Exhaust duct layout refresher

[m2e]

Hi

I haven't worked on a lot of HVAC lately, and now have to work on a job that require a design of an exhaust system. Somebody proposed a system as shown in the attached diagram. The system has exhaust branch ducts, and are combined into a main, but each branch has an inline exhaust fan before the main. My first impression is that it is not going to work, but if we have either of the two condition, would it works?

1. interlock the two fans so that they always run together
2. have a damper after EF-1 and EF-2

Thanks so much!

 

[MintJulep]

Well, it might work. It depends.

Lot of things that could go wrong however.

 

[RossABQ]

Are the exhaust pressures required in each branch really different? Why two fans? Are there constraints you aren't telling us about, like no room for a single large fan?

The common discharge duct is the only real problem with the system as shown. If the fans are constant speed and always both on, it's not a huge problem with proper T&B, but if they modulate independently, you will likely have control problems.

 

[Wareagl487]

I see no problems with the sketch - done all the time. Just have to properly size the main to handle the combined cfm's of the two fans. Also be sure to specify backdraft dampers on each fan so if one is off, exhaust isn't short circuited back through it into the space.

Andy W.

 

[willard3]

Control problems will kick your butt.

Read about control of fans in parallel.

 

[trashcanman]

This is done all the time for toilet exhaust in hotels. The individual fans discharge into a vertical exhaust riser. Each fan has a backdraft damper. The riser is sized as if all fans run at once.

 

[m2e]

To clarify a bit, I don't think space is an issue, there should be room for one big fan. Air flow are of course different for the two branches, but pressure shouldn't have to be different. The reason I wanted to find an answer is because somebody suggested this design, but I'm not sure whether to reject it or not. I guess what I'm trying to get at is the principle of the ductwork design. In this case, whether we can merge the two exhaust branch into a main. (or merge two inlet branch for supply fan)

I don't seem to remember clearly whether they're doable or not. Or maybe doable but not a good engineering practice.

Is this kind of setup discussed in the ASHRAE handbook, or somewhere else?

 

[willard3]

snip
This is done all the time for toilet exhaust in hotels. The individual fans discharge into a vertical exhaust riser. Each fan has a backdraft damper. The riser is sized as if all fans run at once.
snip

You know, what I notice unfailingly about exhaust fans in hotel/motel bathroom is how poorly they work.......

 

[SAK9]

You have not mentioned how critical the airflow rates are.If engineered correctly, the system will work.Fundamentally there is nothing wrong with it.

If the two zones in question are not required to work at the same time,this is the simplest arrangement you could use.

 

[ChrisConley]

We're looking at doing something similar, but we're actually exhausting the main riser as well. I'm looking at the little fans as 'booster' fans, we're also providing a VFD on the main riser fan.