History

Pre-history (1712)

Thomas Newcomen’s engine, which was the first practical steam engine, first ran in 1712.In the Newcomen engine, steam is first drawn into the cylinder. Then, cold water is injected, the steam condenses and the atmospheric pressure drives the piston downwards, hence “atmospheric engine”.

The disadvantage was, that the cylinder cooled down every time cold water was injected, and steam was needed to heat it up again. The efficiency was estimated as 0.5%.

James Watt’s condensing engine (1772)

James Watt realised the disadvantages of the Newcomen engine, and invented the external condenser. This was a vacuum container linked to the cylinder. It was cooled by injection water, and usually located in a water bath to keep its temperature low. An “air pump” was attached to the condenser, to keep it at very low pressure. Once the cylinder was filled with steam. the condenser valve was opened. the steam then rushes into the condenser, changes to water and the vacuum is maintained until the cylinder is completely evacuated – but still hot.

       

a. Watt’s condensing engine                         b. Steam expansion (Farey, 1827)

Fig. 2: Watt’s condensing engine and expansion cycle

James Watt also used a double acting cylinder, where steam was drawn in one one side whilst a vacuum existed on the other – the boiler replaced the atmosphere! This increased and smoothened the power output, whilst the external condenser improved the efficiency to 3 – 3.5%. Not very much, though.

The expansion cycle (1782)

James Watt realised that, with vacuum on one side and atmospheric (or rather boiler) pressure on the other side, the steam still contained energy at the end of the stroke. Ideally, the steam would have a pressure similar to the condenser pressure at that point, this would mean that all the energy of the steam is converted.

This principle was however (a) kept secret by Watt, and (b) had the severe disadvantage that the power output varied during one stroke. This was no good for the pumping of water, which the engines were used for. The expansion was therefore limited to a factor of 1:1.5 or 1:2. Watt kept this principle to himself, and Farey only learned about it during a visit to Russia in the 1820’s. In the context of this development, James watt and John Southern started to use the pressure-volume diagram to describe the process. they also developed a measurement device which plotted the nowadays generally applied diagram, Jun. (1822).

By then however the condensing engine had been replaced by high-pressure steam engines which could deliver more power, and the whole thing was more or less immediately forgotten.Today, there is to my knowledge only one condensing engine in a museum which is still operational in Crofton / England. It uses the so-called Cornish Cycle, i.e. it is not double acting but the steam is moved from one side of the piston to the other to maintain the cylinder temperature.

The maximum efficiency was determined by James Watt as 3.08 times the efficiency without expansion, or 19.6%. But, for all practical purposes this was forgotten when the Condensing Engines disappeared.

Recent history (1886, 2011)

Well, nothing much more was reported on the Condensing Engine. There was one mention in 1886, when an “Atmospheric Engine” was patented and apparently also manufactured for use in e.g. residential buildings where steam engines were not allowed (danger of boiler explosions). A detailed description (although in German) is given in Dingler (1886).

The engine had a power output of 0.79 kW (1.07 BHP) with a speed of 125.7 rpm. It did not employ steam expansion, as far as I can see. We could however not find any more information or further mention of this engine. The expansion of steam was subsequently forgotten, and apparently the only number which was remembered was the 3% efficiency. The Condensing Engine was therefore dismissed by one and all as inefficient, and a thing of the past, and of interest to historians only.

Well, not quite. There was one paper published by Chinese researchers in 2011 (Wang et al. 2011) which described a rather interesting micro-application of the Newcomen cycle. A harmonica-like small metal tube was evacuated, and some droplets of a fluid inserted. As the tube heats up, the fluid evaporates and the tube expands (remember that i litre of water evaporates into 1700 litres of steam). When cooled down again, the fluid condenses, and the tube goes back to its original length. The work done by expansion / contraction is used to generate electricity with a Piezo-element. No moving parts, completely self-contained. Neat.

Literature

Dingler 1886 (Ed.), Ueber Neuerungen an Kleindampfmaschinen. Polytechnisches Journal. Band 259 (S. 1–9). Dingler 1886

Farey J., 1827, A treatise on the Steam Engine. Historical, Practical and Descriptive. Farey 1827

Jun. H.H.(1822). Account of a steam-engine indicator. Quarterly Journal of Science, vol. 13. 95.

Wang, Y., Zhou, Z., Zhou, J., Liu, J., Wang, Z., & Cen, K. (2011). Micro Newcomen steam engine using two-phase working fluid. Energy, 36(2), 917-921.