Document RpvgV05JGZDVmEExp0QxwoV2a

American Society of Heating and Ventilating Engineers Guide, 1937 In discontinuous driers, e.g., compartment driers, the drying operation is given by the equation: (2a) In the continuous drier, the heat consumption per unit time is: 91 -g" = Gsi(tt -- h) + G(rs + h -- (flj -- Hi) + S(t'j -- <'i) (51 + v>i) + B (3) | | Equation (3) assumes continuity of operation. For charge or batch S | operations, the total time of the drying cycle may be broken up into all number of periods, sufficiently short so that over each period average } ? ;I I Chapter 41--Drying Systems into contact with the stock with sufficient intimacy so that the air leaving the drier is saturated, or nearly so. Counter-current as against parallel jjoW 0f 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. 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 decreases, provided the increase in moisture carrying capacity of the air, TEMPERATURE. OEG FAHR I1 values of /, t' and H may be employed provided the third term of the right hand member of the equation is modified to read: S' (<", - <,) (s' - Wi) and in the second term f' be replaced by SI II g 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 (2a) may be approximated in a similar manner. The first term of the right hand member of equation (3) 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 752 Fig. 8. Heat Content of Gases Above 32 F in Btu per Pound due to high temperature, is actually utilized. To secure maximum thermal efficiency in drying, a high drying temperature and high satura tion of the outlet air is imperative. Ventilation Phase The technique of attack of the ventilation phase of a drying problem is best made clear by an illustration. Assume that a material containing 40 per cent moisture is to be dried until this quantity of moisture is reduced to 5 per cent by weight. The material will stand an air tempera ture of 150 F and it is possible to provide sufficiently good contact between'the material and the drying air so that the effluent air can be 753 -/