The Cornish Boiler
The old rusty boiler now enjoying its retirement in a tranquil bushland setting at Napoleon Reef would during its younger days have travelled far and met many people. It would be proud to trace its ancestry back to the 1702 when Thomas Savery, an Englishman, was the first person to employ a steam boiler for doing useful work, and it would have been amazed if it could have gazed into the future and see the modern steam boilers now in 21st century power stations. But it would be comfortable with one constant of the three hundred years of boiler evolution. Coal – to feed the fire.
Its name is a Flat End Cornish Boiler. The Cornish Boiler was invented by Richard Trevithick, a Cornish mining engineer, who, in 1803 had invented the high pressure steam engine requiring steam pressure at about fifty pounds per square inch. Its design was so successful it, and the later Dished End Cornish Boiler, was manufactured well into the twentieth century. It should not be confused with the later Lancashire Boiler which has two furnace tubes instead of one.
The Cornish Boiler is of a type known as a Fire Tube Boiler. It is also known as an internally fired boiler. Boilers may be divided into two main classes, namely the fire tube and water tube.
In a fire tube boiler the hot gases generated by the fire flow through one or more steel tubes which pass through a drum of water, so that the water surrounds the tube externally.
The principle of water tube boilers involves the separation of water masses into small sections (in pipes) so that each section can be exposed to the maximum amount of heat. The water circulates through the pipes and the flowing hot gases surround the tubes externally.
Only boilers of the water tube type meet the need for the higher pressures and temperatures required by power stations. It is not economic to build fire tube boilers capable of high pressures, and because of the physical proportions of their shell and ends their maximum safe working pressure seldom exceeds 250 p.s.i.
The Cornish Boiler is horizontal, cylindrical, with a single furnace tube running from the front to rear. The furnace tube is of a diameter sufficient to allow for a fire grate and ash pit to be placed within the tube. The tube is generally slightly larger than half the diameter of the boiler shell. The placing of a furnace tube of ample size within the boiler necessitates a relatively large shell diameter and although the furnace tube acts as a stay for the lower part of the end plates, the upper parts need supporting with gusset stays.
The fire grate is situated at the front of the furnace tube A with the ash pit underneath the grate. The grate bars are supported at the front and back by a flanged support riveted to the furnace tube and sealed by a brick wall B at the rear end of the ash pit.
To reduce the waste of heat the boiler is set in brick. The hot gases pass from the fire F through the tube to the back end where they divide and return through the side flues C to the front end. They then unite in the bottom flue D and flow under the boiler shell to the rear and then to the chimney stack.
The movement of the gases is created by draught; in the case of the Cornish Boiler by natural draught. Draught may defined as the difference in pressure between hot (lighter) gas inside the chimney and a cooler (heavier) air on the outside. It is this difference in pressure which forces air and gases through the boiler. The control of cold air passing through the fire bed is done by means of a damper in the flue leading to the stack. The furnace door is perforated to allow the passage of air across the top of the fire.
Boilers, including the Cornish Boiler, have a minimum of six essential boiler mountings. Boiler mountings are devices fitted to boilers so that they may be safely and properly operated. They are considered to be part of the boiler. They are:
- Spring loaded safety valve. A means of preventing steam pressure from rising above the safe working level.
- Steam stop valve. It is placed on the steam outlet in a vertical or horizontal position to enable the boiler to be shut off from the main steam pipe. It is not used for regulating the steam flow but is kept either fully open or shut.
- Feed check valve; is a form of stop and non-return valve arrangement used to control the admission of feed water into the boiler. It permits feed water to enter while preventing its escape.
- Blow down valve; provides a means for discharging mud, scale and other impurities which enter the boiler with feed water, and also for emptying the boiler. The blow down valve is placed at the lowest part of the boiler where the mud and sludge tends to accumulate.
- Water level gauge. It is of the greatest importance that those in charge of boilers should know with absolute certainty the height of the water level in the boiler. The principal means of ascertaining the water level is by means of glass water gauges.
- Pressure gauge. The difference in pressure between inside and outside the boiler is determined by the use of a pressure gauge.
Fuel. Coal was the most common fuel for boilers but other fuels used were wood, sawdust, tan bark and even straw. Thoroughly dried wood has a heating value about 50% of coal. It is assumed the boiler at Napoleon Reef was fueled by coal.
The Cornish boiler required an attendant who would need the knowledge and inclination to carry out a variety of tasks whilst in charge of the boiler. The boiler was hand fired and the attendant would have had a standard set of tools to maintain a fire to ensure an even supply of steam at the correct pressure. The tools would have been similar to the following: a rake, a 25 mm iron bar 2.5 metres long with a 220 x 120 mm blade welded on the end, used for moving the fuel on the grate; a slice, a 37 mm iron bar, 2.5 metres long with a chisel point, used for breaking up masses of fuel and lifting the fuel on the grate; a pricker, a 25 mm round iron bar with a single or double pricker blade 22 mm wide welded on the end, used for cleaning the grate bars and air spaces; a square mouthed shovel; a lazy bar which was 18 mm flat steel bent to shape to fit over the fire door hinge and the door catch. It was used as a fulcrum for the rake, slice and pricker when pushing live coals back or drawing clinker forward.
The boiler attendant would have required the knowledge to bring a boiler up to steam pressure from cold after carrying out a check of all boiler mountings of which he would need to know the purpose and function of each and be able to replace blown glass tubes in the water gauge. He would be able to correctly light a fire in the furnace tube, understand different firing methods and know how to clean a fire of clinker and coarse ash which would not pass through the grate into the ash pit which must be cleaned regularly. His most important duty was water tending, as shortness of water is fatal to any boiler. When bringing the boiler up to pressure he would need to avoid water in the steam pipes as it can result in water hammer which can result in deaths and serious injury to personnel. Overall, as steam boilers can be very dangerous if not operated correctly, he would need to be aware of all safety aspects of its operation.
Boilers such as a Cornish boiler were used, apart from heating, to drive engines. (Modern boilers drive steam turbines). The purpose of any steam engine is to change heat energy into mechanical work and a Reciprocating Steam Engine did this. These engines had one or more cylinders and pistons with the admission of steam under pressure controlled by slide valves. The boiler at Napoleon Reef may have been used to run a stamp, or stamping battery, to crush ore. It may also have been used to run water pumps or other machinery.
The boiler’s origin is not certain but H & T Danks, a company established in 1840 in the town of Netherton in England, was a major builder of Cornish boilers for many years and it is possible the boiler at Napoleon Reef originated there.
Compiled by David Scott