Apartment Building Dryer Exhaust

[carlosgw]

I have a three story apartment building wih drayers in each apartment. I want to have a dryer exhaust shaft with a booster fan on the roof.
I think it is fairly typical but I have some questions.
It will be a sheet metal vertical duct in a shaft with a constant speed fan at the top. The 4" individual dryer vents will penetrate the vertical duct and extend up 22". There will be a cleanout at the bottom.
Should I have a drain at the bottom (I do not want one)?
I have a vent at the bottom to allow constant up air flow and air flow when no dryers are operating, but, I am thinking of reducing this vent or eleiminating it as the rooffan should draw a reduced air flow through the dryers which are not operating.
I had to guess on the dryer CFM based on a max. pressure drop requirement I found in one installation manual. Does anyone have better info on residential dryer air flow?
In my case there will be 5 dryers per floor for three floors.

 

[DrRTU]

I have faced the same problems. Which IMC code are you working to? Do you have a submittal on the dryers? Is this a new installation? What is the shaftwall & floor rating?

Steve

 

[bigwilly104]

Dryers dont shut a damper or stop air flow when they turn off so you are going to get air flow at all times through the duct. You want to use a explosion proof motor though with the exaust fan. Dryer lint can ignite easily and when it does it flashes. I would put spring loaded lead element fire dampers at each floor I think that would be code anyway. Good Luck.

 

[carlosgw]

New York State.
I know what dryer they are using but it, like any other dryer info I have found, does not list the CFM.
One hour shaft. I do not need fire dampers with the 22" projections of the duct into the shaft.
The shaft is actually a sheet metal duct in the 1 hour shaft.
The fan motor is not in the air stream and the fan is aluminum (standard rooftop fan).
I am expecting an air flow when the dryers are all off, but a reduced air flow (hopefully very reduced).

 

[AbbyNormal]

I would say 160 to 200 CFM is typical, dryers change over the years.

 

[DrRTU]

Under IMC 2000, and 1 hr walls, I have used standard sheetmetal duct (smacna MP design) as the riser. Seal longitudinal and transverse seams. Dryers were connected to the riser without sub ducts. NFPA 54 does not allow the use of a FD on a fuel burning device vent. I believe the sub duct will trap lint. NFPA shows a plain metal duct passing a 1hr. wall without a FD or SD. Note: UL does not have a detail of plain metal thru a 1hr. wall as a rated assembly. I would install a cleanout at the bottom of the shaft. The cleanout is at the ceiling of basement or parking garage ceiling. I keep the 4” run out as short as possible by having the shaft next to the laundry area. I size the shaft / fan for the following: dryers at 150 CFM each, size the shaft at 1200 to 1500 fpm, 50 % of the dryers in use. I like to use Greenheck kitchen exhaust fans series C type fan at top of the shaft. This is an upblast fan with motor out of the steam and is good with dirt loading on wheel. No backdraft damper. Fans run 24-7. I have not seen a stall or unstable flow issue with zero dyer operation. Pay close attention to NFPA 54 and IMC section on venting, you do not want to get roped into a distance to combustible issue.

The IMC and NFPA limit you to 25 feet of ducting unless you can prove the dryer has “extended” capacity. A few dryers do have extended capacity but most dryers are owner provided. The codes vary upon year.

We have done a few 3 story buildings that use separate 4” runs to a roof mounted vent hood. I like this design except you run out of fan capacity on the lower floor.

I try to vent each unit via a separate vent to sidewall. When all venting is in the unit it forces the unit’s occupant to maintain (clean) the vent system. Remember dryer fires are one of the leading causes of residential fires. Make sure building ownership cleans the lint traps.