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I Forge Iron

Small smithy ventilation


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All very basic and unscientific I am afraid.

I bought the largest (600mm / 24") Axial Plate fan available from MachineMart that had a variable speed controller.

The difference in price between the largest and smallest (300mm / 12") at the time was from 100 pounds to 200 pounds. The extra 100 pounds I paid covered a lot of calculations. Probably about double that now twenty years later, but still cheap insurance and comfort. The blockwork buttress/chimney and sound baffled air inlet I had built probably cost four or five times the cost of the actual fan so I felt no need to skimp on power.

I think it is capable of shifting the total volume of building every couple of minutes when its running flat out. It is rarely on much more than tick-over but good to be able to give it full blast when the smokey old diesel fork lift is around.

Unless your building meets PassivHaus standards I understand the positive pressure will find its own way out...line of least resistance is the hearth chimney, an open sky light or an open door(s).

The advantage of the skylight or chimney is that any sound is directed up rather than across the fence to your neighbours....provided you don't have a domed/reflecting cowl on the flue and the skylight opens to a vertical position.

Alan

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12 hours ago, Alan Evans said:

I am not a ventilation professional.

I read that the Fire Brigade find it is quicker to use positive pressure ventilation to clear a building of smoke and fumes than to use the same size fan as an extractor.

This is on the basis that every cubic metre of air moved by the fan is a cubic metre of fresh air into the building. Used as an extractor, the fan is moving mainly smoke to start with, but then every subsequent cubic metre is a mix of fresh air and smoke, so it is much less efficient.

I also found that positive pressure ventilation has three other advantages for the forge.

If you have a flue above your hearth, positive pressure assists the flow of smoke and fumes up the chimney. An extractor fan in the wall will have the opposite effect.

It has the effect of blowing any sound back into the forge which may help reduce any noise problem for your neighbours...think of the way sound carries downwind rather than upwind...

I set mine up so it blows fresh air directly onto us working on the hammer or presses.

Alan

 

Well, I am a ventilation professional, for what that is worth.  The positive pressure system has both benefits and drawbacks. The former you have described well.  Note however, that overall facility smoke or fume purge is not the target use of a blacksmith shop ventilation system.  If you get contaminated that far you presumably have other problems.  The more standard exhaust system (which incidentally is also used for atrium smoke control, a similar high ceiling space) is designed to extract fumes before they reach the breathing zone.  The air replacement is still with clean air (provided your air inlets are not set adjacent to your exhaust), as all the air exhausted must be replaced by something.  The key advantage for an exhaust system is that you can target the location of the air being exhausted.  This isn't as much an issue if your only building opening is the hood above your forge (where positive pressure will work as you have described), but if you have a gas forge (as the OP indicated) you may not have a hood at all.  In that case I still recommend that the smithy be exhausted from a high point (the roof fan selected is a good option) and ventilation openings be brought in at the code required high and low position.  Note that some engineers even have high and low fume extraction systems (common for car maintenance shops for instance), but in this case your fumes should all be hot and rise up to the smithy peak.

Please note that in my original post I indicated that the exhaust fan system can interfere with the chimney effect in a hooded forge and needs to be used with care primarily as a supplement for heat control.  In this case the building openings need to be large enough to ensure that the makeup air preferentially enters via these openings (low static loss across the openings) and the stack should be drawing strongly before the exhaust is turned on.

Also note that the 10 ACH is a good rule of thumb for the minimum to provide adequate ventilation.  It is not a caveat and the code minimums for your area as regards combustion equipment ratings and occupied space ventilation requirements should still be followed.  I personally have on the order of 30 ACH in my shop, utilize a passive turbine vent year round  and keep a CO monitor in my working area.  For a sidewall exhaust there is minimal cost in adding excess ventilation, and plenty of side benefit.

As far as being concerned about the quantity of air for a fan, they are sized with two parameters: airflow and external static pressure (i.e. 1,000 CFM at 0.25 inches WG).  These are available in a fan chart or table for a specified fan.  For a sidewall fan the static losses are minimal, and provided you have large unobstructed openings for makeup air (or can make them by opening windows and doors) you should be fine with something on the order of 0.15 to 0.3 inches of WG external static at the fan.

 

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11 hours ago, Latticino said:

Well, I am a ventilation professional, for what that is worth. 

:) It is because you had posted your qualification earlier that I prefaced my contribution so that the OP could treat mine with the suspicion it deserved!

My experience of the awful system that was installed at my college forge...metres of horizontal flue ducting, collecting from a number of hearths and in-flue extractor fans which seemed to fail frequently.  Led me to to look for the simplest most naturally aspirated system. In-flue extractor fans seem to be in the worst place...amongst the concentration of any dust and corrosive fumes...for long term service.

My first hearth, a standard Alcosa side blast in a  3 x 4 metre (9' x 12') wooden building initially used the Ø10" flue it came with. When I installed power hammers I converted the hearth to a bottom blast with a long air-slotted base plate which gave me a 600mm (2') heat to make efficient use of the power. The flue could not cope, so I made up a 300mm (12") square flue which doubled the area. At one time when I had neighbour difficulties and needed the door closed I was working with air-fed face shields from the compressor...horrendous.

