A Feed Worm and Air Blast is a two-stage fuel delivery mechanism that meters pulverised solid fuel into a boiler furnace using a rotating screw conveyor (the feed worm) and then disperses that fuel into the combustion zone with a high-velocity air jet (the air blast). The arrangement appeared in fixed boiler practice from the 1890s onward, with engineers like James Howden refining forced-draught variants for marine service. The worm sets the mass flow of fuel; the blast atomises and projects it into suspension so it burns in flight rather than on a grate. The result is faster steam raising, higher combustion efficiency, and the ability to fire low-grade fuels like lignite dust or sawmill waste.
Feed Worm and Air Blast Interactive Calculator
Vary screw speed, worm displacement, fuel density, air ratio, and blast velocity to size fuel feed, primary air flow, nozzle diameter, and jet momentum.
Equation Used
The feed worm is modeled as a positive-displacement screw: each revolution advances a fixed fuel volume. Multiplying displacement by RPM and bulk density gives fuel mass flow. The selected air/fuel ratio sets primary air mass flow, and the blast velocity converts that air flow into an equivalent round nozzle diameter.
- Feed worm meters a fixed displaced volume per revolution.
- Fuel is dry pulverised solid with steady bulk density.
- Primary air density is 1.2 kg/m3.
- Nozzle is treated as a single equivalent round opening.
How the Feed Worm and Air Blast Actually Works
The feed worm is just an Archimedean screw turning inside a close-fitting tube. Pulverised fuel — typically coal dust ground below 75 µm, or wood flour, or rice husk — drops from a hopper into the worm's inlet, and each rotation advances a fixed volume of fuel toward the discharge. Because the worm meters by displacement rather than gravity, the feed rate stays linear with shaft RPM down to about 5 RPM, which is what lets the operator control firing rate cleanly. Clearance between the flight tip and the trough wall must sit between 0.5 and 1.5 mm — tighter and the worm jams on tramp material, looser and fines pack into the gap and back-flow up the screw, which kills your feed accuracy.
At the discharge end, the fuel meets the air blast. This is a primary-air jet from a centrifugal fan or a steam ejector, delivered through a venturi or annular nozzle at 30 to 60 m/s. The blast does two jobs at once: it picks the fuel out of the worm's discharge so it cannot pile up and choke the screw, and it atomises the dust cloud into the furnace so each particle is surrounded by enough oxygen to burn in suspension. Get the fuel-air ratio wrong on the primary air — typically you want around 1.8 to 2.2 kg of primary air per kg of fuel — and you either get rope flow (fuel falls out of the airstream and slumps to the furnace floor) or you get scorching at the nozzle from over-aerated combustion starting before the flame front establishes.
Common failure modes are predictable. A worn worm flight (more than 10% loss off the nominal flight diameter) under-feeds at any given RPM, so steam pressure sags even though the indicator says firing rate is fine. A blocked or partially eroded blast nozzle makes the fuel jet asymmetric, and you get one-sided furnace heating that warps tube banks over a season. Wet fuel — anything above about 12% moisture for coal dust — clumps in the hopper and bridges over the worm inlet, starving the feed entirely. The fix in service is almost always upstream of the worm itself, in the drying or hopper agitation stage.
Key Components
- Feed Worm (Screw Conveyor): Helical screw, typically 75 to 250 mm in flight diameter, rotating at 5 to 60 RPM inside a fitted trough. Meters pulverised fuel by positive displacement so feed rate stays proportional to shaft speed. Flight pitch usually equals flight diameter for free-flowing dust, dropping to 2/3 pitch for sticky or high-moisture material.
- Hopper and Agitator: Holds 30 minutes to several hours of pulverised fuel above the worm inlet. A slow paddle or vibrator prevents bridging — without it, fines pack into a stable arch and the worm runs empty under load, which the operator only notices when steam pressure drops 30 seconds later.
- Primary Air Fan or Steam Ejector: Delivers the blast air at 30 to 60 m/s through the dispersal nozzle. Sized to give 1.8 to 2.2 kg air per kg fuel at full firing rate. Centrifugal fans dominate fixed plant; steam ejectors appear in older marine installations where electrical primary-air supply was not yet standard.
