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VFNTILATIUN IN A DRY FiLN wall,.Il' MENIl(ll~t A \ No. IP1205 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsian i^ft.Jw^t u EV4Sanrafj V777ILATIO:" IS A DRY KIL:F  By W. LARL LOJG:jOAOLJH, Engineer A n."' point of view for an old problem often helps to solve the crblem. The enrinecring point of view, as distinguished from. the empiric'il one, 'ill be some.hat ner in v.'ork on the problems of ventila tion ;L..d circulation In a dry kiln, and makinE use of it sho.Uld help matters. In conformity '.7ith his point of viewr and method of attack, the enic.i r first determines the la;s that govern an action and then applies t?.., td since the r:neral principles covering ventilation in a dry kiln are 'ell know our first task is merely to select the proper ones. Althou:n it is trae that the laws governing the flo7 of air are pcrh. s less xnd.rstood than those of almost every othcr branch of cncn. 'r> for air flo', at pressures as 1o7 as those found in the d&r kiln a t.horou:.. uindcrstaunding of thormodynu:"ics is unnccc's'ry. .hc s' J forila comr:onlly used for the flow of 7.atcr may be used for the flowi of Pir without introducir.Eg iny errors of practical m.gnituide; con trary.r to an ideca th.rt is all too prevtlnt, thcrc is no Mystcry about the 6 The .:riar purposes of this article are to dir.:ct morc Attcnr.tion to the 'rhblom of ventilation in a dry kiln and to urgc considration of the problem 7ith the analyticz,.l method of the cngincer. Carrying out th.cso purposes .ill require sho'iine how the uci'hts of air arc .affectcd by various conditions, such as tc mpcrc tur rclntivo humidity, rund b.aro nactric prcssuro. The n, xt stop ill then be to sh)" hor the ch s in might of .ir affect vc.ntilition in dry kilns of various typcs, Morc .vcr, t.: function of ventilation and also the bcari:.g v:,.tilr.tion hs'. o:n clrculrtion in a kiln will be discussed. The mrattor of circilrtion *ill b csidcr.cd in a later article. Since this nrticlo deals pri.rily "ith vcntiltlon, a cler .dcst :di:.. of the difference bctc. ventilation r.d circullatio: is dcsir'.bl. Ventilation is the roncunal of go'd : ir in a sbst..t.i.lly closc 3 cc, such as a dry kil;n, b; the (1.issior. of fresh air o a (oiC "rour.d in circl., nd it al .. Involvs the idc. ':f a cy.cl as d ist.. shed from a rcpl.acing mov..,cnt. It is crcly the tr.vli of thc air within a closed space,:.d, ulic v .:t il.tion, in its'. If hs noth'. to do .ith cith.;r the d.iszion f fresh "ir fror. the outside :r the expulsion of usedup air from the inside. r:'bKls.ed. in 'ood Products, IMovember nd Decer.ber, 1931. __7 WEIGHT OF AIR The weight of dry air in pounds per cubic foot may be calculate,' from the formula 0.0028862 b 1 + 0.0021758 t (1) where b is barometric pressure in inches of mercury and t is temperature in degrees Fahrenheit. A sliderule calculation gives the weight per cubic foot of air at 160 F. and a standard barometric pressure of 29.92 inches of mercury as 0,0641 pound per cubic foot. To find the weight of moist air, formula (1) is modified by the subtraction of a quantity to take care of the effects of the pressure exerted by water vapor, thus: 0.0028862 b 0.001088 e 1 + 0.0021758 t (2) where b and t have the same meanings as in formula (1) and e is the vapor pressure in inches of mercury. For example, the weight of a cubic foot of saturated air at 160 F. and a barometric pressure of 29.92 inches is by substitution 0.002S862 x 29.92 0.0010gg88 X 9.6500 0.05637 pound per cubic foot. 1 + 0.0021758 x 160 Fortunately, it is rarely necessary in drykiln problems to cal culate the weight of air under various conditions. Every handbook on the subject of heating and ventilating gives in tables and curves the weights of air in almost every condition of temperature, relative humid ity, and barometric pressure. The two formulas just presented will serve their purpose if they show how the factors involved affect the weight of air; the higher the barometric pressure the more the air will weigh, and the higher the vapor pressure and the temperature the less it will weigh. Figure 1 shows how the weight of dry air changes with tem perature, how the weight of water vapor necessary to saturate a cubic foot of dry air and vapor mixture increases with temperature, and finally how water vapor displaces dry air in a mixture of saturated air and thus reduces the weight of the dry air and vapor mixture per unit of volume. Air at constant pressure, expanding as it is heated, changes its volume almost exactly in proportion to its absolute temperature; for kiln temperatures, add 460 to the thermometer reading to get the absolute R1205 2 temperature on the Fahrpnheit scale. Expressed as a formula for use with ordinary conr.ercial