Modification and Characterization of Dry Material Feeder Delivery of Red and Violet Dye Mixtures

Abstract

Uniform delivery of dry material for stable concentrations of aerosols in inhalation exposure chambers is essential in inhalation experiments. Several physical characteristics, including compressibility, adhesiveness, and electrostatic charging, may slow or even stop delivery from dry material feeders to jet mills for generation in exposure chambers. This paper characterizes a modified dry material feeder using different helix sizes, actuation rates, nozzle types, and dye mixtures. The basic design of this dry powder feeder used to deliver precise quantities of dye mixtures is inadequate for aerosol exposure research (Cheng et al., 1985). To improve delivery, a “push-pull” type solenoid (actuator) was mounted to the feeder and connected to a microprocessor-controlled timer for actuation. Without the actuator, the coefficient of variability (CV) for delivered dye mass increased with smaller size helixes and reached 35, 34, and 121% for 1/2–, 3/8–, and 1/4-in helixes, respectively. With the actuator, the CV was reduced to 2.5, 4.8, and 8.8% for the same helix sizes and red dye mixture. The mean red dye mixture mass flow rate with the actuator increased with smaller size helixes and delivered 2, 4, and 24 times more dye for 1/2. 3/8–, and 1/4-in helixes, respectively. Similar results were obtained with the violet dye mixture. The optimum actuation rate was determined to be 90 cycles/min. The use of the stainless steel nozzle improved feed rates by presumably limiting any additional amount of electrostatic charge on dye particles. This may be demonstrated when a polyvinyl nozzle with a 3/8-in helix, using red dye mixture, has a CV for delivered mass of 10.2% compared to 2.8% for a stainless steel nozzle.

Our modification, the addition of an actuator with a striking rate of 90 cycles/min to the dry powder feeder, greatly decreased the amount of variability in mass feed rates for all helixes and dye mixtures, while increasing the dye mass flow rates. This modification is recommended when dry material characteristics are such that accuracy and precision of mass flow rates cannot be optimized.