exhaust fan sizing for long duct
exhaust fan sizing for long duct
(OP)
Hi, I am sizing an exhaust fan for a washroom located 20 meters below ground in a subway station. The horizontal run of the exhaust ducg is 80 meters, plus 8 elbows. The duct ends up at the exterior wall above ground. The exhaust air is 300CFM.
My calculation shows me the pressure drop of the whole duct is 0.5 in.w.c., plus pressure drops caused by fire dampers, control dampers, exterior wall louvers, the total pressure drop is 1.2 in.w.c.
This is the first time I size a small exhaust fan for such a long duct. I don't know will it work.
My question is, can a 1.5 in.w.c. Centrifugal inline fan do the job? Is a second boost fan needed?
Would anybody who has the experience ask my question?
Thank you in advanced.
James
My calculation shows me the pressure drop of the whole duct is 0.5 in.w.c., plus pressure drops caused by fire dampers, control dampers, exterior wall louvers, the total pressure drop is 1.2 in.w.c.
This is the first time I size a small exhaust fan for such a long duct. I don't know will it work.
My question is, can a 1.5 in.w.c. Centrifugal inline fan do the job? Is a second boost fan needed?
Would anybody who has the experience ask my question?
Thank you in advanced.
James





RE: exhaust fan sizing for long duct
RE: exhaust fan sizing for long duct
Thanks.
RE: exhaust fan sizing for long duct
RE: exhaust fan sizing for long duct
RE: exhaust fan sizing for long duct
it is strange layout for washroom in subway station to go 80 meters horizontaly with 8 elbows then go up 20 meters.
this 20 meter will help you too. is it a homework?
RE: exhaust fan sizing for long duct
My calculation seems OK. But I am not sure if it works after installation because I haven't done it before. I have asked tech reps in Greenheck, Loren Cook, and Twin city, and couldn't get clear feedback. I need somebody who can tell me if I am using the right approach. Any question and comments are welcome.
I have to the similar design in sever other service room in other subway station renovation jobs.
J
RE: exhaust fan sizing for long duct
at first glance, solution mentioned seems a bit awkward.
have you considered how to deal with condensate that will certainly be formed along that duct? that could be quantities sufficient to create surrounding for large fungus growth and create serious health problem.
as engineers we have to take all what we considered important into account, so limiting this problem to issue of pressure drop only is in my opinion unacceptable.
RE: exhaust fan sizing for long duct
RE: exhaust fan sizing for long duct
RE: exhaust fan sizing for long duct
You can 'over'size the duct to meet a lower pressure drop if the fan options are too limited. This might be a good economical option anyway over the life of the system.
Unless there is a shower in the washroom or your duct runs in unconditioned space that is ventilated to outdoors, I don't think that there will necessarily be a lot of condensation. If you do think this might be a problem, I would consider providing a drop leg and access door to the bottom of the vertical section for cleaning out and Al duct as suggested. A drain and trap if things look really probable, or even insulate the duct.
A centrif fan will likely be fine, as they do not normally have the hump at the low flow/high dP zone like an axial. This part of the curve can cause the fan to jump randomly back and forth between two operating points. Pressure specs are typically better too.
RE: exhaust fan sizing for long duct
In terms of condensation, that only occurs if the air from the space is cooled as it travels through the duct. In your case, I doubt that the air you will be extracting will be that close to the dew point.....