I am sandblasting under my car, in the garage, in a plastic cocoon I have made to shield in the dust and sand. It is working well on the sand but I need to pull the dust out quicker than I can with two shop vac's rigged up to the TENT. I already have a 4" pipe through the wall I can exhaust dust to the outside if I rig up a filtered pick-up box inside the tent and hook it to some kind of exhaust fan through piping. It is winter in Ohio so I can't just open the doors and vent out the garage doors. This project will take all Winter so the temp can drop to below zero and I heat my garage when I work out there. I want to rig up a semi permanent exhaust system.

My question: Is my theory on this correct?

If my working space is 10' x 10' x 4 feet ( 400 cubic feet).
Would a 400 CFM exhaust fan would remove all the air from that space in one minute.
If So, would a 600 CFM take 45 seconds, a 800 CFM would take 30 seconds ETC ETC.

I'm trying to figure out what Fan/CFM to look for and the cheapest way to do this. I think I need some sort of an IN-Line type Exhaust that I can hook up to 4" furnace type duct work and shoot it outside via my pipe through the wall.

Thanks
Ray

Correct in thinking, but you have a low ceiling = 4 ft, unless you're using the numbers for an example.

Yes, 400 CFM. I would suggest your best bet may be to set a 20" box fan on the inlet of the tent, and one on the outlet, cut a hole for each fan and duct tape the plastic to the frame and have the outlet at the garage door with the rest of it blanked off so only the fan shows in the door.

I don't think you will be able to use a 4 inch duct for 400 CFM, too small.

Consider this: a 4" dia duct has a cross section of roughly 12 sq in. (12.56, but for easy math, assume 12) so to pass a 1 cu ft "chunk" of air would require the movement of 12 linear feet. Therefore, 400scfm x 12 / 60 = 80ft/sec = 54.5 mph....pretty darned fast.

I agree with Everett...just hook up a box fan. I suspect that a 20" fan moves a whole lot more than 400 CFM. I don't think you need one to push and one to pull. Just one to pull.

Before you do anything, consider this: Every cu ft of air you exhaust, must be replaced. If your garage is 20'x20'x10', it contains 4000 cu ft of air. If you exhaust at 400scfm, you replace ALL the air in 10 minutes. That's exhausting HEATED air and replacing it with air from the source!! If your source is outside air, in 10 minutes, the warm garage will become the same temp as outside, which, if I remember my meteorology about Ohio in the winter, is pretty darned cold. That's not too different from just working with the garage doors open!!

Even if you limit the exhaust air to the coccoon, it will still become the temp of the outside air, only in much, much less time.

Consider what happens if the source of replacement air is heated air (like from your house) If your house is 2000sf with 8ft ceilings, it contains 16000 cu ft of air. If you suck out the air from your house at 400scfm, it would only take 40 minutes before your entire house would be the temp of the outside. Now if your house furnace is on, it will pick up some of the slack, but no way can it keep up, so maybe with your furnace on, it would take 50 minutes. Not too much different than having your furnace on with the windows wide open!! Pretty darned expensive.

Bottom line, if you try to recycle the air (like your furnace does) you would need to filter the air, or separate the particulate in some other way. A filter would clog in minutes. A separator (cyclone or settling box) is impractical.

A salimander heater rigged up to blow into the coccoon could serve double duty, if coupled with an exhaust duct about 2-3 times the size as the heater nozzle. It would heat the air inside the coccoon at the same time as it forces air out the exhaust duct. Now your coccoon becomes a pressure vessel, and plastic sheeting is no longer adequate. But that's not the big issue. THE PRODUCTS OF COMBUSTION WOULD PROBABLY KILL YOU!!!!

Therefore, IMO, you either need to live with the dust, or live with the cold....or wait till spring to do your blasting.

Sorry to be the bearer of bad news.

Thanks guys for the response.

Sounds like I either need to stick with the vac's I'm using now ( which seem to work OK, just not enough air movement) or add another vac to the mix or just add a smaller exhaust fan to the 4 inch pipe, which just may be enough to make up the difference I need. I had envisioned something great that would pull all the air out incredibly quickly, but did not think about the in-flow side/air exchange part that would pull replacment air from outside to in. ( darn physics !!)

Thanks
Ray