. Applied thermodynamics for engineers. and the ratio of expan-sion. In a non-conducting cylinder, maximum efficiency would be realized whenthe ratio of expansion became a maximum between the pressure limits. Withoutexpansion, increase of initial pressure very slightly, ifat all, increases the efficiency. Thus, in Fig. 201,the cyclic work areas ahcd, aefg, ahij, would all beequal if the line XY followed the law pv = the actual law (locus of points representingsteam dry at cut-off) is approximately, the work areas increase slightly as the pressure in-creases; but the necessary heat absorp
. Applied thermodynamics for engineers. and the ratio of expan-sion. In a non-conducting cylinder, maximum efficiency would be realized whenthe ratio of expansion became a maximum between the pressure limits. Withoutexpansion, increase of initial pressure very slightly, ifat all, increases the efficiency. Thus, in Fig. 201,the cyclic work areas ahcd, aefg, ahij, would all beequal if the line XY followed the law pv = the actual law (locus of points representingsteam dry at cut-off) is approximately, the work areas increase slightly as the pressure in-creases; but the necessary heat absorption alsoincreases, and there is no net gain. The thermody-namic advantage of high initial pressure is realized onlywhen the ratio of expansion is large. By condensing the steam as it flows from the engine, its pressure may be re-duced from that of the atmosphere to an absolute pressure possibly 13 lb. cyclic work area is thus increased; and since the reduction of pressure is ac- Fig. 201. Art. 459. — Non-expansive MULTIPLE EXPANSION 319 companied by a reduction in temperature, the potential efficiency is increased. Figure 202 shows, however, that the percentage gain in efficiency is small with no expansion, increasing as the expansion ratio increases. Wide ratios of expansion are from all of these standpoints essential to efficiency. We have found, however, that wide ratios ofexpansion are associated with such excessive lossesfrom condensation that a compromise is necessary,and that in practice the best efficiency is securedwith a rather limited ratio. The practical attain-ment of large expansive ratios without correspond-ing losses by condensation is possible by multipleexpansion. By allowing the steam to pass suc-cessively through two or more cylinders, a total Fig ^^02 Art 459—Gain due expansion of 15 to 33 may be secured, with condensa-te Vacuum. tion losses such as are due to much lower ratios. 460. Condensation Losses in Compound Cylinders. The ran
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