Spray dryers or towers, as they are sometimes called, atomize or spray the feed material into the drying chamber in fine droplets. They are continuous processing machines that come in a range of configurations. Traditional vertical configurations are broadly grouped into two categories depending on the method of introducing the feed. The first of these, tall-form dryers, use nozzles to atomize the feed. They are so termed due to the relatively narrow spray angle of the nozzle and the relative velocity of the droplets, requiring a tall drying chamber with a proportionally small diameter to provide sufficient residence time to achieve drying. The second group, which are shorter and fatter, use rotary disk atomizers (also referred to as centrifugal atomizers) to generate the spray. The angle of the rotary atomizer spray is flat with a wide spray pattern, requiring a large diameter tower that is relatively short. Other spray dryer configurations include box spray dryers, which rely on the same operating principles but extract the product from the bottom of the dryer housing by means of a conveyor, and pulse-combustion spray dryers.
Of all drying systems, spray drying is perhaps the simplest in principle but remains one of the most difficult to achieve the desired product characteristics. The theoretical design of the system is simple compared to many other dryers, but the mechanical designs are art!