A Steam Soot Sucker is a steam-jet ejector fitted to a boiler smokebox or flue that uses a high-velocity jet of motive steam to entrain flue gas and loose soot, dragging fireside deposits out of the tube nest. The convergent-divergent motive nozzle is the heart of it — it accelerates steam to roughly Mach 2 and creates the suction that pulls soot-laden gas through the throat. The job is to keep tube surfaces clean between full sweep-outs so heat transfer stays at design figures. A neglected boiler can lose 15-20% of its evaporation rate to soot before anyone notices.
Steam Soot Sucker Interactive Calculator
Vary steam pressure, nozzle throat, steam temperature, entrainment ratio, and diffuser angle to see choked steam flow and soot-gas pickup.
Equation Used
This calculator sizes the motive steam passing through the soot sucker throat using the standard choked nozzle mass-flow equation, then multiplies by the entrainment ratio to estimate soot-laden flue gas draw. Keep the diffuser half-angle at or below about 7 deg to preserve pressure recovery.
- Dry saturated or superheated steam is modeled as an ideal gas.
- Nozzle throat is choked and discharges with Cd = 0.95.
- Steam properties use gamma = 1.30 and R = 461.5 J/kg-K.
- Gauge pressure is converted to absolute pressure using 1.01325 bar atmosphere.
The Steam Soot Sucker in Action
The mechanism is brutally simple in principle and unforgiving in detail. Motive steam at boiler pressure — typically 6 to 10 bar gauge for a heritage marine or industrial boiler — enters a convergent-divergent nozzle and expands to supersonic velocity. That high-velocity jet discharges into a mixing chamber connected to the smokebox or flue duct. The static pressure in the jet core drops well below atmospheric, and flue gas carrying entrained soot particles gets pulled into the mixing chamber, accelerated alongside the motive steam, and discharged either to atmosphere through a stack stub or back into the main chimney where the natural draught carries it away.
The geometry has to be right or it does nothing useful. The motive nozzle throat must be sized to pass the design steam mass flow at the available supply pressure — too small and you starve the jet, too large and you collapse the supersonic expansion and lose entrainment. The diffuser downstream of the mixing chamber recovers static pressure so the mixed flow can discharge against the back pressure of the chimney. If the diffuser angle exceeds about 7° per side the flow separates, recovery collapses, and the suction at the inlet falls to almost nothing. You see this on neglected units where someone has welded up an eroded throat without restoring the original profile — the unit hisses but pulls no gas.
Failure modes are mostly erosion and fouling. Wet motive steam erodes the nozzle throat within a season → you want at least 30°C of superheat at the nozzle inlet, or a properly sized separator immediately upstream. Soot itself can build up in the mixing chamber if the unit is left idle for long periods with the steam supply leaking, because cold condensate forms a slurry with deposited particles and sets like concrete. The other classic failure is a cracked motive pipe at the smokebox penetration, where thermal cycling fatigues the joint and you lose pressure before it ever reaches the nozzle.
Key Components
- Motive Steam Nozzle: A convergent-divergent (de Laval) nozzle that accelerates dry saturated or superheated steam from boiler pressure to roughly Mach 2 at the exit plane. Throat diameter is the critical dimension and is typically 4 to 12 mm on heritage units. Erosion of the throat by 0.2 mm shifts the design mass flow by enough to drop entrainment ratio by 10-15%.
- Suction Chamber: The annular space surrounding the nozzle exit where flue gas enters from the smokebox tap. Cross-sectional area must be 4-6× the nozzle exit area so gas can accelerate without choking the inlet. A common mistake is to fit a smaller bore stub which throttles the suction and gives the impression the nozzle is undersized.
- Mixing Chamber and Throat: A constant-area or slightly tapered section where motive steam and entrained flue gas exchange momentum. Length is typically 6-8× the throat diameter — too short and momentum exchange is incomplete, too long and friction losses dominate. The internal surface should be ground smooth; pitting from wet steam doubles friction loss.
- Diffuser: The expanding cone that recovers the kinetic energy of the mixed flow as static pressure so the discharge can overcome chimney back pressure. Half-angle must stay between 3° and 7° — outside that band flow separates from the wall and pressure recovery fails. Length is usually 10-15× the throat diameter.
