Multiple Inline Exhaust Fans Sharing Discharge Duct
Multiple Inline Exhaust Fans Sharing Discharge Duct
(OP)
What are your thoughts on using one discharge duct for several inline exhaust fans? Is this an acceptable practice? Anybody run into problems doing this?





RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
As long as you're talking about General Exhaust, not Acid Exhaust, or VOC exhaust, etc where a specific stack exit velocity is needed, no problems. The system just needs to be properly designed. There are also Code requirements for separating Toilet exhaust from others, restrictions on kitchen hood exhausts, etc. Guess maybe we need to know a little more what you are contemplating?
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
1. Backfeed of idle fans. It would usually take more than just a backdraft damper to completely prohibit a live exhaust discharge airstream from traveling down an idle fan's duct.
2. Because of 1, I've seen some people add automatic control dampers. This is good for prohibiting backflow, but startup is more complex now. Damper opening and fan speed ramping would have to be coordinated to keep the fan from windmilling as the dampers open prior to the motors starting. This can tend to trip them on overcurrent.
I'd avoid a ganged discharge plenum if at all possible.
-CB
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
If you have different fans, the problem is even more complex.
Don't do it.
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
M&I also has applications for such multiple in-line fans (using vane-axial type for large applications)
Which is essentially what you are doing. I see people putting 1/3 on one VFD and 2/3 on another VFD with a 20-fan arrangement.
Doable in my book. Dig out these manufacturers.
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
Have you considered using one variable speed fan at the end of the duct, and using automatic dampers at the rooms?
B.E.
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
Think about the O&M costs, also. Multiple fans costs more than single fan to maintain. If multiple fans, each must be sized to maintain duct static to discharge velocity. I think you'd have to oversize fan HP to make this work (to keep from having small HP openings for exhuast end up being supply ducts).
I think a VFD would be the best application for general exhaust, and could be run off duct static pressure. Each damper would open based on independent room temperature; when open drops the static which runs the VFD (and starts fan, if not continually operating). Any damper open closes the control circuit. Highest Hz would be all dampers open, lowest would be set based on duct resistance guessed at design and set at balance. Minimum static would be set above standard for low leak damnpers. If it leaks and temp gets higher, the damper opens (dropping duct static).
RE: Multiple Inline Exhaust Fans Sharing Discharge Duct
Size individual branch exhaust fans at CFM/ESP of branch ductwork only. Size main exhaust fan at sum of all branch CFMs and ESP of main duct only. Gravity backdraft dampers will also work. Be aware if you're going through a plenum and what kind of airstreams you're combining.