Welding Tips and Tricks

I'm helping my son who is building a new makerspace (a collaborative work space) that includes a metal shop. There will be three MIG stations along one wall, and he asked me to design a fume extraction system. (I'm a retired mechanical engineer.) I'm thinking of either a single hood along the wall, with a fume extraction fan at each station, or three individual hoods. There will be baffles in each of the ducts to prevent cross flow to stations not currently in use. The fumes will be directed to a single vent to the outside. A friend who is a master welder gave some great advice, but the thing I'm still missing is some kind of guidelines to size the fans. Is there any industry standard CFM number? Any other advice?

Personally, for multi-station booths next to each other, I like a single high flow fan.
It should also have a hood over the grinder area and one over the main welding table.
Each area has a shut off flap.

But if you look at single use machines, the CFM isn't very high. Only around 100.

Pic below is blurry, but low-high is only 95-108 CFM.

Thanks for the excellent tips, MinnesotaDave.

We're planning to design and build our own hoods and vent to the outside, rather than use a machine like the one you showed.

I see prices all over the place. There's an $80 ventilation fan from Harbor Freight that is 1590 cfm:

https://www.harborfreight.com/8-inch-po ... 97762.html

and this fan from Kemper that is 1,140 cfm (Kemper doesn't list a price, but there's one for sale on eBay for $995):

https://kemperamerica.com/fans/wall-mou ... aust-fans/

Both of those are WAY more than 100 cfm, even if I split the flow into three or four stations. Are those about the size of a single fan you would recommend?

What i can't understand is why the Kemper is more than 10X the price of the Harbor Freight fan. Besides the cfm removed, what are the differences between different fans? The Kemper is 1.5 hp, while the Harbor Freight one is 165 W (0.22 hp). Do you have any idea why one uses so much more power than the other?

The rated cfm is at a specific static pressure. So harbor freight can claim a million cfms with 0 static pressure. The minute you hook it up to a pipe check it with an anemometer. You will be surprised.

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I’d recommend using a standard roof top exhaust fan.
https://www.grainger.com/product/DAYTON ... chBar=true
Like this or even a bit larger.

as far as I know, all of the cfm talk is only talk. Purchased a new fan hood so my wife can cook and prevent the smoke alarms from activating and from what we were told by the salesperson we bought the best for smoke removal. I thought the way we were told, the darn thing would be sucking the pots off the stove at low speed. NOT. The house still fills with smoke (great cooking...huh?) The new hood sounds like a 747 at full tilt, but surely won't do the job. I believe actually seeing one in action will tell the story. I believe that you could set up with a full house fan and get the job done. I'm sure there are people n here that have experienced the effects of fume removal. Just my nickel. Keep us posted.
Bill

my wife use the smoke alarm as cooking timer.........

If you wish to design this professionally I would recommend the ACGIH ventilation design manuals available here https://www.acgih.org/forms/store/Comme ... on=Feature

Ken

If you wish to design this professionally I would recommend the ACGIH ventilation design manuals available here https://www.acgih.org/forms/store/Comme ... on=Feature

Ken

Sorry for the dupe. I received an error the first time I tried to submit the reply.

motox wrote:my wife use the smoke alarm as cooking timer.........

Thanks to all for the great info.

I guess the thing I was missing was the difference in power. As I understand it, the higher the motor power, the better the machine can overcome a given static pressure drop to deliver a desired flow rate.

The $470 Dayton 10" roof ventilator is 0.125 hp, while the $80 Harbor Freight 8" fan is 0.22 hp (165 W), almost twice the power. The Harbor Freight should have more oomph, but is 1/6 the price. Is it a matter of quality and durability?

Holy crap that roof ventilator is expensive! I got this bad boy on Amazon for like $120. 2300 CFM. Can't beat it.






They also sell a 10" version. They're no joke, they move a lot of air. My mistake on the price. $106 shipped on Amazon.

The only thing I didn't like about it was the direction of airflow. It used to push the air out of the "bellmouth", when it should be pulling it in from that side, so I opened it up and reversed the motor orientation, which sucked because of the thick orange powder coating.

Oscar, this is the bigger version of the one I saw listed from Harbor Freight for $80.

If you want to compare fans, you need to try and come up with a cfm per amp at a given static pressure. Looking at the harbor freight unit they don't give you the static pressure rating so tough to do. Fantec has all kinds of fans and they give you all the numbers. They make a great product but you will pay more. All the radon pumps you see are fantec. They run 24/7 for years and years.

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BillE.Dee wrote: Re: What is recommended CFM for fume extraction?

Do you have a tight house? What goes out has to come in from somewhere. To get maximum efficiency from a fan there must be an adequate inlet air supply from somewhere. The "747" is just the poor fan working its butt off with no air to feed it. Try opening a window or door a bit and see what happens.

