Dust Collection Information

# Dust Collection Information

To aid in the response to questions from the Dust Collection and Air Filtration discussion group, we've created this informational section.

Intro

Intro

Since creating the Dust Collection and Air Filtration discussion group, we've noticed some of the questions tend to be somewhat similar in nature. To aid in the response to these questions, we've created this informational section. We'll continue to add relevant information to frequently asked questions, so stop back from time to time to review.

For clarity, lets define System, CFM, Static Pressure, and Velocity:

The System is the DC system itself. It consists of the hood, the ductwork, the collector (usually a cyclone for our purposes), the fan, and the after filters if used. Each of these components works together to collect the dust at it's source, transport it to a desired location, and separate it from the air, and recycle or exhaust the cleaned air.

CFM is a volumetric measurement of air flow - Cubic feet per Minute.

Static Pressure is an expression of the resistance of the system. It's a sum of all the entry, acceleration, friction, collector, and filter losses. It's expressed in inches of water column.

Velocity is the speed at which the air is traveling through the ducts. It is calculated by dividing the airflow in CFM by the cross sectional duct area in square feet.

 This discussion group and FAQ section are intended for hobby/home woodworking dust-collection systems. Anyone with a commercial shop, who is subject to governmental regulations, or who has allergies or another needs for an industrial system should hire a professional engineer to design, specify, and certify the performance of an industrial dust-collection system.

FAQs

FAQs

What is the relationship between static pressure and CFM? A fan works much like a pump; it operates at a given volume AND static pressure. For example, 600 CFM @ 6". The relationship between the two is inverse - that is, as pressure drop increases, volume will decrease. But it is not a linear relationship. For a good basic explanation of a fan curve, visit http://www.oneida-air.com/fancurves.htm. Review the System curve page while you are there.

Will larger filters on my DC increase performance and why? Larger filters will increase the performance, in some cases significantly. Filters are rated in terms of resistance at a given air velocity. Increasing the filter area reduces the filter face velocity, which lowers the initial resistance, or pressure drop. Final dirty pressure drop remains the same, but the filter will hold more dust before reaching that point - meaning fewer cleanings. Finally, the efficiency of the filter to clean dust goes up as face velocity goes down. So this is a win-win situation: Lower pressure drops, longer time between cleanings, better filtration efficiency.

Will my CFM change if I put a bigger motor on my DC? No. In order to change the CFM of a system, you must either change the fan RPM, or change the total static pressure of the system, or both. With a direct drive fan, you are left with only one option. Some examples of things to do to reduce the static pressure of the system would be to use long radius stamped steel elbows. Use low angle ( 30-degree) branch entries. Use low angle transitions. Use bell mouth hood inlets, where applicable. Increase the filter area, or clean the filters. Or increase the size of the ductwork.

What are the pros and cons of venting the cyclone outlet outside versus filtering and recirculating the air? On one hand, venting outside eliminates ALL the fine dust from re-entering the shop environment. No filter can do as well. Venting outside is less expensive to install, as no filtration costs are included. No filter pressure drop also means more CFM at the hood for your horsepower dollar. The best argument for recirculating that the exhaust has to be made up with outside air. If your shop is heated and especially if it is air conditioned then bringing the outside air to room temperature consumes a lot of energy. Also, venting outside can be noisy and if you have neighbors close by they may object to the noise. Finally, venting outside without sufficient makeup air can be VERY DANGEROUS if you have gas fired appliances (such as a water heater or a furnace) in your shop. Sufficient outside air MUST be allowed freely into the shop, or the flue gasses will be drawn into the air. The byproducts of combustion contain CO, which will kill you before you realize what is happening.

I am installing a DC in my shop. Do I still need an air cleaner? A dust collection system and an air cleaner serve two distinctly different functions. The dust collector is designed to catch the dust at it's source, filter it out of the air stream, and either filter and recirculate the cleaned air or exhaust it outside. An air cleaner is designed to keep the entire volume of air in the shop moving, while filtering it at the same time. The air cleaner will not function as a DC, nor will the DC as an air cleaner. So, you really do need both. But if budget is short, invest in a good DC first. Capturing as much of the dust at its source as possible removes most of the airborne contaminate. The air cleaner finishes the job.

What type of ductwork should I use for a DC system? The best ductwork available is stamped steel, longitudinally welded duct. Unfortunately, it is also the most expensive. Many home shops are successfully using 6" PVC pipe for the main runs, and branching to either 5" or 4", depending on the volume required. Stay away from using short raduis elbows and tee fittings. Use only long radius elbows, the lowest angle wyes you can, and smooth low-angle transitions. This has a significant effect on the overall static pressure drop of the system, and is necessary to ensure good airflow. Also, 30 guage metal will not withstand the pressures that a DC should run at - it will collapse. Use the heavier 26 guage if you are trying to use hardware store pipe and fittings.

What is the recommended tape for sealing joints? The expensive stuff. Cheap cloth tape will not hold up well. An HVAC supply store will carry aluminum tape, which is much tighter sealing and stronger. Another alternative is hardcast, or a polyurethane duct sealing caulk.

What is the difference between Shop Vac and DC? A shop vac is designed to move a small amount of air at high static pressures. Typical performance is roughly 100 CFM at anywhere from 50 to 100 " static pressure. A DC requires much higher volumes - typically 500 to 1000 or more CFM, and runs at pressures in the 5" to 10" range.

Can I use my Central Vac for dust collection? A central vac is more similar to a shop vac than a DC - it will run at lower volume and higher pressure that a DC system.

Can I use my DC for my central vac? Similar to the shop vac / DC question above, the DC will not generate sufficient static pressure to use the smaller pipe sizes normally seen in a central vac.

Can I use a furnace blower for a homemade dust collector? A furnace fan doesn't work too well for dust collection. It wont generate sufficient static pressure to get the velocity needed to move material. Typical HVAC velocities are 500-1000 FPM, and they run in the .25" to 1.5" static pressure range. Dust collection requires velocity in the 3500-4000 FPM range, and typical systems run in the 4" to 8" static pressure range - which a furnace fan will not produce. Also, the furnace fan is not designed to handle material in the airstream. It has been suggested in our Dust Collection forum to use the forward curve fan as a general exhaust or as an air cleaner.