The Serpollet tricycle used a compact steam generator with externally heated flattened tubing. A cutaway below makes the spiral passage and furnace arrangement visible. The accompanying calculator explores passage area and liquid-filled capacity; it does not size a functioning boiler.
Boiler of the Serpollet Tricycle Interactive Calculator
Compare the rectangular passage area and the tube length corresponding to two liquid-filled volumes. These are geometric study inputs, not documented original dimensions or operating water inventories.
Equation Used
- Constant rectangular internal passage; actual flattened and corrugated tubes have a different section.
- Both entered volumes are comparison capacities; results are sorted from smaller to larger.
- Wall thickness, bend geometry, phase change and pressure drop are omitted.
- The study range and defaults are illustrative, not a reproduction of the original boiler.
The calculator preserves its four geometric results. The furnace view illustrates the historical arrangement; it does not predict boiling, flow velocity, pressure or structural suitability.
How the Serpollet generator is arranged
In the historical spiral arrangement, flattened tubing sits within a furnace. Water enters one end and passes through the heated passage toward the steam outlet. The small passage and hot tube distinguish this construction from a large water-filled shell boiler.
The 1888 Scientific American Supplement shows an exterior, vertical section and plan view. The animation exposes the flattened spiral above the fire, with an outer connection and a raised inner connection. Wall thickness, tube corrugations and the number of turns are simplified so the route remains readable.
Reading the diagram
Blue and pale markers indicate the direction through the tube. Their speed and colour transition are explanatory, not calculated boiling locations. On the right, the cross-section changes with the two passage dimensions, while the length bars compare the two entered capacities.
Historical application
The Musée des Arts et Métiers identifies its Serpollet steam tricycle as an 1888 vehicle by Armand Peugeot and Léon Serpollet. Its collection record places the compact generator in the development of early steam automobiles. The calculator is a geometry study of the passage concept rather than a reconstruction of that museum vehicle.
Passage area and geometric capacity
For a rectangular internal passage of height h and width w, area A = hw and wetted perimeter P =2(h+w). The hydraulic diameter is Dₕ =4 A/P =2 hw/(h+w). This diameter characterizes a noncircular passage; it is not the tube outside diameter.
For a liquid-filled volume V, constant-section length is L =V/A. With V entered in mL and A in mm², the numerical quotient gives L in metres. The two capacity inputs are sorted before reporting lower and upper lengths.
Actual steam generation requires thermal and two-phase flow information absent here. Geometric capacity alone gives no prediction of steam output, pressure, temperature, dry-out, combustion demand or tube-wall strength.
Worked geometric example
Using the illustrative defaults h =2 mm and w =20 mm gives A =40 mm² and Dₕ =80/22 =3.636 mm. A liquid volume of 200 mL fills 5 m of that ideal passage;400 mL fills 10 m.
Doubling either passage dimension doubles area and halves the length required for the same liquid capacity. Doubling both dimensions quadruples area and reduces each length to one quarter. These are volume relationships, not predictions of the amount of water present during steaming.
What this comparison can establish
The calculator helps distinguish cross-sectional area, hydraulic diameter and geometric liquid capacity. Two passages with equal area can have different wetted perimeters and hydraulic diameters. Their pressure drops and heat-transfer performance still cannot be compared from these values alone.
The historical thin passage should not be confused with a pressure-rated design. This illustration omits material properties, wall geometry, joints and operating conditions and therefore supplies no construction or pressure rating.
Questions about the calculator
Does the animation show actual boiling speed?
No. The moving markers only explain the connected flow route. Their speed is independent of the capacity calculations.
Are the default dimensions historical measurements?
No. They are illustrative inputs retained for comparing geometry. The historical article describes a much narrower slit in its experimental generator; surviving arrangements and sources should be consulted for historical identification.
Why are the two output lengths sorted?
Either comparison capacity can be larger. The lower output always corresponds to the smaller entered volume and the upper output to the larger.
Is hydraulic diameter the size of a round replacement tube?
No. It is four times area divided by wetted perimeter. Matching hydraulic diameter does not make different passages equivalent in area, flow or heat transfer.
Can these inputs determine burner power or steam pressure?
No. Those calculations require additional thermodynamic, material and operating information. This tool calculates only geometry.
References and further reading
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