In order to select a dryer with proper diameter and length, the total moisture load to be evaporated must first be determined. Total moisture will depend on final moisture desired, ambient temperature and humidity, as well as the amount of recycle and amount of chemical heat, if applicable. In addition, different materials have different specific heat values and vastly different coefficients of surface heat diffusion. Because these variables are somewhat interdependent and difficult to measure or predict, the selection of rotary dryers for some materials cannot be calculated, although some engineering analysis is required for proper size.

Selection of the Drum Diameter. Drum diameter generally is selected on the basis of velocity of airflow. Empirically, 500 ft/min (FPM) velocity yields a total airflow in cubic feet per minute (cfm) that typically is consistent with the theoretical amount of air needed to dry and carry off evaporated moisture. The value of 500 FPM should not be exceeded because of dust carryover. A larger drum diameter yields lower velocities with less carryover as well as increased drying volume, but practical limits of diameter are generally established by price, space limitations and consideration of particle breakage and attrition when falling in large diameter drums.

Selection of Dryer Length. Because of the variables mentioned above, and such individual considerations as flighting pattern, speed of rotation, efficiency of seals and retention time, length must generally be related to diameter. According to empirical data, dryer length will range from five to eight times the diameter. There may, of course, be some exceptions.