Boiler classification: watertube designs (multi-drum bent tube, D/A/O, packaged, once-through, forced circulation, sub- vs super-critical) and special designs (fluidized bed, HRSG, black liquor, waste heat, biomass, HP/HT hot water)

Misjudge how a boiler circulates and you can starve the riser tubes of cooling water and burst them under fire. On a black-liquor recovery unit, a tube leak onto molten smelt can detonate the furnace — a physical explosion, not a chemical one. On a hot-water heating boiler, forgetting the ASME pressure/temperature boundary means you built the wrong code vessel entirely. When you size up any boiler on the plant floor, a chief engineer asks four things: how was it assembled, how does its water circulate, what shape encloses its furnace, and what is it built to burn? Get those four right and the rest of the design falls into place.

The lava-lamp analogy for natural circulation

Everyday picture — a lava lamp is natural circulation. Before the formulas: think of a lava lamp. The wax at the bottom heats, becomes less dense than the surrounding liquid, and rises; as it rises and cools near the top it becomes denser again and sinks back down to be reheated. A boiler's downcomers and risers do exactly this with water and steam — cool, dense water sinks in the downcomers while the hot, bubble-laden mixture rises in the risers — except the boiler's "heat" is furnace radiation and the "wax" is water turning partly to steam. No pump is needed as long as there is a density difference to drive the loop; kill the density difference (by raising pressure toward the critical point) and the lava lamp effect stops, exactly as it would if you heated a lava lamp until the wax and liquid were the same density.

Packaged and shop-assembled boilers

Packaged, shop-assembled, and field-erected. A packaged watertube boiler leaves the factory complete — burner, forced-draft (FD) fan, automatic controls, steel casing, on a skid-type foundation, bottom-supported — with capacities from 2300 kg/h to over 65 000 kg/h and pressures usually to 1700 kPa (sometimes over 6200 kPa). Its main advantage is low cost: manufacturing and shipping are both cheaper when the whole unit is built and moved as one piece. A shop-assembled boiler is built to the purchaser's exact specification rather than ordered "off the shelf" — the steam-generating section (tubes, drum, header connections) is fully assembled in the shop, but refractory/casing may or may not be installed and burner/draft equipment (including the stack) may ship as separate assemblies for final erection and testing on site.

Field-erected boilers

A field-erected boiler is too large to ship in one piece at all: large waterwall, superheater, and economizer sections are built in the manufacturer's plant, then shipped for on-site erection — hanging the drums and headers, welding/rolling the tube sections, installing burners, ductwork, refractory, insulation, and casings, installing drum internals only after all tubes are attached, then adding external auxiliaries (draft fans, air heaters, safety devices, controls), and finally on-site testing, approval, and commissioning. Most field-erected units exceed 150 000 kg/h, though smaller field-erected designs exist for special applications.

How boiler output is rated

Boiler rating methods. Provincial regulations recognize three rating methods: (1) one square metre of heating surface equals 10 kW — the heating surface is the total area of all surfaces through which heat transfers from the furnace or combustion gases to the water, and for a curved surface the side with the greater radius is the one measured; (2) for an electric boiler, the rating is the maximum kW rating of the heating element; (3) where neither applies, an hourly boiler output of 36 MJ is equivalent to 10 kW.

The steam-capacity rating trap

The exam trap: steam capacity in kg/h is NOT a true measure of thermal energy supplied, because steam quality, steam temperature, and feedwater temperature all change how much heat that mass actually carries. Steam generator capacities are instead described as maximum continuous steam output (kg/h) at design temperature and pressure; where a boiler supplies an entire electric generating unit, output is often quoted as the generator's power (for example, "a 300 MW boiler").

Worked example: rating from heating surface

Worked example — rating from heating surface. A boiler has a total heating surface of 240 m². Use method (1): one square metre of heating surface equals 10 kW.

• Formula: $\text{Rating (kW)} = A_{heating} \times 10$
• Substitute: $\text{Rating} = 240 \text{ m}^2 \times 10 \text{ kW/m}^2$
• Result: $2400$ kW — the figure a chief uses to size relief and feedwater capacity, not the kg/h nameplate.