BillE.Dee wrote:as far as I know, all of the cfm talk is only talk. Purchased a new fan hood so my wife can cook and prevent the smoke alarms from activating and from what we were told by the salesperson we bought the best for smoke removal. I thought the way we were told, the darn thing would be sucking the pots off the stove at low speed. NOT. The house still fills with smoke (great cooking...huh?) The new hood sounds like a 747 at full tilt, but surely won't do the job. I believe actually seeing one in action will tell the story. I believe that you could set up with a full house fan and get the job done. I'm sure there are people n here that have experienced the effects of fume removal. Just my nickel. Keep us posted.
Bill

Re: What is recommended CFM for fume extraction?[/quote] Do you have a tight house? What goes out has to come in from somewhere. To get maximum efficiency from a fan there must be an adequate inlet air supply from somewhere. The "747" is just the poor fan working its butt off with no air to feed it. Try opening a window or door a bit and see what happens.

It's not a house. It's a metal shop, 30' L x 18' W x 15' H. The welding bench is at one end of the shop, and we're going to vent the fan to the outside. Venting is necessary to pass inspection. I'm still struggling to find a number SOMEWHERE that says - for MIG welding, you need a minimum of X cfm to remove fumes. I've seen $2000 units made for the purpose that move ~100 cfm, and the $80 Harbor Freight model that is 1590 cfm. I understand that the cfm ratings are with no static pressure drop, but there's a HUGE difference in these numbers. I don't want a fan so powerful that it sucks away the shielding gas, but it needs to be powerful enough to do the job.

Bill[/quote]
What goes out has to come in from somewhere. To get maximum efficiency from a fan there must be an adequate inlet air supply from somewhere. The "747" is just the poor fan working its butt off with no air to feed it. Try opening a window or door a bit and see what happens.[/quote]
You make a good point. Even though it's a big room, I wonder if we need an inlet vent from the main part of the space (70' x 50').

You need to find a copy of the mechanical code book for your jurisdiction. It will have all the tables to get a hard number.

My main shop area is 30' X 36" X 15'6" high. At one end I have a 12" and a 20" 3 blade regular barn fan mounted fairly high on the wall. At the other end are 2 inlet ducts 14" X 25" bringing air down from the attic to 6' high from the floor. On Amazon the 12" fan is rated 6-800 cfm, the 20" around 3300 cfm. With all doors closed the 2 inlet ducts will supply enough air for the 12" fan ONLY. Turn on the 20" fan and it howls and the 12' X 14' oh door whistles and shakes and all kinds of crap sucks out of the wood stove! The 12" fan was usually enough to exhaust with stick or flux core welding unless going at it steady then crack the door a few inches and use the 20" as needed. The both fans are mainly there for spray painting vehicles, dusty stuff and drying out after washing down equipment etc. The ducts from the attic provide some pre heat to the inlet air instead of direct in from the outside in winter. Gets darn cold in Canada sometimes! A plus is that with a dark brown steel roof on sunny days in spring and fall we could actually heat the shop by drawing down warm attic air with the 12" fan. Doesn't give you any formula but it worked well for me. A variable speed fan might be the answer as you could fine tune it once your set up. That way you could go with a fan a bit bigger to make sure you have enough and always back off a bit if needed.

An erv. Will exhaust and intake an equal amount. While mitigating temp issues on the intake air.

tedrussell1955 wrote:It's not a house. It's a metal shop, 30' L x 18' W x 15' H. The welding bench is at one end of the shop, and we're going to vent the fan to the outside. Venting is necessary to pass inspection. I'm still struggling to find a number SOMEWHERE that says - for MIG welding, you need a minimum of X cfm to remove fumes. I've seen $2000 units made for the purpose that move ~100 cfm, and the $80 Harbor Freight model that is 1590 cfm. I understand that the cfm ratings are with no static pressure drop, but there's a HUGE difference in these numbers. I don't want a fan so powerful that it sucks away the shielding gas, but it needs to be powerful enough to do the job.

There's lots of sites with air exchange rates per hr for various applications. A site I just checked recommended 6 - 10 air exchanges per hr for welding shops. At a median of 8 air changes per hr = 30' X 18' X 15'H = 8100 cf X 8 changes per hr = 64,800 cf/hr divide by 60 min = 1080 cfm fan.

tedrussell1955 wrote:It's not a house. It's a metal shop, 30' L x 18' W x 15' H. The welding bench is at one end of the shop, and we're going to vent the fan to the outside. Venting is necessary to pass inspection. I'm still struggling to find a number SOMEWHERE that says - for MIG welding, you need a minimum of X cfm to remove fumes. I've seen $2000 units made for the purpose that move ~100 cfm, and the $80 Harbor Freight model that is 1590 cfm. I understand that the cfm ratings are with no static pressure drop, but there's a HUGE difference in these numbers. I don't want a fan so powerful that it sucks away the shielding gas, but it needs to be powerful enough to do the job.

There's lots of sites with air exchange rates per hr for various applications. A site I just checked recommended 6 - 10 air exchanges per hr for welding shops. At a median of 8 air changes per hr = 30' X 18' X 15'H = 8100 cf X 8 changes per hr = 64,800 cf/hr divide by 60 min = 1080 cfm fan.