Document nmkbN24wDE6gwp1OYMqoBopkm

American Society of Heating and Ventilating Engineers Guide, 1936 81V A80 3N08 81 _ i, i/iMunii-u yodVA aaivMli -AiiaiwnH H O o o o oo d o .740 30 50 7 0 90 110 130 150 170 190 210 230 TEMPERATURE, DEG FAHR F ig . 3. Psychrometric Chart Plotted in Units Convenient for Use in D ryer Calculations Chapter 41--Drying B = total radiation and conduction losses per unit time. w = pounds of water per pound of dry stock, r = heat of evaporation of water. s = humid heat of air, i.e., heat necessary to raise 1 lb of dry air + iT lb of steam 1 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, those in which all moisture evaporated from the stock leaves the dryer as vapor in the effluent air, and those in which part or all of the moisture is condensed from the air in the drying equipment itself. In any continuously operating dryer of the first type the relation between moisture content of the stock and quantity of air required for the drying operation is given by the equation: G (Ht -- Hi) = S(w,-- w,) (1) In discontinuous dryers, e.g., compartment dryers, the drying operation is given by the equation : G (Hi - H) = S' || (la) In the continuous dryer, the heat consumption per unit time is: -g- = Gsi(ti -- t{) + G(rt + ti -- t'i) (Ht -- Hi) -f- S(t'i -- I'i) (V + wi) + B (2) Equation (2) 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 t, t' and H may be employed provided the third term of the right hand member of the equation is modified to read: S' (f'l - t"i) (S' ~wi) and in the second term t'a be replaced by th + t'\ 2 Theoretically these periods should be very short and the equation integrated. Practically the error introduced by using a small number of long periods and employing average values of the variables over each, rarely introduces serious error. The evaluation of equation (la) may be approximated in a similar manner. The first term of the right hand member of equation (2) represents heat lost as sensible heat in the effluent air. In many drying operations this becomes excessive. Each pound of air supplied should remove the maxi mum amount of moisture. This is best accomplished by bringing the air into contact with the stock with sufficient intimacy so that the air leaving the 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 (G), and a corresponding minimum loss,' as sensible heat, in the exit air. Similarly, continuous operation is superior to intermittent operation. 741