Now you try — faded practice. A field-erected boiler has 1850 m² of heating surface. Same method, same multiplier: multiply the area by 10. You should get 18 500 kW. The only judgment call is recognizing this is a heating-surface job (method 1), not an electric boiler (method 2) — the fuel-fired construction rules method 1 in.

Circulation in watertube boilers

Downcomers and risers. In a watertube boiler the tubes form circuits of downcomers (cooler, denser water, removed from the furnace) and risers (hot, exposed directly to the radiant heat of the furnace or to very hot combustion gases). The water in the downcomers is at or slightly below saturation temperature and is much denser than the steam-water mixture in the risers. Positive circulation is produced as the denser water falls in the downcomers and pushes the less dense water and steam upward in the risers — this is the lava-lamp effect described above, running with no pump at all.

The four factors affecting natural circulation

The four factors affecting natural circulation. Four factors influence how strongly this loop runs:

1. Steam-drum height above the mud drum or header — the greater the height, the greater the difference in mass between risers and downcomers, so the greater the differential pressure and the more circulation.
2. Firing rate — the greater the heat applied, the more steam bubbles form in the risers, increasing the mass differential and thus the circulation.
3. Operating pressure — as pressure increases, steam density increases, so there is LESS difference between the density of the downcomer water and the riser steam-water mixture; circulation therefore DECREASES as pressure rises. Natural circulation is limited to boiler pressures below about 21 000 kPa.
4. Tube cleanliness — dirty heating surfaces, on either the fireside or the waterside, reduce heat transfer, which means less steam production in the risers and reduced circulation; deposits also physically restrict the flow of water.

Why pressure limits natural circulation

Pressure is the counter-intuitive one and the exam's favourite misconception: more firing rate and more drum height both HELP circulation, but more pressure HURTS it.

The critical-pressure limit. At the critical pressure of 22 090 kPa the densities of water and saturated steam become equal, so there is no density difference left to drive flow, and natural circulation is impossible. Boilers operating at or above this critical pressure are called supercritical (below it, subcritical), and a supercritical boiler cannot have a steam drum in the natural-circulation sense — pumps must do the work that density can no longer do.

The A-type boiler layout

Picture the shapes. Imagine looking at the end of a boiler, at the furnace opening. In an A-type boiler, picture a large drum sitting up high, centred above two smaller drums down at floor level, with curved (bent) tubes sweeping from the big top drum down to each of the two bottom drums — the outline traces a capital "A", and the furnace sits inside that triangle. Most steam production happens in the central furnace wall tubes. The burner, draft fan, and control panel mount on the boiler front.

The D-type boiler layout

In a D-type boiler, picture only two drums: one tube bank curves from drum to drum on one side of the furnace (the water-cooled wall), while the boiler's main generating bank of tubes forms the other side — together the two drums and the two tube banks trace a capital "D" around the furnace. The burner fires either from one end (parallel to the drums) or from the curved side wall (perpendicular to the drums); the back wall opposite the burner, called the target wall, is usually refractory with only some cooling tubes.

The O-type boiler layout

In an O-type boiler, picture an upper and a lower drum joined by tubes arranged all the way around in a ring, so the outline traces a capital "O" that fully surrounds the furnace — symmetrical, but because the tubes ring the outside evenly rather than concentrating near the flame, the O-type exposes the LEAST tube surface to radiant heat of the three shapes.

Circulation and gas flow in A, D, and O boilers

Circulation and gas flow in A/D/O boilers. In each of the three designs, steam bubbles form in the hottest tubes (the risers, in the radiant furnace area and the hottest part of the generating bank) and rise to the steam drum, where steam separates from the water; the water returns to the mud drum through the cooler downcomer tubes. The flue gas flow is not always the same: typically the burners sit at one end of the furnace, combustion gases reach the far end and are directed back through the generating bank toward the burner end — a two-pass design, though more passes are common.

A packaged boiler's cross-flow gas path

Picture a packaged boiler's cross-flow path: gas gives up radiant heat to the waterwall tubes in the furnace, then passes screen tubes that absorb the remaining radiant heat and "screen" the superheater tubes from it, then flows through the superheater bank, then is baffled into two further passes through the generating tubes before it exits.

