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.