Cornish engine
The mine Wheal Vor held one of the earliest Newcomen engines before 1714. Cornwall has no coalfield and all coal used had to be brought in from outside the county. Coal imported from south Wales was expensive for these mining operations. The cost of fuel for pumping became a significant part of total mining costs. Lifting water up from deep shafts requires great amounts of work input. Horse power or waterwheels could supply this energy weakly at best. Horses have limited power and waterwheels need a suitable stream of water nearby. Coal-fired steam power proved more versatile and effective than primitive means. Early Watt engines had a duty of 20 million foot-pounds per bushel of coal. Later Watt engines achieved over 30 million foot-pounds per bushel. Boulton and Watt charged mine owners a royalty based on a share of fuel saving.
Increasing boiler pressure above atmospheric pressure was an essential element of efficiency improvement. James Watt conceived expansive working but found low steam pressure made improvements negligible. In a Cornish engine, intake valve shuts off midway through the power stroke. Steam already in that cylinder part expands through rest of stroke to lower pressure. This captures greater proportion of energy compared to Watt engines. Less heat gets lost to condenser during operation. Insulation of steam lines and cylinder helped retain thermal energy. Steam jacketing the cylinder also reduced heat loss significantly. Few Cornish engines remain in their original locations today. Most were scrapped when related industrial firms closed down operations.
Richard Trevithick devised use of steam pressure above atmospheric pressure in early 19th century. His early puffer engines discharged steam directly into atmosphere without condensation. Trevithick's later engines from 1810s combined high-pressure steam with sub-atmospheric principles. Arthur Woolf developed compound steam engine where steam expanded in two cylinders successively. William Sims built or adapted engines after Trevithick left for South America in 1816. One engine at Wheal Chance operated at pressure above atmospheric achieving duty nearly 50 million. A test between single-cylinder and Woolf compound engine occurred at Wheal Alfred in 1825. Both achieved duty slightly more than 40 million foot-pounds per bushel. Samuel Grose decreased heat loss by insulating pipes, cylinders, and boilers in late 1820s. Duty improved to more than 60 million at Wheal Hope and almost 80 million at Wheal Towan.
Pressurized steam inlet valve opens while low-pressure exhaust valves also open during cycle start. Pressurized steam enters top part of cylinder pushing piston downward. Steam below piston draws into condenser creating vacuum beneath it. Pressure difference drives piston down through stroke. Part way down stroke, pressurized steam inlet valve closes completely. Steam above piston expands through rest of stroke while maintaining partial vacuum below. At bottom of stroke, exhaust valve to condenser closes and equilibrium valve opens. Weight of pump equipment transfers via walking beam pulling piston upward. Steam transfers through equilibrium pipe from above piston to bottom of cylinder when piston reaches top. Next stroke may occur immediately or delay via cataract timing device. Engine is single-acting with steam piston pulled up by weight of pump piston and rodding. Steam supplied at pressure up to 50 pounds per square inch.
Cornish engines became widely used for water supply around the world after development. Thomas Wicksteed installed secondhand Cornish engine at Old Ford waterworks in 1838. Comparison to Watt engine showed favourable results due to higher London coal prices. Main textile manufacturing areas like Manchester and Leeds had too low coal price for replacement. Only late 1830s did textile manufacturers begin moving to high-pressure engines. They usually added high-pressure cylinder forming compound engine instead of following usual Cornish practice. Capital and maintenance costs were higher than Watt steam engine initially. This long delayed installation outside Cornwall region. Best duty was short-lived achievement due to general deterioration of machinery. Leaks from boilers and deterioration of boiler plates meant pressure had to reduce. Forces likely caused machinery breakages with pressures up to 50 pounds per square inch.
London Museum of Water & Steam holds largest collection of Cornish engines globally. Crofton Pumping Station in Wiltshire contains two Cornish engines still working today. One 1812 Boulton and Watt engine is oldest working beam engine in original house. It pumps water to summit pound of Kennet and Avon Canal. Two examples survive at Cornish Mines and Engines museum near Pool town. Poldark Mine at Trenear houses Harvey of Hayle Cornish Beam Engine from about 1840-1850. Last Cornish engine worked commercially at Greensplat China Clay Pit until 1959. Cruquius pumping station in Netherlands has largest diameter cylinder ever built for Cornish engine. That cylinder measures 6 feet diameter with eight beams connected to single unit. Restored between 1985 and 2000 but now operated by oil-filled hydraulic system. Dalton Pumping Station preserves 1879 pair designed to run on superheated steam. Dorothea Slate Quarry installed one last Cornish engine in 1904 remaining within engine house. Trevithick Society formed committee in 1935 to preserve Levant winding engine.
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Common questions
What is a Cornish engine and where did it originate?
A Cornish engine is a type of steam beam engine that originated in Cornwall. This region had no coalfield so all coal used for these engines was imported from outside the county.
How does a Cornish engine improve efficiency compared to early Watt engines?
Cornish engines achieve higher efficiency by shutting off the intake valve midway through the power stroke to allow steam expansion. This design captures greater energy proportions while reducing heat loss to the condenser compared to earlier models.
When were high-pressure steam principles first applied to Cornish engines?
Richard Trevithick devised the use of steam pressure above atmospheric pressure in the early 19th century. His later engines from the 1810s combined this high-pressure steam with sub-atmospheric principles to drive piston movement.
Where can you find working examples of Cornish engines today?
Crofton Pumping Station in Wiltshire contains two Cornish engines still operating as of the current era. The London Museum of Water & Steam holds the largest global collection of these historical machines.
What was the maximum duty achieved by late Cornish engines at Wheal Towan?
Samuel Grose improved duties to almost 80 million foot-pounds per bushel at Wheal Towan during the late 1820s. This represented a significant increase over the 30 million foot-pounds per bushel achieved by later Watt engines.