Document 5pBbRj7x1evq2g6naQVbaZ80

American Society of Heating and Ventilating Engineers Gm UXCe- 1931 -M This wet-bulb or evaporation temperature is dependent upon the j-fl bulb temperature and the moisture content, or upon the total heat fjja air as indicated in the previous paragraph. The effect of air velocity depends upon whether the flow of parallel to the surface or perpendicular to the surface elements, flow of air parallel to a horizontal surface where W 0 093 (1 230 ) ~ e) (approximately) F)f v> = pounds evaporated per square foot per hour. v -- velocity of atmosphere over surfaces in feet per minute. s' ~ vapor pressure of the water corresponding to its temperature. e = vapor pressure in the surrounding atmosphere. For transverse flow, as across a tubular surface, the rate of evaporation is nearly doubled. ! These relationships are indicated graphically on the chart, Fig. 2. t Since the difference in vapor pressures is substantially proportional to the difference between the wet and dry-bulb temperatures (i.e., the wet-; bulb depression) the rate of evaporation is also for case two substantially proportionate to the wet-bulb depression. In case two, the rate of sensible heat transfer, from the air to the liquid to produce evaporation is substantially the same as the "rate of heat transfer with the same type of surface, without moisture being present, but with the same temperature differences. In other words, the rate of heat transfer depends upon the temperature difference only, whether the surface is wet or not. For example, it has been shown that the rate of heat transfer with air flowing across staggered coils (transverse flow) may be represented by the formula: where Vt 0.0447 -f j'66 (8) m Ut = heat transfer expressed in' B.t.u per hour per square foot per degree difference in temperature between steam and air, for transverse flow. At a velocity of 400 ft. per minute, Ut = 5.8, and at a velocity of 800 ft. per minute, Ut = 9.3. Referring to Fig. 1, showing the rate of heat transmission by evapo ration for different air velocities, it will be noted that for transverse flow there are 560 B.t.u. per hour per square foot transferred per inch difference of vapor pressure at.a velocity of 400 ft. per minute and 910 B.t.u. per hour per square foot per inch difference in vapor pressure at a velocity of 800 ft. per minute. One inch of vapor pressure difference cor responds approximately to 95 deg. difference between the wet and dry bulb temperature. Dividing by 95, the value of-5.9 B.t.u. per square foot per degree difference in temperature is obtained for a velocity of 400 ft. per minute and 9.55 B.t.u. per square foot for a- velocity of 800 ft. per minute. 452 Chapter 29--Air Conditioning of Buildings - P^; . noted that for these two cases the heat transfer by evapo- degree difference in temperature corresponds almost exactly ||m99'Pe'' \ransfer by convection coils. The similarity may be noted KSthithe formula for heat transfer in parallel flow where with gl^cbmparmg . 1:'. a."- - j TftT p 0.026 + -- p (9) jPipHi at transfer by evaporation with parallel flow. The relationship will to be very close in both cases and would indicate that the heat fly evaporation is actually brought about by a process of con- ^^S^^^difference in form of the two formulae may be due in part to fSPindre in observation at the higher and lower velocities. 113lte`"in cooling air and condensing out the moisture therefrom the heat pfS'&j-ansfer is considerably more rapid than when the air is dry and no IpSrinlisture is condensed. In general the rate of heat transmission on the is increased an amount which is proportionate to the latent heat r^P^femoved as compared with the sensible heat removed.. That is, if the iiftriatent heat removal was 50 per cent of the sensible heat removed, then lfithe conductivity of the surface in contact with the air would be increased g&J '(approximately 50 per cent. iU CONDITIONING FOR INDUSTRIAL PROCESSES jpNI^S^Maiiy materials take moisture from or give it up to the surrounding Igfes'iair depending upon its temperature and humidity. As moisture is ab- il'fK'fp&s/.VcVuribthedatrheevemrsaatleorifaml woiislltuorredifnloawrilcyobmeecsoma ecosroreftsepr,omndoirnegpclhaasntigceainndpphlyiasbiclea.l KInequalities. Economy requires that these physical qualities be maintained |,^?constant in order that high speed machinery may be permanently adjusted the desfoirer dinpsrtoadncuec,tiolono.ms were run by man power the conditions in a M9"?PV;Lancastershire cellar were ideal for the manufacture of woolens and Eg&.Hworsteds. Perhaps they would still be satisfactory with a single modern doom installed. But, if hundreds of these looms are crowded into a single heat from the machinery--which may be sufficient to warm the .".building in severe weather--will raise the temperature and lower the cf.j;relative humidity, thereby nullifying all advantages of climate. Thus, "Xtxj.; the development of the modern manufacturing plant has created a de- mand for industrial air conditioning which in turn has contributed its paTrthetorethaerepmroacnesysinodf udsetvrieelsopwmheicnht. require a definite and unvaried hunudity so that at some periods the normal quantity of moisture in the air " must be increased, and at other times lowered. These include the con fectionery, the artificial silk, and the printing and lithographing industries Other industries require not only a constant relative humidity, but also a fairly uniform temperature. For example, the modern automatic wrap ping machines used for wrapping chewing gum, food products, confec- ; tionery, and machine made cigarettes, require exact conditions of heat jiod moisture in order to function satisfactorily without frequent ad- " justnients. . 453