Tip: Airflow and Forging in a Confined Space

[Robert Simmons]

I have read a few comments from people who, after feeling dizzy, decided to purchase a CO detector for their enclosed shop and were surprised at the high numbers that the detector would have even though their garage door or shop door were wide open. At the same time my CO detector remains at 0 without my garage door open but only a roof vent fan on.

Now I cant stress enough the importance of a CO detector. Whether working with coal or propane CO buildup can send you to the morgue. Your body detects that you need to breathe by the amount of CO2 in your body but CO will push out oxygen WITHOUT triggering a gasping response. This is why pooled gasses like Argon from welding and CO have killed people that haven't even thought there was a problem.

Back in the dark ages of my youth, over 17 years ago, I was a volunteer firefighter and I learned something important about air and heat and thought it would be interesting to pass it on. One large open window or door does not make airflow. To achieve airflow you must have an entry point and an exit point and something moving the air. So two doors open on opposite sides of the shop wont do it either because there is no driver. One fan in one of two openings in the shop is only marginally better because the air blown in can pool and back pressure and go back out the same door it came in. So setting a box fan in the door of your shop will do little more than keep you cool.

The optimum setup is a large, powerful fan mounted high (on the roof if possible) and an opening on the other side of the shop as LOW as possible. If you have a door to your shop, install a roof fan that moves 1000cuft of air or more per minute opposite the shop door then make a little quarter door by cutting your shop door near the bottom and hinging it so the quarter door can be open when the main door is closed. A better alternative is keep all doors closed and put in a gable vent as close to the floor as possible. Gable vents made for the roof of the house usually have bug screens and can even be louvred for automatic open and closing.

What happens when the fan is on is that there will be negative pressure in the shop generated by the air being pulled out of the shop so the atmospheric pressure in the shop will drop. The air must come in from the other side so it is sucked in strongly. Now you have air flow. You place the vent near the floor so gasses that are heavier than air can flow back out the vent (such as argon) and cant pool high enough to be dangerous (think of a glass with a hole cut in it so it cant overflow and drown what is inside of it.)

This configuration will keep your CO meter pegged at 0 while you have all doors closed. You can even run the AC just make sure the AC isnt the only inlet for air in the shop. If you open the garage door much at all, the low pressure zone will be compromised and the air flow will again become chaotic and ineffective at getting rid of the nasties.

Incidentally it works for the car as well, isntead of all windows open, open only the front driver's side and rear passenger side and note the massive difference in air flow. The dynamics are the same for the house and so on.

Anyway, just thought Id offer that tip.

[Robert Simmons]

One more tip I thought of. If you have a wall AC unit, it may or may not deliver enough air to compensate for what is being drawn out yet you may not want a normal open gable vent because warm air might be drawn in in the summer. One solution to minimize the warm air is to get a gable vent that opens depending upon air pressure in the shop. There is no motor driving such a vent, it opens via the low pressure in the shop and closes by gravity. The louvred vent will supply only the extra air that you need to supplement the AC. A friend of mine has his weld and fab shop set up this way and it stays comfy and fume free.

[dkunkler]

The optimum setup is a large, powerful fan mounted high (on the roof if possible) and an opening on the other side of the shop as LOW as possible. If you have a door to your shop, install a roof fan that moves 10cuft of air or more per minute opposite the shop door then make a little quarter door by cutting your shop door near the bottom and hinging it so the quarter door can be open when the main door is closed.

Good advice Robert. Did you mean 1000 CFM instead of 10 CFM ?

[Robert Simmons]

Good advice Robert. Did you mean 1000 CFM instead of 10 CFM ?

Yes. Silly typo that I didnt catch, thanks. I have one of the following in the roof of my garage:

http://www.homedepot.com/h_d1/N-5yc1vZ1xhf/R-100073799/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

[ThomasPowers]

We use swamp coolers out here and they push a LOT of air in---usually have at least a 1/3 hp continuous duty motor doing nothing but spinning the squirrel cage. Set one up to exit around the working area and you are guaranteed to have good airflow *away* from you.

(Unlike airconditioners they require an open vent to the outside somewhere in the building.)

When you get single digit humidities a swamp cooler can sure put a chill on you working in 95 deg heat!

[Robert Simmons]

We use swamp coolers out here and they push a LOT of air in---usually have at least a 1/3 hp continuous duty motor doing nothing but spinning the squirrel cage. Set one up to exit around the working area and you are guaranteed to have good airflow *away* from you.

(Unlike airconditioners they require an open vent to the outside somewhere in the building.)

When you get single digit humidities a swamp cooler can sure put a chill on you working in 95 deg heat!

That would invert the situation and the shop would have a higher ambient presure than the surrounding air. As a result, the air will seek any crack it can to escape. It might go under a toor, through a small hole, you name it. It wont, however form a cohesive flow to the vent unless the shop is absolutely air tight elsewhere. Dont get me wrong, it can work but it wont be as good as negative flow. Instead I would supplement the swamp cooler with a roof fan that can pull more air than the cooler can put out and a secondary louvred vent to make up the difference. Having the fan be able to pull more means that no matter what order they are turned on, the shop will quickly reach negative pressure.

