A taller stack
  3. A higher flue gas temperature in the stack
  4. A lower average flue gas temperature in the stack Correct answer.

Explanation

Stack draft depends on the difference in density between the hot gas inside the stack and the outside air. Cooling the gas reduces that density difference and therefore reduces the draft available to pull air through the furnace.

Question 48Combustion, Fuels, and Draft

On a balanced-draft unit, flue gas oxygen reads high at low fire but normal at high fire. What is the most likely cause?

  1. The flame scanner is misreading the flame signal at low fire
  2. Air in-leakage into the furnace or breeching at low fire Correct answer.
  3. The heating value of the fuel gas has increased sharply
  4. The induced draft fan is running too fast at low fire

Explanation

At low fire the furnace is under more negative pressure, so any casing or breeching openings draw in more outside air. That leakage appears as extra oxygen in the gas, but at high fire the larger gas volume reduces the pressure difference and the effect fades.

Question 49Combustion, Fuels, and Draft

Flue gas analysis shows 5 percent oxygen but also 600 ppm carbon monoxide. Which explanation fits best?

  1. The fuel is being over-atomized by excess steam
  2. The thermocouple reading the flue gas temperature is faulty
  3. Poor mixing leaves fuel-rich pockets that quench before burning out Correct answer.
  4. Five percent oxygen is below the level needed for any combustion at all

Explanation

Averaged oxygen can look generous while local zones in the flame are starved of air or cool enough to freeze carbon monoxide before it finishes burning. The result is measurable CO alongside apparently ample excess oxygen, which is a mixing and residence-time problem rather than an air-quantity problem.

Question 50Combustion, Fuels, and Draft

Boiler draft is normally measured in which units?

  1. Inches of water column Correct answer.
  2. Inches of mercury
  3. Feet of flue gas
  4. Pounds per square inch gauge

Explanation

Draft is a very small pressure difference, so it is read in inches of water column on a manometer or draft gauge; one inch of water column equals about 0.036 psi. Typical furnace draft values range from a fraction of an inch to a few inches of water. Gauges graduated in psi are far too coarse to resolve these small differences.