STEAM ENGINE. A steam engine is a heat engine in which the working substance is steam. By a heat engine is meant a machine for doing mechanical work through the agency of heat: it does this by taking in heat at comparatively high temperature, converting part of the heat into another form of energy, and re jecting the remainder of the heat at a lower temperature. The working substance is the vehicle by which heat is taken in and rejected. In a steam engine, the substance takes in heat mainly in a separate vessel—the boiler—in the process of being vapor ized; it does work by expanding under pressure and thereby con verts part of the heat that is taken in ; finally the remainder of the heat is rejected, either by allowing the steam to escape into the atmosphere, as in locomotives, or by condensing it at a com paratively low temperature and pressure, as in marine and many other engines. This gives a broad distinction between non-con densing and condensing steam engines. The latter have the great advantage that they allow the effective expansion of the steam to be carried much farther ; the substance rejects heat at a lower limit of temperature, and this enables the engine to convert into work a larger fraction of the heat which it has received. That fraction expresses what is called the efficiency of the engine as a contriv ance for converting heat into work. The addition of a condenser, while it increases the efficiency, of course complicates the mechan ism: it requires a supply of cooling water or some equivalent means of keeping down the temperature by absorbing the heat which the steam gives up in the act of being condensed, and also a pump or other means of removing the condensed substance to gether with any air that may be present. But the advantage which it brings about in respect of efficiency is so great that all engines of large power, where economy of fuel is an important factor, and where the use of a condenser is practicable, are of the condensing kind.
Given the upper limit of temperature, at which heat is taken in, the efficiency which the engine may attain is determined by the lowness of the temperature at which heat is rejected. Similarly, when the lower limit of temperature, at which heat is rejected, is assigned, the efficiency which the engine may attain is in creased by raising the temperature at which the working sub stance takes in heat. To secure high efficiency there must be a wide range through which the temperature of the working sub stance falls, as a consequence of expansion within the engine, from the level of temperature at which heat is received to the level at which heat is rejected. Thus in the steam engine the most efficient performance, that is to say the greatest output of work in relation to the heat supplied, is secured by keeping the con denser as cold as the available cooling water will allow, and at the same time using a high boiler pressure, so that the working substance is very hot while it receives heat in the act of changing from water into steam. For this reason, modern practice tends
towards higher and higher boiler pressures, as the mechanical difficulties of boiler construction and high pressure working are overcome.
After conversion into steam the working substance may take in a supplementary supply of heat on its way from the boiler to the engine, by passing through a superheater, in which its tem perature is raised above that of the boiler. A common form of superheater is a group of parallel pipes with their surfaces exposed to hot gases of the boiler furnace.
Steam engines are classified into two general types according to the manner in which the steam does work during its expansion. In the first, or piston-and-cylinder type, the steam, in a confined space, namely the part of the cylinder behind the piston, en larges the volume of that space by pushing the piston forward. It does work by exerting a static pressure on the moving piston: the movement of the steam itself is of no significance. In the second class, to which belong all kinds of steam turbines, the action is less direct. The pressure of the steam is first employed to set the steam itself into motion, forming a jet or group of jets, the momentum of which causes work to be done on a moving part of the machine, either by the impulsive action of the jet or jets on revolving vanes, or by the reaction on revolving guide blades during the formation of the jets, or, as in Parsons' tur bine, by a combination of impulse and reaction. In any turbine the action of the steam is kinetic, in contrast with its static action in an engine of the piston type. In both types there is progressive expansion of the steam from the high pressure and relatively small volume at which it is admitted, to the low pressure and relatively very great volume at which it is discharged. The principle already stated, that a large range of temperature and pressure, between admission and exhaust, is essential to efficiency applies equally to both types. In practice, the turbine has a notable advantage over the piston-and-cylinder engine in this respect, that it allows the last stages of the expansion, when the volume of the steam has become very great, to be effectively utilised, to a degree which is impracticable in the other type, because of the enormous size which the cylinder would have to assume and the waste of work that would be caused by piston friction, if the steam were ex panded down to a very low pressure by the piston methbd. The turbine method escapes these difficulties ; mainly for that reason it has become the most efficient way of converting heat into work, on a large scale, through the agency of steam. It has further ad vantages in compactness, in simplicity of working, and in the facil ity with which it can be adapted to take steam of exceptionally high initial pressure and high superheat.