STEAM. Steam is water in a gaseous state. It liquefies at a temperature of 100° C., or 212° F., under a pressure of one atmosphere at the sea-level, namely, 14.7 pounds per square inch. High pressure and low pressure steam once meant steam used at pressures above or below this point. An engine which exhausted its steam into the open air — that is, at ordi nary atmosphere foressure— was known as a high-pressure engine; an engine which ex hausted into a condenser, where a partial vacuum was maintained, was called a low pres sure engine. But these terms have lost their former significance. Absolute steam pressure is the total pressure computed from the zero of an absolute vacuum, as distinguished from relative pressure at sea-level. Ordinary steam gauges indicate pressure above that of the at mosphere. To this must be added the pressure of the atmosphere in order to obtain the abso lute steam pressure. The temperature of steam is sometimes expressed in degrees °absolute,* meaning the number of degrees above "abso lute zero", which is, on the Fahrenheit scale, — 460° ; that is, 460 below the zero mark on a Fahrenheit thermometer.
Saturated steam is steam at its greatest dens ity and pressure at the particular temperature existing: that is, any increase in pressure or any lowering of temperature would cause con densation—a part of the steam would return to the condition of water. If saturated steam holds in suspension less than 3 per cent of unvaporized water, it is known as dry steam; if 3 per cent or more of water is present it is termed wet steam. Dry steam as it issues from a jet into the air is colorless: wet steam issues as a white cloud. The presence of free water in steam is highly objectionable. It rep resents a loss of heat without production of useful work, and to that extent reduces the efficiency of the boiler and increases the cost of the steam actually used. Further, it adds to the amount of water of condensation to be cared for; increases the wear of valves and valve-seats; washes away lubrication; and, in the case of turbines erodes the blades almost as if they had been subjected to a sandblast.
When a space is saturated with steam the pressure exerted is known as saturation pres sure. If the content of a space which is satu rated with steam be reduced, some of the steam is condensed to a liquid state, but the pressure remains the same. If the volume of the space • be increased, provided that it contains sufficient liquid, more water evaporates and the pressure exerted by the water vapor soon attains the same value as before the change of volume. If the temperature of a space containing water, and its saturated vapor be raised, the satura tion pressure of the vapor is increased; if on the other hand the temperature falls, some of the vapor is condensed, and the saturation pres sure is less.
Superheated steam is saturated steam heated until it resembles a perfect was. The super heating of steam is accomplished by "drying* it of the water vapor, or by relieving it of some of the pressure upon it. The former method is the one used in steam practice. The drying cannot be done while the steam is in contact with the water in the boiler, for any increase of heat applied to the latter goes to forming more saturated steam, the temperature remaining relatively the same — that is, according to the pressure. While saturated steam begins to con dense at once upon loss of heat superheated steam retains its gaseous form until it has lost all its heat above 212°. This quality of steam makes it of great value in engines, the expansion in the cylinder being wholly gaseous, and without leaving any water of condensation — the source of the largest loss of efficiency in engines. The superheating of steam is com monly accomplished by passing it through a grid of pipes open to the fire. In ordinary en gine practice the effect of superheated steam is secured by "wire-drawing* it — the term sig nifying the causing it to pass through a (rela tively) very small opening. This is done on an engine by partly closing the throttle valve. The temperature remaining the same, the small fall in pressure results in superheating the steam allowed to pass through the valve.