A locomotive boiler is a fire-tube boiler: hot combustion gases pass from the firebox through tubes surrounded by water, then into the smokebox and chimney. Steam collects above the waterline. The calculator estimates fuel heat input, useful heat and equivalent evaporation at a standard reference condition.
Locomotive Boiler Interactive Calculator
Follow the separate combustion-gas and water/steam paths. Calculate the fuel heat balance and equivalent evaporation from and at 212°F.
Equation Used
- Steady heat balance.
- Efficiency is entered as an overall measured or assumed value.
- The loss result includes all losses, not stack loss alone.
- Flow markers illustrate paths, not fluid velocities.
The balance index compares the two entered percentages. It is not a physical valve-lead, steam-cutoff or port-timing calculation.
Three distinct flow paths
The rear firebox is surrounded by a water jacket. Hot gases enter the fire tubes and travel forward to the smokebox; the smokebox is outside the water-filled pressure shell. The waterline in the cutaway stays above the firebox crown and tubes.
Steam collects in the upper space and is drawn through the dome and dry pipe. Exhaust steam from the cylinders reaches a separate blastpipe in the smokebox to assist draught. An auxiliary blower can provide draught when the locomotive is stationary; cylinder exhaust is not the only way to maintain it.
The view omits a superheater and many fittings to keep these paths legible. It is an explanatory saturated-steam arrangement, not a construction drawing.
Applications
Locomotive-pattern fire-tube boilers are associated with steam locomotives and related mobile steam machinery. Preserved railway equipment provides examples of the firebox, tubes, smokebox and chimney arrangement. Boiler details, working pressure and steam conditions depend on the individual design.
Heat balance and equivalent evaporation
Multiply fuel mass rate in lb/h by heating value in Btu/lb for heat input. Useful heat is that input multiplied by entered efficiency. The difference is total loss, including losses beyond the stack.
Equivalent evaporation divides useful heat by 970.3 Btu/lb, the reference latent heat for evaporation from and at 212°F. It is a standard comparison quantity. Actual steam mass rate instead uses the delivered steam enthalpy minus feedwater enthalpy, including pressure and superheat where applicable.
250 lb/h fuel example
At 13,300 Btu/lb and 65% efficiency, heat input is 3.325 MMBtu/h. Useful heat is 2.16125 MMBtu/h and total losses are 1.16375 MMBtu/h. Equivalent evaporation is approximately 2,227.40 lb/h.
The same useful heat does not imply the same actual steam mass at every pressure or feedwater temperature.
Interpret the output correctly
This is a steady heat-accounting calculator. It does not size heating surfaces, stays, shell thickness, grate area or safety valves, and does not simulate water-level transients. The diagram keeps gas and water spaces distinct. The entered overall efficiency determines the heat split.
Questions
Do gases travel through the water space?
No. They remain inside the firebox and tubes before entering the smokebox.
Is the smokebox pressurized with boiler steam?
No. It is the combustion-gas collection space beyond the front tube sheet.
Is equivalent evaporation the actual steam flow?
No. Actual flow requires the relevant feedwater and steam enthalpies.
Technical references
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