In a spray drying process , high flow rates of drying air
are generally obtained by the use of centrifugal fans.
Usually a two-fan system is used, the main fan situated
after the powder recovery equipment and the
supply fan located in the inlet duct to the drying
chamber. Two fans enable better control of the pressure
in the chamber. With a single fan after the cyclone,
the whole drying system operates under a high
negative pressure. The operating pressure in a drying
chamber determines the amount of powder in the
exhaust air and hence the capacity of the cyclones
and their collection of efficiency. In special cases,
more fans may be used in a drying process , for example
, a centrifugal fan for the powder pneumatic
transport or small fans for blowing cool air to potential
hot spots in the drying chamber an d atomizer. A
typical centrifugal fan is illustrated in Figure 10.21.
The pressure developed by a fan de pends upon the
blade design. The most common type of fan in spray
drying ha s backward-curving blades, as shown in
Figure 10.21. Such blades are also used for the sup ply
fan. If the powder–air ratio is high, the backwardcurving
blades may cause problem s with deposit formation
on the backside of the blades. In such cases,
the us e of a blade profile intermediate between the
backward curving and the radial is recommend ed. It
gives better self-cleaning properties. In Figure 10.22
are shown the typical characteristic curves for radial
and backward-curving blades for constant fan speed
and air density.
The fan energy consumption E (in W ) may be
calculated by means of the following relationship
between the volumetric airflow rate V (m 3/s) and the
total pressure drop that must be overcome, DP (P a):