Smallest Practical Air Flow Volume Controlled by Dampers

[EnOm]

Hi
What is the smallest practical airflow one can work with when designing duct work. Say I have a laundry room which a min exhaust rate of 15 CFM. Is it practical to try and exhaust this rate by placing a damper in the duct work? The smallest airflow amount specified on a plan I've seen so far is 30 CFM.

Thank you.

 

[EnergyProfessional]

it depends on the duct size. With a 3"duct you may be able to control 15 cfm, but not with 10" duct.

but don't put dampers into the lint-path. that is against code since lint will trap, block the flow and may ignite.

 

[317069]

15 cfm, is it the exhaust from dryer machine or you want to install an exhaust fan in the room itself.
if it is dryer machine, then just ducted to outside, if it is for room itself then you go with what is available in the market, and do not put a damper in exhaust stream

 

[EnergyProfessional]

317069 is correct, if this is only one single exhaust, you don't need to balance with a damper if you select the correct size fan. you won't get exact 15 cfm...

15 cfm sounds like a weird requirement, how did you come up with that? typically spaces have a cfm/ft² or a cfm/fixture requirement.

 

[ChasBean1]

If you designed the duct for your minimum flow of 15 cfm (about a 2" duct), good luck. You might want to look into this some more.

 

[EnOm]

Thank you all for the responses.

HerrKaLeun

317069: I now realize that mentioning a laundry room as an example wasnt the best choice. No it is not the exhaust from the dryer. It is based on 0.12 cfm/ft2. A better example would be a storage room. If I have a storage room of say 100 ft2. If I go with 0.12 cfm/ft2 then I'd get an exhaust airflow rate of 12 cfm. If I were selecting a local exhaust fan then I'd just go with the smallest fan available in the market, if I were to insist on changing the air in that room to begin with. In this case I am insisting because the room has no windows and is located in the basement. The storage room is part of a group of other spaces being exhausted by a central exhaust fan, so I'm looking for the smallest realistic value I can specify as an exhaust flow rate. I don't think 12 cfm is realistic. As I said before, the smallest I've seen is 30 cfm in a (4" X 4") duct.

ChasBean1: I have not finished designing yet, still in the process

Best Regards

 

[317069]

0.12 cfm/sq.ft is the minimum, is there any problem if you use 30cfm or 50 cfm for example

 

[EnOm]

317069:
Thank you for the response. Well there is not exactly a problem, that is something I can consider. But I still want to know what is the minimum realistic airflow amount I can achieve. I'd like to have a solid answer if I'm asked if about the flow rates I specified. Also, I was told that I have an upper limit on the HVAC electrical loads on this project because of all the fancy lighting and other equipment. I'm giving the priority right now for the AC system. If I end up with an extra margin at the end I might consider raising exhaust rates..etc. So I still don't have a clear answer. Is there anything less than 30 CFM in a 4X4 duct?

 

[317069]

your point is not the fan itself as long as you are using a central fan which will have for sure more than 12, 30, 50, 100 cfm,
you point is the exhaust duct design which it depends on the application itself and there is no limits other than duct design rules and equations,
in your case I think 5" duct would work fine.

 

[EnOm]

317069:Yes exactly. My concern is with the duct design, and not the fan. The duct design limits are exactly what I was asking about.
Anyhow, I'm not going to specify anything less than 25 cfm. I don't want to risk going lower than that without being 100% sure that it's possible. Thanks for the help

Best Regards