Atomization
Producing droplets of specific size and surface area by atomization is
a critical step in the spray drying process. The degree of atomization,
under a set of drying conditions, controls the drying rate, & therefore
the required particle residence time, & therefore the dryer size. All of
the atomizing techniques can give good average particle size control,
but there are major differences in the particle size distribution
created. The most commonly employed atomization techniques are:
Pressure Nozzle Atomization
• A spray is created by forcing the fluid through an orifice. The
energy required to overcome the pressure drop is supplied by the
feed pump.
• The narrowest particle size distribution is possible with this
technique. Must be used when minimization of "fines" is important to
the product.
• The average particle size produced for a given feed is primarily a
function of the flow per nozzle, the nozzle orifice pressure drop.
• Can control the spray angle.
• The most energy efficient of the atomization techniques.
• Requires routine changing of the internal pieces, usually made of
tungsten carbide. Changing schedule depends upon the application.
Centrifugal Nozzle Atomization
• A spray is created by passing the fluid across or through a rotating
wheel or disk. The energy required for atomization is supplied by the
atomizer motor.
• A broad particle size distribution is generated.
• The average particle size produced for a given feed is primarily a
function of the diameter of the wheel and the RPM.
• Requires relatively high gas inlet velocity to prevent wall buildup, which
can increase the amount of fines produced.
• Can generally be run for longer periods of time without operator
interface.
• Usually the most resistant to wear. Requires periodic changing of wheel
inserts, usually made of tungsten carbide.
• Control of wall buildup is minimal, due to direction of spray (horizontal)
and broad particle size distribution, forcing the dryer to be relatively
large in diameter.
