The engine had to be turned on again, because the water no longer being pumped by it made life difficult for the nearby South Crofty Mine. Therefor the engine had to be restarted and it worked until 1954.
Mr. Greville Bath was a member of the Cornish Engines Preservation Society, which handed this and other engines over to the National Trust in 1967.
On the day the engine did it's last stroke in 1954, the drivers engraved their names into one of the windows with a diamond, I don't remember into which one, but find out yourself!
The steam pressure the engine was operated at was about 50 lb per square inch (3.5 at). It operated with five strokes/min., 6.5 strokes/min. were the maximum it could do.
The engine is exceptionally heavily constructed: The beam weights 52 tons, the cylinder cap 1.5 tons, all in all it gets to 125 tons for the engine (inc. boilers and fittings). Additionaly, the pumps and all the equipment in the shaft were counted: an iron pump rod (89 tons), the plunger pumps with pump chambers (272 tons), and balance bobs (45 tons). Altogether this is a grand total of 684 tons.
These numbers are very impressive, but they don't show the kind of different materials involved. The pump rod consisted of iron and wood, for example.
The engine pumped water from a depth of 1700 ft. (510m) at Taylor's shaft. To do this, seven plunger pumps are attached to the pump rod. This arrangement was good for 90 gallons (409 l) per stroke, or 450 gallons (2045 l) per minute (5 strokes per minute). The total water load for one stroke equals to 84.7 tons of water, resulting to 26.9 lbs. per square inch (1.89 kg/cm^2 if I calculated right!) of piston area.
These numbers were taken from [1]. The conversions into metric messures might be not very precise, it should be sufficient for all metric-thinking people.
On my visit to the engine, one of the last engine drivers showed and explained the engine to the visitors. He told us that the engine was operated by a team of three drivers working in 8 hour shifts. The engine had to work 24 hours a day, and I assume 7 days a week, in order to keep the mine from being flooded.
To provide the steam for the engine, five Cornish Boilers had to be operated, I don't know by how many men. The consumption of coal is not recorded in my books.
When the engine ran, it made five strokes per minute, which was found to be the optimum. The events happening stroke for stroke are as follows:
- the piston is driven down by the pressure of steam, that makes the pump rod go up in the shaft.
- the piston is pulled back up by the weight of the pump rod, which simultaneously operates the seven pumps, thus pumping water 'up'. That is, the lowest pump brings water up to level 1, the 2nd pump brings water from level 1 to level 2, the ...(optimized)...7th pump brings water from level 6 to the surface.
- now the piston rests for some seconds, in order to let the water settle in the pumps.
- this delay is controlled by a device called cataract, which now opens the inlet or steam valve together with the exhaust valve.
- the entering steam pushes down the piston, which pulls down the indoor side of the beam.
- attached to the indoor side of the beam are two plug rods with tappets, which on their move up and down can close valves. They close the steam valve after approx. one third of the stroke.
- at the same time the steam valve was opened, the exhaust valve was opened by the cataract, too. This is closed by the tappets when the stroke is at an end, so the piston is now at the bottom of the cylinder.
- having all valves closed, the engine remains in this position for some seconds, before the second cataract starts the piston to go way up, the outdoor stroke. It does this by opening the equilibrium valve, which connects the room above the piston with the one below it.
- the pump rod pulls the outdoor beam, the piston moves up, and the steam from above the piston floodes to the room under the piston.
- another tappet closes the equilibrium valve, leaving a portion of steam above the piston to bring the piston to a gentle halt.
- now, after the delay, the cataract again opens steam and exhaust valve, which connects the chamber under the piston with a steam condenser. Condensing steam creates a vacuum, thus two forces make the pistone move down: the vacuum of the condenser sucks from the bottom, the new steam pushes from the top.
- during the movements up and down, the two catharacts are initialized by the plug rods, in order to be able to run their delay when the piston arrives at the top or the bottom position.
Interesting details are of course the starting and the halting of the engine. During startup, all valves have to be operated by the driver, until he determines the engine to be solid, that is, no air is in the cylinder nor in the condenser. He then enables the automatic control of the valves by the plug rods.
To stop the engine, the automatic opening of the steam valve by the cataract has to be prevented. David Edmondson, editor of the Kew Bridge Steam Museum's page, provided the informtion, that a little pin is simply inserted into the mechanism to achieve this.
Roland Wagener , Ortsmuehle 3 , D-44227 Dortmund , Germany
roland@heeg.de