When defining bulk density, the key is to understand all materials have a range of possible densities and some materials can have a broad range. Bulk density and flow rate determine the size and speed of a rotary airlock. When sizing equipment, one must understand the range of densities and related material properties (particle size distribution and shape, stickiness, etc.). Different processes can increase or decrease the bulk density of a material through compaction or aeration. Different materials fill the airlock pockets to different volumetric capacities (i.e., fill factor) as a function of rotational speed and as a function of how the product’s bulk density actually changes as it is aerated by the airlock’s normal leakage. As an example, we had a request for sizing an airlock processing a specific rate (lb/hr) of wood waste. Wood byproduct density may vary from 10 to 30 lb/ft3, depending on particle size and shape, moisture content, and the level of compaction vs. aeration. If a relatively high value for the bulk density is used (e.g., 20-25 lb/ft3), a 10 in. diameter airlock would be selected, but conversely, using 10-15 lb/ft3 would result in a 12 in. diameter airlock size selection.