An internally fired Lancashire boiler places two furnace flues inside the water-filled cylindrical shell. Combustion gas travels through those internal flues, then through passages formed by the external brick setting toward the chimney. Galloway water tubes crossing the flues add water-connected heating surface and stiffen the large flues.
Internally Fired Flue Boiler Interactive Calculator
Trace the twin internal furnace flues and the external return setting, then evaluate a measured thermal balance using an independently supplied steam-minus-feedwater enthalpy rise.
Equation Used
- Steam mass flow and steam-minus-feedwater enthalpy rise refer to the same condition.
- Entered efficiency is useful heat divided by firing heat input.
- Equivalent evaporation uses the historical 970.3 Btu/lb reference from and at 212°F.
- No blowdown, stored-energy change or transient term is included.
Enter mass flow, enthalpy rise and efficiency from the same measured or specified condition. The calculator does not infer steam state from pressure.
Follow the internal and external gas passes
Fuel burns on grates at the front of the two internal furnaces. Gas moves rearward through the large furnace flues while boiler water surrounds them. Conical Galloway tubes cross the flues and remain in communication with the surrounding water. After leaving the internal flues, gas is routed through external bottom and side passages in the brick setting before reaching the chimney.
The animation opens the shell and setting so both furnace passages are visible. Orange markers show gas direction and blue markers identify the separate steam outlet. Marker speed, drawing dimensions and the number of Galloway tubes are explanatory only.
Use measured state information for the thermal balance
Enter a measured steam mass flow and the corresponding steam-minus-feedwater specific enthalpy. Obtain the enthalpy difference from appropriate property data for the actual inlet and outlet states; pressure alone does not define enthalpy when quality or superheat is unknown. The useful heat result is mass flow multiplied by that entered enthalpy rise.
Efficiency is also entered, not predicted. It converts useful heat to a firing-heat input for the same stated condition. The historical construction drawing does not establish a modern rating or safe firing range.
Thermal balance and unit conversions
With steam flow m in lb/h and entered enthalpy rise Δh in Btu/lb, useful heat is Q=mΔh Btu/h. Equivalent evaporation divides Q by 970.3 Btu/lb, the reference evaporation from and at 212°F. Thermal boiler horsepower divides the converted heat rate by 9.8095 kW per boiler horsepower. Required firing heat is Q/η for entered useful efficiency η.
Supply the enthalpy rise for the actual feedwater and steam states. Gauge pressure alone does not establish those states.
Example with a stated enthalpy rise
At 6,000 lb/h, Δh=1,100 Btu/lb and η=78%, useful heat is 6.60 MMBtu/h. Equivalent evaporation is about 6,802 lb/h, thermal output is about 197 boiler hp, and firing heat input is about 8.46 MMBtu/h.
These values are arithmetic consequences of the entered measurements, not a capacity claim for a particular boiler. Changing efficiency changes firing heat but not useful heat or equivalent evaporation.
What the drawing and thermal balance establish
The cutaway illustrates internal furnace flues, surrounding water, Galloway cross-tubes and an external return setting. Its simplified passages are not a complete construction drawing or pressure-vessel design.
The useful-heat calculation omits blowdown and heat storage. Combustion and other losses are not calculated separately; their overall effect is represented only through the entered efficiency. Displayed input ranges are not operating limits. Actual operation and inspection require the boiler documentation and qualified personnel.
Questions
Does pressure set the steam enthalpy?
Not by itself. The steam state also depends on saturation, quality or superheat, so the calculator asks for the matching enthalpy rise directly.
Are Galloway tubes gas tubes?
No. They cross the large gas flues and communicate with the surrounding boiler water.
Does the efficiency slider predict combustion efficiency?
No. It is an entered useful-efficiency value used only to back-calculate firing heat.
Is boiler horsepower shaft horsepower?
No. It is a thermal unit. NIST gives 1 boiler horsepower as 9.8095 kW.
Construction and unit references
N. Hawkins, Maxims and Instructions for the Boiler Room, figures 18–20 and accompanying text: Lancashire boiler furnaces, flues, Galloway tubes and external gas routing.
NIST Special Publication 811: 1 boiler horsepower equals 9.80950 kW. Enthalpy rise must come from suitable property data for the actual states; this calculator does not supply steam properties.
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