In this article, the term "industrial duct" is used to refer to ducting used under negative pressure (suction) for the removal of dust or fumes generated during the production process. The design of industrial ducting that is under negative pressure and containing particulate requires a different set of engineering parameters than those associated with the design of duct used in the conditioning of the workspace. The former uses a fan to create suction; the latter utilizes fans to pressurize the duct and blow the conditioned air into the facility.
One of the main differences is the type of branch (laterals) parts used. Because the duct in an HVAC system is pressurized, branch ‘T’s (where the lines are 90 degrees to each other, forming a “T”) is commonly and correctly used. However branch ‘T’s are defined by a negative system where the air is being ‘pulled’ into the system, the air will seek the least path of resistance, and a ‘T,’ which would require the air to make a 90-degree turn, creates so much resistance that the 90-degree line sees little if any suction. Key is the material in the air stream adds even more complexity to the use of ‘T’s. The material (if it is even conveyed) comes into the duct at 90 degrees and is required to make a 90-degree turn. Anyone driving a car knows what happens when you try to make a 90-degree turn at full speed.
Therefore, an industrial dust collection system uses branches with the lateral branch entering at either 30 or 45 degrees. While there will always be a debate over which degree is better, and it only stands to reason that the 30-degree is a more gentle entry, the fact is that either will work just fine. However, always consider that that 30-degree branch requires a 60-degree elbow to complete a 90. A 45-degree branch, of course, requires a 45-degree elbow, and 45s are more commonly used in systems to make other turns in the system. Therefore, the 45-degree branch and the accompanying 45-degree elbows keep you from having multiple-degree elbows such as the ’30s and ’60s. It is simply a matter of convenience and economics.
Note: This reference to using a 45 (or 60) to turn the line back to 90 may raise the question as to why you would turn the duct at 90 degrees after the discussion on ‘T’s. First, there is nothing wrong with running the lines at 90 degrees to the trunk line. In fact, it is much easier to run lines to your machinery this way than trying to ‘angle off’ and ‘hit the point’. But the point is that you don’t want the lateral line to enter the trunk at 90 degrees.
A second difference is the fact that industrial duct systems need to be tapered main systems, meaning that, as more air enters the main trunk line, the larger the duct needs to be. This, again, is different from an HVAC system where the duct is pressurized, and the only consideration for size is (in general) the total amount of air. (These systems do use different size duct to remain efficient and economical, but would work with one size as long as it accommodated the total volume.)
The tapered main system keeps the duct at the appropriate size to keep the velocity of the air at a level commensurate with keeping the dust entrained in the air and thus keep it moving but without speeding it up. If the duct is too large, the material will fall out and lay in the bottom of the duct. If the duct is too small, the dust will be kept moving, but the fan size required to pull the larger air volume through the smaller opening will increase. This is inefficient, and the dust will move faster and, in the case of abrasive material, wear the duct – especially in the branches and elbows where it has to turn.
A properly sized system follows the parameters where the size increases as the total CFM increases. Refer to a volume and velocity chart or call US Duct to receive an Excel-based sizing chart.
A third main difference and consideration is the duct style and ‘Class’. A discussion on the ‘Class of Duct’ is too in-depth to discuss further here. However, suffice it to say that the design of an industrial duct system must consider:
The type of duct
- Lock seam
- Welded seam
- Spiral duct
The gauge of duct needed to address:
- The abrasive nature of the dust
- The static pressure under which the system will operate
- Positioning and installation of outer angle rings, as well as the welding of the rings to the duct, can impact the gauge required.
The joining method of the duct:
- Flanged for heavier and large size duct
- Welded for 100% air-tight applications
- Clamp Together- general dust collection and fast installation
- Big End/Small end (one piece fits inside the other) for spiral
The SMACNA (Sheet Metal Air Conditioning National Association, the governing body of the sheet metal union contractors and the established authority for proper duct installation and design) has an entire book of criteria that needs to be considered based upon size of duct, pressures and outer reinforcement. This guide should be consulted when determining the gauge needed.
Fourthly is the use of flex hose. While flex hose makes for a very quick installation, the internal ribbing dramatically increases the static pressure required to suck the dust. You should restrict the amount of flex hose used to the amount needed to make the final connection.
Fifth is the radius of elbows. This, like the degree of branches, will be eternally debated. In general, US Duct believes that 1xD elbows in any diameter are too tight and restrictive and should be limited to situations requiring a ‘tight’ fit. 2.5 x D elbows, of course, provide better flow but, in general, are an over-kill except in highly abrasive applications or where the existing fan does not have enough available static.
Finally, new NFPA regulations have dramatically impacted the design of industrial duct by adding requirements for backblast dampers that prevent an explosion in the collector from migrating back into the building. These requirements are too complex to be properly considered here. There are multiple considerations ranging from the explosive nature of the dust to the collector’s ability to address the explosion to the installation of spark-arresting equipment. Numerous books from the NFPA address different kinds of dust. However, while they all set forth the proper design, they refer the designer and owner to the authorities having ‘local jurisdiction’- meaning that you should consult local fire inspectors, building permit officials, and insurance personnel. All of these can provide you with the accepted local standards, which may vary from location to location.