THEORY OF THE STEAM ENGINE AS A HEAT ENGINE Properties of Steam.—Steam-engine theory is an application of the principles of thermodynamics (q.v.) to a machine using steam as its working substance. The relevant properties of the substance have been briefly described in the article STEAM. They include the pressure, temperature, and density (or volume per unit of mass) also (per unit of mass) the entropy 4 and the total heat I. For definitions of these quantities reference should be made to that article. Numerical values will be found in Callendar's or other steam tables, over a sufficiently wide range of conditions to meet the purposes of the engineer.
Given steam in any initial state, we have to consider what hap pens when it expands, doing work, as in the cylinder of a steam engine. It is easy to imagine steam expanding without turbulence in an ideal cylinder which is a perfect non-conductor of heat. Under these conditions no heat is being taken in, and there is no change of entropy : such expansion is said to be adiabatic. Adia batic expansion, and adiabatic compression, although never strictly realized owing to the influence of the conducting cylinder walls, are ideal actions important in the theory of the steam engine.
During adiabatic expansion the substance is doing work at the expense of its stock of internal energy, and its temperature falls. Steam expanding adiabatically becomes partly condensed : the substance then becomes a mixture of saturated steam with water at the same temperature, and the energy or the total heat of one pound is to be found by considering what fraction is present as water and what fraction is steam. Such a mixture is often called wet steam. The degree of wetness reached at any stage of adia batic expansion is readily calculated by taking account of the fact that the expanding mixture keeps its entropy unchanged.
From the general principles of thermodynamics it is easy to assign an upper limit to the efficiency of a steam engine, when the temperatures are known at which the working substance takes in and rejects heat. By "efficiency" is here meant the ratio of the work done to the quantity of heat supplied. Suppose, to simplify
the problem, that there is no superheating, that is to say no supple mentary taking in of heat after the steam has been formed at the constant temperature of the boiler. Suppose also that the only rejection of heat is at the constant temperature of the con denser. Then the principle of Carnot (see THERMODYNAMICS) shows that under ideally favourable conditions of working the fraction of the heat supplied which is converted into work cannot exceed when the temperatures are expressed on the absolute scale.