ļ Algorithms for Psychrometric Calculations (Skeleton Tables for the Thermodynamic Properties of Moist Air) T. Kusuda Computer algorithms to obtain thermodynamic properties of saturated and unsaturated moist air are presented in this paper. The saturated moist air properties are calculated by the methodology developed by J. A. Goff and S. Gratch for their ASHRAE tables (1967 Book of Fundamentals, The American Society of Heating, Refrigerating and Air Conditioning Engineers). Sample calculations were performed using a computer program based upon the algorithms presented herein and the results are attached. Key words: Computer algorithm; psychrometrics; saturated and unsaturated moist air; thermodynamic 1. Introduction Very accurate values of moist-air properties are required for many engineering problems. Most notable examples are those required in psychrometric calorimetry for measuring the capacity of various air conditioning apparatus, moisture transfer analyses in the cold storage warehouses, and analyses of simultaneous transfer of heat and moisture affecting the physiological responses of biological bodies. Although numerous psychrometric formulas and charts currently exist, the thermodynamic properties of moist air calculated by Goff and Gratch (1945) [1]1 are still considered most accurate and extensive. Their calculations are based upon the theory of statistical mechanics whereby interactions of major molecular components in the moist air are taken into consideration. Thus the calculation methodology employed by Goff and Gratch to produce the now well-known ASHRAE tables of moist-air properties should be valid beyond the ranges within which their calculations had been made. For example, the barometric pressure up to 3 atm and the temperature to 400 K can be covered. In addition, it is believed that the methodology is valid for a mole fraction composition of dry air different from that used in the original calculations. Although the basic principle of calculation procedures for obtaining the moist air properties is described in the 1949 paper of J. A. Goff [2], it is not readily adaptable for machine calculation. purpose of this paper is, then, to list step-by-step procedures for computer-oriented engineers to be able to calculate the accurate values of moist air properties based upon the Goff paper mentioned above. Since the paper is not intended for elaborating the thermodynamic principles inherent to the calculation procedures, those who wish to familarize themselves with that account should refer to references [1] and [2]. The complete National Bureau of Standards program called PSYCHR has been written to follow the calculation procedures described in this paper. This program has successfully reproduced the ASHRAE table of saturated moist air for 29.92 in of baro The metric pressure. Additional programs called CF, DBWBW, DBDPW, DBRHWB, coupled with PSYCHR, can be used to calculate thermodynamic properties of unsaturated moist air by inputting values of the barometric pressure, dry-bulb temperature, and any one of several humidity indices such as wet-bulb temperature, dew-point temperature, relative humidity, and humidity ratio. The program has been further employed to calculate the thermodynamic properties of moist air at various degrees of saturation and at various barometric pressure levels. Sample results of such calculations are attached to this algorithm. It is believed that the step-by-step account of the calculation methodology and the sample calculations presented herein should be very useful in future reference work relating to accurate psychrometric calculations. Figures in brackets indicate the literature references on p. 10. air-water interaction factor hw enthalpy of saturated water (Btu/lb of H2O) rs = vapor pressure of liquid water in moisture saturated air (inches Hg) = Sentropy of saturated moist air (Btu/F, lb of dry air) P Sa entropy of dry air (Btu/lb, F) * If the new Kelvin Temperature Scale is to be used, 273.16 in this algorithm should be replaced by 273.15. Detailed discussions on this subject are presented beginning on p. 9. |