Ventilation - How to. I need some good help / advice

[Avadon]

Alan, can you describe what is going on in the pictures? I guess I don't understand what air is going where. I couldn't find a "straw bale building" thread. can you link me to it?

 

Do you just mean that the intake to the shop is not pulled in by an exhaust van or by the heat movement of the forge, but rather fresh is forced in via a fan?

[Frosty]

Positive pressure ventilation is exactly what it says. Positive means more pressure inside so it gets blown out your exhaust vents.

 

Frosty The Lucky.

[Avadon]

Positive pressure ventilation is exactly what it says. Positive means more pressure inside so it gets blown out your exhaust vents.

 

Frosty The Lucky.

 

and you create positive pressure by just using fans to bring it in? Then does that mean you don't use exhaust fans? Or does that mean you use fans to bring it in and expel it?

[Alan Evans]

I could not work out how to quote the actual straw bale thread on the iPhone last night, herewith the links.

 

The photos in my earlier post (#25 this thread) show the hearth flue going out through the wall beside the fan which blows air into the shop. The others show the brickwork "chimney breast" outside the shop which forms the baffled air duct. The inlet is at ground level. Sound does not carry so well either up-wind or down-hill. The Stainless Steel flue passes out of the wall and continues up vertically beyond the eaves. The flue was built first and the block work air duct was a later addition which enclosed it. There is no connection between the flue and the ventilation air apart from inside the shop obviously, when some of the ventilation air will find its way up the flue.

 

The thread on straw building I referred to is 6 down the list on this "Building, Designing a Shop" forum and is entitled "Straw bale, anybody?" The last paragraph of my post #23 outlines the logic of the principle.

 

 

This is quoted from my post #19…

 

 

"I built my forge in a village and used mass for sound attenuation. The key to vibration transmission reduction is isolation of the hammer inertia blocks with an air gap from the floor and walls. Otherwise the structure is just a cubic sounding board. I used concrete block cavity walls with a 100mm (4") gap back filled with rockwool fibre insulation. The ceiling had two layers of plaster board and a acoustic wool board above with fibre cement sheets above an air gap above that.

 

After vibration transmission comes the air born noise and the clue is in the phrase...sound can only get out if the air can. Because we blacksmiths work with furnaces and fires (my big gas furnace is outside and we carry the bars in closing the door when forging) being air tight is a bit of a problem. I use a positive pressure ventilation fan and have a skylight. Passive ridge ventilation units would have been better, my brother in law had those installed in his service garage. Sound doesn't tend to come down, (think jet engine test beds) so if you can exhaust it up a chimney your neighbours will hear it less in normal atmospheric conditions."

 

and this from post #23...

 

 

"The skylight vents the heat and its window swings open to vertical so the noise goes up.

 

The positive pressure ventilation inlet is at ground level and comes up through a baffled (for sound) flue inside the blockwork chimney breast that is up the outside of the forge, the fan blows air into the forge and it finds its own way out through the imperfect air tightness of the building, the skylight, the hearth chimney or any open door.

 

I set it up that way because I read that the fire brigade use a big fan which blows clean air into a building as the quickest most efficient way to clear it of smoke. If the fan runs at so many cubic metres a minute you can be sure that all of them are moving clean air in. If you suck out at the same rate you may be sucking out some clean air along with the smoke, logical really."

 

Alan

[Avadon]

Thanks Alan!! That makes a lot of sense and really helped progress my thinking a great deal. Thanks for sharing that. :)

[Alan Evans]

You are welcome!

 

Two plus points to my positive pressure system I forgot to mention:-

 

The fan blows fresh air over the area where we are standing when forging, which is great for personal health and comfort!

 

The positive pressure of course helps push any smoke or fumes coming off the gas furnace or coke hearth up the the flue/smoke hood. An ordinary extractor system of course has the opposite effect and sucks the fumes back out of the flue into the room, or at least slows its progress up the flue.

