exhaust fan sizing for long duct 1

[baecs]

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

 

[chicopee]

What does the fan performance curve tells you? I am assuming that your total pressure drop is for 300CFM.

 

[baecs]

Hi, here is the fan curve and data sheet.

Thanks.

 

[willard3]

Find an Engineer who does ventilation and understands air systems; you are probably in over your head.

 

[baecs]

I would like find someone who could offer a suggestion in this forum.

 

[317069]

your calculation told you you that you need 1.2 in.wc and your fan is 1.5 in.wc, what is the problem.
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?

 

[baecs]

No, it is not a homework. It is a subway station renovation project. The washroom is located in 20 metres below ground. The exhaust air duct starts from the washroom, through a few rooms at the same floor, then rises up 10 meters to the concourse level, the horizontal run of the duct in concourse level is 20 meters, then run duct up 10 meters to the ground floor, the horizontal run of the duct at the ground floor is about 40 meters.

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

 

[Drazen]

it is really not sarcasm what is stated that you need someone more experienced for that task.

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.

 

[baecs]

I know it is an unusual design. It is for a very old subway station. There is no way to build ventilation shaft at this time. The only way I can think about is to run a very long duct.

 

[MechEngNCPE]

For that fan you need a 1/3hp motor. If it provides 1.5" and you only need 1.2" then you should be fine. Is this fan protected from moisture? Aluminum?

 

[KiwiMace]

Often best practices have to take a back seat as we are stuck with the building we are given.

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.

 

[Waramanga]

I dont think this problem could be over anyones head if they had a reasonable text book next to them. dont forget to check if your K factor for fitting losses are for total pressure or static pressure. If its for total pressure you need to take the velocity pressure at the fan off the total pressure drop. In any case, find a book with a good example to follow.
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.....