- Dispersal Nozzle (Venturi or Annular): The throat where worm discharge meets the air blast. Geometry sets the spray cone angle, typically 15 to 25° half-angle. Erosion-resistant lining (chilled iron or ceramic) is mandatory because fuel particles at 50 m/s will sandblast a mild-steel nozzle through in weeks.
- Drive Gearmotor: Variable-speed unit, usually a worm-reducer plus VFD-controlled motor, giving 5 to 60 RPM at the screw shaft. Torque sizing must allow for a 3× starting load if any fuel has settled and compacted in the trough overnight.
Who Uses the Feed Worm and Air Blast
You see Feed Worm and Air Blast arrangements wherever a boiler needs to burn powdered or finely divided fuel without a traditional grate. The technology underpins most modern pulverised-fuel power generation, but it also shows up in heritage steam plant, biomass boilers, and industrial process heating where the fuel is a by-product. The mechanism's strength is fuel flexibility — anything you can grind to dust and dry below 12% moisture, the worm-and-blast can fire.
- Power Generation: Pulverised-coal boilers at stations like Drax in North Yorkshire (now converted to biomass pellets), where each burner front carries a worm feeder paired with a primary-air dispersal nozzle for every mill output line.
- Marine Steam (Heritage): Howden-pattern forced-draught firing arrangements on early 20th-century steamers, including auxiliary boilers on preserved vessels at the Scottish Maritime Museum at Irvine.
- Sawmill and Wood Processing: Sawdust-fired package boilers at operations like the Hull-Oakes Lumber sawmill in Oregon, where worm feeders take dust direct from the cyclone and blast-fire it into a refractory-lined combustion chamber.
- Cement Manufacturing: Rotary kiln burner pipes where pulverised coal or petcoke is metered by a screw feeder and conveyed pneumatically into the kiln hood at plants such as Hanson Cement Ribblesdale in Lancashire.
- Biomass District Heating: Wood-pellet and miscanthus-fired district boilers across Scandinavia, including municipal plants in Växjö, Sweden, where worm-fed pneumatic burners modulate over a 4:1 turndown for heat-load tracking.
- Agricultural Drying: Rice-husk-fired hot-gas generators at parboiling mills in West Bengal and the Mekong Delta, using a simple worm-and-blast burner to fire husks straight from the dehulling line.
The Formula Behind the Feed Worm and Air Blast
What you actually need to size is the fuel mass flow rate the worm delivers at a given shaft RPM, because that sets your boiler firing rate and, indirectly, your steam output. At the low end of the typical operating range — say 10 RPM on a 150 mm worm — you are at minimum stable firing, where flame stability becomes marginal and you may need to support combustion with oil. At the high end, 50 to 60 RPM on the same worm, you hit the worm's volumetric ceiling and any further demand has to come from a larger screw or a parallel feeder. The sweet spot for most installations sits around 40 to 60% of maximum RPM, where the worm is fully filled, the dispersal cone is well-formed, and turndown response stays linear.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁfuel | Fuel mass flow rate delivered by the worm | kg/s | lb/s |
| ρbulk | Bulk density of the pulverised fuel | kg/m³ | lb/ft³ |
| ηfill | Trough fill efficiency (fraction of theoretical screw volume actually carrying fuel) | dimensionless | dimensionless |
| D | Outside diameter of the screw flight | m | ft |
| d | Diameter of the central shaft | m | ft |
| P | Pitch of the screw flight (axial advance per revolution) | m | ft |
| N | Rotational speed of the worm shaft | rev/s | rev/s |
Worked Example: Feed Worm and Air Blast in a sawmill biomass boiler
You are sizing the pulverised sawdust feed rate across three worm speeds on a recommissioned 1948 Cleaver-Brooks dust-fired package boiler being returned to service at a heritage sawmill operation in Oregon's Willamette Valley, where the boiler raises 6,000 lb/hr of saturated steam at 100 psig for kiln drying. The worm has a flight outside diameter of 150 mm, a 50 mm central shaft, a pitch of 150 mm, and the dried sawdust has a bulk density of 220 kg/m³ with a measured trough fill efficiency of 0.35.
