Document JRQZRZ55wBa98pG4jBQ2497K

40 CHAPTER 3 1951 Guide Thermodynamics 41 vapor present. The probable reasons for this inaccuracy are due to the effect of: 1 Chemical solution of gas molecules in the water vapor. 2 The finite size of the molecules causing interference with the free passage of other molecules toward the boundaries of the system. 3. Intermolecular forces of attraction and repulsion. Many attempts have been made to develop an equation of state which would predict the true states of real gases and vapors. The Van der Waal, Maxwell,, and Beattie-Bridgman equations are probably the best known. Unfortunately, these expressions rapidly become much too com plicated to be used in everyday calculations and, therefore, engineers find it more convenient to use tables of thermodynamic properties for specific working substances, as these can be prepared by physicists using the best laboratory equipment and all the refinements of mathematics. Mechanical engineers have long been familiar with such tables for the properties of steam. Tables of the properties of moist air, as prepared by Goodenough and others, have been available for some time, but the latest and most precise of such tables are those which have resulted from a co operative research agreement between the American Society op Heating and Ventilating Engineebs and the Tovme Scientific School of the University of Pennsylvania. These properties are published herein as Table 2, and are taken from a research report by Goff and Gratch.4 Table 2, which experimentally and mathematically takes into account deviations from perfect gas behavior, such as those listed above, makes the applica tion of the Gibbs-Dalton Rule a less frequent necessity. In Table 2 there are 15 columns of figures, each column being headed by a suitable symbol. In the following sub-paragraphs brief explanations are given of the data in Table 2 under the appropriate column headings. 1(F) = Fahrenheit temperature defined in terms of absolute temperature T by the relation, T = t -f 459.69 (24) Absolute zero of temperature may be defined as the receiver, temperature which will enable a Carnot Cycle engine to transform into work all the energy it receives in the form of heat. Wa = humidity ratio at saturation. Saturation is the condition at which the vapor phase (moist air) may exist in equilibrium with a condensed phase (liquid or solid) at the given temperature and pressure (standard atmospheric pressure in the case of Table 2). At given values of temperature and pressure, the humidity ratio W can have any value from zero to W. y. = specific volume of dry air, cubic feet per pound. the difference between the volume of moist air at saturation, per pound of dry air, and the specific volume of the dry air itself, cubic feet per pound of dry air. vm = specific volume of moist air at saturation per pound of dry air, cubic feet per pound of dry air. ha = specific enthalpy of dry air, Btu per pound of dry air. The specific enthalpy of dry air has been assigned the value zero at 0 F, standard atmospheric pressure. The energy unit Btu is related to the foot-pound by definition, as follows: 1 Btu ** 778.3 ft-lb. h,, =* ht -- ht, the difference between the enthalpy of moist air at saturation, per pound of dry air, and the specific enthalpy of the dry air itself, Btu per pound of dry air. h0 =* enthalpy of moist air at saturation per pound of dry air, Btu per pound of dry air. . * specific entropy of dry air, Btu per (pound) (Fahrenheit degree). It will be Complied by John A. Goff and S. Cratch.