Integral-furnace design — what A, D, and O share

Integral furnace — what A, D, and O share. Because the furnace walls of all three shapes are themselves water-cooled tubes that are part of the boiler's own circulation circuit — not a separate refractory box — A-, D-, and O-type boilers are all called integral-furnace boilers. This is what lets these designs run hotter, cleaner, and more compactly than a refractory-walled furnace.

What a steam generating unit includes

The full unit, not just the boiler. A steam generating unit is the complete assembly that turns water into high-temperature, high-pressure steam for a large turbine: the boiler (where water becomes steam — the term "boiler" is often loosely used for the whole unit), the superheater (raises steam above saturation temperature by adding sensible heat once the moisture has evaporated), the reheater (found in many larger units — reheats steam that has already expanded partway through a turbine and cooled, back up toward its original superheat temperature),

More components: economizer, air heater, draft fans, ash removal

the economizer (preheats feedwater with the combustion gases leaving the boiler; feedwater is normally heated to within about 20 °C of the boiler water temperature — a feedwater temperature of 280 °C–300 °C is typical for a drum temperature of 320 °C), the air heater (preheats combustion air to about 350 °C using gases leaving the economizer, improving combustion efficiency; where pulverized coal is used, coal pulverizers are part of the fuel-burning equipment), the draft fans (a forced draft fan supplies air through the air heater to the pulverizers and burners; an induced draft fan exhausts the combustion gases, cooled to around 170 °C, from the air heater to the stack), and — in solid-fuel units — ash removal equipment, since ash accumulating in the furnace-bottom hopper is removed pneumatically or hydraulically.

Gas-side temperatures, front to back

Trace the temperatures front to back. A typical coal-fired steam generator supplying superheated steam at about 11 300 kPa to a 300 MW turbine-generator shows this profile: forced-draft air enters at roughly 27 °C–28 °C and is heated to about 280 °C in the air heater before reaching the burners and pulverizers; the furnace runs at about 1650 °C; flue gas downstream of the primary superheater is about 1100 °C; after giving up more heat in the reheater it drops to about 843 °C; after the economizer it is about 380 °C; and after the air heater it exits to the precipitators and stack at about 170 °C.

Water/steam-side temperatures, front to back

On the water/steam side, feedwater enters the economizer at about 254 °C and leaves it (heading to the steam drum) at about 280 °C; the saturated water/steam mixture in the drum and generating tubes sits at about 320 °C (the saturation temperature for this pressure); the superheater — typically a convection stage followed by a radiant stage — raises the steam to about 538 °C; and the reheater brings steam returning from the high-pressure turbine back up to about the same 538 °C before it goes on to the intermediate- and low-pressure turbine stages. Note the economizer water exit (280 °C) always stays below the drum's saturation temperature (320 °C) — if it reached saturation, the economizer would start making steam, which a once-through convection feedwater heater is not built to do.

Worked example: reading a temperature drop

Worked example — reading a temperature drop. A unit's flue gas is 1100 °C downstream of the primary superheater and 843 °C leaving the reheater. How much heat did the reheater section extract from the gas, expressed as a temperature drop?

• State what's given: gas in = 1100 °C, gas out = 843 °C.
• Subtract: $\Delta T = 1100 - 843 = 257$ °C.
• Plant-floor reading: that 257 °C drop is heat the reheater pulled out of the gas and put into the returning HP-turbine steam — exactly the energy that raises the reheat steam back toward 538 °C. If a fouled reheater stopped absorbing that heat, the gas downstream would run hotter than expected and the reheat steam temperature would fall short.

An oil/gas-fired unit, for contrast

An oil/gas-fired unit, for contrast. A steam generator designed to burn oil or natural gas might run at 12 700 kPa with steam superheated AND reheated to 538 °C and a capacity of 450 000 kg/h. Such units often place an attemperator (a device that injects water into the steam to control its temperature) between a primary and secondary superheater stage, plus two stages of reheater — an arrangement chosen because gas/oil firing gives finer, faster control of furnace heat than solid fuel.

