Design of a Circular Slot Hood for a Local Exhaust System and its Application to a Mixing Process for Fine Particles and Organic Solvents.

Abstract

The characteristics of airflow (pressure loss and entry loss factor) were measured around a circular slot hood for its application to a local exhaust system. Centerline velocity, defined as the ratio of air velocity on the centerline of the slot hood to average slot face velocity, was found to be independent of the airflow rate. The relationship between the centerline velocity and the ratio of centerline distance to slot width was also found to be independent of the slot size. The empirical centerline velocity equation for the circular slot hood was thus constructed to design the local exhaust system. Recommended values for airflow rate into the circular slot hood and the average slot face velocity were found to be 20.14 m3/min and 8.55 m/sec, respectively. The optimum air velocity at a capture point was also found to be 5 % of the average slot face velocity, i.e. 0.43 m/sec, and the effective ventilation with the hood was achieved with these values. The local exhaust system with the circular slot hood was installed for a mixing process of fine particles and organic solvents in a magnetic coating works. The effectiveness of the circular slot hood was confirmed by measuring the concentrations of airborne particles and vapors before and during the operation of the local exhaust system.

Capture velocity with slot entry to conical hood

“…A study in Japan looked at creating a circular slot hood that was to be used in a mixing process (Iwasaki et al 1997). This design was successful in lowering both fine particle counts and organic solvent levels.…”