The formula
which expresses the efficiency in this ideal operation shows the importance of making T, high and T2 low. No real engine attains this limit of efficiency, for no engine completely expands the steam down to the condenser temperature in a strictly adiabatic manner. Further, no engine has an adiabatic compression stage corresponding to da : the use of a separate con denser makes that impracticable.
The action in an engine cylinder may be more appropriately idealized as in fig. 13 where AB represents the admission of the steam at
BC its adiabatic and complete expansion to
and CD its rejection to the condenser. Under these conditions it is easy to show that the work done, which corresponds to the area of this ideal diagram, is equal to for each lb. of steam admitted to the engine, where L is the total heat (per lb.) on admission, and I, is the total heat (per lb. of mixture) in the condition C, when expansion is complete. This applies whatever be the state of the steam on admission, whether superheated or not. The quantity II—I2 is called the adiabatic heat-drop, and measures the greatest amount of work theoretically obtainable from each lb. of steam. In practice about 70 per cent. of the adiabatic heat-drop may be realized in favourable cases. Tables of the adiabatic heat-drop, for steam in various assumed initial states as to pres sure and temperature, expanding to vari ous assumed condenser pressures, have been compiled, and are of great service in problems of engine design.
The process exhibited in fig. 13 becomes a complete cycle when, in addition, account is taken of the action by which the feed-pump restores the condensed steam to the boiler. Thus completed the process is called the Rankine cycle. Its efficiency is necessarily somewhat less than that of the Carnot cycle of fig. 12. Superheating, which is not in practice carried beyond a temperature of 400° C, and rarely so far, adds to the heat-drop by increasing
But its chief effect on the efficiency of the process is indirect. By tending to keep the steam in a drier state it greatly reduces the losses that arise through exchange of heat between the working substances and the cylinder walls : in effect superheating makes the expansion more nearly adiabatic than it would otherwise be, and therefore gives a better approach to the ideal conditions of fig. 12. In a turbine superheating is beneficial by reducing the friction of wet steam on the blades.
In exhibiting graphically the action of an engine under assumed conditions of working we may adopt various alternatives to the pressure-volume or "indi cator" diagram. One interesting form takes for its two co-ordinates the temperature and the entropy : another (introduced by Mollier) takes the entropy and the total heat. Both of these diagrams are instructive in allowing the action to be traced through its several stages and in exhibiting the differences which result from varying the conditions of supply and of condensation. They show the direct influence of superheating, and the amount of wetness to be expected at any stage in the expansion. They allow, in some
cases, measurement from a chart to take the place of numerical calculation; but their greatest merit is that they enable the opera tion of the working fluid to be visualized. No account of such diagrams can, however, be attempted within the limits of this article.
In modern steam engines, especially of the turbine class where a very large range of expansion may be effectively carried out, efficiency is aimed at by using the best possible vacuum, to make
low, and by raising the pressure of supply to make T, high. Pressures approaching 000 lb. per square inch are not uncommon; occasionally that figure is exceeded, and even at the highest pressures some addi tional high-temperature heat is taken in by superheating. Re heating at one or more stages during expansion is resorted to, in order to prevent the expanding steam from becoming unduly wet. Another device, which is also applied in the most economical large-scale turbine plants, is to remove a portion of the expanding steam at each of two or three stages, and apply it in heating the feedwater on its way back from the condenser to the boiler. This progressive heating of the feedwater, by "bleeding" the turbine of steam which has already done more or less work, is called "cascade" feed-heating. Its effect is to make the whole cycle approach more nearly to the ideal cycle of Carnot, for the pro gressive feed-heating is nearly reversible and serves as a substi tute, in this respect, for the adiabatic compression which makes that cycle differ from the cycle of Rankine. (See also TURBINE : STEAM ; LOCOMOTIVES ; MARINE ENGINEERING.) BIBLIOGRAPHY.-Robison, System of Mechanical Philosophy, vol. II. (1822) ; Stuart, Descriptive History of the Steam Engine (1825) ; Muirhead, Life of James Watt; Dickinson and Jenkins, James Watt and the Steam Engine (1927) ; Parsons, The Reid Lecture (Cambridge, 1909) ; Ewing, The Steam Engine and other Heat Engines (4th ed., 1926) ; Ewing, Thermodynamics for Engineers (1920) ; Dalby, Steam Power (1915) ; Callendar, Properties of Steam (192o) ; Callendar, Steam Tables (1915, enlarged ed., 1924) ; Moss, Heat Drop Tables (1917) ; Dalby, Valves and Valve-Gear Mechanism and The Balancing of Engines. (J. A. E.)
is an instrument which by utilizing the elasticity of a metal indicates the pressure of steam in a boiler or other vessel. The action is explained in the article PRESSURE GAUGE. The gauge must be fixed above the highest water level of the boiler, and stand away from it to avoid heating. A gauge is unduly hot if it cannot be touched by the hand without dis comfort, and the U-tube or syphon is therefore used for connec tion to the boiler. In order to avoid straining the action, a gauge is generally graduated to twice the working pressure. Then the pointer stands vertically at the normal working pressure. High pressure steam-gauges are those graduated for pressures between 200 lb. and I,000 lb.