Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/647

1314 3It is the purpose of this paper to apply these final data to the development of rational formulae for the solution of all problems pertaining to the phenomena of atmospheric moisture as related to psychrometry and to air conditioning. Original data are given in proof of fundamental relations as well as in determination of errors in standard psychrometric instruments. The author hopes these results may prove to be of permanent value.

4In order to establish a logical basis for the presentation of these data and the derivation of the rational formulae, the established principles and laws governing atmospheric moisture will be reviewed and the present methods of determining atmospheric humidity discussed.

VAPOR PRESSURE AND LAW OF PARTIAL PRESSURES

5Water vapor exists in the air purely as a mixture in relation to its other elements. This vapor, according to Dalton's law, is capable of exerting a certain maximum vapor pressure dependent entirely on its temperature and regardless of the presence of other gases or vapors. For example, assume 1 cu. ft. saturated with vapor of alcohol at 100 deg. cent, having a vapor pressure of 1697.6 mm., and add isothermally to this 1 cu. ft. saturated with water vapor at 100 deg. cent, having a vapor pressure of 760 mm. This will give 1 cu. ft. of the mixture saturated with both water vapor and alcohol vapor at 100 deg. cent., having as a total pressure the sum of the two separate saturated vapor pressures, or 2457.6 mm. Similarly, an equal volume of a third saturated vapor might be added without affecting the other two. But if, on the other hand, it is attempted to include isothermally an additional amount of either of the saturated vapors, a corresponding condensation of the particular vapor added would result. In the same manner, an unlimited amount of a gas, such as air, could be added isothermally to a cubic foot of water vapor without affecting its condition of saturation, giving a combined pressure equal to the gas pressure plus the vapor pressure.

6The established temperature-pressure relationship of saturated water vapor is shown by curve (1) on the charts, Figs. 1 and 2. This is the well-known temperature-pressure curve of steam.

PARTIAL SATURATION

7When the temperature of a definite weight of saturated vapor is increased isobarometrically, it is said to be superheated. Its