. Applied thermodynamics for engineers. n, ^ i_oPV ^ = constant; which holds fairly well for limited ranges of pressure when the initial dryness , but which gives a curve lying decidedly outside the true adiabatic for any con-siderable pressure change. The error is reduced as the dryness decreases, down toa certain limit. Zeuner found that an exponential equation might be written in 260 APPLIED THERMODYNAMICS the form PV^ = constant, if the value of n were made to depend upon the initialdryness. He represented this by n = + X, for values of X ranging from to , and fou
. Applied thermodynamics for engineers. n, ^ i_oPV ^ = constant; which holds fairly well for limited ranges of pressure when the initial dryness , but which gives a curve lying decidedly outside the true adiabatic for any con-siderable pressure change. The error is reduced as the dryness decreases, down toa certain limit. Zeuner found that an exponential equation might be written in 260 APPLIED THERMODYNAMICS the form PV^ = constant, if the value of n were made to depend upon the initialdryness. He represented this by n = + X, for values of X ranging from to , and found it to lead to sufficiently accu-rate results for all usual expansions. For a compression from an initial dryness x,n = + a:. Where the steam is initially dry, n = for expansion for compression. There is seldom any good reason for the use of exponentialformulas for steam adiabatics. The relation between the true adiabatic and thatdescribed by the exponential equation is shown by the curves of Fig. 173, after. 0 5 Fig. 173. Arts. 394, 395 10 Vo Adiabatic and Saturation Curves. Heck (40). In each of these five sets of curves, the solid line represents theadiabatic, while the short-dotted lines are plotted from Zeuners equation, and thelong-dotted lines represent the constant dryness curves. In I and IT, the twoadiabatics apparently exactly coincide, the values of x being and InIII, IV, and V, there is an increasing divergence, for x = , and 0. CaseV is for the liquid, to which no such formula as those discussed could be expectedto apply. 395. Adiabatics and Constant Dryness Curves. The constant dryness curvesI and II in Fig. 173 fall above the adiabatic, indicating that heat is absorbed duringexpansion along the constant drt/ness line. Since the temperature falls duringexpansion, the specific heat along these constant dryness curves, within the limitsshown, must necessarily be negative, a result otherwise derived in Art. 373. Thepoi
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