Agar Road, Pool, Near Redruth, TR15 3NP
Tel: (01209) 315027
In the heart of Cornwall's richest mining district there is a rare opportunity to see two fine Cornish beam engines preserved in their imposing houses. One of them a 30 inch whim (winding) engine at Mitchell's Shaft, East Pool Mine, can be seen in action. The other is a 90 inch pumping engine at Taylor's Shaft, a giant of an engine, the largest left in Cornwall, which was still working as late as 1954.
The East Pool Whim is a late example of a steam winding engine of the beam type, built in 1887 to the design of a local engineer, E. W. Michell, after whom the adjacent shaft, 1,500 feet deep, was named. From 1887 until 1921, tin was extracted from this shaft, until a large underground movement of rock destroyed the shaft and the old part of the mine was abandoned. The company sank the new Taylor's Shaft on the other side of the main road, where operations continued until 1945.
The engine was manufactured by Holman Brothers of Camborne, the only one of the type made by the firm and the last rotative beam engine made in Cornwall.
It is a double-acting engine, i.e. the steam acts alternately on the top and bottom sides of the piston and is then exhausted to a condenser instead of being discharged into the atmosphere. Although only worked at a steam pressure of about 40 pound per square inch it was a highly economical engine, doing a day’s hoisting on a few hundredweight of coal.
The cylinder is 30 inch in diameter and the piston has the unusually long stroke of 9 feet. There is an outer cylinder, known as the 'case', the space between the two being filled with steam at full boiler pressure. This kept the inner cylinder much hotter and thus greatly increased the engine's efficiency. This particular engine was designed to run at 17 revolutions per minute compared with the more usual 10 to 12 revolutions per minute of most Cornish rotative beam engines. Having no gears and being a condensing engine it ran very quietly.
Unfortunately it is not practical to work the engine by steam and it is driven by an electric motor. In its silent smooth action it looks just as it did when powered by steam, but it lacks the gentle heat, the indescribable smell of steam and hot oil associated with every sort of steam engine.
The boiler was made by Messrs. Ruston and Hornsby in 1926. It is a Cornish pattern boiler of Trevithick's type and identical to the two which used to work at this Engine House. It was formerly at the Poor Law Institution of the Truro Union where it provided steam for the laundry.
In the boiler house is a model showing the working of East Pool mine circa 1905.
Like so many of the shafts in Cornish mines Michell's was not perpendicular but was steeply inclined and crooked as it followed the vein in depth. Instead of riding in true cages as in a modern vertical shaft the men rode in wheeled vehicles known as gigs. These could be changed quickly for long box-like skips in which the ore was hoisted.
The steel wire winding ropes were wound round the two drums in opposite directions, so whichever way the engine rotated one vehicle was raised and the other lowered. The weight of the vehicles themselves was thus in balance, considerably lightening the load on the engine and making for economy in working.
In the middle chamber you can still see the hand windlass, for lifting heavy parts during repairs, and the spare piston rings still hanging on the walls.
This is one of the largest and most impressive of the world-famous Cornish pumping engines and is one of the last that were built.
It was designed by a local engineer, Nicholas Trestrail, and built in 1892 by the famous firm of Harvey & Co. of Hayle. The cylinder is 90 inch or 7 feet 6 inch in diameter, the piston stroke is 10 feet and the stroke of the pump rods in the shaft 9 feet. The very large cast-iron beam or bob is 33 feet 3 inch in length and weighs over 52 tons, the total weight of metal in the engine being 125 tons.
This great engine was built to pump water from the deep Carn Brea Mines (near the hill of that name) but after the closure of those mines it was re-erected at its present site in 1924 and was thus the last Cornish engine to be put up anywhere in the world.
East Pool Mine closed in 1945 and as the engine was then likely to be scrapped it was purchased for preservation by Mr Greville Bathe of Florida, an engineer-historian. He most generously gave the engine to the Cornish Engines Preservation Society who handed it over to the National Trust in 1967.
The engine's active life did not, however, end with the closure of East Pool, for the cessation of pumping there caused a lot of water to percolate into the workings of the neighbouring South Crofty Mine and that company was compelled to work the engine to prevent their own mine from being flooded. The engine was therefore put to work again and continued to pump from a lesser depth (1,034 feet) until, in 1954, it was displaced by the new electric pumps at South Crofty which eventually displaced three large Cornish pumps. The engine’s last stroke was made on 28th September 1954.
At Taylor's Shaft the engine originally pumped from a depth of 1,700 feet. As the pump rams or poles in the shaft were 18 inches in diameter it pumped about 90 gallons per double stroke or 450 gallons per minute when running at the usual speed of about 5 strokes per minute.
