What is adequate ventilation?

So I'm in the process of setting up a small fabrication shop in an
extra garage(as a business) and am currently trying to sort out how
much ventilation I need. Up here in The Great White North of eastern
Ontario, daytime its not getting much warmer than 10* Celcius (50F)
and dropping as low as freezing overnight. We're in the final push to
insulate, drywall, and seal up before the winter, and keeping the
garage door and windows open is no longer an option.
I'm having no luck finding any specific tables or official info re:
how much ventilation is needed. I bought a wall mount fan made by
CanArm that can suck out 1850 CFM. I didn't notice till I got it to
the shop, but the box had a chart on it that gave various examples of
times req'd for total air exchange. The example calculation was for a
garage 36x36x10, so 12960 cubic feet. The required air exchange time
for a GARAGE was stated to be 5 min.
12960 CF / 5 min = 2592 CFM.
Look up the model that draws equal to or more than 2600 CFM and Bob's
yer uncle.
The chart on the box suggests that a WELDING SHOP should exchange its
air every 2 min.When the ceiling goes in this week, the volume of the
shop will be 8410 CF (29x29x10). My fan will only do it in 4 and a
half min. For small work I've already built a fume hood over my
2.5x4ft welding table that probably draws about 250CFM.
So the concern is work done in the middle of the shop on trailer
frames etc. --BUT-- I'm the only welder, so the worst case scenario
is me stick welding and my partner MIG tacking things together at the
same time. This is not a production welding shop with rows of welding
booths, each one manned with an arc welder running all the time.
I'm mounting the fan in the attic gable and building a large duct in
the attic to connect the fan directly to the 22x30" opening in what
will be the ceiling(approx. centred in the middle of the shop).
Do I really need to get my air exchange rate down to 2 minutes? Buy
the bigger Fan?
Thanks in advance.
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In Ontario you probably can't afford to do the full air exchanges that they are recommending. The cost of heating the place will drive you to bankruptcy.
Typical shop area should have perhaps 2 to 3 exchanges per hour. If you have solvents or oils, double that. Welding doubles that if not more. That's where they get the 2 minutes to 5 minutes exchange number.
They key to getting a reasonable compromise between staying warm and reasonable air quality is to make sure you suck out the bad air and leave the good air. Picture a shop with two sections separated by a 7'x8' doorway. Fresh (and heated) air on one side, exhaust fan on the other. Do your welding in the doorway. All the smoke and fumes go into one space, you get to breathe decent air while welding.
See if you can set up welding in on corner, set the exhaust fan up in that area. Adding some baffles to direct the airflow through the weld area helps a lot. Repeat: A LOT.. If you set things up properly, you should be able to do just fine with a few hundred CFM
If you insist on welding dead center, about the only thing you can do is use one of the portable exhaust fans or a moveable exhaust duct. 'Smog Hog' is the brand name, fume extractor is the generic.
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I would come up with a blower from a furnace and couple that to a high efficiency filter. Take a look at Lowes or Home Depot for filters about 4 or more inches thick.
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I'm a simple layman here but rather than a huge exhaust in the center of the space that would suck all of your heated breathable air out too why not use smaller flexible ducts over your work area.
I'm thinking about the 6 or 8" flexible tubes/ducts hung at hieghts that allow one to walk under them and positioned over the work area. These would be less apt to suck non contaminated air and could be used with a considerably lower CFM. One other poster mentioned something like this. Truly it puts an "exhaust hood" over each work area. Couple each duct to a separate 600-800 CFM fan (my guess here) and you can turn on only the ones where work is being done at any time. Such in line duct fans are pretty cheap and can easily be wired to a wall switch.
Again..I am only a layman but this sounds like a reasonable way to clear the air. (pun intended)
Reply to
Krazy Old Man
Have you looked into an air-to-air heat exchanger? It might be just what you need. Would probably save a lot of money in the winter, and almost as much in the summer if you run AC
Reply to
Pete Snell
When I worked for a company which designed ventilation for industrial facilities (among other things) we used the ASHRAE Industrial Ventilation Handbook as guidance. They recommend "source capture" to the greatest extent possible, because the closer you can get the exhaust duct/hood/ whatever to the source the less air you need to exhaust or filter. Even an old outdated copy of this book which you could probably find cheap would give you the rundown on the methods available and recommended flow rates for each. The 2 minute exchange rate is based on no source capture methods; no one does this in places that need heat or air conditioning.
Exactly what you really need depends on the nature of welding to be done, but for the situation you describe I would be thinking about a pair of movable ducts for welding exhaust (or filtration), and/or a booth with hood, and overall exchange rate set by other uses of the space. The exhaust or filter decision needs to be based on the number of hours per week the system is used; with high usage the filter will be cheaper.
Reply to
Glen Walpert
Hi everyone
I'm recounting exactly what someone else said - possibly here on SEJW or maybe visiting a college in a cold place... Never seen this either...
Locally extract generously over the welding area. Plentifully supply air from the outside under the welding area. So you have two ducts - one extract and one supply. The idea is that the air you are extracting is the supplied cold air + the welding smoke. The heated air in the room is entrained as little as possible.
As I said - I am recounting about a scheme I have never seen. I hope this is helpful - sounds fairly convincing.
Richard Smith
Reply to
Richard Smith
Sounds like a plan. Having a blower with filter to scrub the heated air independent of the above is worthwhile if one can scrounge the blower. Buying new, not so much.
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What you are describing is exactly what a commercial kitchen hood does. The supply air is set really close to the exhaust volume, but leaving a slight negative pressure under the hood. Look up "make up air hood". If the temperature differentials are extreme, it is sometimes best to temper the supply air, though this certainly adds to the cost. Any local stainless manufacturer will be VERY familiar with all the issues
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Thank you, gentlemen.
I appreciate your input. I've decided on the setup I'm going to use. I'm going to use aspects of almost all your suggestions. In particular the fresh air intake near the extraction point, and the use of more elephant hoses(6" flexible vinyl duct) for at source extraction than I had originally planned. In addition to the fume hood over my fab table, I'm keeping the big central extraction vent in the ceiling , but it will be modular. I'm making an insert for the opening which will have 2 elephant trunks hanging from it for local extraction while working on big stuff, but will always have the option to run it wide open for things like painting or using it in the summer as a "whole house" fan to exchange for cooler air overnight . For the winter, additional "fresh" air will be drawn down from the attic (which will be slightly warmer than directly from outside) through a few carefully placed vents through the ceiling. Two vents in particular will be located either side of the stove pipe in the corner to take advantage of the excess heat in that area. The wood stove we have is very efficient and is capable of heating an area more than twice the size of the shop coupled with R22 Roxul insulation all around, so I think we're good in that respect.
When its all done and the drywall is up maybe I'll post some pictures of my "system".
Thanks again. -Mark
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