Boulton and Watt steam engine

The Watt steam engine (alternatively known as the Boulton and Watt steam engine) was the first type of steam engine to make use of a separate condenser. It was a vacuum or "atmospheric" engine using steam at a pressure just above atmospheric to create a partial vacuum beneath the piston. The difference between atmospheric pressure above the piston and the partial vacuum below drove the piston down the cylinder. James Watt avoided the use of high pressure steam because of safety concerns. Watt's design became synonymous with steam engines, due in no small part to his business partner, Matthew Boulton.

The Watt steam engine, developed sporadically from 1763 to 1775, was an improvement on the design of the 1712 Newcomen steam engine and was a key point in the Industrial Revolution.

Watt's two most important improvements were the separate condenser and rotary motion. The separate condenser, located external to the cylinder, condensed steam without cooling the piston and cylinder walls as did the internal spray in Newcomen's engine. Watt's engine's efficiency was more than double that of the Newcomen engine. Rotary motion was more suitable for industrial power than the oscillating beam of Newcomen's engine.

In 1699, the English mechanical designer Thomas Savery invented a pumping appliance that used steam to draw water directly from a well by means of a vacuum created by condensing steam. The appliance was also proposed for draining mines, but it could only draw fluid up approximately 25 feet, meaning it had to be located within this distance of the mine floor being drained. As mines became deeper, this was often impractical. It also consumed a large amount of fuel compared with later engines.

The solution to draining deep mines was found by Thomas Newcomen who developed an "atmospheric" engine that also worked on the vacuum principle. It employed a cylinder containing a movable piston connected by a chain to one end of a rocking beam that worked a mechanical lift pump from its opposite end. At the bottom of each stroke, steam was allowed to enter the cylinder below the piston. As the piston rose within the cylinder, drawn upward by a counterbalance, it drew in steam at atmospheric pressure. At the top of the stroke the steam valve was closed, and cold water was briefly injected into the cylinder as a means of cooling the steam. This water condensed the steam and created a partial vacuum below the piston. The atmospheric pressure outside the engine was then greater than the pressure within the cylinder, thereby pushing the piston into the cylinder. The piston, attached to a chain and in turn attached to one end of the "rocking beam", pulled down the end of the beam, lifting the opposite end of the beam. Hence, the pump deep in the mine attached to opposite end of the beam via ropes and chains was driven. The pump pushed, rather than pulled the column of water upward, hence it could lift water any distance. Once the piston was at the bottom, the cycle repeated.

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