The Cahall Vertical Water Tube Boiler is a small-footprint steam generator built around vertical water tubes connecting an upper steam drum to a lower mud drum, patented by Samuel Cahall in the late 1880s. It solves the headroom and weight problem facing horizontal fire-tube boilers in launches, sawmills, and traction service. Hot flue gases pass across the tube bank while water rises by thermosiphon, separating dry steam in the top drum. A typical 50 HP Cahall evaporates around 1,500 lb/h of water at 100 psig.
Cahall Vertical Water Tube Boiler Interactive Calculator
Vary boiler rating, firing level, and working pressure to see estimated Cahall steam evaporation and thermosiphon circulation.
Equation Used
The calculator applies the article's Cahall rule of thumb: nominal evaporation is about 30 lb/h per boiler HP, so a 50 HP unit at 100% firing gives 1,500 lb/h. Feedwater gpm assumes liquid water at about 8.345 lb/gal.
- Uses the article rule that a 50 HP Cahall evaporates about 1,500 lb/h at nominal firing.
- Evaporation scales linearly with firing percentage over the practical slider range.
- Pressure is shown as operating context and converted to bar; no steam-table correction is applied.
How the Cahall Vertical Water Tube Boiler Works
The Cahall packs heating surface into a tall, narrow shell instead of a long horizontal one. Water sits in a lower mud drum and rises through a curtain of vertical or near-vertical water tubes into an upper steam drum, while flue gas from the grate sweeps up across the outside of those tubes. Heat transfer happens tube-wall to water, just like a Babcock & Wilcox — but the geometry is rotated 90°, so you can drop the boiler into a steam launch hull or a sawmill corner where a horizontal return-tube boiler would not fit.
Circulation runs by thermosiphon. Hotter water and steam bubbles in the tubes facing the fire are less dense, they climb into the steam drum, and cooler water from the back-side tubes falls back down to the mud drum to replace them. That loop is the whole reason the boiler steams cleanly — kill the circulation and the front tubes overheat in minutes. If feedwater scale builds inside the lower bends of those tubes, you starve the upflow, the tube metal runs dry on the fire side, and the tube blisters or splits. Cahall users in the 1890s lost tubes for exactly this reason on hard well water, which is why the design relies on regular blowdown from the mud drum and clean feed.
Tube-to-drum joints are expanded and beaded — not welded on the originals. The bore tolerance on the drum tube-sheet hole matters: too loose and the expander cannot get a gas-tight seal at 100 psig, too tight and you crack the ligament between holes during expansion. Heritage rebuilds typically hold drum holes to ±0.13 mm of nominal tube OD plus the expander allowance, and any tube that won't pass a hydro at 1.5× working pressure gets re-expanded or replaced before the boiler goes back into service.
Key Components
- Upper steam drum: Horizontal cylindrical drum, typically 24 to 36 inches in diameter on a 50 HP unit, that collects steam released from the tube bank. Holds the steam-water interface, the safety valves, and the main steam stop. Water level sits at roughly the centreline ±25 mm — drop below the lower gauge cock and the top tube row uncovers.
- Lower mud drum: Smaller drum, 12 to 18 inches in diameter, sitting below the firebox and acting as the downcomer return point and sediment trap. The blowdown valve hangs off this drum because scale and sludge settle here. Blowing down 5 to 10 seconds every watch keeps the lower tube bends clear.
- Vertical water tubes: Bank of seamless steel tubes, commonly 2 to 2.5 inches OD and 0.105 to 0.120 inch wall, expanded into both drums. Tube length runs 4 to 8 feet depending on rating. The tubes form the primary heating surface — count and length set the evaporation capacity.
- Firebox and grate: Internally fired below the mud drum or in a side pocket, with a cast-iron grate sized for the fuel. Burning anthracite at 33,000 kJ/kg HHV releases heat that the tube bank must absorb before flue gas exits at 250 to 350 °C — any hotter and you are wasting fuel up the stack.
- Brick or refractory casing: Wraps the tube bank to force flue gas across all tube rows rather than bypassing the back of the bank. A cracked baffle is a hidden killer — gas short-circuits, front tubes over-fire, rear tubes run cold, and steam output drops 20 to 30% with no obvious symptom from the gauge.
