In a co-current dryer (Fig. 2.1.1, Fig. 2.1.2), the spray
is directed into the hot air entering the dryer and both
pass through the chamber in the same direction. Cocurrent
dryers are the preferred design for heatsensitive
products because the hottest drying air
contacts the droplets at their maximum moisture
content. Spray evaporation is rapid, and the
temperature of the drying air is quickly reduced by
the vaporization of water. The product does not
suffer from heat degradation because the droplet
temperature is low during most of the evaporation
time. Once the moisture content reaches the target
level, the temperature of the particle does not
increase greatly because the surrounding air is now
much cooler. Dairy and other heat-sensitive food
products are usually dried in co-current dryers.
Open Cycle Co-current Flow Layout
Fig. 2.1.2
1. feed storage
2. pump
3. drying chamber
4. air heater
5. cyclone
6. gas scrubber
7. separator
2.2 Counter-current flow dryer
In this dryer design (Fig. 2.2.1), the spray and the air
are introduced at opposite ends of the dryer, with the
atomizer positioned at the top and the air entering at
the bottom. A counter-current dryer offers more rapid
evaporation and higher energy efficiency than a cocurrent
design. Because the driest particles are in
contact with