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 — and on a black-liquor recovery unit, a tube leak onto molten smelt can detonate the furnace. When you size up a boiler on the plant floor, a chief engineer asks three things: how was it assembled, how does its water circulate, and what is it built to burn? Get those three right and the rest of the design falls into place.

Assembly classes. A packaged watertube boiler leaves the factory complete — burner, FD fan, controls, steel casing, on a skid foundation, bottom-supported — with capacities from 2300 kg/h to over 65 000 kg/h, pressures usually to 1700 kPa (sometimes over 6200 kPa); its great advantage is low cost. A shop-assembled boiler is built to the purchaser's exact spec, shipping the steam-generating section largely complete while burners, draft equipment, stack, and external controls are erected on site. A field-erected boiler is too large to ship whole, so waterwall, superheater, and economizer sections are made in the plant and welded together on site; most exceed 150 000 kg/h.

Boiler rating methods. Provincial regulations recognize three rating methods: (1) one square metre of heating surface equals 10 kW; (2) for electric boilers, the rating is the maximum kW of the heating element; (3) where neither applies, an hourly output of 36 MJ equals 10 kW. The exam trap: steam capacity in kg/h is NOT a true thermal measure, because steam quality, steam temperature, and feedwater temperature all change how much heat that mass actually carries.

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.

$\text{Rating (kW)} = A_{heating} \times 10$

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.

Downcomers and risers. In a watertube boiler the tubes form circuits of downcomers (cool, dense, removed from the furnace) and risers (hot, exposed to radiant heat). The dense water in the downcomers falls and pushes the lighter steam-water mixture up the risers — this density difference is the engine of natural circulation, with no pump involved. Remove the density difference and the circulation stops.

The four factors affecting circulation. Four factors influence natural circulation. Steam-drum height above the mud drum — higher gives more circulation. Firing rate — more bubbles give more circulation. Operating pressure — higher pressure shrinks the density difference, so circulation DECREASES (natural circulation is limited below about 21 000 kPa). Tube cleanliness — fouling resists flow. Pressure is the counter-intuitive one and the lesson's key misconception.

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. Supercritical boilers operating above this point must therefore use forced circulation — pumps do the work that density can no longer do.

Once-through boilers. A once-through boiler has no drum and no recirculation: feedwater is pumped through one continuous circuit, evaporating (about 85% in the radiant zone) and superheating in a single pass. With nothing recirculating, there is no natural-circulation loop at all — the feed pump sets the flow, which is why this design suits supercritical pressures.

Quick recap before configurations. Before moving to drum geometry, lock in the headline: natural circulation is driven by the density difference between downcomer water and riser mixture, and RAISING pressure SHRINKS that difference and REDUCES circulation — the reason high-pressure, supercritical, and once-through units rely on pumps rather than density. Hold that idea; the next beats are about shapes, not circulation.

Structural configurations (A, D, O). An A-type has two small lower drums or headers and one large upper steam drum, with the steam drum centred above the two mud drums forming the 'A' shape. A D-type has two drums: bent tubes on one side combine with the generating bank on the other to enclose the furnace, forming the 'D'. An O-type has an upper and a lower drum with the connecting tubes arranged in an 'O' that surrounds the furnace — a symmetrical design, but it exposes the LEAST tube surface to radiant heat.

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

Once-through steam generators (OTSG). OTSGs for SAGD bitumen recovery deliberately produce wet steam at 75%–80% dryness (20%–25% moisture) to keep dissolved salts in the water portion and off the tubes; they have no steam drum and run to 20 000 kPa. The wet steam is the point — the moisture carries the salts that would otherwise foul and overheat the tubes.

Fluidized bed boilers. A fluidized bed boiler burns crushed coal (1.6–6 mm) in a bed of inert granular material fluidized by air, running cool at 800–900 degC versus 1600–1900 degC for pulverized coal. That low temperature does two things: limestone added to the bed cuts SO2 by up to about 80%, and the cooler flame cuts thermal NOX. Two sub-types: bubbling fluidized bed (BFB) has a distinct bed level; circulating fluidized bed (CFB) uses more air, has no distinct level, and recycles solids through a hot cyclone for long residence time.

HRSGs. A heat-recovery steam generator (HRSG) — also called a waste-heat recovery boiler (WHRB) or turbine exhaust gas boiler — recovers gas-turbine exhaust heat as its primary heat source. Most horizontal-gas-flow units use natural circulation with both a steam drum and a water drum, and may be unfired (waste heat only) or duct-fired to raise inlet gas temperature. The exhaust gas is the heat source, so the HRSG produces steam from energy that would otherwise go up the stack.

Black liquor recovery boilers — the smelt-water hazard. A black liquor recovery boiler burns the organic content of kraft-pulp spent liquor, and inorganic smelt collects molten on the floor. Water contacting molten smelt causes a violent PHYSICAL (not chemical) explosion — the gases expand extremely rapidly, like a shock wave. To prevent it, liquor must be fired above about 62% solids, and many units carry an emergency rapid-drain system. This is the lesson's safety-critical fact.

Refuse and biomass boilers. A refuse boiler burns municipal waste (mass-burning the fuel as received, or a screened/shredded prepared fuel) and fights severe chloride corrosion: the higher the tube temperature the worse it gets, so all mass-fired units use refractory-lined lower furnace walls, and bimetal tubes (carbon steel with an outer Inconel — nickel-chromium — layer) protect against metal loss. A biomass boiler burns wood, bark, sawdust, and similar materials for process steam and power. Both burn difficult, variable fuels, so corrosion protection and fuel handling dominate their design.

Common misconceptions and exam traps. (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 limit of natural circulation; 22 090 kPa is the critical pressure where it becomes outright impossible — do not confuse the two. (3) The black-liquor smelt-water explosion is PHYSICAL (rapid gas expansion), not a chemical reaction of sodium with oxygen. (4) An OTSG's wet steam is intentional (to carry salts away from the tubes), not a sign of poor performance. (5) BFB has a distinct bed level; CFB has none and recirculates solids through a cyclone. (6) Steam capacity in kg/h is not a true thermal rating — use the heating-surface (or kW) method. (7) The O-type is symmetrical but exposes the LEAST tube to radiant heat — symmetry does not mean most heat absorption. (8) A, D, and O are ALL integral-furnace boilers (water-cooled walls); integral furnace is not a fourth shape.

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