- Safety valve and gauge fittings: Pop safety valve set to working pressure (commonly 100 to 125 psig on small Cahalls), water gauge glass, and pressure gauge mounted on the steam drum. Two independent means of checking water level was the rule even on small heritage units.
Who Uses the Cahall Vertical Water Tube Boiler
Cahall boilers were the answer whenever someone needed real steaming capacity in a small footprint. The vertical layout meant you could fit 40 to 100 HP into the engine room of a launch, the corner of a sawmill, or the chassis of a traction engine without the floor-space penalty of a Lancashire or horizontal return-tube boiler. They were never marine-rated for ocean service like a Yarrow, but on inland water and stationary duty they earned their keep.
- Inland steam launches: Aultman, Taylor & Co. fitted Cahall-pattern vertical water tube boilers to small Ohio River and Great Lakes launches in the 1890s where headroom and trim ruled out a horizontal fire-tube unit.
- Sawmill stationary power: Midwest US sawmills used 60 to 100 HP Cahalls to drive a single-cylinder slide-valve engine running a circular saw, taking advantage of the boiler's rapid steaming from cold against the morning startup load.
- Industrial process heat: Tanneries and small textile bleach houses ran Cahalls for low-pressure process steam at 60 to 80 psig, with the small water content giving fast warm-up at shift start.
- Heritage traction and portable engines: Replica builders and museum restorations have used Cahall-style vertical water tube boilers on portable steam plants where an Aultman-Taylor or Russell engine sits on the same skid.
- Demonstration steam plants: The Coolspring Power Museum in Pennsylvania has displayed vertical water tube boiler examples of this period alongside Atlas and Skinner engines for live steaming weekends.
- Rural electrical generation (early 1900s): Small isolated generating sets coupling a Cahall boiler to a vertical compound engine driving a 25 kW DC dynamo served creameries and ice plants before grid power arrived.
The Formula Behind the Cahall Vertical Water Tube Boiler
What the operator actually wants to know is steam evaporation rate in lb/h or kg/h at a given firing rate. At the low end of the firing range — banking fire, holding pressure overnight — you are evaporating maybe 20 to 25% of rated capacity, just enough to cover radiation losses and gland leakage. At nominal firing, the boiler hits its design evaporation and the safety valve sits quiet just below set pressure. Push past the high end and you start lifting water with the steam (priming) because steam release area in the upper drum can't keep up — a Cahall's small drum diameter is the ceiling on overload capacity. The formula below ties fuel input to steam output through boiler efficiency.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁsteam | Steam evaporation rate at working pressure | kg/h | lb/h |
| ηb | Overall boiler efficiency, fuel HHV to steam enthalpy | dimensionless (0 to 1) | dimensionless (0 to 1) |
| ṁfuel | Fuel mass burn rate on the grate | kg/h | lb/h |
| HHV | Higher heating value of the fuel | kJ/kg | Btu/lb |
| hg | Specific enthalpy of saturated steam at working pressure | kJ/kg | Btu/lb |
| hf | Specific enthalpy of feedwater at inlet temperature | kJ/kg | Btu/lb |
Worked Example: Cahall Vertical Water Tube Boiler in a recommissioned Cahall boiler driving a sawmill engine
You are sizing the steam evaporation rate across three firing rates on a recommissioned 1896 Cahall Vertical Water Tube Boiler being returned to demonstration steaming at the Hesston Steam Museum in LaPorte, Indiana, where the boiler fires on Indiana bituminous coal at 28,000 kJ/kg HHV and supplies saturated steam at 7 bar gauge to a small Russell sawmill engine driving the museum's demonstration circular saw rig. The trustees want evaporation verified at a banking fire rate of 12 kg/h coal, a nominal cruise rate of 35 kg/h, and a high-fire demo burst at 55 kg/h before the public open weekend. Boiler efficiency is taken as 0.68 from a fuel-and-steam audit done at recommissioning. At 7 bar gauge (8 bar abs), saturated hg = 2769 kJ/kg, and feedwater enters at 60 °C giving hf = 251 kJ/kg.