 

Alan

[BobL]

Very interesting responses. Thanks for the help. Do you know which chart you used. There are so many and most I can't seem to readily understand. Also how many cfm rated motor-fan is best for 6"?

Yeah it gets complex very quickly

 

This is the chart I use.

 

It shows the CFM possible for a given pressure differential along different diameter pipes.

The purple shaded area shows the typical pressures possible with conventional impellers ie 6 to 20" of Water Column (WC).

I modified it so that I could include vacuum cleaner type pressures 50 - 90" of water column.

 

The red dot shows that a 6" siam duct can at most transfer 1000 cfm when the pressure differential between the ends is 6.5" of WC

To generate this pressure you will need at least a 2HP motor and a 12" impeller and 6" diam or more openings all the way through the system.

 

I have just finished testing a 2HP/12" impeller from a generic dust extractor unit that the manufacturer claimed was a 1200 CFM with 8" of WC unit, but it was in stock form <600 cfm and 7" of WC. By simple modifications I was able to make the impeller move 1100 cfm and generate 8.1" of WC. This of course is just the impeller, If significant lengths of ducting are used then the flow rates will drop off pretty quickly.

 

To move 1000 cfm through more than say 10 ft of  6" diam ducting a 3 HP/13" impeller is needed.

[Avadon]

You are welcome!

 

Two plus points to my positive pressure system I forgot to mention:-

 

The fan blows fresh air over the area where we are standing when forging, which is great for personal health and comfort!

 

The positive pressure of course helps push any smoke or fumes coming off the gas furnace or coke hearth up the the flue/smoke hood. An ordinary extractor system of course has the opposite effect and sucks the fumes back out of the flue into the room, or at least slows its progress up the flue.

 

Alan

Sorry for the delay, crazy busy work schedule. Does the fan of incoming air disrupt the path of smoke going up the flue at all?  Is the fan inline or blowing across the front of the forge? Or does it even matter? Sorry, I can't tell orientation very well from your picture.

[Avadon]

Yeah it gets complex very quickly

 

This is the chart I use.

Flowratesclean.jpg

 

It shows the CFM possible for a given pressure differential along different diameter pipes.

The purple shaded area shows the typical pressures possible with conventional impellers ie 6 to 20" of Water Column (WC).

I modified it so that I could include vacuum cleaner type pressures 50 - 90" of water column.

 

The red dot shows that a 6" siam duct can at most transfer 1000 cfm when the pressure differential between the ends is 6.5" of WC

To generate this pressure you will need at least a 2HP motor and a 12" impeller and 6" diam or more openings all the way through the system.

 

I have just finished testing a 2HP/12" impeller from a generic dust extractor unit that the manufacturer claimed was a 1200 CFM with 8" of WC unit, but it was in stock form <600 cfm and 7" of WC. By simple modifications I was able to make the impeller move 1100 cfm and generate 8.1" of WC. This of course is just the impeller, If significant lengths of ducting are used then the flow rates will drop off pretty quickly.

 

To move 1000 cfm through more than say 10 ft of  6" diam ducting a 3 HP/13" impeller is needed.

 

That sounds like a lot of air to have to push if your using a 6" duct. I'm guessing that a 4" duct is more reasonable for point extraction or even small hood? Is that right?

These http://www.ebay.com/itm/350252094762?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 fans only do 449CFM not even close to what you are estimating I would need in a 6" duct. Where Do I find these 3HP/13" impellers?

[BobL]

To that end positive pressure ventilation is more efficient than extraction. It is how the Fire Brigade clear a building of smoke.
 

Positive pressure extraction is used for areas that are already contaminated with stuff like smoke or poisonous gasses but it would be far better if a shop was never left to get that way. A much better way to go is to apply high flow negative pressure at the source. This has been well developed in wood working where the point of dust generation by machines and tools are directly attached to dust extractors. Initially developed as chip collectors the dust extractors have been extended to also capture the more health significant invisible dust (ie smaller than 10 microns) which acts more like a gas than chips.  I have access to several different dust particles counters and have studied this is some detail in dozens of woodshops. Capture of invisible dust from machinery at source requires high flows (1000 cfm) but it is not impossible as it is being done regularly by many woodworkers. Whether these flow rates are what is needed to keep the levels of contaminants down in a metal shop is another matter. 

