The air distribution in spray dryers is one of the most vital
parts. There are various systems depending on the plant design and the type of
product to be produced.
DIFFERENT DRYER DESIGNS
Dryer design falls into three categories: co-current,
counter-current and mixed flow. However, the co-current dryer design
best meets the goal of the dairy industry. The goal is to get the best mixture
of the hot incoming air and the concentrate droplets in order to obtain a fast
evaporation.
In dryers with horizontal chamber, the air disperser is
arranged like a plenum chamber, and each nozzle will be surrounded by an air
stream. The same system is also seen in vertical cylindrical dryers, see Fig.
42.
However, the most common is that the air disperser is situated on top of the
dryer ceiling, and the atomizing device is placed in the middle of the air
disperser thus ensuring an optimal mixing of the air and the atomized droplets.
In cylindrical vertical dryers it is also seen that the whole ceiling is
perforated thus creating a plug-flow air stream - numerous nozzles are situated
in the perforated plate in order to ensure that the air is cooled by the
concentrate. This system, however, makes fines return complicated, and the
obtainable air velocity/nozzle position is not optimal for an efficient drying.
It should be noted that an air disperser should have the ability to guide the
air and the atomized droplets in the right direction in order to avoid deposits
in the drying chamber.
On big capacity dryers equipped with nozzles,
the so-called "multi-neck" air disperser is seen, i.e. the dryer is equipped
with 3-5 air dispersers and nozzle units. The center area in the ceiling
between the air dispersers is, however, impossible to keep free from deposits,
and uniform fines return is difficult.
TWO TYPES OF AIR DISPERSERS
Today two different types of air dispersers are used in spray dryers for
food and dairy products:
ROTARY AIR STREAM
The air enters tangentially into a spiral-shaped distributor housing, see
Fig. 43, from where the drying air is led radially and downward over a set of
guide vanes for adjustment of the air rotation. This type of air disperser is
used for rotary atomizers and nozzle atomizers placed in the centre of the air
disperser. Very important is the cooling ring. This can be closed or open
depending on product, at the edge of the ceiling/air inlet, in order to avoid
powder deposits, which gets discoloured and result in scorched particles in the
powder, or even in a fire.
PLUG-FLOW AIR STREAM
The air enters radially through one side and is distributed
through an adjustable air guiding arrangement, see Fig. 44.
This
type of air disperser is used for nozzle atomiz-ers, where a laminar plug-flow
air stream is wanted. As for the rotary air disperser cooling air is also used
here. As the nozzle rods are placed in the middle of the hot air stream,
cooling air is also provided for the nozzles lances to keep the product from
over-heating.