- Steam Supply Valve and Strainer: An isolating valve and a fine mesh strainer (typically 60-100 mesh) on the motive steam line. The strainer catches scale and pipe debris that would otherwise lodge in the nozzle throat and ruin the jet pattern. The valve is usually a globe pattern so the operator can crack it open progressively to warm the unit through before going to full flow.
Industries That Rely on the Steam Soot Sucker
You find soot suckers anywhere a coal- or oil-fired boiler runs at full output for hours at a stretch and the operator cannot afford to drop fires for a manual sweep-out. Marine boilers, locomotive practice, sugar mill flues, and heritage industrial boilers all use them. The common thread is firetube or watertube geometry where soot deposits between the gas-side surfaces and the convective bank, a pressure source of dry steam available on demand, and a chimney with enough natural draught to carry the discharged plume clear of the working area.
- Heritage Marine: Smokebox soot ejector on the Scotch boiler of the SS Shieldhall preserved steamship at Southampton, run for 30 seconds at the end of each watch to clear the tube ends before deposits bake on.
- Preserved Railway: Steam-jet self-cleaning smokebox arrangement on Bulleid Pacifics at the Bluebell Railway, where the blast pipe geometry doubles as a continuous soot ejector during running.
- Sugar Milling: Diamond Power IK-525 retractable soot blower on bagasse-fired Stirling boilers at the Tully Sugar Mill in Queensland, sweeping the superheater pendants every shift change.
- Heritage Industrial: Hand-operated soot blower lance on the Lancashire boiler at the Kew Bridge Steam Museum in London, used during the cool-down at the end of each Sunday steaming.
- Power Generation: Clyde Bergemann sootblower fleet on the coal-fired Drax Power Station boilers in North Yorkshire, where dozens of wall-mounted lances cycle continuously across the furnace water-walls.
- Pulp and Paper: Recovery boiler smelt-bed soot ejectors on the Mondi Štětí mill in the Czech Republic, where black-liquor firing produces aggressive deposits that demand frequent cleaning.
The Formula Behind the Steam Soot Sucker
What you actually want to know is the entrainment ratio — how many kilograms of flue gas the unit pulls per kilogram of motive steam consumed. At the low end of the typical operating range, with a marginal supply pressure or a partially eroded throat, you might see ratios of 0.4 to 0.6 — the unit is working but inefficient and barely justifies the steam it burns. At nominal design conditions a well-built single-stage steam ejector delivers entrainment ratios of 1.0 to 1.5. Push the supply pressure to the high end of design with a clean nozzle and you can briefly see 1.8, but you are then close to choking the diffuser and any further pressure increase costs you ratio rather than gaining it. The sweet spot for heritage and industrial duty is right around 1.2.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ω | Entrainment ratio — mass of suction (flue) gas per unit mass of motive steam | dimensionless (kg/kg) | dimensionless (lb/lb) |
| ṁm | Mass flow rate of motive steam through the nozzle throat | kg/s | lb/h |
| ṁs | Mass flow rate of suction flue gas entrained from the smokebox | kg/s | lb/h |
| Pm | Motive steam absolute pressure at the nozzle inlet | bar absolute | psia |
| Ps | Suction chamber absolute pressure (smokebox side) | bar absolute | psia |
| Pd | Discharge absolute pressure at the diffuser outlet (chimney back pressure) | bar absolute | psia |
| ηnoz | Empirical nozzle and mixing efficiency, usually 0.75 to 0.90 for a well-built unit | dimensionless | dimensionless |
Worked Example: Steam Soot Sucker in a heritage Cornish boiler soot ejector
You are sizing the entrainment performance of a recommissioned single-stage steam-jet soot ejector being refitted to the smokebox of a 1908 Galloways Cornish boiler at the Markham Grange Steam Museum near Doncaster, where the boiler supplies process steam to a small horizontal mill engine driving a museum-display rope race. The trustees want the entrainment ratio confirmed at three operating conditions: a slow Sunday demonstration with motive supply throttled to 5 bar absolute, a nominal cleaning cycle at 8 bar absolute, and a brisk full-pressure burst at 11 bar absolute before the public open day. Discharge is to atmosphere through a short stub above the smokebox door at 1.0 bar absolute, suction-side smokebox pressure sits at 0.97 bar absolute, and the nozzle and mixing efficiency has been measured on a comparable unit at ηnoz = 0.82.
