Hiscox describes the Hazelton boiler as a central vertical drum with closed-ended tubes projecting radially, heated by a grate beneath the tubes and around the drum base. This page includes a general heat-transfer calculation. The section and plan show the central drum and capped radial branches; their dimensions and tube count are illustrative.
Hazelton Boiler Interactive Calculator
Compare assumed heat rate, latent-only evaporation, equivalent BHP and mean heat flux. The model does not rate an actual boiler.
Equation Used
This general heat-transfer model uses entered tube area, effective coefficient and mean temperature difference. Dividing by latent heat assumes saturated water ready to evaporate; it omits feedwater heating and does not solve the historical boiler’s circulation.
- Heating surface is counted on water-tube outside area only; drum area is ignored.
- Uses steady mean gas-to-water temperature difference.
- Overall heat-transfer coefficient is an estimated operating value, not a code rating.
- Latent heat is an entered parameter; the tool has no pressure or feedwater-temperature input.
The historical arrangement
In Hiscox’s entry 192, the central drum contains water and steam. Closed-ended tubes extend outward from that drum, providing additional surface exposed to heat. The grate sits below the radial tubes and around the drum base.
This differs from a tube connecting a lower header to a separate upper steam drum. A closed-ended branch exchanges fluid through its connection to the central drum; it is not a through-flow passage between two headers.
What the illustrations show
The section shows the drum, radial tubes, surrounding furnace and grate. The plan shows a submerged tier of branches, each open to the drum and closed at its outer end. Gold markers indicate heating-gas direction outside the water spaces. The layout and heating motion remain fixed when inputs change. The controls update the heat-balance results; they do not determine tube dimensions, water level, circulation or operating conditions.
Understanding the design and its heat balance
The radial-tube arrangement is an example of increasing exposed surface around a central vessel. Studying it helps distinguish surface area, heat transfer and steam generation: adding area alone does not establish how much heat reaches the water or how well fluid circulates.
The calculator can compare explicitly assumed heat-transfer cases. It does not establish the output of a particular surviving boiler, suitability for a launch or industrial plant, or the condition of historical pressure parts.
Four heat-balance quantities
m = Q/hfg
BHP = Q/33,475
q″ = Q/A = UΔT
Enter outside tube area A in square feet, an effective overall coefficient U in Btu/(h·ft²·°F), mean temperature difference ΔT in °F, and latent heat hfg in Btu/lb. Outputs are heat rate Q in Btu/h, latent-only evaporation in lb/h, equivalent boiler horsepower and mean surface heat flux in Btu/(h·ft²).
U and ΔT are assumptions supplied by the reader. The calculator does not derive them from combustion, radiation, fouling, gas flow or circulation. Area and U must use the same surface basis. Drum heating and other surfaces are excluded from the entered tube-area calculation.
Dividing by latent heat alone assumes that the water is already at the relevant saturation state and all Q supplies evaporation. Feedwater heating, superheating and other energy demands are not included. Actual steam production needs the enthalpy change between entering water and leaving steam, rather than latent heat alone.
Boiler horsepower is a heat-rate equivalent using the tool’s 33,475 Btu/h conversion. It is not engine shaft horsepower, a manufacturer capacity rating or allowable firing rate. The heat-flux result is an average over the entered area, not a local tube-temperature or burnout check.
Illustrative heat-transfer scenario
The displayed preset uses A = 120 ft², U = 6 Btu/(h·ft²·°F), ΔT = 500°F and hfg = 950 Btu/lb. These are calculation assumptions, not operating instructions or measured Hazelton performance.
- Q = 6 × 120 × 500 = 360,000 Btu/h.
- Latent-only evaporation = 360,000/950 ≈ 378.9 lb/h.
- Heat-rate equivalent = 360,000/33,475 ≈ 10.8 BHP.
- Mean heat flux = 360,000/120 = 3,000 Btu/(h·ft²).
Doubling area with the other inputs held fixed doubles Q, evaporation and equivalent BHP, but leaves average heat flux unchanged. Increasing hfg changes only the evaporation result. Neither comparison accounts for a real boiler’s fuel supply or circulation limits.
Use the results within their limits
| Result | What it represents | What it does not determine |
|---|---|---|
| Heat rate | Entered area × coefficient × temperature difference | Fuel efficiency or measured boiler duty |
| Latent-only evaporation | Assumed heat divided by latent heat | Steam output from cold feedwater |
| Equivalent BHP | A conventional heat-rate unit | Shaft power or pressure rating |
| Mean heat flux | Average heat per unit entered area | Local cooling, tube stress or safe firing rate |
Water-level changes, blowdown, pressure changes and pressure-part repairs require equipment-specific assessment and applicable requirements. This tool provides no settings or acceptance criteria for those actions.
Frequently asked questions
Does this reproduce the historical Hazelton tube layout?
It shows the central-drum and closed-ended radial-tube arrangement in Hiscox figure 192. Tube count, dimensions and water level are illustrative.
Is the evaporation result actual boiler output?
It is Q divided by the entered latent heat. It omits the energy needed to heat colder feedwater, along with other energy demands and losses.
Why does area not change mean heat flux?
For this model Q = UAΔT and q″ = Q/A. At positive area, A cancels, leaving UΔT.
Why does latent heat change only one output?
Latent heat is used only in Q/hfg. Heat rate, its BHP equivalent and mean heat flux use the other three inputs.
Is the coefficient a measured Hazelton value?
No. It is an entered scenario parameter. A justified coefficient requires information about the actual heat-transfer surfaces and conditions.
Can the results justify a pressure increase or valve setting?
No. The model contains no pressure-part strength, condition, inspection or pressure-relief analysis.
Do the moving markers calculate circulation?
No. The gold markers indicate external heating-gas direction. Water circulation, heat transfer within individual tubes and dryout are not simulated.
References
- Gardner D. Hiscox, Mechanical Movements, Powers, Devices, and Appliances, entry 192: central drum and closed-ended radial tubes.
- Spirax Sarco: boiler ratings and the distinction between reference evaporation and actual steam output.
- National Board: pressure-equipment inspection, repairs and alterations.
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