Critical and supercritical boilers

Why density, again. Natural circulation depends on the density difference between a column of water and a column of steam/water mixture; that difference shrinks as pressure rises and becomes zero at the critical pressure of 22 090 kPa. Above the critical pressure the densities of water and steam are identical, so a boiler operating at or above it is supercritical and cannot have natural circulation — pumps must provide forced circulation. Boilers below critical pressure are subcritical. A supercritical unit is typically a once-through, forced-circulation design with no steam drum; its other major components resemble a natural-circulation boiler's.

Why forced circulation is added

Why add a pump. Boilers with high pressures or intricate tube paths may need help ensuring good circulation, so they add a pump (or pumps) to the water circulation path. The advantage of forced circulation is positive flow in every tube — no tube starves for lack of internal water flow — and fitting orifices or nozzles of varying size at tube inlets lets the flow in each tube circuit be individually regulated. The disadvantages are higher equipment cost (pumps, piping), more maintenance and operating cost, and the particular difficulty of sealing the circulating pump's glands against leakage to atmosphere. Forced circulation becomes advantageous at pressures of roughly 12 500–13 800 kPa and is absolutely necessary at or above the critical pressure of 22 090 kPa. Forced-circulation boilers fall into two classes: controlled-circulation (recirculating) boilers, and once-through boilers.

Controlled-circulation boilers — tracing the flow

Controlled-circulation boiler — trace the flow. Feedwater enters through the economizer, is preheated close to the boiler water's saturation temperature, and joins the drum; from the drum it flows through the downcomers to circulation pumps. The pumps push the water to an inlet header, where orifices control how much water reaches each individual steam-generating circuit. The steam/water mixture produced in the radiant and convection generating tubes discharges into the steam drum, where separating equipment removes the free water; the saturated steam then passes through a superheater and leaves via the superheated steam outlet. The pump is what replaces the density difference as the driver of flow — everything downstream of the drum looks like a natural-circulation boiler.

Once-through boilers — no drum, one continuous pass

No drum, one continuous pass. A once-through boiler uses forced circulation with no steam drum and therefore no recirculation of water at all — feedwater is forced through one continuous circuit, and the change of state from water to steam happens during that single pass. The feed pump supplies water to the economizer; after the economizer, the heated water flows to the furnace-wall tubes in the radiant zone, where about 85% of the water evaporates (the radiant evaporator section); the steam/water mixture then passes to a final evaporator section, located in a cooler flue-gas zone, where it is completely converted to steam; the steam then flows through a convection superheater and finally a radiant superheater to the steam outlet.

A supercritical once-through example

A supercritical, once-through example designed for pulverized coal and oil firing might run at 16 500 kPa and 538 °C with one reheat to 538 °C, a capacity of 1 197 000 kg/h (approaching 400 MW), and a single furnace about 15 m wide firing through both front and rear walls — with no steam drum and no circulating pumps required at all.

Special boiler designs (Chapter 2)

Canada's SAGD oil boom and the growth of combined-cycle power have made several special-purpose boiler designs common knowledge for a 3rd-class engineer.

Once-through steam generators (OTSGs) for SAGD

Once-through steam generators (OTSGs) for SAGD. Steam-assisted gravity drainage (SAGD) drills a pair of horizontal wells 4–6 m apart to recover heavy bitumen; the OTSG injects steam into the upper well while oil is produced from the lower well. Because SAGD needs maximum heat put into the well, OTSGs deliberately produce wet steam at 75%–80% dryness (20%–25% moisture) — the moisture keeps dissolved salts and organics in the water portion so they do not deposit on and foul the tubes; deposits that do form are removed by periodically pigging the boiler.

OTSG equipment and safe operation

OTSGs are forced-circulation, multi-pass, drum-less units with a flow-control valve and a pressure-control valve on each pass; outputs run 75–200 m³/h of feedwater at pressures up to 20 000 kPa. Because the boiler must always have flow through it to prevent overheating, a blowdown line vents excess steam/water to a pond whenever well demand drops, keeping flow moving even if the wells are shut in.

Fluidized bed boilers

Fluidized bed boilers. A fluidized bed boiler burns crushed coal (1.6–6 mm) suspended by upward-blown combustion air in a bed of inert granular material (ash or crushed rock), running cool at 800 °C–900 °C versus 1600 °C–1900 °C for pulverized coal or oil firing. The lower temperature does two useful things at once: crushed limestone added to the bed combines with fuel sulfur to cut SO2 emissions by up to about 80%, and the cooler flame produces far less thermal NOX.

