Theory of Spray Drying Fundamentals

In contrast, a vaned wheel directs the flow of the liquid feed across the surface of an inner liquid distributor in which liquid slippage over the surface of the distributor occurs until there is contact with the vane or channel. The feed then flows outward due to centrifugal force and forms a thin film across the surface of the vane. As the liquid film leaves the edge of the vane, droplet formation occurs as a result of the radial and tangential velocities experienced. Atomizer wheel characteristics that influence droplet size include speed of rotation, wheel diameter, and wheel design, e.g., the number and geometry of the vanes. 

There are several two-fluid nozzle designs available to produce the conditions necessary for liquid-air contact. A common design is one in which the liquid and air come into contact outside the nozzle. This nozzle is often referred to as an external mixing nozzle and its main advantage is the greater control available over the 

Droplet composition also plays a significant role in droplet evaporation history. Typically, sprays are differentiated into three main types pure liquids, feeds containing undissolved solids, and feeds containing dissolved solids (16). 

If the air temperature exceeds the boiling point of the droplet solution, then a vapor will be formed. As the solid crust forms around each droplet, vapor pressure within the droplet is formed and the resultant effect of this pressure is dependent on the nature of the crust. A porous crust will release the vapor, but a nonporous crust may rupture resulting in fractured particles or fines from disintegrated particles. 

Alternately, the droplet temperature may not reach boiling point levels due to cocurrent airflow or because the residence time of droplets in the hottest regions of the dryer is often very short. In this case, moisture migration occurs through diffusion and capillary mechanisms. 

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