The Watt Steam Engine

The Watt steam engine was the first type of steam engine to make use of steam at a pressure just above atmospheric to drive the piston helped by a partial vacuum. Improving on the design of the 1712 Newcomen engine, the Watt steam engine, developed sporadically from 1763 to 1775, was the next great step in the development of the steam engine. Offering a dramatic increase in fuel efficiency, the new design replaced Newcomen engines in areas where coal was expensive, and then went on to be used in the place of most natural power sources such as wind and water. James Watt's design became synonymous with steam engines, due in no small part to his business partner, Matthew Boulton.

Introduction

In 1698, the English mechanical designer Thomas Savery invented a steam pumping appliance that drew water directly from a well by a vacuum, then sent it up to a higher level by steam pressure. The appliance was also proposed for draining mines, but limited pumping height made this impracticable. It also consumed an inordinate amount of fuel.

The solution to draining deep mines was found by Thomas Newcomen who developed an "atmospheric" engine working only on the vacuum principle. It employed a cylinder containing a moveable piston connected by a chain to one end of a rocking beam that worked a mechanical lift pump from its opposite end. The top side of the power cylinder was open to the atmosphere, steam being introduced at top stroke to the underside of the piston then water sprayed in, condensing the steam and creating a vacuum; thus atmospheric pressure acting on the upper side of the piston drove it down.

It was both powerful and useful and for the first time water could be raised from a depth of over 150 feet. The first example from 1711 was able to replace a team of 500 horses that had been used to pump out the mine. In the next fifty years only a few small changes were made to the basic engine, seventy-five examples of which were at mines in Britain, France, Holland, Sweden and Russia.

The system brought great practical benefits, but still at the price of very high coal consumption as the water jet into the cylinder cooled the walls at each stroke; this meant that when the next charge of steam was introduced it would continue condensing until the cylinder approached working temperature once again.

Separate condenser

A Scottish instrument maker, James Watt, was given the job in 1763 of repairing a model Newcomen engine for the University of Glasgow, and noted how inefficient it was. In 1765, while wandering across Glasgow Green he conceived the idea of a separate condensing chamber for the steam engine. Watt's idea was to separate the condensation system from the cylinder, injecting the cooling water spray in a second cylinder, connected to the main one. When the piston had reached the top of the cylinder, the inlet valve was closed and the valve controlling the passage to the condenser was opened. External atmospheric pressure would then push the piston towards the condenser. Thus the condenser could be kept cold and under less than atmospheric pressure, while the cylinder remained hot.

Watt also realised that the new operating cycle might increase engine speed and the power produced; low pressure steam could now be substituted for atmospheric pressure. If the top of the cylinder was closed off, the steam could act upon the piston during the power stroke; the low steam pressure would not be sufficient to move it in normal circumstances, but it could if acting upon a vacuum.

This led to the fully developed version of 1775 that actually went into production [1]. There was no spray, the condenser being immersed in a water tank and at each stroke the warm condensate was drawn off and sent up to a hot well by a vacuum pump which also helped to evacuate the steam from under the power cylinder. The still-warm condensate was recycled as feedwater for the boiler.

The Partnership of Matthew Boulton and James Watt

The separate condenser showed dramatic potential for improvements on the Newcomen engine but Watt was still discouraged by seemingly insurmountable problems before a marketable engine could be perfected. It was only after entering into partnership with Matthew Boulton that such became reality. Watt told Boulton about his ideas on improving the engine, and Boulton, an avid entrepreneur, agreed to fund development of a test engine at Soho, near Birmingham. At last Watt had access to facilities and the practical experience of craftsmen who were soon able to get the first engine working. As fully developed, it used about 75% less fuel than a similar Newcomen one.

In 1775, Watt designed two large engines: one for the Bloomfield Colliery at Tipton, completed in March 1776, and one for John Wilkinson's ironworks at Willey, Shropshire, which was at work the following month. A third engine, at Stratford-le-Bow in east London, was also working that summer.

Boulton and Watt's practice was to help mine-owners and other customers to build engines, supplying men to erect them and some specialised parts. However, they mainly profited from their patent by charging a licence fee to the engine owners, based on the cost of the fuel they saved. The greater fuel-efficiency of their engines meant that they were most attractive in areas where fuel was expensive, particularly Cornwall, for which three engines were ordered in 1777, for Wheal Bussy, Ting Tang, and Chacewater mines.

Later improvements

Driving the engines by the pressure differential between low-pressure steam and a partial vacuum raised the possibility of reciprocating engine development. An arrangement of valves could admit steam to either end, or connect either end with the condenser. Consequently, the direction of the power stroke might be reversed. The resulting double action gave a very even movement to the beam and made possible the development of rotative engines. Furthermore, the linkage to the beam had, until then, been by means of a chain, which meant that power could only be applied in one direction, by pulling; it had to be made possible for the piston to also push the beam whilst keeping the piston rod vertical; this Watt achieved by developing his parallel motion.

In order to avoid patent rights already claimed by another party, on the use of the crank, he adopted the epicyclic sun and planet gear system suggested by an employee William Murdoch, only later reverting once the patent rights had expired to the more familiar crank seen on most engines today.

Because factory machinery needed to operate at a constant speed, Watt adapted the centrifugal governor (earlier used to automatically control the speed of windmills) linked to a steam regulator valve.

These improvements allowed the steam engine to be used to replace water wheels, thereby freeing British industry from geographical constraints and becoming one of the main drivers in the industrial revolution.

Watt was also concerned with fundamental research on the functioning of the steam engine. His most notable measuring device, still in use today is the Watt indicator incorporating a manometer to measure steam pressure within the cylinder according to the position of the piston; this enabled a diagram to be produced representing the action of the steam throughout the cycle.

Preserved Watt engines

The oldest working engine in the world is the Smethwick Engine, brought into service in May 1779 and now at Thinktank in Birmingham (formerly at the now defunct Museum of Science and Industry, Birmingham). The oldest still in its original engine house and still capable of doing the job for which it was installed is the 1812 Boulton and Watt engine at the Crofton Pumping Station. This was used to pump water for the Kennet and Avon Canal; on certain weekends throughout the year the modern pumps are switched off and the two steam engines at Crofton still perform this function. The oldest rotative steam engine (the third rotative engine ever built) is located in the Powerhouse Museum in Sydney, Australia. A Boulton-Watt engine of 1788 may be found in the Science Museum, London., while an 1817 blowing engine, formerly used at the Netherton ironworks of M W Grazebrook now decorates Dartmouth Circus, a traffic island at the start of the A38(M) motorway in Birmingham.

The Henry Ford Museum in Dearborn, Michigan houses a Watt rotative engine manufactured in 1788 by Charles Summerfield. This is a full-scale working replica of a Boulton-Watt engine. The American industrialist Henry Ford moved the engine to Dearborn around 1930.