Calculate the thermal efficiency of the rotary dryer described in Example 24.3. Assume air was heated from 70 to 260°F before entering the dryer.

Example 24.3

Calculate the diameter and length of an adiabatic rotary dryer to dry 2.800 lb/h (1.270 kg/h) of a heat-sensitive solid from an initial moisture content of 15 percent to a final moisture content of 0.5 percent, both dry basis. The solids have a specific heat of 0.52 Btu/lb·°F; they enter at 80°F (26.7°C) and must not be heated a temperature above 125°F (51.7°C). Heating air is available at 260°F (126.7°C) and a humidity of 0.01 lb of water per pound of dry air. The maximum allowable mass velocity of the air is 700 lb/ft²·h (3,420 kg/m²·h).