Clothes dryer exhaust 1

[cme]

I have 4 dryers connecting to a common duct/exhaust fan.

My 2 schemes of thought of design/control as follows:

Scheme 1: Have a "draft hood" type arrangement (indirect connection) where the dryer exhaust would discharge into the exhaust duct such that there would be a gap between the ducts that would allow plenum air to be induced and make up the difference of either 1 or 3 dryers are on. A sail switch on the dryer duct would start/stop the fan.

Scheme 2: Have the dryer duct connected directly to the exhaust fan and have an air induction damper (motorized) to keep the static pressure at the dryers at zero or slight neg pressure. A wall switch would energize the fan.

The dryers are HD residential and am considering 300 cfm each. One of the Engineers here measured 220 cfm which matches his calculations.

Would welcome feedback.

 

[GMcD]

First, why the fan? Long duct run to outdoors? Are these gas dryers or electric? Gas Codes can affect your intended scheme.

If the dryers are electric, I like scheme #1 as it is simple and reliable as long as you have a corresponding make-up air flow into the room where the dryers are. I hope you also have an in-duct lint trap in an easily accessible location. The lint traps in the dryers won't catch all the lint, and if you don't have a lint trap/screen in the duct before the airstream hits the fan, the fan will get coated with lint and become another midnight phone call.

 

[trashcanman]

I don't think either solution is appropriate. With the "draft hood", there is a definite possibility that not all of the lint would be captured, especially if the hood is horizontal. If vertical, the air velocity would have to be high enough (I don't know how high) to enable the lint to jump from the duct into the hood. You also mentioned that the gap would allow "plenum" air to be induced. The Uniform Mechanical Code prohibits moisture laded air ducts extending into or through ducts or plenums. Also, where is the makeup air coming from?

I also would not recommend any damper (motorized or manual) in the air stream. That is a sure fire lint catcher. The code also prohibits using a screen at the duct termination for the same reason. Lint builds up, dries out, reduces air flow, makes the dryer run hotter, thus setting up a lint fire.

Why not just route each exhaust duct directly outside to a wall or roof cap? If the distance is prohibitive, a booster fan in the duct might work (radial blade or axial, to handle the lint).

In any case, be sure to check with the dryer manufacturer and get their recommendations, so as not to void the warranty. Good luck.

Trashcan Man

 

[Rodale]

If I remember correctly... In Wisconsin, the dryer has to certified for use with a secondary exhaust fan. The State Plan Examiner said he didn't care what the fan was rated to do, the dryer needed to be approved also.

 

[walkes]

Exhausto makes a dryer venting system that will take into account the number of dryers operating to ensure the proper air flow and pressure in your vent.

http://us.exhausto.com/files/pdf/Brochures/3911005.pdf