temperatures, c the volume at any temperature arnd l1, the volume at any different, temperature tD, have the following rel1ation: q = qo 460 +t 460 + to j (5) The relative volume of a given weight of air at different tempera ture., is pictured in Ficare 2, B; air at 10O F., for example, has about 21 r cer.t greater volume rer unit of weight than air at 70 A similar reltion holds for the %'eights w and 1o of a given volume of air at tw7o ter.J.rritues, under constant pressure: tew(.4 ;.c. t+ t W6 = w C' + t (4) .Then water varor is admitted into dry air under constant pressure the vapor dLsplaces the air. The amount of air displaced depends on the pressure :.f the water vapor, and the total pressure of the m.ixtire is therefore unchar.ged; it is merely the constant sum of the pressures ex erted by the dry, air ind the water vator. Thus, becCause water vapor is lighter than dry air under the same conditions of temperature and pressure, th,. weight of air per cubic foot is decreased by an aniount equal to the difference in weiht between the 7ater vapor and the dry air that it dis Ilaces. Finally, at 212 F. and normal barometric pressure, there is no dry air present; vapor pressure alone balances the barometric pressure. FiF:re 2, A, sh'.,s the relation for a wide ranie of temperature. At 1900 F. a pound of dry air with the vapor needed to saturate it occuries 3.23 times as much space as a pound of djry air at 70% (Fig,. 2, A.) This fact has some significance in the determination of the relative size of the inlet and the outlet ducts of a ventilated kiln. F:1ire 3 shows the relationship between the space in saturated air occup'ied by a pound of water in vapor and in liquid form. At 2120 F. the ratio is 1670. Figure 3 gives some idea of the large volume of air th:..t must be put through a kiln in removing. 1 pound of water from it, and how the required amount of air is affected by the kiln temperature. The cirvc brings out plainly the fact that the lo'er the temperature is, the rriter the ventilation r.uist bc. 3 AMOUNT OF WATER REMOVED THROUGH VENTILATION Kiln men often talk about moisture content expressed in percentage of the weight of the ovendry wood, but few have tried to picture what moisture expressed thus means in terms of absolute water. The amount of water to be removed from a kiln in a given time must be known before the required ventilation can be determined. A simple calculation will give us some conception of how much ventilation is required to remove the water evaporated. Suppose that a kiln charge of 35,000 board feet of airdried red oak has an average moisture content of 35 percent when entering the kiln, and that this lumber will be dried down to a moisture content of 10 percent. The approximate weight of a thousand feet of ovendry red oak, which may be calculated from its specific gravity, is 2900 pounds. In drying 35,000 feet from 35 percent to 10 percent moisture content it will be necessary to evaporate 35 x 2900 x 0.25 or approximately 25,400 pounds of water. A gallon of water weighs 8.34 pounds and there are about 50 gallons of water in a barrel. Roughly then, 61 barrels of water, nearly a carload, must be evaporated from the 35,000 feet of air dried oak. VENTILATION IT A DRY KILN Having discussed in a general way the various conditions that affect the weight of air, a preliminary matter, we are now in a position to consider the ventilation in a dry kiln. It is often said that air in a kiln moves by suction. Ordinarily this is a misconception. Air in a kiln moves primarily under the action of its own weight. When a volume of air rows heavier than the air around it, for any reason, it flows downward just as a liquid or any other ras would do under the same cir cumstances. Adequate appreciation of the effect of the weight of air in causing movement of air is necessary for a clear understanding of ven tilation in a dry kiln. Suppose, for example, that the temperature of a kiln is 150 F. while the outside temperature is 60, that the vertical height from inlet to outlet is 25 feet, and that the outlet is closed so that no air can escape from the kiln. The weight of a cubic foot of air at 60 F. is 0.0764 pound; hence the weight at 1500 formula (4) is 0.065 pound. The difference between the weights of a cubic foot of air inside and one outside the kiln is then 0.0114 pound, and the actual difference in the weight of the two 25foot columns of air will be 0.285 pound. The inter val column will be driven up against the cover at the top of the chimncy with a force amounting to 0.285 pound per square foot. When the cover is removed the hot air will flow into the atmosphere because the pressure on the lower side of the cover exceeded the pressure on the upper side. As long as a temperature difference