Dumb DC idea?

[Rick Potter]

Here's the deal..I live in SoCal where I usually work with the shop doors open, so I ducted the 3HP dust gorilla outside. This works great most of the year, but on those few months when it is either very hot or too cold to leave the doors open, I have to open a window when I turn on the DC, otherwise it will starve the DC for air within a short time. If I forget, which is often, I am reminded when the DC effeciency drops noticeably (it is a well sealed building).

Here is the dumb idea. If I install a shuttered vent that pops open under pressure, only reverse it so it opens under negative pressure, that would automatically feed the DC only when needed....right?

My questions are:

1. Has anyone done this, and how did it work?

2. My DC has an 8" exhaust. Since that is under pressure, should I use a larger opening vent, like a 12X12", or larger?

3. For you engineer types out there....just how large should the shuttered intake be for a 3HP dust gorilla?

Now, if this is doable, I am fortunate in that I have a covered porch where the intake could go without worrying about water leakage. However, if the panel approves of this idea, I would start with a shuttered duct mounted in plywood and put in a window for a temporary trial.

Rick Potter

PS: Yes, I know I should just open and shut that window, but I forget (old age), and they are hard to reach. Cracking the door open is a problem because my dog runs for the door anytime I make a move in that direction. She is a shelter dog, mistreated as a pup and has separation issues. This is no problem in the summer when the garage door is open and she knows she has an escape route, but in the winter I almost get knocked down when I open the door.

[Aaron Berk]

I think it sounds like a fabulous idea.

I'm working on mounting a VERY LARGE squirrel cage fan on one end of my shop, and a lever controlled shutter on the other. The fan vents to the swamp out back, and the shutter pulls in fresh air.

I don't know the formulas for deciding make up air, but I would think that if the shutter is opening forcefully and staying wide open, you'd need a bigger one.

So how much do you open your window, how many sq inches of venting? Just get a shutter that is a bit bigger than what you open your window.

[Rod Sheridan]

Rick, a gravity damper is a great idea, although they're restrictive because they don't open fully.

Use a damper 3 or 4 times the exhaust opening area which would put you in the 12" X 12" or larger size.........Regards, Rod.

[Bruce Wrenn]

A barometric damper will work. They come in all kinds of sizes and are used on the exhaust side of boilers, and furnace to help induce draft in the stack. But all I have ever seen are round. Remember that what ever rate your unit exhausts air, your make up air will be the same

[George Bregar]

Seems easier to install a second duct to let in air and wire a blast gate to open when the dc is on. Plenty of info here on how to do it.

[Thomas Canfield]

The inlet shutter to supply the air displaced by the cyclone has to operate at a very low differential pressure which will require a large shutter. Check the air flow and look at a attic fan type shutter.

[Rick Potter]

Thanks for the suggestions guys. I will check out McMaster Carr.

Rick Potter

[David Giles]

Ditto on 2-3X the area of an 8" duct. This will keep the pressure drop down. Consider the duct location and air flow patterns. Putting the duct close to the main users and near the floor lets the air flow from the inlet damper, across the floor, into and out of the machine. This reduces the heated/cooled ambient air loss from the building.

[Aaron Berk]

Sounds like you put some thought into that answer David.

Low to the floor to limit the loss of heated air;) right on. Sounds like a good call.

When I run my squirrel fan (one day:rolleyes:) I'm putting an over sized "doggy door" in my shop door and installing a drop in filter for it. That way I'll have filtered air for when I'm doing finishes.