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?
Would these be identical fans "in battery", operating in unison at the same conditions, or are you considering collecting the exhaust from many diverse size/static fans?
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?
This is general exhaust controlled by t-stats (large storage / mechanical / electrical rooms that call for exhaust/makeup to keep temp 10 degrees below ambient). I would put backdraft dampers at the discharge of each fan. They would not necessarily be running in unison. Some could be off. Airflows would vary. One of these rooms is a battery room so that should probably be ducted separately.
In your case, I wouldn't recommend it. If one room isn't that hot and its fan is ramped down, but another fan for another room is pushing a lot of air, the static in the common discharge might prevent the slow fan from operating properly.
I agree with RossABQ. Flows could be affected with variable backpressure. Couple of issues also:
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.
they apply multiple fans (fan wall technology) very successfully.
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.
Definitely put the battery room on dedicated exhaust.
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).
One design I've seen done is to have each inline fan on a t-stat interlocked with a normally closed motorized damper. Each inline fan is ducted to a main exhaust duct/riser with the motorized damper in the branch duct near the main duct. Inline fan turns on and damper opens - fans turns off and the damper closes. A main exhaust fan located near the end (or at least downstream of all the branch taps) on a VFD exhausts the main riser and is controlled via static pressure sensor like in a typical VAV system for fan speed control. All the main exhaust fan does is keep the main exhaust duct negative, say 0.25" - 0.50" WC negative. Main exhaust fan could also be another inline fan or dome or whatever will work for your application. Main exhaust fan speeds up/slows down as the individual inline fans turn on/off since they would be positively pressurizing the main exhaust 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.
