The number and shape of the vanes differ according
to the product quality and capacity requirements.
The usual shape of vanes is circular, oval, or rectangular
, as shown in Figure 10.6. The influence of
vane shape on droplet size was studied in Ref. [10] .
For high-capacity applications, where the same degree
of atomization is required at higher feed rates ,
the numbe r and he ight of the vanes are increa sed to
maintain the same liquid-film thickness on each vane.
High-cap acity wheels often have two tiers of vanes
[22] (see Figure 10.7) . The largest wheel atomizers
allow feed rates up to 200 ton/h. The curved vane
wheels are somet imes used instead of the standard
straight radial wheels, mainly in the milk industry.
The curved vane wheel produces a powder of high
bulk density, up to 15 % higher than the standard
wheels, be cause of reduced air-pumping effect. The
other way to achieve high bulk density of the product
is to use the so-called bushing wheels, which are
widely used in the chemical industry. The bushings
are made from very hard material , such as silicon
carbide ; thus the wheel can atomize even very abrasive
mate rials [22].
Gen erally, the wheel atomizer produces a spray of
high homogeneity within a wide range of mean droplet
size. The size distribution of droplet s can be controlled
by changing the wheel speed. Feed rate
variation produ ces much less effect. Wh eel atomi zers
are very flexibl e an d can handle a wi de assort ment
of liquids with diff erent phy sical prop erties. Fac tors
influencing the wheel atomize r perfor mance are specified
in Ref . [25] for instance.
The so-called vaneles s disk atomi zers sh ould also
be mentio ned, althoug h their use is limited to specia lized
applic ations in whi ch coarse particles are required
at high producti on rates . A typical cu p-type
disk atomizer is sh own in Figu re 10.8. Liquid is fed
inside the cup and is presse d by centrifugal force
agains t the cup wall, flowi ng down tow ard the cup
edge, wher e it is disi ntegra ted.
The wheel s are conventi onally driven by elect ric
motor s, eithe r direct ly or by belt drives through a
worm and worm wheel gearing system (low-c apacity
wheels) or by helical or ep icyclic geari ng systems
(high-cap acity wheels) .