[fciron]

When I get negative pressure in the shop it messes with the draft on my coal forge. Any suggestions?

[Mainely,Bob]

When I get negative pressure in the shop it messes with the draft on my coal forge. Any suggestions?

Turn things around and pressurize the shop,that should help the draft.
If you were to draw air in from above it would tend to force the heat down and then push any gases hanging around the floor out of the vent(s) down low.Sounds like a good thing during the northern winters.
Any reason to not do it that way?

[fciron]

Turn things around and pressurize the shop,that should help the draft.
If you were to draw air in from above it would tend to force the heat down and then push any gases hanging around the floor out of the vent(s) down low.Sounds like a good thing during the northern winters.
Any reason to not do it that way?

Bob, that's what I do. What I really need to do is extend the chimney.

I assume your suggestion to force the hot air down and out was not addressed to my coal forge.

One reason to draw things out the top, even in winter, is to get the hot combustion products out of the shop as quickly as possible. If you draw cold, heavy air in from above it will probably drop straight down, leaving all that hot CO hanging around to get breathed in.

[Mainely,Bob]

Bob, that's what I do. What I really need to do is extend the chimney.

I assume your suggestion to force the hot air down and out was not addressed to my coal forge.

One reason to draw things out the top, even in winter, is to get the hot combustion products out of the shop as quickly as possible. If you draw cold, heavy air in from above it will probably drop straight down, leaving all that hot CO hanging around to get breathed in.

I meant the heated air that rises up after leaving the heater will be pushed back down off the ceiling and then out the lower vents.Wouldn`t that take the CO with it as it left?
Also the pressure will help the draft and that will push more of the CO/smoke up the chimney too I would think,especially during start up.
Am I missing something?

[Robert Simmons]

I meant the heated air that rises up after leaving the heater will be pushed back down off the ceiling and then out the lower vents.Wouldn`t that take the CO with it as it left?
Also the pressure will help the draft and that will push more of the CO/smoke up the chimney too I would think,especially during start up.
Am I missing something?

Think of it like a glass of water with vegetable oil on top and trying to stuff the veg oil down and out a hole in the glass while the glass is being constantly filled with water to balance what is lost. Because it is less dense than water it will try to float and its boyancy will overcome all but the most powerful forces. CO would do the same as well as the smoke. If you had a low vent and pressurized from above, you would have a very smoky, uninhabitable shop. As you try to force all that down, the flow will be chaotic and mix the air. Some will get pushed out.

If you have a coal forge and chimney, the best bet is to overpressurize the shop from an inlet in-wall fan near the ground and let the pressure assist the draft. At that point make sure other methods of escape for that air are not available except perhaps a small vent in the roof or near the roof. You want the chimney to be the exit point for the major volume but it cant be the only exit point or bad gasses will pool at the top. Putting in a smaller top vent than can handle the whole airflow means some air gets pushed out the top and some out the chimney.

Although optimal air flow is low pressure shop with low air inlet, the next best thing is overpressured shop with chimney and small vent as outlet and all other outlets securely plugged.

[Mainely,Bob]

Think of it like a glass of water with vegetable oil on top and trying to stuff the veg oil down and out a hole in the glass while the glass is being constantly filled with water to balance what is lost. Because it is less dense than water it will try to float and its boyancy will overcome all but the most powerful forces. CO would do the same as well as the smoke. If you had a low vent and pressurized from above, you would have a very smoky, uninhabitable shop. As you try to force all that down, the flow will be chaotic and mix the air. Some will get pushed out.

If you have a coal forge and chimney, the best bet is to overpressurize the shop from an inlet in-wall fan near the ground and let the pressure assist the draft. At that point make sure other methods of escape for that air are not available except perhaps a small vent in the roof or near the roof. You want the chimney to be the exit point for the major volume but it cant be the only exit point or bad gasses will pool at the top. Putting in a smaller top vent than can handle the whole airflow means some air gets pushed out the top and some out the chimney.

Although optimal air flow is low pressure shop with low air inlet, the next best thing is overpressured shop with chimney and small vent as outlet and all other outlets securely plugged.

Got it!You want the upper vent to be balanced with the chimney and the pressure source/intake(the fan) to be down low.
Thank for taking the time to run it down for me.

[fciron]

Got it!You want the upper vent to be balanced with the chimney and the pressure source/intake(the fan) to be down low.
Thank for taking the time to run it down for me.

Thinking on this today and it seems to me that the negative pressure approach is really only applicable to free-standing gas forges. If you've got the forge under an exhaust hood or you have one of those deluxe shops with gas heat and hot water then you are in a similar situation to my coal forge. Make sure that the combustion products are exiting correctly.

[pkrankow]

Exiting from the top only works when the exhaust gasses are significantly hotter than the surrounding air. Once the exhaust is cooled to ambient it is heavier than air.

I agree that having air forced in or out and having additional controllable venting is the ticket. Having a fan blow into a garage-like space through the one very large door tends to cause a swirl that is essentially stagnant instead of removing contaminated air and replacing with fresh air.

Phil