Document wD7BZp6rmQ5BLbdy4VLNaXxv4

American Society of Heating and Ventilating Engineers Guide 1931 With the ordinary sling psych'rometer which is used in measurin i3 moisture content of air, however, there is always some radiation n ^ *4 and, rnay if the wet-bulb be be considera_b_l_e_. riot strongly With strong- -A.------------ vve-e-n-n-t-i-tl-ia-l-at-e1t--ido>,n-t,-h-eh- uoeuwreerovtoredrr,au'edthiatetoT"ra^d" effect is usually less than one per cent. This error depends upon ? vl temperature and'velocity of air movement, and the relationship 0f error to these factors is shown in a paper entitled The Temperature fit Evaporation of Water into Air, presented by W. H. Carrier and D*rli Liridsay, before the American Society of Mechanical Engineers in 1924'il From the foregoing, the following fundamental principles in air co *3 ditioning may be evolved: .. n'M 1. When dry air is saturated adiabatically the temperature is reduced as the absol humidity is increased, and the decrease of sensible heat is exactly equal to the sjm | 'M taneous increase in latent heat due to evaporation. m 2. As the moisture content of air is increased adiabatically, the temperature is reduced''# simultaneously until the air is saturated, when no further heat metamorphosis is possible 3 This ultimate temperature may be teriried the temperature of adiabatic saturation "Sf 3. When an insulated body of water is permitted to evaporate freely in the air assumes the temperature of adiabatic saturation of that air and is unaffected by con-`^ vection, t.e., the true wet-bulb temperature of air is identical with its temperature of il . adiabatic saturation. fg 4. The true wet-bulb temperature of the air depends entirely on the total of thelS sensible and the latent heat in the air and is independent of their relative proportions v In other words, the wet-bulb temperature of the air is constant, providing the totai 1 heat of the air is constant. - It will be obvious in considering the foregoing process, that a formula ij relating the latent heat change to the sensible heat change establishes if the relationship of this theoretical wet-bulb temperature to the tern- |i perature and moisture of the air. This was first pointed out by W. H. f| Carrier in the"Ratioriid Psychrometric Formula: in which r< (W - W) = Cpa (t - t') + CpsW (< - /') (3) (l -- l') = the true wetbulb depression. .- ..... ;, (W -- W) = the moisture absorbed per.pound of pure air when it is adiabatically saturated from an initial dry-bulb temperature lQ and an initial - ' moisture content W. pa = mean specific heat of air at constant pressure between temperature /: . : 1 and - . .. ' -; v Cps -- specific heat of steam at constant pressure between t and t'. V r! = latent heat of evaporation at wet-bulb temperature Knowing any two of the three important values of t, t' or W,,the third may be solved. This is an exceedingly useful relationship, not only in. air conditioning and the drying of materials but in the psychrometry of air or other gases to which it directly applies. It is evident that if the wet-bulb temperature of air of a known dry-bulb temperature but unknown moisture content be determined experimentally with the wet-bulb thermometer, it will be . possible to calculate directly the moisture content of that air from the relationship. 448 Chapter 29--Air Conditioning of Buildings . ^nation (3) a psychrometric chart in common use has been JjPj? e$jee pigs. 1,2, 3, 4, 5, Chapter 26, and Fig. 5,Chapter 30). This useful in all problems pertaining to air conditioning, drying, arid determination of the moisture content of air. rmula has also been derived which will permit the approximate SK^ftion of the vapor pressure from the wet-bulb temperature obserguHla j.jjjg js useful particularly in determining relative humidities for ggP?".' ns other than the standard barometric conditions for which the Ur t is made. The formula for expressing this relationship is: (P - e<) (l - t') c -- e,1 -- 2,800 - 1.3P (4) pcoarrrteiaslppornedsisnugretootfhtehedemwopisotuinret. in the air, which also equals vaHpor Hpressure V = the vapor pressure corresponding to saturation at wet-bulb temperature I'. P = the barometric pressure. t = dry-bulb temperature in deg. fahr. I1 = wet-bulb temperature in deg. fahr. iTotal Heat cooling and dehumidifying air, it is necessary to take into considera- sition not only the sensible heat of the air itself but the latent heat of the "-moisture removed. . gg^j\ir at any temperature and moisture content may. be1 Considered as tfehaving a certain definite total heat in thermal units per unit weight as ^calculated from some arbitrary base, such as zero moisture and zero i?femperature. By using the psychrometric chart, on which are given gSwet-bulb temperatures and corresponding total heats, it is possible to il^jitetermine the total heat to be removed in the process of dehumidification w|$by subtracting the total heat of the cooled air with its reduced moisture pfjfcontent from that of the air in its initial condition as indicated on the chart. 'b '' - &CThis process is simplified greatly by taking into account the fourth psychrometric principle, by observing the fact that^ the wet-bulb temX. perature of the air may be used as the measure of the total heat which jj^jthe air contains. Thus, by referririg to the chart\ having given air of p- known temperature and known moisture content, -a corresponding wetifcvbulb temperature is obtained. Then, under standard barometric con ditions, the quantity desired is determined by reference to the curve of ii^f-total heat. For example, air at 85$^ deg. with 80 per cent relative humidity has exactly the same total heat as air at 101 deg. and'40 per cent S55 relative humidity, because in both cases the wet-bulb temperature is 80 deg. This wet-bulb temperature represents about the maximum wet- bulb temperature ever encountered in the Temperate Zone under extreme /conditions, and is nearly the maximum found iff the Tropics. A normal thigh wet-bulb temperature for theTemperate Zone rnay be taken at 75deg. .TV an calculating requirements for air cooling, as this is seldom exceeded, except for short periods. 449