Bubbling vs. circulating fluidized beds

Two sub-types: a bubbling fluidized bed (BFB) has a distinct, visible bed level, with solids kept in suspension by upward air and combustion gases and complete combustion at 815 °C–875 °C; a circulating fluidized bed (CFB) supplies more fluidizing air, so there is no distinct bed level, and the fuel, air, and bed material circulate continuously up to a hot cyclone that returns solids to the bed, giving a longer fuel residence time and very efficient combustion (bed density, not depth, is what's controlled). Fluidized-bed combustion can also burn biomass, wood products, petroleum coke, and industrial waste — the largest fluidized boiler in Canada, a 183 MW circulating fluidized bed unit, burns petroleum coke and coal.

Heat recovery steam generators and combined cycles

Heat recovery steam generators (HRSGs) and combined cycles. An HRSG — also called a waste-heat recovery boiler (WHRB) or turbine exhaust gas boiler — makes steam from a gas turbine's exhaust heat. Most horizontal-gas-flow HRSGs use natural circulation with both a steam drum and a water drum; they may be unfired (waste heat only) or use auxiliary duct burners to raise inlet gas temperature and boost steam production. In a combined-cycle plant, a gas turbine drives a generator and its hot exhaust passes through an HRSG to raise steam for a separate steam turbine-generator — combined-cycle efficiency can reach 50% or more, well above a simple gas-turbine or steam-only cycle.

Cogeneration

Cogeneration is a related but distinct idea: the simultaneous generation and use of TWO forms of energy from one fuel source — for example, a gas turbine that both turns a generator AND exhausts heat to an HRSG whose steam drives a second turbine as well as supplying process heating steam. Because almost all the fuel's energy ends up doing useful work (electricity plus process heat) rather than being rejected, cogeneration's total energy utilization can approach 80%.

Black liquor recovery boilers — the smelt-water hazard

Black liquor recovery boilers — the smelt-water hazard. In the kraft pulping process, wood chips are cooked in white (cooking) liquor (sodium hydroxide + sodium sulfide); after washing, the resulting weak black liquor (10%–15% solids) is concentrated into strong/heavy black liquor and burned as fuel in a recovery boiler. The boiler burns the organic content while the inorganic sodium chemicals collect on the furnace floor as molten smelt, later dissolved to reclaim the pulping chemicals. The single most dangerous fact in this unit: if water contacts molten smelt — for example from a furnace tube leak — the result is a violent PHYSICAL explosion (not a chemical reaction): the water flashes to steam and the gases expand so quickly that the effect is like a shock wave.

Guarding against the smelt-water explosion

To guard against it, black liquor is fired above about 62% solids (weaker liquor is itself an explosion risk), and many recovery boilers carry an emergency rapid-drain system that can drop the boiler water down to just above the smelt bed within moments of a detected tube leak.

Refuse and biomass boilers

Refuse and biomass boilers. A refuse-to-energy boiler burns municipal waste either by mass burning (fuel used largely as received, dumped onto travelling-grate stokers) or with a prepared fuel (screened, shredded, and magnetically de-metaled first). Refuse combustion is corrosive: chlorides deposit on furnace, superheater, and boiler tubes, and the hotter the tube the worse the metal loss — so mass-fired units line the lower furnace with refractory (pin-studded to the tubes) and often use bimetal tubes (carbon steel with an outer Inconel, i.e., nickel-chromium, layer) to resist it. A biomass boiler burns wood, bark, sawdust, and similar mill or forestry residues, commonly on a travelling, vibrating, or pinhole grate, or in a Dutch oven (a refractory-lined cell attached to the main furnace, good for high-moisture fuel but needing manual raking to clear ash).

HTHP hot water boilers — basics and code boundary

High-pressure, high-temperature (HTHP) hot water boilers. These heat large buildings and institutions as an alternative to steam, avoiding steam traps, condensate-return corrosion, and continuous blowdown/make-up since the system is closed. They typically run 1000–2000 kPa and 175 °C–215 °C. The construction-code boundary matters here: units above 1.1 MPa (160 psig) and/or above 120 °C (250 °F) fall under ASME Section I rules (the same code used for steam boilers); units below both thresholds may be built to the lighter ASME Section IV rules. A high-limit control — a secondary device that should never activate in normal operation — protects against exceeding design pressure/temperature.

