Document zomz24x5MvBmV1XB6n34DzdGg

948 CHAPTER 46 . 1950 Guide and a humidity Hi willcool following this:cooling:line toward the point A; Air leaving withja.humidity Hj will consequently have cooled to fe, the wet-bulb temperature of the- air throughout the dryer being t. When heat is losti.to the; surroundings, the operation is somewhat lower than tj, so that the actual humidity-temperature relation is represented- by the line Bb, having less slope than the adiabatic saturation line. The ratio (ti -- ta)/(ti -- tj) then gives a measure of the evaporative efficiency of the dryer. For the case of dryers containing steam coils maintained at a con stant temperature, the humidity-temperature relation is obviously repre sented by the vertical line Be, assuming the initial and final-humidities to be :Hi ,and: Hi as before. The heat supplied within, the dryer itself is -usually less,"but may.i be greater, than the total heat: requirements of the dryer. If less, the cooling is indicated by some such fine as Bd, and if greater, by a line such as Be having a positive slope. DjytogcSyste108 94 The use of Fig. 8 in practical drying problems is-.as follows: Since the drying conditions of temperature and relative humidity.are-fixed, the corresponding absolute drying.rate:js-read;from. Fig:- 8. ,This value is then multiplied by the correction factor corresponding to the air-velocity employed. The rate so obtained, however, does not include any effects of radiation or of conduction through unwetted surfaces. These effects tend to increase the rate of evaporation so that the chart is conservative.' It has been demonstrated empirically for certain materials that; the rate of drying during the falling-rate period is approximately proportional to the free water content Of the material. Actual calculations of-drying time Fro. 7. Humidity-Temperature Relations in Dryers1 DRYER CALCULATIONS '. : As shown in the foregoing part of this chapter calculations for drying during the constant-rate period are different from those applying to the falling-rate period, and in contrast are subject to relatively simple mathe matical analysis. The constant rate of drying by convection is directly proportional to the temperature difference between air and wet solid, and also proportional to ithe 0.8 power of- the air velocity as shown by Equation 3. Usually 'the'wet surface is assumed to attain the wet-bulb' temperature of the air`passing 'oyer it;- and! evaporation takes place at a constant rate under equilibrium 'conditions!'. This is a conservative assumption, however, and when con duction and'radiation effects occur;-'the constant rate may be increased by '30 to 60 per'cent over that for pure convection. . ! .. Figure 8 permits a ready estimate of the constant drying rate for various 'air temperatures and humidities. The chart is based on the difference between the dry-bulb and wet-bulb temperatures of the entering stream of :air, and on an air velocity of 300 fpm. It may be assumed satirtaetbry for tray drying of any material in the constant-rate drying period. It does not apply to rotary or through-circulation drying. ^ curve'for correcting the air velocity in any given problem is incor porated in Fig, 8. . This curve is based on the variation of drying rate with the p.8 power of thovelocity, as given by Equation 3,/ -f > i ?-fu i; ; > during the falling-rate period for this case require only a knowledge of: the critical moisture content and the constant rate. For other cases of the falling-rate period, calculations are not feasible. Consequently, it is best to determine drying times for design purposes by means of pilot tests. -However, when tests are not feasible, diying times may often! be.estimated approximately from Equations.22 and 23. ' The following nomenclature wifi be used in the discussion of design cal culations: u H = humidity ratio of air, pounds of water vapor per pound of dry air. ' . N = pounds of dry air supplied to the dryer per unit of time. S = pounds of stock dried per unit of time in a continuous dryer. S'- = pounds of stock charged per batch to a discontinuous dryer. 0 = time, hours: ......... - . .......... - O' Q = total heat supplied to the dryer, Btu.