In this column, John J. Walsh discusses combustion basics and reviews some key fire and explosion safety factors often overlooked by dryer manufacturers and users during the dryer’s design engineering phase.
Among bulk solids processing operations, drying is one of the most notorious for its potential to create fires and explosions. Besides causing production losses and damaging property, these events can result in worker injuries or even death. Combustion is an exothermic chemical reaction, meaning that it releases heat to its surroundings. The rate at which the combustion propagates through the fuel source (typically, the material being dried) determines whether an explosion will occur. The combustion rate is determined by several factors: the material’s moisture content, the ratio of particle surface area to particle volume, the concentration of particles and the extent of their disbursement in the drying air, and the degree of air turbulence. When these factors combine to allow the combustion to propagate extremely quickly, the quick rise in air temperature will cause a large pressure wave to form, thus producing an explosion.