With condensing engines, the ideal ratio might be 40 or 50, while the ratio for maxi mum duty would be not above 20, and the best ratio, from the point of view of the treas urer, might 'be not above 12 or 15. The ac curacy with which the designing and construct ing engineer determines the adjustment for maximum financial efficiency is a measure of his ability and skill and a gauge of his success in solving his problem. With each standard con struction, experience usually enables the engi neer to satisfactorily determine the proper solu tion of this problem.
The case of the steam-turbine exhibits here one of its essential peculiarities. The ratio of expansion is fixed by the conditions of its de sign, construction and operation and is neces sarily the ratio of initial to back-pressure if properly constructed. The maximum efficiency of the turbine is obtained at its maximum power and it possesses the same inherent inflexibility as the hydraulic turbine, if of other than the °partial" class, in which latter case power is adjusted to load by varying the number of noz zles or supply-passages in action. It has the same possibilities of adaptation as has the hy draulic with, further, available recourse to in termittent supply, as with the Parsons turbine, a plan unavailable with the hydraulic machine, as an element of regulation. The steam-tur bine, also, is not subject to internal condensa tion as all its elements, when in steady opera tion, maintain a constant temperature. Its economic theory is thus greatly simplified. Its financial theory simply dictates the construc tion of a light, rapidly moving vane and a minimum cost of application to its work, to operation and to maintenance, as a total.
Essential Parts of the Steam-Engine. The steam-engine consists necessarily of two parts—the engine proper, in which the expan sive force of steam is expended and the gen erator or boiler, in which the energy derived from a burning fuel is transferred to water. (The steam generator or boiler is fully dealt with in the article BOILER). The engine proper is made up of several parts of which the chief is a cylinder in which a piston is moved by the expansive force of the steam generated in the boiler. The boiler is connected to the cylinder by means of the steam pipe, in which is a stop valve, also the throttle valve or regu lator, for adjusting the opening for the admis sion of steam to the cylinder, which in some engines is regulated by hand, and in others by a governor. The steam pipe contains some times also the cut-off valve or expansion valve, for cutting off the admission of the steam to the cylinder at any required period of each stroke of the piston, leaving the remainder of the stroke to be performed by the expansion of the steam already admitted to the cylinder. The
cylinder, the heart of the steam engine, may be single or double acting. In a single-acting engine, the piston is forced in one direction by the pressure of the steam, and made to return in the opposite direction when the steam is discharged by the action of a weight or counter poise. In a double-acting cylinder, the piston is forced in either direction by the pressure of the steam which is admitted and discharged at either end of the cylinder alternately. The admission and discharge of the steam take place through openings near the ends of the cylinder, called ports (see illustration), connected with passages called nozzles, which are opened and closed by induction and eduction valves. Some times the induction and eduction valves are combined in one valve, called a slide valve. The valves are contained in the valve chest. In non-condensing engines (conventionally called high pressure engines), the waste steam discharged from the cylinder escapes into the atmosphere through the blast pipe; in locomo tive engines, as well as some others, the blast pipe is placed in the centre of the chimney, so that the successive blasts of steam discharged from it augment the draught of air through the furnace, and cause the combustion of the fuel to be more or less rapid, according as the engine is performing more or less work. The cylinder cover has in it a stuffing box for the passage of the piston rod; in large engines there are sometimes more than one piston rod and stuffing box, and sometimes a tubular piston rod, called a trunk. The cylinder cover is also provided with a grease cup to facilitate the lubrication of the cylinder and piston rod. In many large engines, there is a spring safety valve, called an escape valve, at each end of the cylinder; the chief use of which is to discharge water which may condense in the cylinder, or be carried over in the liquid state from the boiler, by what is called priming. To prevent condensation in the cylinder, it is sometimes enclosed in a casing, called a jacket, the in termediate space being filled with hot steam from the boiler, or hot air from a flue. Out side the jacket, to prevent loss of heat ex ternally, there is a covering of felt and wood or other insulation. Double cylinder engines have two cylinders; the steam being admitted from the boiler into the first cylinder and then filling the second by expansion from the first. The ordinary condenser is a steam and air tight vessel of any convenient shape, in which the steam discharged from the cylinder is liquefied by a constant shower of cold water from the rose-headed injection valve.