The Eclipse return tubular marine boiler illustrated by Hiscox has a firebox feeding a lower bank of direct tubes. Gas enters the rear chamber, reverses direction, and returns through the upper tube bank to the forward smoke outlet. Water surrounds the heating surfaces. The cutaway below distinguishes this two-pass tube arrangement from a boiler with one large furnace flue.
Eclipse Return Tubular Marine Boiler Interactive Calculator
Trace gas through the Eclipse boiler’s direct and return tubes. Estimate steam flow and thermal output from a measured heating surface, specific evaporation and matching enthalpy rise.
Equation Used
- Steady flow with entered evaporation referring to delivered steam.
- Steam and feedwater enthalpies correspond to the measured operating condition.
- No blowdown or transient stored-energy term included.
- Historical arrangement illustrated with representative tube banks.
Use measurements for the same operating condition. Boiler horsepower is thermal output, not engine shaft power.
Follow the two tube banks
Fuel burns above the grate at the front. The lower direct tubes carry combustion gas toward the back chamber. That chamber connects the lower bank to the upper return bank. The return tubes lead back toward the front smoke passage and funnel.
Orange markers follow the outward pass, then become amber on the return and gray at the exhaust. Blue water surrounds the firebox and tube banks; the pale upper space collects steam. The transparent side wall and three representative tubes in each bank expose the routing. Tube count, shell dimensions, fittings and the exact outlet duct are schematic.
The animation follows Hiscox figure 180. Its lower direct-tube bank connects to the upper return bank through the rear gas chamber. A separate steam outlet is shown apart from the combustion exhaust. Flow markers explain direction; their display speed does not measure gas velocity, temperature or residence time.
Use measured evaporation for a thermal comparison
The historical source identifies this as a marine boiler. A folded gas path allows forward and return heating passes within one installation. The drawing explains the arrangement; it does not establish the dimensions, pressure or output of any particular surviving boiler.
Enter its measured heating surface and a specific evaporation figure obtained at the operating condition being compared. Enter the steam enthalpy minus feedwater enthalpy for that same condition. These quantities estimate the thermal output associated with that measured steam flow. They do not determine engine shaft power, fuel consumption or allowable firing rate.
Mass flow and transferred heat
With heating surface A in ft² and measured specific evaporation E in lb/(ft²·h), steam mass flow is m=AE in lb/h. Multiply by 0.45359237 for kg/h.
Let Δh be steam specific enthalpy minus feedwater specific enthalpy, in kJ/kg, for the actual inlet and outlet states. Transferred heat Q in kW is m(kg/h)·Δh/3600. Thermal boiler horsepower is Q/9.8095, using the NIST conversion of 9.8095 kW per boiler horsepower.
Boiler horsepower is a thermal unit, distinct from mechanical shaft horsepower. Dividing arbitrary operating steam flow by 34.5 lb/h assumes a particular equivalent-evaporation basis and is not generally valid. The enthalpy input makes the operating-state assumption explicit.
Hiscox also gives a historical heating-surface rule of A/12 for this pattern. That old rule is not used as a measured thermal-performance result or a modern rating. No universal priming threshold can be inferred from heating surface and evaporation alone.
Example: a stated operating point
For A=120 ft² and E=3.5 lb/(ft²·h), steam flow is 420 lb/h, or 190.51 kg/h. If the corresponding steam-minus-feedwater enthalpy is 2,400 kJ/kg, transferred heat is 127.01 kW, equivalent to 12.95 thermal boiler horsepower.
These are example inputs, not a recommended firing condition. Increasing A while holding E constant increases mass flow proportionally. Increasing Δh while holding measured mass flow constant increases calculated heat output without changing mass flow. A real boiler’s E must be measured or justified for each operating condition.
Model assumptions and limits
The illustration shows the connected combustion path through direct tubes, a back chamber and return tubes, plus the separation between gas passages and surrounding water. It keeps the waterline fixed to explain the two spaces; it does not simulate level-control transients.
The calculator is a steady thermal balance. It assumes the entered flow refers to the delivered steam and uses the corresponding enthalpy rise. Blowdown, stored-energy changes, fuel-side efficiency, gas pressure loss, steam quality and entrainment require additional information.
Tube clearances, pressure limits and maintenance requirements must come from the actual boiler design and operating documentation. The displayed input ranges are calculator ranges, not an operating envelope for a pressure vessel.
Questions about this cutaway
Why are there two tube banks?
The lower bank leads toward the back chamber; the upper bank returns toward the front. They are connected gas passages within the surrounding water space.
Are the colored dots water inside the tubes?
No. They represent combustion gas in fire tubes. Water is outside those tubes in the blue region.
Does faster animation mean a measured gas velocity?
No. Marker speed is illustrative. The numerical outputs report the entered operating point, not a fluid simulation.
Can heating surface predict priming?
No. Heating surface and specific evaporation alone do not determine when water droplets become entrained in the steam. That also depends on water level, steam-space geometry, water condition and operation.
Why does enthalpy change heat but not steam flow?
Flow comes from the entered surface and specific evaporation. Enthalpy rise specifies how much heat each kilogram absorbs between the feedwater and delivered-steam states.
Does the figure predict engine horsepower?
No. Boiler horsepower expresses heat output. Engine shaft power also depends on the engine cycle and its losses.
Sources and unit basis
Gardner D. Hiscox, Mechanical Movements, Powers and Devices, printed page 58, figure 180: the Eclipse firebox, direct tubes, back chamber and return tubes. The drawing supplies arrangement, not a modern pressure or performance rating.
NIST Guide to the SI, Appendix B.8: boiler horsepower to watts. Thermal output otherwise follows the steady-flow mass-times-enthalpy-rise balance.
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