Fluid bed dryer

Fluid bed dryers are applied mainly to further dry product with a high fat content. A much occurring combination is an atomising tower followed by a fluid bed dryer.

In the dryer, hot air is blown from below through a kind of sieve plate. The air speed is chosen so that the product to be dried is raised from the sieve plate and stays suspended in the air. Fluid beds are also used as cooling devices to cool dried (hot) product by blowing cold air through it.

Some fluid bed dryers have a vibrating sieve plate, those dryers are known as vibrating bed dryers.

Fluid bed dryers: more fires than explosions

Below the sieve plate there is normally no dust present. Only in the case of an emergency stop it may happen that some fine powder falls through the sieve. For this reason the lower side of the dryer must, after such an emergency stop, always be inspected and cleaned.

Above the sieve plate there may be an explosive mixture present. But in many cases the quantity of powder is too high (just above the sieve plate) or too low. In practice, in such dryers there are many more fires than explosions.

Ignition sources in a fluid bed dryer

Ignition is nearly always caused by smouldering product, either originating in the dryer (as well over the sieve as under the sieve in a “clean”, but very hot environment), or imported together with the product to be dried.

Protect your fluid bed dryer against explosions

Is your installation part of a larger unit, that is also equipped with a powder tower and protected by CO detection? Then the CO detection is installed in such way that also the fluid bed is monitored.

Because fluid bed dryers are often installed in the middle of a building and moreover (vibration bed) often move horizontally with a large amplitude, explosion venting is seldom or never possible. The protection mode most applied is explosion suppression.

Protect your fluid bed dryer against fire

As a consequence of the dust concentration in a fluid bed dryer there might arise, except for an explosion, also a fire; except for explosion protection, a good fire detection- and extinguishing system (e.g. by steam injection) is of great importance. A fire in the fluid bed might easily lead to an explosion in adjacent equipment.

In- and outlets of a fluid bed dryer

The product in- and outlets: here a supplementary protection is not necessary if the in and outlet are equipped with an explosion proof rotary valve or if a permanent presence of sufficient product in the in- and outlet can be guaranteed so that explosion propagation becomes impossible. If this is not the case, in both cases a chemical barrier would also be the best solution here.

The air inlet: as the sieve plate works as a flame retarder, a barrier on the air inlet is generally not necessary, provided the room below the sieve stays clean.

The air outlet: explosion propagation to the cyclone or the filter is certainly possible here. As the ducts concerned are mostly large in diameter, the best solution here is a chemical barrier.