Drying in a Continuous Tunnel Dryer. A rate of feed of 700 lbm dry solid/h containing a free moisture content of X1 = 0.4133 lb H2O/lb dry solid is to be dried to X2 = 0.0374 lb H2O/lb dry solid in a continuous-counterflow tunnel dryer. A flow of 13 280 lbm dry air/h enters at 203°F with an H2 = 0.0562 lb H2O/lb dry air. The stock enters at the wet bulb temperature of 119°F and remains essentially constant in temperature in the dryer. The saturation humidity at 119°F from the humidity chart is HW = 0.0786 lb H2O/lb dry air. The surface area available for drying is (A/LS) = 0.30 ft2 /lbm dry solid.
A small-batch experiment was performed using approximately the same constant-drying conditions, air velocity, and temperature of the solid as in the continuous dryer. The equilibrium critical moisture content was found to be XC = 0.0959 lb H2O/lb dry solid, and the experimental value of kyMB was found as 30.15 lbmair/h · ft2. In the falling-rate period, the drying rate was directly proportional to X.
For the continuous dryer, calculate the time in the dryer in the constant-rate zone and the falling-rate zone.