An Internal Fired Cylindrical Tubular Boiler is a horizontal fire-tube boiler in which the furnace fire burns inside one or more large flue tubes running through a water-filled cylindrical shell. It solves the problem of getting high heating surface and good thermal efficiency from a single self-contained pressure vessel without an external brick furnace. Hot gases pass through the internal flues, then return through smaller fire tubes or side flues, transferring heat to the surrounding water. The Lancashire and Cornish patterns delivered 7-9 lbs of steam per lb of coal at 80-160 psig and ran mills, ships, and locomotives for over a century.
The Internal Fired Cylindrical Tubular Boiler in Action
The shell is a riveted or welded cylinder, typically 5-9 ft diameter and 20-30 ft long, mounted horizontally in a brick setting. One or two large flue tubes run end to end through the shell — 3 ft diameter is common for a Lancashire pattern, a single 4 ft flue for a Cornish. The grate sits inside the front end of that flue, so the fire is genuinely *internal* — the entire furnace is surrounded by water on all sides except the firedoor end. Combustion gases travel down the flue, exit at the back, then double back along external side flues built into the brickwork, and finally return underneath before reaching the chimney. That triple pass is where the heating-surface area comes from.
Water level sits roughly 4-6 inches above the crown of the internal flue. If you let it drop below that crown the flue overheats, loses strength, and collapses inward — the classic furnace-crown failure that killed boilers and stokers in the 1870s and 1880s. That's why two independent water gauges and a fusible plug in the flue crown are mandatory. The flat ends of the shell are stayed with longitudinal through-stays because flat plate under pressure wants to bulge; you would be amazed how fast an unstayed 6 ft flat end deforms at 100 psig. Galloway tubes — short tapered water tubes crossing the flue diagonally — get fitted on later Lancashire boilers to add 15-20% more heating surface and break up gas stratification in the flue.
If the tube-to-tubeplate joints leak, you'll see it as white crusty deposits at the back tubeplate and a steady drop in steam pressure under load. If the flue itself bulges, it's almost always low water or scale buildup on the flue crown insulating the metal from the water. Scale above 1/16 inch on the fire side of the flue raises metal temperature by 200°F or more and you'll lose the flue inside a season.
Key Components
- Cylindrical Shell: The main pressure vessel, typically 7 ft diameter × 28 ft long for a Lancashire boiler, rolled from 5/8 to 7/8 inch steel plate and riveted or welded. It holds the water and steam space and carries the hoop stress from working pressure up to 160 psig.
- Internal Flue Tubes: One large flue (Cornish, ~4 ft diameter) or two parallel flues (Lancashire, ~3 ft each) running the full length of the shell. They carry the fire and first-pass gases, and are usually built from Adamson or Fox corrugated rings to resist collapse and absorb thermal expansion without distorting the shell.
- Firegrate and Firebridge: The grate sits inside the front of the internal flue, typically 6 ft long × the flue diameter wide. The firebridge at the back of the grate forces gases up against the flue crown to maximise radiant heat transfer to the water surrounding it.
- Galloway Tubes: Short tapered conical water tubes — typically 9 inches at the top, 7 inches at the bottom — crossing the internal flue diagonally. They add heating surface, promote water circulation, and act as additional flue stays. A typical Lancashire boiler carries 4-8 Galloway tubes per flue.
- Longitudinal Stays: Through-rods, 1.5 to 2 inches diameter, tying the front and back flat ends together to resist the pressure trying to push them outward. Stay pitch is typically 14-16 inches; spacing wider than that lets the flat plate bulge measurably between stays.
- Fusible Plug: A bronze plug filled with low-melting tin alloy threaded into the flue crown. If water drops below the crown, the plug melts at around 450°F and dumps steam into the firebox to alert the stoker before the flue itself fails.
- External Side and Bottom Flues: Brickwork passages built into the boiler setting that route the gases for the second and third pass along the outside of the shell. They convert otherwise wasted exhaust heat into additional shell heating surface, lifting overall efficiency from around 55% on a single-pass design to 70-75%.
Who Uses the Internal Fired Cylindrical Tubular Boiler
These boilers ran the 19th and early 20th century industrial world. Anywhere you needed steady saturated steam at moderate pressure with a coal grate and a chimney, this is what got specified. They are still found running in heritage settings because the design tolerates rough water, rough coal, and rough firing — qualities that were essential in a textile mill in 1880 and are essential in a museum demonstration today.
- Textile Mills: Lancashire boilers at Quarry Bank Mill in Cheshire originally supplied steam at 100 psig to a beam engine driving the spinning floor — a typical pair of 7 ft × 28 ft Daniel Adamson Lancashires.
- Marine Propulsion: Cornish-pattern internal fired boilers powered early steam launches and tugboats; the SS Sicamous on Okanagan Lake used Scotch marine boilers, a direct descendant of the internal fired tubular form.
