Vent hoods are the point where gasses are suctioned into your ventilation system. They are a very important component in the performance of your baghouse and discharge system, yet designers often pay little attention to their design and location.
To understand hooding design, first we need to understand the purpose of your ventilation system. Or more precisely, what it’s not supposed to do. With a few exceptions in process equipment, the ventilation system is NOT intended to suction dust. Its purpose is just to keep your process equipment or your enclosure under negative pressure. Some dust will inevitably end up in the baghouse, but it’s not intended to act like a vacuum cleaner.
Hoods have two basic functions: They should minimize material carryover and they should minimize pressure losses at the pick-up points. These two objectives can be accomplished with proper hooding design.
In a nutshell, the larger the hood, the lower the amount of dust suctioned. And the more tapered, the lower resistance to flow. There is obviously a physical and cost limitation, so rule-of-thumb guidelines are presented in design manuals to make them reasonably sized. The typical recommendation is to form a square box as wide as the enclosure allows before tapering to the round ductwork.
Hooding position is also important to minimize material carryover. Even a properly designed hood will end up suctioning excess material if it’s placed at the wrong location. In general, hoods must be placed away from the source of dusting, as shown below.
Properly designed and properly positioned hoods can make a big difference in your systems’ performance to reduce wear on filter bags and discharge system overload.
Call us to discuss your application with one of our engineers today!
We’re happy to announce that we will be at the following trade conferences:
IEEE-IAS/PCA 2015 – CEMENT CONFERENCE: April 26-30, 2015; Toronto, Canada – Sheraton Centre Toronto; Booth #111
AIR QUALITY WORKSHOP – Kansas City, MO June 16 Chicago, IL Sept. 9
FRAC SAND SUPPLY & LOGISTICS CONFERENCE 2015: September 24-25, 2015; San Antonio, TX – JW Marriott Hill Country Resort; Booth #32
AIR QUALITY WORKSHOP – Orlando, FL Oct. 7th, Houston, TX Nov. 3rd
DUG EAGLE FORD 2015 – DEVELOPING UNCONVENTIONALS, GAS & OIL: October 25-27, 2015; San Antonio, TX – Henry B. Gonzales Convention Center Booth #3023
We talked about the importance of proper enclosures in conveyor transfer points. However, there are many cases where enclosures are properly designed and there’s adequate ventilation, yet dusting soon becomes a problem. The ventilation system always takes the blame, but is the problem really due to poor ventilation?
Engineers in charge of material handling design often design chutes and loading points without considering the long term impact on the ventilation system.
Take the loading of a belt. If the chute and loading point are poorly designed, the enclosure seals are soon worn out. A resulting gap between the enclosure and the belt adds to the open area originally considered by the designer, which proportionally reduces the ability of the ventilation system to suction fugitive dust emission. In more extreme cases, transported material even shoots out of the enclosure, leading to excess dusting and material accumulation nearby. Yet the ventilation system is blamed for the mess.
So what to do? Two simple solutions in conveyor loading include muckshelves and rockboxes. These minimize impact and wear on the conveyor and center the material loaded on the conveyor, extending the life of the enclosure seals and maximizing the performance of the ventilation system.
Rockboxes and muckshelves are not part of your ventilation system, but can sure have an impact on its performance.
Call one of our expert engineers to discuss your application and possible improvements.
Dr Vent Goode
Enclosures should efficiently cover the area of dust generation. Although they don’t look like they are even a part of your ventilation system, they are absolutely required, and their design is extremely important for the overall performance of your air pollution control equipment.
If you could completely enclose a source of dust generation, no ventilation would be required simply because there would be no openings where dust could escape. From this statement we can generally conclude that the more you enclose a source of dust generation, the more you reduce dust emissions problems. This is true, but there are limitations that need to be considered.
Take a belt conveyor transfer point, where dust is generated mainly where the material impacts the conveyor being loaded. An enclosure contains the cloud of dust, but the transfer cannot be sealed because openings are required for material to enter and exit the enclosure. So your installer hangs rubber skirting down to the belt surface in an attempt to eliminate all openings. This is a problem because the skirting then drags on the conveyed material, creating an additional point of dust generation and greatly affecting the performance of your air pollution control equipment.
Proper design guidelines recommend a 2” clearance between the skirting and the material conveyed. Sure, that’s an opening, but it’s a necessary opening. The ventilation system is designed to pull ambient air through these controlled openings, and it is this incoming ambient air that keeps fugitive dust from escaping. The incoming air sweeps fugitive dust to the baghouse, which is exactly what your ventilation system should be doing, keeping fugitive dust from escaping, not suctioning conveyed material.
Enclosure design is critical for the performance of your system and can be the main reason for poor performance in a ventilation system. Details vary greatly depending on the application and sometimes these details can be counter-intuitive, so count on experienced IAC engineers to help you optimize your installation for maximum performance.
Dr. Vent Goode