This simple boiler was designed to power the oscillating steam engine project, but could be used to run any engine of a similar capacity.
The boiler shown has the minimum of fittings for easy construction and is fired using solid fuel tablets available from model shops.
Of course, any boiler should be properly constructed and tested to ensure it is safe and it is the builder's responsibility to check the boiler for quality issues before steaming.
This style of boiler is known as a "pot" design, being simply an enclosed cylinder above a heat source. There were no water tubes, fire tubes or super-heaters to be concerned with. The chosen fuel was model steam, solid fuel tablets; these being clean and easy to operate. Operating pressure was defined as 30psi so a hydraulic test would be conducted to twice working pressure (60psi).
The pressure vessel was designed around a piece of 50mm diameter copper tube of 1.2mm thickness. Although over-thick for a boiler operating at these pressures, it was a standard available size and removed the need to roll a shell from sheet material.
The end caps were made from the same material and were supported by a single boiler stay. Again, this stay was not necessary for strength, but it assisted in the production of parts, the assembly of the boiler for silver soldering and also provided a means to fix the pressure vessel to the firebox.
Atop the boiler were two bushes, one for a safety valve and the other for a steam take-off point.
An optional feature was the water level plug on the front end plate. This was not a necessity, but would make filling the boiler to the correct level easier, particularly when starting with an unknown quantity of water inside.
The smoke stack was not designed to be functional as far as the fire was concerned but it was designed to double as a condenser; particularly important if the steam plant was to be fitted to a model boat to prevent oil getting into the lake. Exhaust steam rising from the smoke stack would also add to the realism of the steam plant.
Copper was chosen for the pressure vessel, it being the best material due to its mailability, conductivity and ease with which it could be joined. Bushes for fittings should be made from bronze where possible. Brass not being recommended as it can suffer from a process known as dezincification over a long period of time; but for a boiler operating at these pressures brass could be considered suitable with an awareness that this "corrosion" should be checked for periodically.
Silver solder is the only process suitable for joining the pressure vessel components. Soft solder not being strong enough at elevated temperatures from a safety and reliability point of view.
The firebox was made from 0.5mm steel sheet but brass or copper could also be used. Aluminium is less good due to its lower melting temperatures. BBQ aerosol paint was used to finish the firebox housing.
Some calculations were made on the proposed copper tube to see if it was suitable to be made into a pressure vessel and to ensure a safe design.
The formula used was P = (2T x s) / D
Where :
P = working pressure
T = Thickness of the copper tube (inches)
D = Internal diameter of tube (inches)
s = Maximum safe stress of copper **
Therefore : P = (2x0.047" x 2500) / 2" = 118 psi
So the 1.2mm thick, 50mm diameter copper tube could be used up to 118psi and still be within a tenth of the copper tensile strength. It is worth running this calculation on any proposed boiler material.
The engine was found to perform best on 20 - 40 psi, any more than this and the cylinder was lifted from the port face. So this copper tube would be more than strong enough for this pressure.
** Copper tensile strength taken as 25,000 pounds force.
** Chosen factor of safety = 10, so Copper strength reduced to 25,000 / 10 = 2,500 pounds.
It was planned to make the boiler end plates from the same thickness of copper, with a central stay through the middle, also made from copper. The stay diameter was calculated as follows.
Working pressure 40psi.
Copper tensile strength 25,000 pounds force.
Area supported by stay = Pi x R2 = Pi x 12 = 3.141 square inches.
Multiplied by boiler pressure = 3.141 x 40 = 125psi x factor of safety (10) = 1,250 pounds.
- So the stay would need to have a breaking stress of 1,250 pounds force.
This was divided by the tensile strength of copper to give the area required of the boiler stay to ensure adequate strength = 1,250 / 25,000 = 0.05 square inches.
So boiler stay radius would therefore be sqroot of (0.05 / Pi) = 0.126" or 3.2mm radius
As a first boiler build there was a requirement to purchase some essential items.
First of all some sliver solder and flux for the construction of the pressure vessel.
Secondly a valve to be used to set engine speed and a safety valve could also be purchased to save time.
Consider also any screws and washers that might be needed to complete construction along with some copper tubing and unions to make connections.
Possibly also some taps and dies for the required threads and some stainless springs and bronze balls if a safety valve is to be made; and don't forget some fuel tablets for firing.
A steam test proved that the boiler design would run the simple oscillating engine project at a continuous 700 to 800 rpm.
4 tablets of fuel would provide enough steaming for 15 to 20 minutes of run time at these speeds.
General plans for the boiler can be viewed by clicking on the image to the left. However thread sizes, and final dimensions should be varied to suit the materials and tools to hand.
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