Page:Elementary Text-book of Physics (Anthony, 1897).djvu/266

253 '''233. The Steam-engine.'—The steam-engine'' in its usual form consists essentially of a piston, moving in a closed cylinder, which is provided with passages and valves by which steam can be admitted and allowed to escape. A boiler heated by a suitable furnace supplies the steam. The valves of the cylinder are opened and closed automatically, admitting and discharging the steam at the proper times to impart to the piston a reciprocating motion, which may be converted into a circular motion by means of suitable mechanism.

There are two classes of steam-engines, condensing and non-condensing. In condensing engines the steam, after doing its work in the cylinder, escapes into a condenser, kept cold by a circulation of cold water. Here the steam is condensed into water; and this water, with air or other contents of the condenser, is removed by a pump. In non-condensing engines the steam escapes into the open air. In this case the temperature of the refrigerator must be considered at least as high as that of saturated steam at the atmospheric pressure, or about 100°, and the temperature of the source must be taken as that of saturated steam at the boiler-pressure. Applying the expression for the efficiency (§ 231), $$e = \frac{S - R}{S},$$ it will be seen that, for any boiler-pressure which it is safe to employ in practice, it is not possible, even with a perfect engine, to convert into work more than about fifteen per cent of the heat used.

In the condensing engine the temperature of the refrigerator may be taken as that of saturated steam at the pressure which exists in the condenser, which is usually about 30° or 40°: hence $$S - R$$ is a much larger quantity for condensing than for non-condensing engines. The gain of efficiency is not, however, so great as would appear from the formula, because of the energy that must be expended to maintain the vacuum in the condenser.

234. Hot-air and Gas Engines.—Hot-air engines consist essentially of two cylinders of different capacities, with some arrangement for heating air in, or on its way to, the larger cylinder. In one form of the engine an air-tight furnace forms the passage