Spray Drying Extraction Cyclone
The high efficiency cyclone (figure 9) is the most commonly used item of plant for separating the dried powder from the exhaust air and is often preferred where multiple or difficult products are produced due to the relative ease of cleaning down and low cost. Although over 99% collection efficiency is possible, back up after the cyclone in most locations is usually necessary to meet emission regulations. Back up air cleaning is usually provided by either a wet scrubber or a “policeman” bag filter.
Where only a single material is being dried or products are manufactured in campaigns, the ‘bag filter’ or ‘baghouse’ may be used without the need for a cyclones. The bag filter has a much higher efficiency rating than cyclones and meets all emissions regulations. The most common type of bag filter employed would be the reverse jet bag filter. This type ensures that the bags operate at maximum efficiency at all times as the compressed air jet timings can be adjusted to suit the application.
Where the investment capital is plentiful a CIP capable bag filter can be employed giving all the collection benefits of the bag filter with the wash capabilities of a cyclone.
Cyclone Dead Zone
Powders enter a cyclone tangentially, air and entrained powder spin down the length of the cyclone wall, while air is pulled tangentially in the opposite direction creating an inner vortex. Where the two vortex meet at the bottom of the cyclone a dead zone is created, when the powder entrained in the air hits the dead zone it is spun out free of the vortex while the air is pulled from the system. Cyclones must be sealed so powder exits through a rotary valve or similar device.
Bag filters work very simply, air is pulled through the filter drawn by the exhaust fan powder collects on the bags while air is allowed to pass through the filter medium. A compressed air jet dislodges the powder from the bag and drops into a hopper and out of the system.