Drying in a Continuous Tunnel Dryer. A rate of feed of 700 lb_m 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 lb_m 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 H_W = 0.0786 lb H₂O/lb dry air. The surface area available for drying is (A/L_S) = 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 X_C = 0.0959 lb H₂O/lb dry solid, and the experimental value of k_yM_B was found as 30.15 lb_mair/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.