Spray Drying Basics 3

Spray Drying Basics Diagram 2

Drying Chamber

Drying Chambers are of similar construction to storage silos, large open vessels, there volumes being determined by the required heat exposure time for the feed materials being dried. The air forced through the chamber is heated by the burner and pushed by the fan. Feed solution is atomised into the chamber to form droplets, on contact with the hot air liquids evaporate leaving a powder.     

On initial contact with heat the visible powder particles are case hardened, a hard outer surface with a molten inner. As the particle falls down the length of the chamber contact with the hot air draws the liquid from the rest of the particle. A small amount of retained liquid within the particle is usually desirable, this is called residual moisture. The amount of time it takes for the particle to fall the length of the chamber is called residence time.

Air Removal

Air is removed from the chamber from various ducting arrangements depending upon the chamber design. Simple designs remove both air and entrained powder from the base of the chamber.    

The type of chamber shown in the diagram is a co-current design where the liquid spray and the hot drying air both flow in the same direction, usually downwards. The vast majority of spray dryers are co-current but there are many variations and specials available some of which can be seen in “Types of Chambers”. 

Tower-3
Tower-01

Whatever the design of chamber, a degree of powder will be entrained within the airflow leaving the chamber. On the basic design shown in the diagram all air and powder is removed from the base of the chamber and conveyed to a powder / air separator. High powder collection efficiencies are essential for separating the powders from the air to reduce loss of revenue and to conform with clean air regulations.

Two methods of separator may be employed, the choice of which is often dictated by the type of operation the spray dryer has to perform and cost.