PARTICLE SUSPENSION IN A ROTATING DRUM CHAMBER WHEN THE INFLUENCE OF GRAVITY AND ROTATION ARE BOTH SIGNIFICANT

In recent years, rotating chambers have been found to be an effective method of retaining particles suspended in the air for an extended period of time. Rotating drum chambers have the potential of providing a stable atmosphere of well-characterized inhalable particles for periods lasting from hours to days for use in inhalation toxicology studies. To aid in planning for the use of rotating drum chambers in inhalation studies, we created a model that describes (a) the concentration of particles in the chamber under various conditions and (b) the particle sizes for which gravity and rotation influence particle dynamics. Previous publications describe the suspension/deposition of particles when the rotational effect is dominant, but do not describe particle suspension/deposition when gravitational settling is significant as occurs when such drum chambers are operated at optimal conditions for retaining the highest fraction of particles over time. By using the limiting trajectory of particles, the fraction of particles that remain suspended in a 1-m diameter rotating drum chamber was derived for forces of gravity only, rotation only, and gravity plus rotation. For particles between 0.5 and 1 mum in diameter and for suspension times of < 96 h, there was no loss of the suspended particles for drum rotation rates from 0.1 to 10 rpm. For 2- and 5-mum diameter particles, > 98% and 91%, respectively, remain suspended after 96 h under optimal rotation of the drum chamber. Optimal rotation rates were independent of particle size for particles < 10 mum in diameter (agreeing with Gruel et al. [1987] even though we predicted suspended fractions higher by > 30% for 10-mum particles after 96 h). For 20-mum diameter particles and suspension times < 96 h, the maximum suspended fraction occurred for drum rotation rates between 0.3 and 0.5 rpm. The particles > 2 mum can be selectively removed from an airborne particle size distribution in time periods of < 15 h when the rotational rate is > 5 rpm.