- Cement and Lime Works: Rugby Cement's heritage centre runs a recommissioned 1898 horizontal return tubular boiler feeding a Robey mill engine on the rawmeal grinding line.
- Brewing and Distilling: Bass Brewery at Burton upon Trent ran banks of Lancashire boilers supplying mash-tun heating steam and engine drive for the maltings until the 1960s.
- Sawmills and Logging: Hesketh sawmill in New Zealand and Roots Brothers mills in the US Pacific Northwest used internal fired tubular boilers to feed Corliss engines driving 60-inch circular headsaws.
- Pumping Stations: Kew Bridge Steam Museum's Lancashire boilers feed the 90 inch Cornish pumping engine at 50 psig for demonstration days.
The Formula Behind the Internal Fired Cylindrical Tubular Boiler
The figure that matters most for sizing or recommissioning one of these boilers is the equivalent evaporation rate — the pounds of water per hour the boiler turns into steam, normalised to a standard reference condition (212°F feed, atmospheric exhaust). At the low end of typical firing, say 4 lb of coal per sq ft of grate per hour, you'll see the boiler loafing along at maybe 50% of its rated output and very high efficiency because gas velocities through the flues are low and heat transfer time is long. At the high end — 25 lb/sq ft/hr forced firing — output climbs but efficiency collapses because gases blow through the flues without time to give up their heat, and you start losing 8-10% up the chimney. The sweet spot for a Lancashire is around 12-15 lb of coal per sq ft of grate per hour.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| We | Equivalent evaporation rate, referenced to 212°F feed and atmospheric steam | kg/h | lb/h |
| Wa | Actual evaporation rate measured at the boiler | kg/h | lb/h |
| hg | Specific enthalpy of saturated steam at working pressure | kJ/kg | Btu/lb |
| hf | Specific enthalpy of feedwater at supply temperature | kJ/kg | Btu/lb |
| 970.3 | Latent heat of vaporisation of water at 212°F (1 atm) — the reference constant | kJ/kg (≈ 2257) | Btu/lb |
Worked Example: Internal Fired Cylindrical Tubular Boiler in a heritage sugarmill Lancashire boiler
You are predicting the equivalent evaporation rate of a recommissioned 1894 Daniel Adamson Lancashire boiler being returned to demonstration steaming at a heritage sugarmill museum on the Clarence River in northern New South Wales, where it will supply saturated steam at 120 psig to a horizontal mill engine driving a small cane-crushing roller train. The shell is 7 ft diameter × 28 ft long with two 3 ft Adamson-ringed internal flues and 6 Galloway tubes per flue. Total heating surface is measured at 720 sq ft. Feedwater enters at 80°F. Measured actual evaporation under steady firing trial is 6,200 lb/h.
Given
- Wa = 6200 lb/h
- Working pressure = 120 psig
- hg at 120 psig saturated = 1190.4 Btu/lb
- Feed temperature = 80 °F
- hf at 80°F = 48.0 Btu/lb
Solution
Step 1 — compute the heat absorbed per pound of steam at the nominal 120 psig working condition. This is the difference between the steam enthalpy leaving the boiler and the feedwater enthalpy entering it:
Step 2 — compute the nominal equivalent evaporation rate at the rated 6,200 lb/h actual evaporation:
Step 3 — at the low end of typical firing (around 4,500 lb/h actual, what you'd see during a quiet morning warm-up before the engine takes load):
That feels like a boiler quietly ticking over — safety valves silent, chimney showing only a thin grey haze, fireman tending the grate every 8-10 minutes. Step 4 — at the high end of forced firing the same boiler can be pushed to roughly 8,500 lb/h actual evaporation for short periods:
That's the boiler working hard — safety valves lifting, dense black smoke, fireman feeding every 3-4 minutes, and gas exit temperature climbing past 600°F because the flues no longer have time to extract the heat. You will burn 30-35% more coal per pound of steam at this end of the range than at nominal.
Result
Nominal equivalent evaporation works out to about 7,300 lb/h at 120 psig with 80°F feedwater. That's enough to comfortably feed a 150 IHP horizontal mill engine running a small cane-roller train with margin for the bagasse-conveyor auxiliary. The low-end warm-up figure of 5,300 lb/h shows the same boiler loafing at around 70% of nominal — efficient and quiet — while the high-end forced rate of 10,000 lb/h shows what you can squeeze out before efficiency falls off the cliff. If your measured equivalent evaporation comes in 15-20% below the predicted 7,300 lb/h, the three most common causes are: (1) heavy scale on the waterside of the internal flues — anything above 1/16 inch insulates the metal and drops heat transfer dramatically, (2) air leakage into the brick side flues drawing cold air into the gas path and dropping mean ΔT, or (3) wet steam carryover because the steam space is too small or feed surges are throwing water into the offtake — check for priming at the stop valve before blaming the heating surface.
