Industrial Ventilation Design
May 17, 2021
Industrial Ventilation Design
Is it the dust collector, the fan or the duct system that is improperly designed or installed? We receive phone calls and emails all the time stating, “The system doesn’t have enough suction.” or “The duct collapsed.” The reality is most of the time it is the result of HVAC contractors installing heating and ventilating duct design principles in an industrial ventilation application.
In HVAC systems, you are dealing with positive pressure applications moving only tempered air. In industrial ventilation, we live in a negative, high-pressure world that sucks (no pun intended), unwanted hazardous material to some means of filtration/disposal system.
HVAC contractors blow, typically working in the 500 – 1000 fpm range, and do not have to be concerned with conveying velocities. Their duct design in most cases allows for 22 – 26-gauge duct with branches that come off the bottom of the header at a 90-degree angle to a diffuser.
Industrial ventilation is the opposite. Air is sucked into the duct, so we are dealing with negative pressure. Velocities are much higher, 2500 fpm and up because we need to transport unwanted contamination through the duct. These pressures associated with high velocities require 14 – 20-gauge duct material. Is there heat or moisture? If so, don’t forget the density correction factor otherwise the design is destined to fail.
So why do some companies use their heating and air conditioning contractor to install an industrial ventilation system? Perhaps we are the red-headed step-child of the HVAC industry or perhaps some people don’t understand the difference in what we do. The purpose of this article is to point out the most common mistakes we see and help you identify them in your own plant.
A lack of sufficient velocity is where we see most issues start. To quote Maverick from the movie Top Gun “I feel the need for speed.” Without velocity, we can’t convey what we set out to capture. When conveying velocities are too low, the material will settle. Wood dust generated from shapers and routers can be very heavy and requires a conveying velocity in excess of 4,000 fpm. By comparison pharmaceutical dust and fine powders can be conveyed at 3,000 fpm and welding fume particulate around 2,500 fpm. Over time, the material build-up can eventually clog the duct, decreasing the airflow much like a clogged drain.
The next common mistake is the branch entry into the duct header. When doing industrial ventilation duct design and moving material through the duct, system branches should enter the main duct header from the side of the duct (3 or 9 o’clock) at a 30- or 45-degree angle. When the material comes in from the bottom, air current eddies are created. Material frequently builds up at the branch entry point and will eventually lead to a decrease in airflow.
Dust collection systems typically start at about -6” w.c. and go up in pressure from there. The walls of the duct must be strong enough to resist the pressure pulling the inward. SMACNA standards classify your duct based on the material your conveying and the pressure you are operating at. Foundries working with an abrasive material will need a heavier gauge duct to withstand the abrasive materials being conveyed as compared to a welding fume extraction system. Given the choice between hanging 16 gauge, 20” diameter duct in 10-foot sections or using quick connect, clamp together duct you will learn that the quick connect duct is the way to go. Standard duct will get HEAVY FAST at higher gauges and diameters!
Quick connect or clamp together duct comes in 5’ sections, is seam welded with rolled ends and uses an outside band clamp to join pieces together. This is what gives it the structural ability to withstand up to -80” w.c.
The additional benefit is the ease in which the duct system can be continually modified over time as duct locations move.
Elbows are bad from both a pressure and abrasion standpoint and so they should be eliminated as much as possible. The biggest NO-NO is an elbow right at the fan. Discharge or inlet it does not matter, they should be 6 – 8 duct diameters before you have an elbow at the fan. If you must… consider a turning vane box, but if the system is properly laid out, you should be able to eliminate many elbows.
Clean Air Company does industrial ventilation design, installation, and service for all your indoor air quality needs. Get your ducts in a row and Breathe Clean Air