Steam

STEAM, the vapour of water. In the pure state it is a dry invisible gas. Often, however, as in a jet escaping from the spout of a kettle or the funnel of a locomotive, it is mixed with minute particles of water which are produced by condensation of portions of the gas. In such a mixture the suspended particles of water constitute a visible cloud. Any mixture of steam with water, whether in such a cloud or in the working chamber of an engine or turbine, is often spoken of as wet steam.

Its properties are most conveniently described by imagining an experiment in which steam is formed by applying heat to a small quantity of water contained at the bottom of a large upright cylindrical vessel. Suppose that the vessel is fitted with a piston which rests on the water to begin with and can rise when the fluid below it is made to change from water into steam by applying heat. Imagine further that the piston is frictionless and carries a definite weight so that, as the piston rises, the fluid continues to be subjected to a constant pressure, say, p lb. per square inch.

Saturated Steam.

When heat is applied, no steam is formed until the temperature of the water is raised to a value T which depends on p. Steam then forms, raising the piston, and this goes on without further rise of temperature until all the water is converted into steam, when it occupies a certain volume V. During this stage the steam is said to be saturated. T is the tern perature of saturation corresponding to the pressure p, and is the lowest temperature at which steam can exist in stable equilibrium at that pressure.

Superheated Steam.

Suppose now, after all the water has turned into steam, that we go on applying heat. The temperature will rise and the volume will increase beyond V if we keep p con stant, or the pressure will increase beyond p if we then fix the piston so as to keep V constant. In either case the steam is said to be superheated.

Steam is superheated when its temperature is raised in any manner to a value which exceeds the temperature of saturation corresponding to the actual pressure. Thus for example steam may change from the saturated to the superheated condition by being compressed (without loss of heat), or by passing (without loss of heat) through a throttle valve into a region of lower pressure. When steam is so "throttled" its temperature falls to

some extent, but remains higher than the temperature of satura tion corresponding to the reduced pressure.

Properties of Steam : Callendar's Tables.

The physical properties of steam have been the subject of systematic experi mental enquiry by Regnault and many later observers. Our modern knowledge of them is largely due to H. L. Callendar who. with the help of formulas the basis of which is partly theoretical and partly empirical, has rationalized the experimental data and has compiled comprehensive tables for the use of engineers. Callendar's tables, published in 1915 and in an enlarged form in 1924, set forth all the important properties throughout a suitable range of pressures and temperatures. From the principles of thermodynamics (q.v.) it is known that certain relations hold between various properties, in steam or any other vapour; the values stated by Callendar, besides being founded on the best available data, are consistent with these relations. They apply to the superheated as well as to the saturated state; saturation is to be regarded as only a limiting case.

So long as steam is saturated the relation of temperature to pressure is definite. But steam may be superheated to any temper ature above the saturation temperature at which it is formed in the boiling of water, and the temperature then becomes an inde pendent variable. This affects certain other properties with which the steam engineer is concerned, namely:— The volume V, The internal energy E, The total heat I, The entropy cb, all of which are to be reckoned per lb. of the substance. Each of these quantities has a definite value for steam or for water in any assigned state of pressure and temperature. Steam tables usually follow a convention, according to which quantities such as the energy or the entropy are treated as zero for water at o° C; the tabulated numerical value accordingly expresses the amount by which the quantity in question has changed when the sub stance passes from that zero condition to the actual state.