Given
- Pm,nom = 8.0 bar absolute
- Pm,low = 5.0 bar absolute
- Pm,high = 11.0 bar absolute
- Ps = 0.97 bar absolute
- Pd = 1.00 bar absolute
- ηnoz = 0.82 dimensionless
Solution
Step 1 — at the nominal cleaning cycle of 8.0 bar absolute motive supply, compute the motive-to-discharge pressure ratio and its square root:
Step 2 — the suction term captures how much driving pressure the smokebox side can support. With Ps = 0.97 bar and Pd = 1.00 bar, suction is below discharge so we work with the absolute-pressure formulation referenced to motive — at nominal:
That sits in the realistic band for a heritage single-stage soot ejector pulling against atmospheric discharge — ratios above 1.0 generally need a slight smokebox depression or a properly tuned chimney venturi to support them.
Step 3 — at the low end of the operating range, 5.0 bar absolute motive, the jet is weaker and the ratio falls:
The small drop from 0.77 to 0.73 hides what you actually feel in operation — the absolute mass flow of soot extracted falls by roughly a third because both the motive steam mass flow and the entrained gas mass flow scale with motive pressure. At 5 bar the unit clears loose flake but leaves baked-on deposits stuck to the tube ends.
Step 4 — at the high end, 11.0 bar absolute, push the same calculation:
Above about 9 bar the ratio plateaus — you are mass-flowing more steam through the throat and entraining more gas in absolute terms, but the ratio itself stops climbing because the diffuser starts to choke. That is why heritage practice settles around 7-8 bar for cleaning cycles even when the boiler can comfortably supply more.
Result
At the nominal 8. 0 bar motive supply the ejector pulls an entrainment ratio of about 0.77 kg of flue gas per kg of motive steam, which on a 30-second cleaning blast at perhaps 0.15 kg/s motive flow shifts roughly 3.5 kg of soot-laden gas — enough to visibly clear a slug of black smoke from the chimney for a few seconds. Across the operating range the ratio barely shifts (0.73 at 5 bar, 0.77 at 8 bar, 0.78 at 11 bar) but the absolute soot extraction roughly doubles between the low and high points because mass flow scales with pressure. The sweet spot for this Cornish boiler installation is 7-8 bar — past that you burn steam without measurable cleaning gain. If your measured ratio falls noticeably below 0.7 in service, the three usual culprits are: (1) wet motive steam eroding the convergent-divergent nozzle throat oversize, which collapses supersonic expansion, (2) a partially blocked steam strainer upstream throttling supply pressure below the nameplate value, or (3) a misaligned diffuser cone where a previous repair left the half-angle outside the 3-7° band and the flow separates from the wall.
When to Use a Steam Soot Sucker and When Not To
A steam soot sucker is one of three common ways to keep fireside surfaces clean on a working boiler. The other two are mechanical rotary soot blowers driven by an external motor with a steam lance, and old-fashioned manual brushing during cool-down. Each suits a different scale of plant and a different operating regime.
| Property | Steam Soot Sucker (steam-jet ejector) | Rotary Soot Blower (Diamond/Clyde Bergemann type) | Manual Brush Sweep-Out |
|---|---|---|---|
| Cleaning cycle duration | 20-60 seconds per blast | 2-5 minutes per lance | 4-12 hours including cool-down |
| Steam consumption per cycle | 3-8 kg of motive steam | 20-80 kg per lance traversal | Zero (boiler is shut) |
| Capital cost (heritage scale) | £400-£1,200 fabricated unit | £8,000-£25,000 per lance installed | Negligible (brushes and rods) |
| Performance on baked-on deposits | Poor — clears loose flake only | Good — direct jet impingement at ~10 bar | Excellent — manual chipping reaches anything |
| Maintenance interval (nozzle/throat) | 1 season between throat re-machining | 12-18 months between lance overhauls | Brush replacement annually |
| Suitable boiler scale | Small to medium heritage boilers, <500 kg/h steam | Industrial watertube, 10-1000 t/h steam | Any size during scheduled outage |
| Operator skill required | Low — single valve operation | Medium — sequencing and overlap control | High — confined-space and safety procedures |
Frequently Asked Questions About Steam Soot Sucker
Counter-intuitive but common. At full boiler pressure the motive steam mass flow through a fixed throat scales linearly with absolute pressure, but if your discharge stub is short and dumps into the same chimney that the boiler is already pressurising under hard fire, the discharge back-pressure Pd rises with firing rate. The diffuser then has less pressure ratio to work against and entrainment collapses.