continues, air will continue to flow from the atmosphere through the kiln and into the atmosphere again, thus ventilating the kiln. R1205 This illustration points out two things. One is that the greater t'.t difference in air temperature between the inside and the outside of the kiln, the greater will be the ventilating action. The second is that the' lower the intake with respect to the outlet, the greater the difference b't: ".n the weights of the two columns of air and consequently the greater the ventilation. An experimental kiln 13 feet high was built at the Forest Products Laboratory, so that cold air could enter at the top, middle, and bottom of the kiln wall and could leave the kiln through outlets built at dif fernt cl'vations in the kiln chimney. In operation, air is permitted to ent,.r in turn at the there. inlets and each time to escape in turn at one of the three outlets* Anemomuter readings in the inlet, taken when the tem,'crature of the kiln was 150 F., tend to show the effect of chimney height on ventilation. WMhen the air was taken in at the bottom and allowed to escape at the top the anemometer gave a wind velocity of 360 feet per minute. Reversing the process, allowing the air to enter at the top and dischare at the bottom, made the air velocity so slight that it would i.:.t turn t. anenomet..r over. Yet inferring from. this that kilns that take air in close to the roof and discharge it near the bottom will not dry lumber is not correct. A satisfactory location of inlets and outlets in a kiln depends vEry largely on the amount of ventilation rcquircd to remove the moisture :vaporated from the ,ood. Obviously, the ventilating system in a kiln used for drying the grcn sapwood of softwoods should be designed so as to obtain a rapid turnover of air in the kiln. In kiln drying airdried hard.;oods, however, the location of the inlet and outlet ducts is relative ly unimportant bcausc only a small mount of ventilation is then necessary. If a kiln can dinchar;e the moisture evaporated from the lumber that is all th1cit ncd be .xpccted from a ventilating system. The standard formula "2 ..h (5) v",,. re V is the velocity of the air movere.r.t caused by difference in den sit.", the acceleration of gravity, and h the head producing the V1ir flo", sho"s the factors that must b.' considered in the manipulation of ventilation darmers d'uring the drying operation. Removing large quanti ties of air trnls to lower the relative hur.idity in the kiln, while a hiih rate :.f evaporation tcnds to increase it. Th. higLcr the, temperatures ar., the relative b':.lty, the riter t:.c ?. air than the amount netded to reduce, the relative humidity to the value desir, '.. 'The function of ventilation in a dry kiln is to remove the moit:r. that evaporates from the lumber. After the moisture has boon converted into vapor it must escape from the kiln in some way, either from the doors or through properly designed and adjusted ventilators. The quantity of air moved for this purpose is perhaps larger than we orAinarily think it. For example, our calculation showed that 25,400 pounds of water had to be evaporated from 35,000 board feet of oak in drying it from a moisture content of 35 percent to one of 10 pcrcen:t. Since lumber can not be dried to low moisture values if the air leaves the kiln in a saturated condition, let us suppose that the average tem perature and relative humidity of the air as it leaves the kiln of 150 F. and 50 percent, respectively. The weight of saturated vapor per cubic foot at this temperature is 0.01030 pound. At 50 percent relative humid ity the weight of the vapor will be just half of this or 0.00506 per cubic foot. Then 5,020,000 cubic feet of dry air must leave the kiln in order to carry away the evaporated water. Supposing that it takes 12 days to dry this kiln charge, an average of 292 cubic feet of air must leavc th. kiln per minute, either through cracks around the doors or through the ventilators. Possibly the air inlets and outlets in a given kiln do not func tion properly, making it easier for the air to escape from the kiln through cracks at the tops of the doors than through the ducts provided for the purpose. Now the pressure of the heavier outside air forces the lighter warm air out of the kiln. If a certain volume of heated air is forced out, it is forced out by an equal volume of cold air coming into the kiln, and if the air enters through leaks at the bottom of a door the temperature near the door will be lower than that in the center of the kiln. Some operators often think it more expedient to make the doors tighter than to provide larger air ducts of better design. Spending money on new doors is no solution of the problem of producing uniform temperatures in the kiln. The procedure