Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/666

54⁠Motsture-Absorbing Capacity of Air. For determining the moisture-absorbing capacity, or moisture deficit of air, having a known temperature and moisture content, we have per pound of pure air

[11]

$(W'-W)={\frac {(C_{pa}+C_{ps}W)(t-t')}{r'}}$

55⁠Per Cent Adiabatic Saturation. The per cent of adiabatic saturation is

[12]

${\frac {W}{W'}}={\frac {r'W'-C_{pa}(t-t')}{r'W'-C_{ps}(t-t')W'}}$

or if we neglect $C_{ps}(t-t')W'$

[13]

${\frac {W}{W'}}=1-C_{pa}{\frac {(t-t')}{r'W'}}$

56⁠Specific Heat of Air. The equation for the experimental determination of the specific heat of air by the evaporative method is

[14]

$C_{pa}={\frac {r'(W'-W)-C_{ps}(t-t')}{(t-t')}}$

57⁠For engineering purposes however, it is preferable to determine any unknown value directly from the psychrometric charts, which afford to a great degree of accuracy a simple graphic solution of any problem of psychrometry.

DERIVATION OF THE RATIONAL PSYCHROMETRIC FORMULA FOR VAPOR PRESSURE

58⁠The present empirical psychrometric formula in use by the United States Weather Bureau was first deduced by Professor Ferrel,^[1] while the constants of the formula were deduced from a series of experiments by Professor Marvin and Prof. H. A. Hazen. The error in Broch's and Regnault's values for vapor pressures which they adopted alone would require its revision. Moreover, the opportunity for error in their methods has already been pointed out. However, considering the difficulties of their experimental method and the correction necessary for radiation in the wet-bulb reading, the results obtained are remarkable for consistency and accuracy. There are errors in the form of the equation, however, as well as in

1. Annual Report, Ch. Signal Officer, 1886, Appendix 24, pp. 233 259.