 

 

That sounds like a lot of air to have to push if your using a 6" duct. I'm guessing that a 4" duct is more reasonable for point extraction or even small hood? Is that right?

These http://www.ebay.com/itm/350252094762?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 fans only do 449CFM not even close to what you are estimating I would need in a 6" duct. Where Do I find these 3HP/13" impellers?

 

It depends what you are doing. If you want to trap gasses and dust at source then the more flow you can get the better.

BTW the maximum a 4" duct can pull with a 2 - 4 HP motor driving a 12 to 16" impeller is about 450 cfm 

 

There are many 2 and 3HP with 12 or 13" impellers in use in woodworking that can pull 1000+ cfm through 6" ducting. In the case of the 2HP systems they usually have 4.5" intakes and 5" outlets so they need to be modified to achieve the 1000 CFM through 6" ducting. My 3HP 13" impeller system has an 8" intake and a 6" outlet so no mod is needed

 

If you just want to vent a shop ie no ducting or are prepared to use large (1 sq ft) cross section ducting maybe square or rectangular,  then something like a squirrel cage fan may be suitable.

I just replaced the 350 CFM squirrel cage fan in my fume hood with a variable speed squirrel cage fan that can draw up to 2600 cfm for short periods. I cannot run it at this rate for too long as it overheats but I can run It indefinitely at 1000 cfm. At 1000 cfm it uses just over half a HP so it is very cheap to run and much quieter than a woodworking dust extractor.

[Avadon]

Positive pressure extraction is used for areas that are already contaminated with stuff like smoke or poisonous gasses but it would be far better if a shop was never left to get that way. A much better way to go is to apply high flow negative pressure at the source. This has been well developed in wood working where the point of dust generation by machines and tools are directly attached to dust extractors. Initially developed as chip collectors the dust extractors have been extended to also capture the more health significant invisible dust (ie smaller than 10 microns) which acts more like a gas than chips.  I have access to several different dust particles counters and have studied this is some detail in dozens of woodshops. Capture of invisible dust from machinery at source requires high flows (1000 cfm) but it is not impossible as it is being done regularly by many woodworkers. Whether these flow rates are what is needed to keep the levels of contaminants down in a metal shop is another matter. 

 

 

 

It depends what you are doing. If you want to trap gasses and dust at source then the more flow you can get the better.

BTW the maximum a 4" duct can pull with a 2 - 4 HP motor driving a 12 to 16" impeller is about 450 cfm 

 

There are many 2 and 3HP with 12 or 13" impellers in use in woodworking that can pull 1000+ cfm through 6" ducting. In the case of the 2HP systems they usually have 4.5" intakes and 5" outlets so they need to be modified to achieve the 1000 CFM through 6" ducting. My 3HP 13" impeller system has an 8" intake and a 6" outlet so no mod is needed

 

If you just want to vent a shop ie no ducting or are prepared to use large (1 sq ft) cross section ducting maybe square or rectangular,  then something like a squirrel cage fan may be suitable.

I just replaced the 350 CFM squirrel cage fan in my fume hood with a variable speed squirrel cage fan that can draw up to 2600 cfm for short periods. I cannot run it at this rate for too long as it overheats but I can run It indefinitely at 1000 cfm. At 1000 cfm it uses just over half a HP so it is very cheap to run and much quieter than a woodworking dust extractor.

 

Now that makes sense why they were all maxed out at 450cfm in the 4" impellers on ebay. Thanks for all this good information.

[Robert Yates]

Squirrel Cage ! Man way over thinking  things High school welding shops do it all the time and it works good small flex tube with a duct work hood and your done !

 

Sam