is wrong in principle. This type of kiln must be ventilated, whether some moisture is removed through condensation or not. If air does not enter the air intakes satisfactorily and if the doors are made air tight, what will displace the vapor in the kiln? It is better for the air to enter around the kiln doors than not to enter the kiln at all. On the other hand, the local chilling caused by outside air entering a dry kiln improperly is bad in every respect. Tking an ex treme case for illustration, suppose that the kiln drying the 35,000 feet of oak just mentioned has no ventilating ducts, all the air enter ing through leaks around the doors. Then the heating coil nearest the door will have to supply ten times as much heat as the other coils in order to warm the entering air and keep the temperature of the kiln charge uniform. It can not supply so much heat, and in consequence the lumber near the door chills and fails to dry. Although this illustration ,is somewhat extreme, the general fact is true of all leaky kilns. Lack of uniformity in the heating requirements along a kiln rnpk3s it practically 31205 6 iTmos.iie to maintain a uniform temperature throughout the charge. The solution of the problc is to provide arple intake ducts designed to distribute the air along the length of the kiln. In this way the heating r' .:irc::,r. nts of each unit length of the. coils would at least tend to be the sn. To prevent air from entering around the doors without providing a ifl.ce herere it can enter is poor practice. Because of the effect of tcmpcraturc on the density of air it is LrcGpibic to establish ;7s groat a difference between the weights of insi i' and of outside air when the dry kiln is operLted at low tonpcrat.ires as r.'hen a hightu=perature sch,'dule is followed. In other .ords, it is difficult to ustablish a rLpid r.te of air discharge frc, a ventilated kiln 'when s'ch a L:iln is ocratUd at low temperatures. For ,xar.plc, to c:cr'.it cir "'t the s.re rate the crosssectionrl. area of ventiltor s.pace will have to be about onethird larger for a kiln t<cpcrrturc of 2l?0 F. tChna fcr one of 150. Hcnc(, it is difficult to operate a vcntil:tud ':iln sLtisfactorily when the species of wood bLing dried demands a lotcmper ature s ch c duelo. .tter vapor is lighter thTi the air it displaces. Hence,th n hihor the relativCe humidityy the grL.o'ter the rate of ventilation. At a tzmpcra ture of 1500 F. and with oth:r conditions constant, the draft causoc by saturated air is 60 percent greater than that caused by dry air. .ZCI _^? VETILATIOK ON CIR .UTION IN A DRY KFi: Ventilation, of course, plays some part in the circulation )f a natraldraft kiln, but often too much is said about where the air leaves arnd 7"here it enters Lni its effect on circulation; this effect is small. The f1riztion of ventilation is to get out of the kiln the moist,:re evar orate!1 fr:. the lumber. If it also induces a certain amount of circula tion so mui.ch the better, but that is not the proper function Af ven:tila tion; ircr.3asint the ventilating characteristics of the kiln for the ,rine purpose of sti=alatinE the internal circulation is Fener'illy ExTrensivo anr.''. ? ;.V :s is an infficierit mieth:d movinrE. air. ;7j':.:i.wARi The most iport.At fact to keep in r.in' vhen consiLerirn ven.tila tion in a dry kiln is that the air in the kiln moves inder the. action of its :'r. weight. Both the desi.. r a.. the o:crat:r sl.culi underst . id this clearly. 3tter kilns *.ed better dr.'i. should come from proper attention to this one fct. R12. .5 7 0.09 0.08 a 0.07 0 .0 _ ____ _ _ ___ o 0.06 IL U U w 0.05 w o 0.04 0.03 0.02 /c 0.01 0  0 40 80 120 160 200 240 ZM1632rzF TEMPERATURE (*F) Figure i.Weight at normal atmospheric pressure and at various temperatures of  A.A cubic foot of dry air. B.A cubic foot of a saturated mixture of air and vapor. C.The vapor necessary to saturate a cubic foot of dry air. 360 320 . 280 IL 0 F A 240 200 0 o // U 160 w B 120 000 80 40 01 l*J4 ii3 (F 0 40 80 120 160 200 240 ZM 43 a 3fTEMPERATURE (F Figure a.A.Volume at various temperatures of i pound of dry air plus the volume of the vapor required to saturate the &ir, expressed as a ratio to the corrempo0ding total volume of dry air a2d 0por at 70 F. and normal atmospheric preaeure. B.Volume of i pound of dry air at various temperatures expressed aa a ratio to the volume at 7o0b F. at normal atmospbhric pressure. 160 140 120 0 0 0 2  _ o IO 0 100 x .J U V) 80 0 60 40 20 0 0 40 80 120 160 200 240 "MlG321F TEMPERATURE (OF.) Figure 3.Ratio of the volume of i pound of water vapor to the volume of i pound of water at the same temperature. The volume of the water vapor is independent of any air that may be present. UNWVERL TVY OF FLORIDA 3l 12610il88648I3 I 3 1262 08866 4833 1 