I1J, 
 
 FUNCTION AND CALCULATION 
 
 Of VENTILATION IN DRYING 
 
 COMPARTMENTS 
 
 Information Reviewed and Reaffirmed 
 March 1954 
 
 DtPOSi 
 
 No. R1265 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST SERVICE 
 FOREST PRODUCTS LABORATORY 
 Madison 5, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
Digitized by the Internet Archive 
 
 in 2013 
 
 http://archive.org/details/calculatOOfore 
 
FUNCTION AND CALCULATION OF VENTILATION 
 IN DRYING COMPARTMENTS 
 
 3y 
 0. W. TORGESON, Engineer 
 
 In kiln drying work, ventilation should not be confused with circula- 
 tion. Ventilation refers to the passage of air into and out of tne drying 
 compartment to dispose of the evaporated moisture. Circulation refers to 
 the internal movement of air which is necessary to carry heat to the drying 
 surfaces and to- replace tne heat lost through the roof, walls, and doors. 
 Ventilation is necessary to maintain scheduled relative humidity conditions, 
 while circulation is needed so that these conditions can "be maintained, as 
 uniformly as practical, within all parts of the compartment. 
 
 In kiln drying green lumber, a great deal of heat would be wasted if 
 all the air necessary for a satisfactory rate of air circulation were drawn 
 from the outside, heated, humidified, and then discharged after having passed 
 through the load only once. For that reason, most of the circulation should 
 be provided by recirculation and the amount of fresh air taken in should be 
 sufficient to absorb only the evaporated moisture when raised to kiln condi- 
 tions. 
 
 Modern kilns are equipped with fans to recirculate the air, but these 
 may act also as exhaust fans by taking in and discharging air through vents 
 located on both vacuum and pressure sides. The desired humidity is maintained 
 by opening or closing these vents either manually or automatically. A sepa- 
 rate exhaust fan and motor, automatically operated by a hygrostat or by a 
 wet bulb thermostat, can be used also. Steam sprays are used ordinarily to 
 humidify the air in case of overventing or if, because of leakage, the amount 
 of evaporated moisture is insufficient to maintain the desired humidity. In 
 an automatic system the sprays should not come on until after the vents are 
 closed or the exhaust fan is shut off. 
 
 The problem of estimating the amount of venting needed involves a 
 knowledge of the drying rate, and of the outside air conditions as well as 
 those within the kiln. The basis of the calculation is that the amount of 
 incoming air must be such that, when heated to the kiln temperature and 
 mixed with the evaporated moisture, the relative humidity is raised to that 
 called for by the kiln schedule. 
 
 The first step is to estimate the amount of moisture loss per minute. 
 This value when divided by the difference in moisture associated with one 
 
 Report Ho, R1265 _1- 
 
unit of inside and outside air, gives the amount of fresh air to be taken in 
 per minute. By using 1 pound of dry air as the unit, the complication due 
 to volume changes is eliminated. Expressed in terms of an equation: 
 
 A= R 
 
 V l- V 2 
 
 Where : 
 
 A = Amount ©f dry air to be taken in per minute (pounds) 
 
 R = Drying rate (pounds of water per minute) 
 
 V-j_ = Amount of vapor per pound of dry air in kiln (pounds) 
 
 Vp = Amount of vapor per pound of dry air outside (pounds). 
 
 The last step is to convert "A" to volume of air by multiplying by 
 the volume (cubic feet) of 1 pound of dry air plus its associated vapor at 
 the temperature and' humidity of the outside air. 
 
 Air properties in tabular form are given usually for either dry air 
 or fully saturated air and those for partly saturated air must be estimated 
 or computed. Figures 1 and 2 present these values graphically and can be 
 used with sufficient accuracy for computations such as these. To illustrate 
 their use in computing the volume of outside air to be taken in, certain 
 assumptions will be made and the corresponding values and results tabulated. 
 To show that the needed amount of outside air varies with outside air condi- 
 tions, two possible winter and two possible summer conditions will be assumed. 
 The winter and summer conditions will be represented by relative humidities 
 of 90 and $0 percent, respectively, under a winter temperature of 20° F. and 
 a summer temperature of 80° F. As the need for venting at any one set of air 
 conditions varies directly with the drying rate, only one rate will be given 
 in the illustration. A rate of 0.5 pound per minute will be assumed as being 
 that possible from 1,000 board feet of 1-inch sugar maple during the initial 
 stage of drying under a temperature of 130° F. and a relative humidity of 80 
 percent. It will be assumed also that all the evaporated moisture is mixed 
 with the incoming fresh air. 
 
 The results of the computations given in table 1 show that 108 cubic 
 feet is the maximum volume of fresh air needed under these particular assumed 
 conditions. Fresh air at 80° F. and 90 percent relative humidity, when heated 
 and humidified to 130° F. and 80 percent humidity, increases in volume from 
 lluO cubic feet per pound of dry air to 16.9 (fig. 2). Using this ratio of 
 expansion, the 108 cubic feet of fresh air is increased to- 130, which, if 
 passed through 1,000 board feet of h/k lumber piled on 1-inch stickers to 
 a width of h feet, results in an air velocity of 6 feet per minute through the 
 load. This is approximately only 2 percent of the air velocity common in 
 present day fan kilns. 
 
