PHYSICAL SEPARATION OF AIRBORNE ASBESTOS FIBERS BY SIZE.

摘要

The hazardous properties of airborne fibers are generally thought to be related to fiber diameter and length. The purpose of this study was to develop a technique for physically classifying airborne fibers on the basis of diameter and length, keeping the separated fractions in the suspended state. This would permit either fraction to be further classified, monitored by real-time instrumentation and/or used in size-related inhalation studies. Other fiber classification methods have involved the deposition of fibers onto a substrate which precludes these advantages.; In this study diameter classification was achieved by application of inertial forces, and length classification was attempted by application of electrical forces. In both cases the classification device used was an opposing-jet aerosol classifier. Diameter classification was sharpest for liquid particles, less for isometric solid particles and least for fibers. Length classification was not achieved.; Particle losses in the classifier were greatest for liquid particles, less for isometric solid particles and least for fibers. This trend is attributed to decreasing adhesiveness in going from liquid to isometric solid to fibers.; Classification efficiency is defined as the number of particles in the lower outlet flow divided by the combined number of particles in both the upper and lower outlet flows of the classifier.; In classifying both liquid and solid isometric particles, some runs were made with the classifier inlet flowrates purposely out of balance. By using an excessive flow of clean air through the lower inlet, classification efficiency was reduced for all particle sizes tested, and by using a deficient lower inlet flow, classification efficiency was increased.; It has been suggested that an approximately monodisperse aerosol could be separated out of a polydisperse aerosol by being drawn through two opposing-jet classifiers in series. As the classification efficiency usually comes no closer than a few percentage points to 0 and 100% when the classifier flowrates are carefully balanced, the twice-classified aerosol would be contaminated with a low level of over- and undersize particles. To minimize this problem, the classifier that removes the oversize fraction could be operated with a deficient lower inlet flow, while the classifier that removes the undersize fraction could be operated with an excessive lower inlet flow. The undesired fractions would thereby be more completely removed than if the inlet flows were balanced.