Given
- D = 0.150 m
- d = 0.050 m
- P = 0.150 m
- ρbulk = 220 kg/m³
- ηfill = 0.35 dimensionless
- Nnominal = 30 RPM
Solution
Step 1 — compute the swept cross-sectional area of the screw annulus:
Step 2 — at nominal 30 RPM, convert to rev/s and compute fuel mass flow:
That is roughly 720 lb/hr of dry sawdust, which matches the heat input you need for 6,000 lb/hr of steam at typical sawdust calorific value (around 18 MJ/kg) and 75% boiler efficiency. Comfortable working point.
Step 3 — at the low end of the typical operating range, 10 RPM:
At 109 kg/hr you are at minimum continuous firing — flame stability gets twitchy because the dispersal cone is under-fed and the air blast starts dominating the fuel jet. Some installations bring oil pilots in below this point. At the high end, 60 RPM:
653 kg/hr is the worm's volumetric ceiling on this geometry. In practice you will not reach it cleanly because ηfill drops off above 50 RPM as fuel can no longer settle into the flights between the inlet's gravity feed and the next thread coming round. Expect actual delivered flow at 60 RPM to come in 10 to 15% short of the calculated number.
Result
At nominal 30 RPM the worm delivers 0. 0907 kg/s, or roughly 327 kg/hr of pulverised sawdust — exactly the firing rate the heritage Cleaver-Brooks needs for its 6,000 lb/hr steam duty. The 10 RPM low-end gives 109 kg/hr (minimum stable firing, flame may need pilot support) while the 60 RPM high-end theoretically gives 653 kg/hr but realistically tops out near 555 kg/hr as fill efficiency collapses. If your measured fuel flow comes in 20% or more below the predicted number at a given RPM, the most likely causes are: (1) hopper bridging starving the worm inlet so ηfill is actually closer to 0.20 than 0.35, (2) flight wear that has opened the tip-to-trough clearance beyond 1.5 mm and is allowing back-flow up the screw, or (3) wet fuel above 12% moisture clumping on the flights and reducing effective bulk density at the discharge.
Feed Worm and Air Blast vs Alternatives
Worm-and-blast is one of three main ways to get solid fuel into a boiler. The other two — gravity-fed travelling grates and direct pneumatic conveying without a screw — each fit different fuel types, firing rates, and turndown requirements. The comparison below shows where each one earns its place.
| Property | Feed Worm and Air Blast | Travelling Grate Stoker | Direct Pneumatic Conveying |
|---|---|---|---|
| Typical firing rate range | 100 to 5,000 kg/hr per feeder | 500 to 50,000 kg/hr per grate | 1,000 to 200,000 kg/hr per line |
| Fuel size requirement | Pulverised, <75 µm to 3 mm | Sized lump, 6 to 50 mm | Pulverised, <75 µm |
| Turndown ratio | 4:1 typical, linear with RPM | 3:1, limited by grate speed | 5:1 with variable-speed feeders |
| Combustion efficiency | 80 to 88%, suspension burn | 75 to 82%, grate burn | 85 to 92%, full suspension |
| Capital cost (relative) | Medium | High (mechanical grate) | Very high (mill + classifier + lines) |
| Maintenance interval | 6 to 12 months (worm flight wear) | 12 to 24 months (grate bar replacement) | 3 to 6 months (pipe erosion) |
| Fuel flexibility | High — any dryable solid | Low — sized solids only | Medium — pulverisable solids only |
| Best application fit | Mid-size biomass and dust-firing boilers | Coal-fired industrial and utility boilers | Large utility pulverised-coal stations |
Frequently Asked Questions About Feed Worm and Air Blast
You are almost certainly seeing hopper bridging and collapse. Pulverised fuel forms a stable arch over the worm inlet, the screw runs empty for 20 to 40 seconds, the arch collapses under its own weight, and you get a brief surge of fuel that over-fires the boiler. Steam pressure rises, then sags as the next arch forms.