Given
- ηb = 0.68 —
- HHV = 28,000 kJ/kg
- hg = 2769 kJ/kg
- hf = 251 kJ/kg
- ṁfuel,low = 12 kg/h
- ṁfuel,nom = 35 kg/h
- ṁfuel,high = 55 kg/h
Solution
Step 1 — compute the enthalpy rise the boiler must deliver per kg of steam:
Step 2 — at nominal firing of 35 kg/h coal, compute the useful heat into water and divide by Δh to get steam rate:
That's the design sweet spot — about 580 lb/h of saturated steam, enough to keep the Russell engine happy on the saw rig with a touch of margin against the safety valve. Pressure sits steady, the fire breathes evenly, and the firebox doesn't push the stack temperature past 320 °C.
Step 3 — at the low end, banking fire at 12 kg/h coal:
91 kg/h is enough to cover radiation, gland leakage, and the occasional injector lift while the saw is idle between cuts — what you would expect in the lunch break. You can hold pressure here all afternoon on a small fire.
Step 4 — high-fire demo burst at 55 kg/h coal:
In theory, 416 kg/h. In practice on a small Cahall with a 30-inch steam drum you'll start priming above roughly 350 kg/h — water carries over with the steam because the drum's release area can't keep up, and you'll see the gauge glass dance and the engine cylinder cocks throw water. Don't sustain high fire for more than a few minutes of demo unless you've verified the drum internals.
Result
Nominal evaporation comes out at about 265 kg/h (≈580 lb/h) of saturated steam at 7 bar gauge — exactly where a recommissioned 50 HP-class Cahall should sit on Indiana bituminous. Low fire holds 91 kg/h for steady banking; high fire reaches a theoretical 416 kg/h but the boiler will prime before you get there, so the practical ceiling on this drum size is closer to 340-360 kg/h. If your measured evaporation comes in 15-25% below predicted at nominal firing, the most common causes are: (1) a cracked or slumped refractory baffle letting flue gas short-circuit past the back tube rows so half the heating surface is cold, (2) scale build-up inside the lower tube bends choking thermosiphon circulation and dropping the effective heat transfer coefficient, or (3) feedwater entering well below 60 °C because the feedwater heater is bypassed, which inflates the Δh denominator and steals capacity from the figure.
Choosing the Cahall Vertical Water Tube Boiler: Pros and Cons
The Cahall lives in the same design space as the Babcock & Wilcox horizontal water tube and the Stirling bent-tube boiler. Each one solves the small-to-medium stationary steam problem differently, and the choice comes down to footprint, water content, response time, and how clean your feedwater is.
| Property | Cahall Vertical Water Tube | Babcock & Wilcox Horizontal Water Tube | Stirling Bent-Tube Boiler |
|---|---|---|---|
| Footprint (floor area per HP) | Small — vertical layout, ~0.4 m²/HP | Large — long horizontal drum, ~0.9 m²/HP | Medium — two or three drums, ~0.6 m²/HP |
| Working pressure ceiling | 100 to 150 psig typical | 150 to 250 psig | 200 to 600+ psig (utility scale) |
| Steam-up time from cold | Fast — small water content, 20 to 40 min | Moderate — 60 to 90 min | Slow — large water mass, 90+ min |
| Tolerance to scaling feedwater | Poor — lower bends scale and starve circulation | Fair — straight inclined tubes easier to clean | Good — bent tubes flex with thermal cycles, easier blowdown |
| Maximum useful capacity | ~150 HP before priming becomes the limit | ~1,000 HP per unit | 10,000+ HP at utility scale |
| Tube replacement complexity | Moderate — vertical access through drum | Easy — straight tubes, expand from headers | Hard — bent tubes need matched bending |
| Typical era and survivors | 1888-1920, scarce heritage survivors | 1880-present, many in service | 1900-present, dominant utility design |
Frequently Asked Questions About Cahall Vertical Water Tube Boiler
You're almost certainly priming. On a small drum Cahall, when steam release rate exceeds what the drum's free surface can disengage, water carries over with the steam into the main. The boiler reads as if it's making steam, but a chunk of what's leaving the drum is liquid water — it does no useful work in the engine cylinder, and the gauge pressure drops because real steam mass flow has actually fallen.