Internal Fired Cylindrical Tubular Boiler vs Alternatives
The internal fired cylindrical tubular boiler sits in a specific niche — moderate pressure, moderate output, very tolerant of rough operation. Compared to a water-tube boiler it is slower to raise steam and limited in pressure, but it is also vastly simpler to operate and maintain. Compared to an externally fired boiler it gives more heating surface per unit floor area but demands more careful water management.
| Property | Internal Fired Cylindrical Tubular (Lancashire) | Water-Tube Boiler (Babcock & Wilcox) | Externally Fired Return Tubular (HRT) |
|---|---|---|---|
| Working pressure range | 80-200 psig practical limit | 200-2000+ psig | 80-150 psig |
| Time to raise steam from cold | 3-6 hours | 30-60 minutes | 2-4 hours |
| Thermal efficiency at nominal load | 70-78% | 82-88% | 65-72% |
| Tolerance of rough/dirty feedwater | High — large water volume buffers contamination | Low — narrow tubes scale and fail rapidly | Medium |
| Heating surface per cubic ft of vessel | 6-10 sq ft/cu ft | 20-40 sq ft/cu ft | 8-12 sq ft/cu ft |
| Stored energy (explosion risk) | Very high — large water mass | Low — small water content | High |
| Capital cost (period equivalent) | Medium | High | Low |
| Service lifespan | 50-80+ years documented | 25-40 years | 30-50 years |
Frequently Asked Questions About Internal Fired Cylindrical Tubular Boiler
Two effects usually combine. First, the internal flues and shell hold thousands of gallons of water — heating that mass from 50°F ambient to 350°F saturation at 120 psig takes a fixed amount of energy that the grate can only deliver so fast. Second, modern museum operation typically uses a much milder fire during warm-up than a Victorian fireman would have driven, both because of thermal-shock concerns on century-old riveted seams and because firing rules now require slow controlled rise (typically 50°F per hour on the shell). Original mill records showing 2-3 hour starts assumed a hot-banking overnight start with the brickwork already at 200°F and an aggressive coal fire from minute one.
Cornish gives you simpler construction and slightly lower cost, but the single large flue limits grate area and therefore steam output for a given shell diameter — practical ceiling around 3,500-4,000 lb/h equivalent evaporation. Lancashire's twin flues roughly double the grate area for only a modest shell-diameter increase, putting practical output up around 8,000-12,000 lb/h. If your downstream load is below 3,000 lb/h and you want the simpler vessel, go Cornish. Above that, the twin-flue Lancashire is what every working mill specified for a reason.
Weeping fusible plugs almost always trace to one of two causes: scale buildup on the waterside of the plug body insulating the tin core from the water and letting it overheat locally, or a worn plug seat in the flue crown letting steam track around the threads rather than actual core melting. Pull the plug, inspect the tin face — if it's pitted or partially recessed, replace it; the alloy degrades slowly above 380°F even without melting through. If the threads or seat are damaged, you need to retap the flue crown and fit an oversize plug, which is a notifiable repair under most insurance regimes.
The reference is a normalisation, not an operating condition. 970.3 Btu/lb is the latent heat of water at standard atmospheric conditions, picked over a century ago as a fixed yardstick so engineers can compare boilers running at different pressures and feedwater temperatures on a single common scale. A boiler producing 6,200 lb/h at 120 psig from 80°F feed and one producing 6,200 lb/h at 60 psig from 200°F feed are doing very different amounts of actual work — converting both to equivalent evaporation lets you compare them honestly.
Realistic gain is 12-18% in heating surface and roughly 8-12% in equivalent evaporation, plus better water circulation in the flue which reduces hotspot risk on the crown. Whether it's worth the work depends on whether the flue is already out for inspection — fitting Galloways into an in-situ flue means cutting holes through the corrugated rings and rolling new tubes into them, which is heavy boilermaker work. If the flue is already pulled for retubing or repair, adding Galloways is straightforward. As a retrofit alone, the labour rarely pays back unless you are output-limited.
Pressure drop under load with good combustion almost always points to either steam leaks downstream of the stop valve (check engine glands, throttle stem, and feedwater heater coils) or wet steam carryover from the boiler robbing apparent capacity. Wet steam typically traces to feedwater surging — an injector cycling on and off rather than running continuously throws cold water into the steam space and induces priming. Fit a continuous feed regulator or run a second injector at lower output for steadier feed. Also check the dryback or anti-priming pipe inside the steam offtake; if it's corroded through, the boiler effectively becomes a wet-steam producer regardless of water level.
Practical ceiling for original riveted Lancashire shells is 160-180 psig working, even when hydro tests pass at 1.5× that. Riveted longitudinal seams are the limiter — they have a documented joint efficiency of around 70-85% versus 100% for modern fully welded shells, and rivet fatigue under pressure cycling is cumulative over decades. Hydro testing only proves the shell holds at one point in time; it does not address the cyclic-fatigue history. Heritage operating pressures are usually held at 60-80% of the original working pressure for that reason, regardless of test results.
References & Further Reading
- Wikipedia contributors. Fire-tube boiler. Wikipedia
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