Diagnose it by watching the worm motor current — if it pulses between near-zero and full load on the same period as the pressure cycle, that's bridging. Fix is upstream: a slow paddle agitator in the hopper, a vibrator on the hopper wall, or a steeper hopper cone (mass-flow geometry, 60° or steeper to horizontal).
Two smaller worms win on turndown and on redundancy. A single 250 mm worm running 5 to 60 RPM gives you 12:1 mechanical turndown but only about 4:1 useful turndown because flame stability collapses at low fuel flow. Two 150 mm worms in parallel let you shut one down entirely at low load and run the other at a comfortable mid-speed, so you stay in the stable combustion window across an 8:1 firing range.
The cost penalty is real though — you pay for two gearmotors, two VFDs, and two dispersal nozzles. Single-worm makes sense if you run flat-out almost continuously. Twin-worm makes sense for district heating, kiln drying, or any duty with daily load swings.
Check the fill efficiency assumption first. The textbook 0.35 only holds for free-flowing dry dust delivered to a fully flooded inlet. If your hopper outlet is restricted, or the worm is inclined more than 15° from horizontal, ηfill drops to 0.20 or lower, and 30% short is exactly what you'd see.
The other common miss is bulk density. Operators often use a handbook value (220 kg/m³ for sawdust, say) when the actual fuel as delivered is fluffed up to 160 kg/m³ because the pneumatic conveying line aerated it. Take a sample, weigh a known volume, and recompute.
Aim for 30 to 60 m/s at the nozzle throat. Below 30 m/s the fuel jet does not maintain suspension — particles fall out of the airstream within a metre of the nozzle and slump to the furnace floor as unburnt fines, which then either clinker or get carried up as ash and foul the convection bank.
Above 60 m/s you over-penetrate the furnace, the flame front pushes against the back wall, and you get tube damage from impingement. You also accelerate nozzle erosion sharply — wear rate scales roughly with velocity cubed, so a 70 m/s jet wears the throat about three times faster than a 50 m/s jet.
Pre-drying is mandatory below about 12% moisture for the worm-and-blast configuration to work. Above that, two failures stack up: the dust cakes on the flights and back-flows up the screw, and the wet particles refuse to ignite in suspension because too much combustion heat goes into evaporating bound water before the particle reaches ignition temperature.
For green chips at 40 to 50% moisture, you want a moving-grate or fluidised-bed firing system instead — the longer residence time on the grate or in the bed gives the fuel time to dry and ignite. Worm-and-blast is the right answer only after a flue-gas dryer or rotary drum dryer has taken the fuel below 12%.
Asymmetric dispersal nozzle wear. Pulverised fuel at 50 m/s erodes the inside of the nozzle preferentially on the side facing the worm discharge, because the fuel jet exits the worm with a slight directional bias and impinges that wall. Over six months you can develop a 2 to 3 mm scallop that deflects the spray cone off-axis by 10 to 15°.
The hot side of your furnace is where the deflected cone is now pointing. Pull the nozzle, measure the throat profile against the as-built drawing, and replace it if any dimension is more than 1 mm out. While you are in there, check the worm tip clearance too — uneven flight wear can also bias the discharge direction.
For most pulverised-fuel worm-and-blast burners, stable unsupported combustion holds down to about 25 to 30% of maximum continuous rating. Below that, three things go wrong: the dispersal cone becomes under-fed and the air blast over-aerates the flame, furnace temperature drops below the fuel's ignition threshold (around 600°C for coal dust, 350°C for wood dust), and flame radiation back to the burner front is no longer enough to ignite incoming fuel.
The fix is a small oil or gas pilot rated for around 5% of MCR, kept lit whenever firing rate drops below 30%. Modern installations interlock the pilot to flame-detector signal so it cuts in automatically.
References & Further Reading
- Wikipedia contributors. Pulverized coal-fired boiler. Wikipedia
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