2.8. Dust Characteristics

Red oak wood was selected to study the effects of a single source of dust. Rough sawn red oak boards were purchased from a sawmill and reduced into approximately 12.7 mm (0.5 in.) wood chips using a chipper. The wood chips were bagged and transported to a controlled environment chamber at Penn State University where they were spread out on the floor and dried at 33 °C (92 °F) for 72 h. The dried wood chips were then transported to the Agricultural and Biological Engineering Building at Penn State University for further size reduction using a Munson SCC-10-MS rotary cutter (Munson Machinery Co., Utica, NY, USA) with a 1.6 mm (0.06 in.) diameter screen. The Munson grinder outlet had a chip collection basket at the bottom, and the top had a high-volume centrifugal dust collection system fitted with a fine mesh filter bag secured to a plastic 208-L (55 gallon) drum. Typical operation of the grinder involved collecting the coarse ground material and discarding the fine dust. However, the fine dust was needed, thus we collected fine dust from the 208 L (55 gallon) drum, then reprocessed the coarse particles through the grinder multiple times for further size reduction. The fine dust was then run through a mechanical sieve fitted with an 80-mesh filter cloth, and the fines were collected and bagged for use. The fine dust was placed on a metal pan and dried in an oven at 55 °C (131 °F) for 72 h to a moisture content of 4.8% (d.b.) before being used in the dust generator. Figure 9 shows a microscope image of the sieved red oak dust on a 50-micron grid. Figure 10 shows a graph of the sieved red oak dust particle size distribution. The mean particle length was 21 microns (n = 100 particles, min 2.1, max 160.6, SD 20.3).

Image showing red oak dust particle shape and size on a 50-micron grid.

Red oak wood dust particle size distribution (n = 100 particles). Blue bars indicate frequency of each particle length, and the orange line indicates cumulative percentage.

During the six-week study, fine dust was brushed from all surfaces inside the controlled environment chamber and collected in a storage bin. The collected dust was then passed through a coarse sieve with 3.2 mm (0.13 in.) openings and 42% open area to capture feathers and very coarse material generated by the birds, followed by the 80-mesh filter cloth to separate coarse from fine dust. The fines were then oven dried for 72 h before being reused. A small volume of coarse dust particles was mixed with the fine dust to improve flowability of the powder and prevent packing. The coarse dust particles did not remain airborne but settled on the floor directly under the outlet within 2 m (6.4 ft) of the dust generator outlet.