 If both the kiln temperature and humidity are lowered until the amount 
 of vapor per pound of dry air is the same as that of the outside air, then 
 the amount of needed venting as computed above becomes infinitely large. 
 This, of course, is a fallacy because, as the amount of venting increases, 
 more and more of the evaporated moisture passes directly out of the kiln, and 
 
 Report No. R1265 -2- 
 
less and less remains to "be mixed with the incoming air. For that reason 
 the amount of evaporated moisture to he mixed with the fresh air should be 
 reduced by a percentage based somewhat on the ratio of ventilation to circu- 
 lation rates, and the limit is reached when items B and C in table 1 become 
 zero, meaning that the air passes directly through the kiln with no recircu- 
 lation occurring. For this reason the accuracy of the results as computed 
 is greatest where the required venting is only a small part of the total air 
 circulation through the load, but becomes increasingly erroneous as the com- 
 puted value approaches the desired circulation rate which represents the 
 upper limit of venting. 
 
 To summarize: 
 
 (1)- Ventilation refers to the disposal of the evaporated moisture by 
 venting. 
 
 (2) Circulation refers to the rate of air movement through the load. 
 
 (3) The amount of needed ventilation is governed by the drying rate 
 and by the difference in the amount of vapor per pound of dry air within and 
 without the kiln. Ordinarily the needed ventilation i3 only a small part of 
 the desired circulation and recirculating fans should be provided which are 
 independent of the venting system. 
 
 (4) The fresh air requirements as computed here are greater than 
 needed because some of the evaporated moisture is vented without being re- 
 circulated. The greater the amount of venting, the greater becomes the 
 error. The limit is reached when the needed ventilation is equal to tho 
 desired circulation rate. In this case, the proper kiln condition is at- 
 tained simply by heating the outside air to the kiln temperature and no re- 
 circulation is necessary or possible. "Where the desired kiln condition is 
 such that the unit amount of vapor in the kiln is less than that outside, a 
 condensing system must be employed to remove the evaporated moisture, 
 
 (5) Best economy can be secured by automatically controlling the vent 
 dampers or ventilating fan to prevent overventing and, therefore, unnecessary 
 heat losses. 
 
 Report No. F.1265 -3- 
 
Table 1 
 
 
 : 1 
 
 : 2 
 
 : 3 
 
 4 
 
 
 
 : Kiln 
 : condition 
 
 : Outside conditions 
 
 
 : 130° F. 
 : 80 percent 
 : relative 
 : humidity 
 
 : 20° F. 
 50 percent 
 relative 
 humidity 
 
 ' 20° F. 
 90 percent 
 : relative 
 humidity 
 
 80° F. 
 50 percent 
 : relative 
 . humi di ty 
 
 80° F. 
 
 90 percent 
 
 relative 
 
 humidity 
 
 (A) Amount of 
 vapor per pound 
 of dry air 
 (Pounds) 
 
 : (Figure 1) 
 0.085 
 
 (Figure 1) 
 : 0.001 
 
 (Figure l) 
 : 0.002 
 
 (Figure 1) 
 0.011 
 
 (Figure 1) 
 0.020 
 
 (B) Amount of 
 vapor per pound 
 of dry air to 
 be added 
 (Pounds) 
 
 
 (1A - 2A) 
 
 0.084 
 
 (1A - 3A) 
 0.083 
 
 •(1A - 4A) 
 0.074 
 
 (1A - 5A) 
 0.065 
 
 (C) Amount of 
 evaporated 
 moisture added 
 to fresh air 
 per minute 
 (Pounds) 
 
 0.5 
 
 • 
 
 
 
 . 
 
 (D) Amount of ! 
 jiry air to be , .. 
 taken in per 
 minute 
 (Pounds) 
 
 ! 
 
 (1C + 2B) 
 5.95 
 
 (1C * 3B) 
 6.02 ; 
 
 (1C * 4B) 
 6.75 
 
 (1C * 5B) 
 7.69 
 
 (B>. Volume of : 
 1 pound of dry 
 air plus vapor 
 (Cubic feet) 
 
 
 (Figure 2) 
 12.10 
 
 (Figure 2) 
 12.11 
 
 (Figure 2) 
 13. 83 : 
 
 (Figure 2) 
 14.03 
 
 (F) Amount of : 
 fresh air to be : 
 taken in per : 
 minute per ; 
 1,000 board : 
 feet : 
 (Cubic feet) 
 
 
 (2D x 2E) 
 72.00 : 
 
 (3D x 3E) 
 73.00 
 
 (4D x 4E) 
 93.00 
 
 (5D x 5E) 
 
 108.00 
 
 Report No. R1265 
 
0.40 
 
 Z * 3S596 F 
 
 20 30 40 50 60 10 80 
 RELATIVE HUMIDITY (PERCENT) 
 
 figure 1. --Weight of vapor (pounds) per pound of dry air at various 
 combinations of temperature ir.d relative humidity and a 
 barometric pressure of 29-92 inches of mercury. 
 
UNIVERSITY OF FLORIDA 
 
 kj 
 
 kj 
 
 k. 
 
 k> 
 k" 
 
 k. 
 
 kj 
 k. 
 
 <^> 
 ex. 
 
 k, 
 
 kl 
 — i 
 
 10 
 
 z m 38595 V 
 
 10 30 40 50 60 10 80 
 
 RELATIVE HUMIDITY (PERCENT) 
 
 Figure 2. --Volume of air and vapor (cubic feet) per pound of dry air 
 at various combinations of temperature and relative hu- 
 midity and a barometric pressure of 29.92 inches of mercury,