dust collection and ventilation
FWIW -
Woodshop dust collection has had gallons of ink spilled in discussions.
Suggested reading - Bill Pentz's website
www.billpentz.com. Zandor's (I can neither remember nor spell his last name) book on woodshop dust collection.
critical concepts and ideas - the chips one normally can sweep up using a broom and dustpan are not a hazard to one's health. The health hazards come from very fine dust.
Air movement depends on friction, turbulence, and pressure differences. The greater the friction, the less air movement. Round and smooth in more efficient than rectangular and rough. The greater the turbulence, the less air movement. Trees make great windbreaks. The greater the pressure differences between two locations, the greater the air movement. Compressed air is great for dusting out corners. Preserving pressure differences (eliminating leaks), reducing turbulence (straight runs, gradual turns), and reducing friction (smooth walled ducting and flex) combine to make the most effective use of one's dust collection dollar.
Bags do not filter the very fine dust effectively, because they have insufficient surface area to do so. Cartridge filters mated to the volume of air being moved can be effective, but the chips need to be separated out of the airstream first. Appropriate cyclone separator design and building are the most effective way to do that. Refer to cyclone design at bill pentz's site, and clear-vue cyclones
www.clearvuecyclones.com.
The better the cylone design, the less friction and turbulence, the more efficient the system. Cyclones in which the inlet is angled to the same inclination as the deflector ramp have less turbulence than those in which the deflector ramp has to re-direct the airstream downward. Cyclones in which the inlet leads to the centerline of the upper cylinder have less turbulence than those in which the inlet stops at the cylinder wall. The upper cylinder height is dependent upon its diameter, and the angle of inclination of the ramp. Cone height is dependent upon cylinder diameter, with two natural "break points" at which the airstream easily turns around and goes up, leaving all but the finest dust behind. The smallest ratio of cylinder diameter to cone height should be 1:1.65. the second "break point" comes at 1:3.
Duct sizing for the volume of air to be moved is important. A small duct will carry only a small volume of air. If one works with MDF on a routine basis, one needs to move a sufficient volume of air to capture the dust before it gets out into the shop. That volume must also be moving quickly (~4000 fpm) enough that the chips are kept entrained in the air stream until separation. If the chips fall out in the ductwork, piling and plugging - dangerous situations - ensue.
Air movement depends upon the size of the impeller, and the ductwork leading to the impeller. Four impeller designs are, in order of decreasing efficiency, Airfoil, Backward Curved, Backward Inclined, and Straight. Most hobbyiest impellers have straight blades, because such impellers are the easiest and least expensive to manufacture. A three hp motor driving a 15 inch backward curved impeller can effectively move 800 cfm through six-inch ductwork that is smooth inside. In order to move that 800 cfm, however, the inlets must be sized for the ductwork. There is no way a 5 inch inlet is going to allow the passage of 800 cfm. A 4 inch inlet restricts the airflow even more.
allen norris
