The Challenge

Ring or column dryers are used to drive moisture from wetted particles like cornstarch. A pre-heated air stream moves the material through a vertical column where the dried particles lose moisture and are transported to the top of a column. Further drying is done by moving the particles through one or more rings connected to the column. The dried material is then pneumatically conveyed to down-stream separating equipment such as cyclones or dust collectors.

As the moisture content of the material decreases, the risk of explosion increases. The flammable dust cloud usually ignites as a result of an electrostatic discharge or exposure to the high temperatures of materials clinging to the dryer wall. Once ignited, the explosion grows rapidly due to the configuration of the dryer and the turbulence of the air stream.

As is the case in many applications, the main concerns are (1) detecting an explosion in one or more areas of the process; (2) suppressing the explosion before it grows in intensity; and (3) preventing the explosion from spreading to other processes.

The hazard

If the material being handled by a process is combustible, then the dust it generates is likely to be explosive.  In flash drying systems the material being processed becomes more sensitive to ignition as it passes down the dryer and loses its moisture.  The minimum ignition energy of the  dried product can be low enough to present the threat of ignition by static discharge.  Additionally material adhering to the walls of the dryer can absorb enough energy to self ignite.  the resulting deflagration generates high flame speeds and rapidly produces destructive pressures due to the pressure piling effects of the duct geometry.

protection system considerations

Protection systems for flash dryers must provide adequate agent concentration throughout the process. Additionally if cyclones are employed, smooth internal walls are needed to maintain air separation efficiency.  Fenwal suppression systems accommodate this by not having protrusions into the process and by incorporating flush spreader assemblies for suppressant dispersal, where needed.

protection system description

In an explosion suppression system for flash dryers, explosion pressure detectors are mounted at several locations along the dryer duct and on each of the air separation vessels.  They detect the pressure excursion from an impending explosion and signal a control panel.  The control unit triggers high rate discharge extinguishers while simultaneously shutting down the process.  Extinguishers are designed to rapidly discharge and quench the fireball before maximum pressures are created and mitigate flame propagation.  Similarly extinguishers mounted on the air separators quench the fireball and mitigate explosion propagation to interconnected equipment.