Diagnostic check: tap a manometer into the discharge stub and watch what happens to chimney pressure when you open the firehole door briefly. If discharge pressure swings by more than 50 mbar with firing changes, you need a separate discharge route or a longer stub clear of the main chimney plume.
Dry saturated will work for a season or two, then your throat will be oversize and your entrainment ratio will be in the floor. Even 2-3% wetness in the motive steam delivers liquid droplets at supersonic velocity into the throat wall, and they cut metal like a waterjet — you can lose 0.1-0.3 mm per season on a mild-steel nozzle.
Rule of thumb: aim for at least 30°C of superheat at the nozzle inlet, or fit a baffle-type separator immediately upstream of the supply valve. On Cornish and Lancashire boilers without a superheater, a separator is non-negotiable. On locomotive practice the steam is usually superheated already so the issue rarely arises.
It comes down to where the deposits actually form. A smokebox-mounted ejector pulls loose particulate out of the gas stream and clears the tube ends, but it does almost nothing for baked-on deposits midway down the tubes or on superheater pendants. If your boiler runs hard for long periods and you see deposits more than 1.5 mm thick on tube interiors at the cool end, you need lance-type blowers that put a direct jet onto the surface.
Decision rule: ejectors for boilers under 500 kg/h evaporation that fire intermittently, lance blowers for anything continuously fired above 1 t/h. The cost step between the two is roughly 20× so the decision matters.
Half the predicted ratio almost always means the motive jet is not reaching design velocity. The single most common cause on a refurbished unit is the diffuser half-angle being wrong — if a previous repair built up an eroded throat with weld and the operator filed it back by eye, the half-angle is usually too steep, the flow separates from the wall in the diffuser, and pressure recovery falls to almost nothing.
Check it with a straight-edge and a vernier protractor. Half-angle must be 3-7°. Anything outside that band, you re-machine or you live with the loss. The second-most-common cause at this magnitude is a leaking flange between the suction stub and the smokebox — if atmospheric air can short-circuit into the suction chamber, the unit pulls air rather than smokebox gas and your useful entrainment crashes.
You can but you should not. Continuous running consumes motive steam at a rate that will measurably hurt your boiler evaporation budget — a typical heritage unit pulls 0.1-0.2 kg/s, which on a small Cornish boiler is 5-15% of total steam production. More importantly, continuous running provides no cleaning benefit beyond the first 30-60 seconds because once the loose particulate has been swept out, the ejector is just pumping clean flue gas around.
Industrial practice is 30-second blasts at intervals matched to the firing pattern — typically once per shift on a coal-fired Lancashire, every 4 hours on a hard-driven oil-fired marine boiler.
Cold air testing is misleading because the density and sonic velocity of the test fluid are nothing like steam at 8 bar and 200°C. Air at room temperature is denser than steam, so for the same nozzle pressure ratio you get more momentum and apparent suction during the test, even though the unit is undersized for steam duty.
The other trap is that compressed air at workshop pressure (usually 6-7 bar gauge) is well below the design motive pressure. A unit designed for 8 bar absolute steam produces a healthy supersonic jet on workshop air but will run subsonic when fed real steam at the wrong condition. Always validate with a flue-gas O2 reading or a smokebox manometer rather than trusting a cold test.
References & Further Reading
- Wikipedia contributors. Injector — Steam-jet ejector and related steam entrainment devices. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
