Each application is unique and must be treated as such. The following is an overview of the criteria that should be considered and the principles that must be incorporated in designing an effective system.
Material and Volume of Air (Cubic Feet per Minute)
Each material has physical characteristics that impact the collection and filtering process. Foremost in the collection is the amount of air needed to collect the material. Heavier material will, of course, need more air to capture it.
A certain amount of CFM (cubic feet of air in a minute) must be present to address the dust, the amount of dust and the area in which the dust is produced. More air is needed to pick up wood chips than welding fume; more air for a wood planer than for a chop saw; and more air over the top of a mixing vat than for a spot welder. Often, the CFM required is dictated by the pick-up or hood size already present on the machinery.
Material and Velocity (Feet per Minute)
Once the material is collected, it is important to consider the velocity with which the material needs to be conveyed to keep it from "dropping out" into the duct. This is easily understood by considering the different velocities of wind that are required to move a leaf across a parking lot, as compared to the velocity needed to move a heavier object.
To use two extremes as an example, a wood chip typically requires a velocity of 4500 FPM (feet per minute) to keep it moving, while sawdust remains mobile at 3500 FPM. On the other end of the spectrum, welding fumes will move easily with 1500 FPM and not "drop out" into the duct.
Material and Abrasiveness
Increasing the velocity for the sake of a higher velocity is not necessarily a desirable thing. The faster you have to move the air and dust, the higher the pressure and therefore the greater the horsepower required to move the greater volume of air through the same confined space. Abrasiveness can become an issue as well; materials become more abrasive at higher velocities.
Material and Kst Value (propensity and magnitude)
Much attention should be given to ensuring that material does not explode and protecting the worker in the case where it does. Any material of an unknown nature should be tested and the advice of an expert obtained. In general, a system collecting explosive dust should have an automatic blast gate to prevent a deflagration in the collector from returning to the work area. If the cleaned air is returned to the work area, then an abort gate is required. As for the ducting, all ductwork between the collector and these gates is required to be bolted, flanged and of a gauge equal to the collector.
When it comes to safety, early detection and prevention is key. A spark detection system utilizes infrared detection that signals the release of water mist into the duct to extinguish sparks. These systems are typically installed in the trunk line just prior to the backblast, and require a minimum of 30 feet (based on velocity) of straight duct.
Collecting air with a hood involves more than just creating a range hood-style apparatus close to the source and hoping that the air will be drawn in. Air behaves as a fluid and therefore has dynamics that work against air in front of the hood being drawn in. A good way to think about this is to visualize pushing a bucket down into a large tub of water. The water flows in over the side even if the bucket is forced down quickly. Likewise, air will come in from the side of a hood before it will enter the face. Adding a perimeter flange can limit this effect as can the addition of baffles. Bell mouths also can serve as effective hoods.
With so many factors to consider when designing a dust collection system, it's best to consult an expert. US Duct can help design a system that suits your application and meets your specific requirements.