Although the amount of water pumped per stroke was only 90 gallons, or 900 pounds in weight, the weight of the total column of water moving in the shaft at each stroke was 84.7 tons, equivalent to 26.9 pounds per square inch of piston area. This is a very heavy load for a Cornish engine which, if it is to work at maximum efficiency, should not be loaded to more than 15 pounds per square inch of piston area.
In the classical arrangement of the Cornish pump, the engine lifts the pump rods and the latter, by virtue of their weight, force the water up on their succeeding down or outdoor stroke. For reasons of strength, the wooden rods in the shaft have to be made of a great size, in this case they are 20 inches square at surface, tapering down to 16 inches square at the bottom of the shaft. The total rod weight, including the connecting iron plates, the poles, etc. is more than twice the weight of the column of water in the shaft. To avoid excessive strain on the rods, and very heavy coal consumption, it is necessary to balance out as much of the excess weight as possible. This was done by having a balance beam or bob on the surface, three more underground working in big chambers cut out of the solid rock and a water balance near the bottom of the shaft.
The surface balance bob is interesting as it was made of steel plates riveted together instead of being made of the usual cast iron, and it bears the date 1911 and the name of the maker, the Charlestown Foundry Co. at St. Austell. Historically, this is very interesting as it originally formed part of the last Cornish engine to be built, though it was later utilised as the balance bob for this much larger engine.
The engine of this massive pumping plant worked at a steam pressure of 50 pounds per square inch, steam being generated in five large Cornish boilers which stood in the adjoining house of which only the walls now remain.
Before describing what the visitor sees on entering the house it is necessary to describe very briefly the unique Cornish single-action steam cycle.
When the engine is at rest the pistols is at the top of its stroke and the pump rods are down. To start the engine the driver opens the exhaust valve which puts the cylinder space beneath the piston into communication with the condenser. He then gently opens the steam valve, admitting steam at a pressure of 40 to 50 pounds per square inch above the piston which is thereby forced down and the great rocking beam draws the pump rods up. At a pre-determined point in the stroke, often a third or even less, the steam valve is closed and the remainder of the stroke is performed by the expansion of the steam. After the engine has completed this, the 'indoor' stroke, another valve, the equilibrium, is opened, thereby allowing the steam, now reduced to approximately atmospheric pressure, to pass to the underside of the piston which is thus placed in equilibrium. The pump rods then take charge and their weight, acting upon the rams in the shaft, forces the water up to the surface. During the descent of the rods, or 'outdoor' stroke of the engine, the piston is again lifted to the top of the cylinder. To prevent it hitting the cylinder cover the equilibrium valve closes just before the top end of the stroke so that the remaining steam is trapped. thus cushioning the rising piston and bringing it to rest.
Before the next 'indoor' stroke begins the exhaust valve is again opened, thereby allowing the expanded steam beneath the piston to go to the condenser which, by condensing the steam, creates a vacuum beneath the piston and this alone generates considerable horsepower. After a short pause the steam valve again opens and the cycle is repeated, the piston descending under the pressure of the live steam above and the vacuum beneath.
When the engine is started all these valve movements are controlled by the driver working the handles of the gear manually but as soon as he judges that the engine is 'solid', i.e. all the air has been expelled from the cylinder, the condenser and from beneath the pump rams, the automatic valve mechanism can be engaged and the engine then works by itself. The cataract or water dash-pot gear in the 'cockpit' or basement of the house can be adjusted to give anything from one to ten or more double strokes per minute, thus matching the amount of water to be pumped at different seasons of the year. Taylor's engine was, however, a very large and heavy one and it was found in practice that its maximum speed was about 6½ strokes per minute.
Among items of interest in the bottom chamber is a diagram showing the arrangement of the pumps in the shaft, a spare water valve or clack and a model demonstrating the working of the whole mine complex at Taylor's.
There are a number of unusual features about this great beam, one being that the top rib, which is in tension, is made much deeper and therefore stronger than the bottom one which, being in compression need not be as large. It will also be noted that the indoor nose pin is free to float in brass bearings, which is the usual plan adopted in large Cornish engines, whereas the outdoor pin is fixed in the nose of the beam. The reason for this was the unusual arrangement of iron shaft rods with which the engine was equipped when first built, but which were later replaced by conventional wooden ones as being more satisfactory. This non-standard feature of the engine has misled some model makers who copied it slavishly when building a miniature.
In common with all these pumping engine's, the indoor nose of the bob is fitted with a cross member known as the catch wing. If the pistol is making the correct length of stroke the wing comes down to within an inch or two of the heavy timber beams alongside the bob. If, however, with slight variations of steam pressure the piston travels too far the wing will bump on the spring beams and on hearing the sound the driver will close the throttle valve slightly.
The other and more important purpose of the catch wing is to try to arrest the piston and stop it smashing the cylinder bottom if one of the wooden shaft rods breaks. The power of the engine is, however, so great that if this happens the spring beams may be completely smashed as indeed did happen on one occasion.
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