The Lancashire arrangement places two furnace tubes inside a horizontal shell. Water surrounds the tubes and steam collects above it. Gases leaving the rear of the furnace tubes travel back under the shell, then return along external side flues to the chimney.
Cylindrical Double Flue Boiler Interactive Calculator
Follow the twin furnace tubes and external return flues, then estimate steam output from fuel energy, efficiency and the steam-minus-feedwater enthalpy rise.
Equation Used
- Steady-state energy accounting with supplied efficiency; no warm-up storage term.
- Enthalpy rise is supplied, not inferred from pressure or temperature.
- Efficiency and heating value use the same HHV or LHV basis.
- Gas arrows show the historical three-pass route; particle speed is illustrative.
Use a consistent heating-value basis for fuel energy and efficiency. Enter the enthalpy difference for the actual steam and feedwater states.
Two internal tubes and three gas passes
The cutaway shows a fire at the entrance of each furnace tube. Both first passes run from the front to the rear. The gases then reverse through a bottom passage and divide between two external side passages before reaching the rear chimney.
The two side routes are unfolded below the shell in this drawing so that the arrows can be followed. They are outside the water-containing shell. The pale upper region is steam space; blue water surrounds the furnace tubes. The blue feed line and steam outlet are separate from the combustion-gas circuit.
Estimate the energy available for steam generation
Fuel rate and heating value determine thermal input. The supplied fuel-to-steam efficiency determines how much of that input becomes useful steam energy. Dividing useful energy by the steam-minus-feedwater enthalpy difference gives an estimated mass flow.
Use the same heating-value basis when entering efficiency and fuel value. An efficiency quoted on a lower-heating-value basis should not be paired directly with a higher heating value.
A steady-state heat balance
For fuel flow m in kg/h and heating value H in MJ/kg, input thermal power is mH/3.6 in kW. Useful power is that result multiplied by efficiency expressed as a fraction. Unrecovered power is input minus useful power.
Estimated steam flow in kg/h equals useful power multiplied by 3600 and divided by the enthalpy increase in kJ/kg. The enthalpy increase is steam outlet enthalpy minus incoming feedwater enthalpy.
This is energy accounting, not a boiler structural or operating-limit calculation. It does not solve warm-up, blowdown, transient storage, combustion chemistry or gas flow velocity.
An illustrative 500 kg/h fuel input
At 28 MJ/kg, 500 kg/h of fuel provides 3888.89 kW. With 70 percent fuel-to-steam efficiency, useful power is 2722.22 kW and unrecovered power is 1166.67 kW.
For a 2400 kJ/kg enthalpy rise, estimated steam flow is 4083.33 kg/h. These are assumed example values, not ratings for the historical boiler shown.
Heat output does not determine pressure limits
A fuel-to-steam energy balance estimates heat flow and steam production. It does not determine the pressure that the shell and furnace tubes can withstand or the condition of their heated surfaces.
Actual allowable pressure, inspection, test procedures and protective settings depend on the specific boiler’s documented design and applicable requirements. The heat-balance sliders do not supply that information.
Double-flue boiler questions
Do the gases mix with the water?
No. Furnace-tube walls and the shell separate combustion gases from the water and steam circuit.
Are the side flues inside the shell?
No. They are passages in the external setting. This schematic unfolds them below the shell for visibility.
Does the animation show calculated gas speed?
No. Particles show direction. At zero fuel input the firing and flow demonstration stop.
Does changing efficiency change the gas route?
No. It changes the energy split and steam estimate. It does not reconfigure the passages.
Can this set a test or relief pressure?
No. The energy estimate does not determine test or relief pressure; those require the specific boiler’s design and applicable requirements.
References
Spirax Sarco, Shell Boilers, Lancashire section and figure 3.2.2, explains the two furnace tubes and bottom-then-side gas passes. Crofton Beam Engines, The Boilers, distinguishes this route from the Cornish arrangement. Energy outputs follow the stated steady-state heat balance.
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