This is a special case of the compound steam engine, in which each charge of steam is used to push three separate pistons against atmospheric pressure in three consecutive expansion processes.
It was developed primarily for marine applications (small ships and boats) as it offers a good power to weight ratio and excellent efficiency compared with single-stage steam engines. The high power to weight ratio comes as a result of using high pressure steam (above around 75 psi/ 5 bar), and then extracting the energy from the steam in multiple expansion stages, which maintains high temperatures in the higher pressure cylinders. It contrasts with early steam engines which relied on the use of huge volumes of low-pressure steam. These were appropriate for land-based or fixed machines, but not for mobile engines.
Engines capable of using high pressure steam only became feasible towards the end of the 18th century, as advances in metallurgy and machining allowed engineers effectively to contain and seal the high pressure steam. The first such engines were starting to appear around 1870, but they did not reach their full potential for a further 20 years or so. By 1890, however, triple expansion engines were so successful in terms of reliability, economy and power output, that they led directly to the substitution of sail by steam in the large ocean-going vessels over the following decade or two.
First to use these big engines were the cargo ships such as the Inchdune serving the tea trade, but soon after, the new engines permitted the development of large passenger liners which could reliably cross the Atlantic using the shorter, but more northerly Arctic routes where winds were often insufficient to propel a sailing ship. For example, the Cunard liner Campania
About the same time (1890s) the military stared using these engines to power heavy cruisers and battleships. The triple expansion steam engine remained the engine of choice for large ships until the 1940s, but is now used primarily on ships for the tourist trade. Modern commercial ships use either large diesel engines or gas turbines.
How it works
In essence, a steam engine works by filling a high pressure cylinder with steam directly from the boiler. As this steam pushes the piston back against a load, it expands and loses enthalpy (pressure and temperature). One of the challenges for steam engine makers is that the steam cools as it expands, and this in turn makes the heavy metal pistons and cylinders cool down. When the piston is re-filled with fresh steam, some of the energy from this steam is absorbed as the heavy metal piston and cylinder are heated up. This leads to inefficiencies
One way around this problem is to reduce the pressure drop in the cylinder, but this means losing more enthalpy at the end of the stroke when the steam is vented to atmosphere.
The answer was to take the exhaust steam from the first (high pressure) cylinder and use it in a second (intermediate) cylinder. This operates at a lower temperature than the high pressure cylinder but allows the engine designers to extract more useful energy from the steam. This idea can be extended to a third cylinder (or sometimes two separate low pressure cylinders).
This is the basic principle behind the triple expansion engine.
In order to minimise the temperature loss in this cylinder, the designers have allowed the steam to expand by only a third of its full potential. This keeps the steam hot, and improves the thermal efficiency of the cylinder.
As the piston reaches the end of its stroke, the exhaust steam is fed to the intermediate cylinder—larger than the first to ensure a similar force—and is used to push back this second piston against atmospheric pressure. The designers can balance the expansion of the steam in the first cylinder against the diameter of the second piston, so that the same charge of steam can be made to do the same amount of work as it expands into the second cylinder.
At the end of this expansion process, the steam still has enough enthalpy to do further work, and the steam is fed to a third cylinder where it once more does some useful work pushing back a large piston. Again, the size of the third cylinder is balanced against the expansion in the first two cylinders and the amount of work needed to balance the power output. If the designers have done their job well, the enthalpy remaining in the steam after this third expansion is minimal, and the steam can be vented to atmosphere with no significant loss of energy.
In larger engines, the diameter of the third cylinder is too large for practical manufacturing constraints, so the steam vented from the second cylinder is fed to two identical low-pressure cylinders. Although such engines have four cylinders in total, there are only three expansion stages, so they are also called triple expansion cylinders.
The trick in designing the engine is to extract the maximum energy from the steam, while also keeping the flows of steam consistent and trying to balance the power output from the three separate cylinders. In each of the three expansion stages, the temperature differentials are minimised, which also minimises the inefficiencies of the engine.
In theory, more expansion stages will improve the efficiency still further, but practical difficulties mean that three stages is the optimum for cost-effective engines.
Sources and further reading