Diagnostic check: open the cylinder cocks at the engine. If they throw water rather than dry steam during high fire, you're priming. The fix is to back off the fire, raise the water level slightly less aggressively, and check for oil or solids in the boiler water — surface tension contaminants make priming worse at lower steam rates than you'd expect.
It comes down to three things: floor space, feedwater quality, and target working pressure. If you have less than about 6 m² of floor and headroom is generous, the Cahall wins on footprint. If your feedwater is hard and you can't run a softener, the B&W is more forgiving because the inclined straight tubes are easier to clean and circulation doesn't depend on a tight thermosiphon loop in the same way.
For pressures above 125 psig, lean toward the B&W. The Cahall's drum design and tube-to-drum expanded joints were specified for lower pressures, and pushing the working pressure up means re-rating the drum, which a heritage hydrostatic test may not pass.
Yes, and it's the single biggest hidden efficiency leak on small water tube boilers. Every 20 °C above about 280 °C of stack gas costs you roughly 1% of overall efficiency because that heat is leaving up the chimney instead of going into the water. At 420 °C you're probably at 0.60 to 0.62 efficiency rather than the 0.68 you'd get at a properly-tuned 280 to 300 °C stack.
The two real causes on a Cahall are (1) the refractory baffle has slumped or cracked so flue gas bypasses tube rows, or (2) the tube bank exterior has built up a layer of soot above 1 mm thick — soot is an excellent insulator and dramatically cuts heat transfer to the tubes. Soot-blow or hand-brush the tubes during the next cold inspection before you blame the design.
On softened municipal feedwater with TDS below 200 ppm, a 5 to 10 second mud drum blowdown every 4 hours of steaming is the working rule. The point isn't to remove dissolved solids — softening already did that — it's to clear the sediment trap of any precipitates and oxide flakes that settle between the tube ends in the lower drum.
Skip blowdown for a full day on hard or unsoftened water and you'll find the lower bends of the front-row tubes start scaling within 100 hours of steaming. That scale chokes the upflow side of the thermosiphon loop, which is the failure mode that puts a tube on the floor faster than anything else on this boiler.
This is a classic feedwater symptom, not a fire-side problem. As you draw steam the boiler keeps swallowing cold or barely-warm feedwater. If your feedwater heater is undersized or bypassed, the hf term in the evaporation equation grows — every kg of steam now needs more enthalpy because you're starting colder. Effective evaporation drops 10 to 15% even though fire-side input is unchanged.
Check feedwater inlet temperature with a thermocouple at the boiler check valve. If it's below 50 °C on a unit designed for 70 to 90 °C feed, route the feed through an exhaust steam heater or bring the open heater back online and you'll see the steady-state steam rate recover within 20 minutes.
Only if you re-size the grate and accept a lower peak rating. Dry hardwood runs around 18,000 to 20,000 kJ/kg HHV against bituminous coal at 28,000 kJ/kg, so per kg of fuel you're getting roughly 65 to 70% of the heat. To hold rated steam output you need to burn 40 to 50% more wood by mass per hour — which means a bigger grate area, more frequent stoking, and higher induced draft.
The grate on a coal-spec Cahall typically isn't big enough to burn the required wood mass without choking. Heritage operators usually accept a derate to about 70% of original rated evaporation when running wood, rather than chasing the original number.
The boiler predates code-acceptable fusion welding by 30 to 40 years — Samuel Cahall's patent work and the production runs at Aultman, Taylor in Ohio happened in an era where pressure-vessel welding wasn't trusted, wasn't inspectable, and wasn't legal under the early state boiler codes. Expanding the tube end with a roller expander and then beading the protruding lip provided a mechanical seal that could be re-rolled if it leaked.
For a heritage rebuild, stick with expanded-and-beaded joints if you want the boiler to remain authentic and inspectable under heritage steam rules. The drum hole tolerance matters: keep tube OD to drum hole clearance at 0.4 to 0.8 mm before expanding, and verify with a hydro at 1.5× working pressure before lighting up.
References & Further Reading
- Wikipedia contributors. Water-tube boiler. Wikipedia
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