Document NG7y2VJqVL7rpaZpp7pON87xw

V:/' 1068 CHAPTER 47 / - { = air temperature, Fahrenheit 1955;G|ii{le^'^ i' = stock temperature, Fahrenheit. . t" = average stock temperature over short time interval, in a batch dryer'^FaEr* : enheit. t = wet-bulb temperature, Fahrenheit. ' r frtffl : *i = specific heat of the stock, Btu per pound. ," Qr, = total radiation and conduction losses, Btu per hour. 1 : ' t- IF = pounds of water per pound of dry stock. '\ X = heat of evaporation of water, Btu per pound. ... iT c, = humid heat of air, .e., heat necessary to raise 1 lb of dry air + H lb of steam IF deg. Subscript (1) designates conditions at the point where the material in question (air or stock) enters, and (2) where it leaves the dryer. Air dryers may be divided into two classes, batch and continuous. In any continuously operating dryer, the relation between moisture content of the stock and quantity of air required for the drying operation is given by the equation A. (Hi - Hi) = 5(TF, - Wi) (24) ^Industrial Drying Systems 1069 fjVthe dryer is saturated, or nearly so. Counter-current, as against parallel, flow of air and stock gives rise to optimum operating conditions, resulting ; in a minimum quantity of air required (jV?), and a corresponding minimum loss, as sensible heat, in the exit air. Similarly, continuous operation is superior to intermittent operation. Despite the fact that the sensible heat loss increases with the rise in temperature of the air, the percentage of heat lost from this source de creases if the increase in moisture carrying capacity of the air (due to high temperature) is actually utilized. To secure maximum thermal effi ciency in drying, a high drying temperature and high saturation of the outlet air are imperative. The second term of the right member of Equation 26 represents the latent heat of evaporation of the water plus the heat to raise this water to the temperature of evaporation. The third term of the equation represents heat to raise the temperature of the stock plus the water which remains unevaporated in the stock. The changes taking place in the air during the drying process can be illustrated on the skeleton psychrometric chart, Fig. 9. The case illus- where Hi is constant. In discontinuous dryers, the drying operation is given by the equation .dtF NAHi - Hi) = S' de (25) where Hi is a variable during a portion of the cycle. In the continuous dryer, the heat consumption per unit time is ? = - h) + N,(\i + t, - r,)(H, - Hi) + S(V, - t'.)(s, + IF,) + Q,, (26) 0 --4s ORY BULB TEMPERATURE Fig. 9. Changes in Air During Drying Process Equation 26 assumes continuity of operation. For charge or batch operations, the total time of the drying cycle may be broken up into a number of periods, sufficiently short so that over each period average values of l, t' and H may be employed, provided the third term of the right hand member of the equation is modified to read: S'd'i-t't) (s, - IFi) (rated is typical of tunnel and rotary dryers where heat is applied to the air at one point only. After the first adjustment stage, during which both ne material and the dryer reach the working temperature, the only heat osses from the dryer are those of radiation and conduction from the housg, and these are practically negligible for an insulated dryer. Hence, ne drymg process can be considered to be. adiabatic. and in the second term i'i be replaced by ;; i t'i + t" Iff10A percent outside air is used, the air can be considered to enter at D(0n t u r 8' 9 Prevai,ing outside air condition), and be heated to P int B (the maximum permissible temperature tm or the temperature 2 ,,erTed ky previous test). As the air evaporates moisture,: it cools Theoretically, these periods should be very short and the equation inte grated. Practically, the error introduced by using a, small number ot long periods and employing average values of the variables over each,|S not serious. The evaluation of Equation 25 may be approximated m a similar manner. The first term of the right hand member of Equation 26 represents hea lost as sensible heat in the effluent air. In many drying operations this becomes excessive. Each pound of air supplied should remove the mum amount of moisture. This is best accomplished by bringing the ai into contact with the stock with sufficient intimacy so that the air leaving th 8 6 C0n3^an^ wet-bulb fine BD to point C. The difference between ofe(rSture C0I1tent of air at B and at C represents the moisture pick up in 6 ^r` d'he maximum possible pick up from B to D is never achieved dryers, the actual pick up being anywhere from 10 to 75 per01 the maximum. jder to conserve heat and to control the wet-bulb temperature at on F o dryiog takes place, recirculation is used. The process is shown j_ ; !"' , The outside air at A is mixed with recirculated air until the at *"e *evel is raised to the desired point. The mixture is represented turf>/ filters heat the mixture to the desired dry-bulb tempera- ,nn>.at point S. The moisture is picked up from S to L. Point L is the oition at which air is exhausted.