HTHP hot water boilers — expansion tank, pump, and chemistry

An expansion (compression) tank, built to ASME Section VIII, absorbs the water's thermal expansion as the system heats from ambient to operating temperature; if it is undersized, the boiler's relief valves open and the system loses treated water. A circulating pump (air-cooled to about 163 °C, or water-jacketed to about 204 °C) moves the water, and an air separator prevents trapped air from air-locking flow to a convector or radiator. Chemistry is simpler than a steam boiler's: a sodium-nitrite blend (with small amounts of silicate, borate, sodium hydroxide) forms a passivation layer protecting iron and low-grade stainless steel, with other additives protecting brass, copper, and aluminum.

HTHP hot water boiler shutdown checklist

Before a long seasonal shutdown ends, a fixed sequence of checks applies: (1) review manufacturer start-up recommendations; (2) confirm the blowoff valve is closed and manhole/handhole covers are torqued correctly; (3) open the vent and fill the boiler, closing the vent when water issues; (4) check the expansion tank and altitude gauge show a proper fill; (5) confirm main-burner and pilot fuel valves are closed; (6) check fuel supply availability; (7) confirm make-up air is unobstructed; (8) set the control switch off, then power up the boiler and circulating pump; (9) check the high-limit switch and operating control settings; (10) check the low-water cut-off and high-limit manual reset.

HTHP hot water boiler start-up sequence

Start-up then follows its own order: open valves and start the circulating pump, open pilot/burner fuel valves, purge the furnace of combustibles, light the pilot (if a standing pilot is used), set the burner switch to "on" so the main burner lights at minimum firing rate, slowly raise water temperature while checking for leaks throughout the system, and log time, problems, cut-off temperature/pressure, and any abnormal conditions once the operating control extinguishes the burner at the set cut-off temperature.

Misconceptions: circulation and pressure

1. Raising pressure does NOT improve natural circulation — it shrinks the density difference and REDUCES circulation, which is exactly why high-pressure and supercritical units need pumps.
2. 21 000 kPa is the practical pressure limit below which natural circulation is kept usable; 22 090 kPa is the critical pressure where natural circulation becomes physically impossible — do not confuse the two numbers.

Misconceptions: black liquor, OTSGs, and fluidized beds

3. The black-liquor smelt-water explosion is a PHYSICAL reaction (flash steam expansion, like a shock wave), not a chemical reaction of sodium with oxygen.
4. An OTSG's wet steam (75%–80% dryness) is intentional, to keep salts off the tubes — it is not a sign of poor performance or incomplete boiling.
5. A bubbling fluidized bed (BFB) has a distinct bed level; a circulating fluidized bed (CFB) has none and continuously recirculates solids through a cyclone — do not swap which is which.

Misconceptions: boiler ratings and shapes

6. Steam capacity in kg/h is not a true thermal rating on its own — steam quality, steam temperature, and feedwater temperature all matter; use the heating-surface (or kW) method for a true rating.
7. The O-type is symmetrical but exposes the LEAST tube surface to radiant heat of the three shapes — symmetry does not mean the most heat absorption.
8. A-, D-, and O-type are ALL integral-furnace boilers (their walls are water-cooled tubes); "integral furnace" is a shared property of all three shapes, not a separate fourth design.

Misconceptions: cogeneration, codes, and economizers

9. Cogeneration (up to ~80% total energy utilization, using TWO forms of energy from one fuel) is not the same idea as combined-cycle efficiency (~50%+, a single electrical-efficiency figure) — keep the two numbers and the two definitions apart.
10. The ASME Section I/IV boundary for hot water boilers is 1.1 MPa (160 psig) AND/OR 120 °C (250 °F) — either threshold alone triggers Section I, not just pressure.
11. An economizer's water exit temperature must stay below the drum's saturation temperature; if it reached saturation the economizer itself would start steaming, which the once-through convection feedwater heater is not designed to do.

Source: PanGlobal Third Class, Part B1 (Boilers), Chapters 1–2; SOPEEC 3rd Class Paper 3B1.