Watertube Circulation, Drums and Heat-Recovery Surfaces

Watertube Boilers

In a watertube boiler, water and steam circulate inside small-diameter tubes while hot combustion gases flow across their external surfaces within the furnace and convection banks.

Thermosiphon Natural Circulation

Watertube boilers rely primarily on natural thermosiphon circulation, driven by the difference in mass density between liquid water and a boiling two-phase mixture:

1. Downcomers: Large-bore unheated (or lightly heated) pipes located outside the main gas stream convey cooler, dense liquid water ($\rho \approx 750-900\text{ kg/m}^3$) from the upper steam drum downward to the lower mud drum and bottom waterwall headers.
2. Risers (Steam Generating Tubes): The furnace waterwall tubes absorb intense radiant heat from the flame. Subcooled water entering the tube bottom reaches saturation and boils, generating steam bubbles. The resulting two-phase steam-water emulsion has a significantly lower bulk density ($\rho_{\text{mix}} \approx 200-450\text{ kg/m}^3$).
3. Circulation Driving Head: The hydrostatic pressure exerted by the cold downcomer leg exceeds that of the hot riser leg, establishing a powerful continuous upward flow through the risers:

$$\Delta P_{\text{circ}} = (\rho_{\text{downcomer}} - \rho_{\text{riser}}) \cdot g \cdot H$$

Where $H$ is the vertical height of the boiler circuits. The density difference generally diminishes as pressure approaches the critical point. Natural, assisted/forced and once-through arrangements have design-specific operating limits; no universal 15–18 MPa cutoff selects the type.

Drum Functions & Membrane Waterwalls

• Steam Drum: The upper pressure vessel serves as the central manifold that receives the steam-water mixture from risers, houses internal steam-purification equipment, distributes cold feedwater evenly along its length, provides chemical treatment injection, and establishes the normal operating water level (NOWL).
• Mud Drum: The lower pressure vessel acts as a quiet settling chamber where insoluble precipitated mineral sludge collects away from direct furnace heat, allowing safe blowdown via bottom blowoff valves.
• Membrane Waterwalls: Modern furnace walls are constructed of vertical water tubes connected continuously along their lengths by welded solid steel fins or membrane bars. This forms a membrane enclosure intended to reduce gas leakage and refractory requirements; seals, penetrations and the actual wall still require inspection.

Packaged Bent-Tube Configurations

Shop-assembled watertube boilers utilize bent tubes entering the drums radially, categorized by their cross-sectional profile:

• D-Type Boiler: Features two drums: a upper horizontal steam drum and an offset mud drum directly below it. Waterwall tubes extend horizontally across the floor, vertically up the outer wall, across the roof, and back down into the steam drum, forming the letter "D". The combustion burner fires into the large open furnace space, while convection tubes occupy the space between drums. Highly popular for industrial plants.
• A-Type Boiler: Features one central upper steam drum connected to two smaller lower mud drums by two symmetrical banks of bent tubes, forming the shape of an "A". Symmetrical weight distribution and central furnace firing make it ideal for marine and heavy industrial service.
• O-Type Boiler: Features a central upper steam drum positioned directly over a lower mud drum, with tubes curving outward symmetrically on both sides in an "O" configuration. Its narrow, compact cross-section conforms to standard railway transport clearances.

Steam Drum Internals & Purification

High-pressure steam turbines and superheater tubes demand exceptionally pure steam. If saturated steam leaving the drum carries liquid water droplets (carryover), dissolved salts and silica deposit inside superheater tubes (causing localized tube overheating and rupture) or form hard mineral deposits on turbine blades (causing severe unbalance and catastrophic erosion). Steam-purity and moisture requirements depend on the boiler and downstream equipment; use their specified limits:

1. Primary Cyclone Separators: High-velocity steam-water mixture from the risers enters vertical cylindrical cyclone tubes tangentially. Centrifugal force spins the dense liquid water outward against the cyclone cylinder wall where it drains down into the water mass, while dry steam exits axially upward through the top.
2. Secondary Chevron Mist Eliminators (Vane Separators): Banks of closely spaced corrugated stainless steel plates mounted above the cyclones. Steam is forced through tortuous zigzag paths; water droplets, unable to negotiate the rapid directional changes due to inertia, impinge on the corrugated surfaces and drain back down.
3. Corrugated Scrubber Screens (Wire Mesh Demisters): A fine mesh screen providing final mechanical polishing to trap sub-micron mist particles.
4. Internal Feedwater Pipe: A perforated pipe extending along the drum below the NOWL that distributes cooler feedwater uniformly to prevent localized thermal shock.
5. Continuous Blowdown (CBD) Pipe: Located $25-50\text{ mm}$ below the NOWL, at the designed withdrawal location to control boiler-water concentration, continuously skimming water to maintain water chemistry.
6. Chemical Feed Quill: Injects boiler water conditioning chemicals (such as sodium phosphate and oxygen scavengers) directly into the water mass, discharging away from drum walls.

Heat Recovery Equipment

Heat-recovery equipment can improve performance within the design, fuel and operating limits:

1. Superheaters & Reheaters

• Superheaters: Raise the temperature of dry saturated steam above its saturation boiling point at constant pressure, increasing cycle thermodynamic efficiency and reducing moisture-related erosion in steam turbine blades.
  • Convective Superheater: Located in the convective gas passes. Steam temperature increases as boiler load increases (higher flue gas mass flow and velocity).
  • Radiant Superheater: Located directly in the furnace facing the flame. Steam temperature decreases as load increases (furnace temperature does not rise as rapidly as steam flow increases).
  • Pendant (Vertical Hanging): Tubes hang vertically from overhead headers outside the gas duct; cannot be drained by gravity, requiring slow, careful boil-out during startup.
  • Horizontal Superheater: Fully drainable by gravity, simplifying wet storage and chemical cleaning.
• Reheaters: In power utility cycles, partially expanded steam exhausted from the high-pressure turbine is returned to the boiler reheater, reheated back to initial throttle temperature ($540-565^\circ\text{C}$), and returned to the intermediate/low-pressure turbine stages, drastically reducing exhaust moisture.

2. Economizers

An economizer is a bank of serpentine tubes located in the lower-temperature flue gas duct downstream of the boiler convection bank. It extracts residual thermal energy from exiting flue gas to preheat incoming boiler feedwater before it enters the steam drum.

• Economizers also reduce thermal shock inside the steam drum by minimizing the temperature delta between incoming feedwater and saturated drum water.

3. Air Preheaters (APH)

An air preheater transfers waste heat from the flue gas exiting the economizer into incoming cold combustion air before it enters the burners.

• Recuperative Air Preheaters: Plate-type or tubular heat exchangers where flue gas and combustion air flow continuously on opposite sides of stationary metal partition walls.
• Regenerative Air Preheaters (Ljungström Type): A heavy cylindrical rotor packed with corrugated metal heat-storage baskets slowly rotates (approx. $1-3\text{ RPM}$) through alternating halves of a split casing. Flue gas flows through one half, heating the baskets; as the rotor turns into the opposite half, incoming cold combustion air strips the stored heat from the hot steel baskets.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2.