The new forge building was block built with 100mm (4") blown insulation and initially I thought would be large enough not to need assisted ventilation. But then I built a 1220mm (4') gas furnace which would not go under the hearth canopy. I rapidly installed an opening roof light and the positive pressure fan and we still opened the doors when we were not actually forging.

We then found it was much better to cover the furnace with a sheet of steel for weather protection and leave it outside and carry the workpieces in to the hammer.

Even with the big furnace outside, with large sections like 80mm and 100mm (3" and 4")  the radiant heat coming off a 1220mm (4')  length means the blast of fresh air blowing over us down the line of the hammers and presses is most welcome.

Alan

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Alan,  Actually I was quite impressed with the design and analysis you have come up with.  I am also not a huge fan of in flue extractors (as you can see if you read my posts).  I've been forced to employ them for some large boiler room systems I've designed and typically find them finicky, difficult to tune properly, prone to failure and sensitive to control drift.  I try to avoid them whenever I can, and that is when they are expensive industrial systems with unitary computer controls.

It appears that you also understand the different challenges for a smith as regards solid fuel fired forges (which typically include some sort of capture hood) and gas forges (which often do not).  With a properly designed hood and stack I'm definitely in favor of the natural chimney exhaust and directed makeup air that you advocate (as noted in my fist response to the OP).  However with forges that don't include decent capture hoods I think that powered exhaust is the best choice, at least for the budget hobby smiths that appear to be the bulk of the members online here.  Of course you can still direct your makeup air to advantage.  In my old glass studio I built an insulated enclosure that confined my furnace and glory hole with reasonably small door openings to access the working ports.  That entire enclosure was exhausted and the makeup air for the shop came in from a window and cooler parts of the facility.  The only studio I've been in that was more comfortable is the one down at Corning that is air conditioned.  Proper location of makeup air openings can make your life a lot easier, and a fan to direct that air where it serves best certainly helps as well.

I'm flirting with the idea of designing a combination door system and slot extractor hood for a typical gas forge.  Just a pipe dream right now, but I think it could be an interesting option. 

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I have a door on a wire rope and pulleys to a boat trailer winch on the big furnace. It even has provision for a secondary horizontal sliding door suspended below. The secondary door was used for a few years but the projects for the last decade or so have been better served by a line of firebricks across the bed just inside the door which leave an appropriate gap for the workpiece. I tend to double up the shorter lengths so I can use one piece as a handle while I forge the other so I have a water tank below and some sacking which I wrap around the piece and keep pouring water over the sacking which drains back down into the tank.

The reason for that description is that a friend working up in Birmingham, Ian Moran once told me of a place he worked which used an air curtain to keep the handle pieces cool. It was a length of scaffold pipe set with a row of air blast holes pointing up which took some of the air from the burner supply. He said it was very effective.

Alan Dawson used my big furnace and burner design as the basis for his, and he improved on it by enclosing it in a steel jacket which had a chimney out to atmosphere. Can't quite remember whether it had a cowl overhanging the door to catch any heat from there. 

My burner design is an amalgam of one Tom Joyce from Santa Fe sent me and the college notes borrowed from the glass studio technician at Farnham Art School. The glass studio glory holes were the basis for my first furnace used for heating 2" square aluminium lengths. I went onto heavier construction when I wanted to heat heavier metal.

Alan

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Thank you to everyone for the input. You've helped me to conceptualize ventilation needs and considerations much better. At this point I'm planning on going with a powered exhaust fan at the roof peak. I'll monitor CO levels closely with the idea that I can add additional ventilation (beyond open windows/doors) at a lower level if needed in the future.

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On 8/16/2016 at 1:17 AM, Lou L said:

Guys, if it helps at all, let it be known that I've taken your input to heart....

Hopefully the OP continues to read; he bailed in quite a huff (unnecessarily).  Doesn't require youth to be data spoiled.

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Ive always wanted to ask this question.  I paint cars in a autobody shop and our make up air system for the paint booth will bring the temp inside up to 80f even on the coldest minnesota day. Needless to say thats quite the burner.  And all those fumes and exhaust are pushed into the booth and around me. The way I see it is the paint booth IS the chimney for that burner. Why dont I die from CO poisoning. Is it because of the  amount of air being feed through. Im not sure but it has to exchange the all the air in the booth every 5 minutes. Or does my charcoal respirator mask filter co.

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There are two conventional types of natural gas fired Makeup air units: direct fired and indirect fired.  The former has the combustion operating right inside the breathable airstream while the latter has a heat exchanger where the essentially sealed combustion occurs outside the airstream and the heat gets transferred in through the metal walls of said exchanger.  Needless to say the indirect unit is less efficient as well as being more costly and prone to failure.  Because of this, many industrial facilities use a direct fired makeup air unit, but that is not the case in all locations.  There are some drawbacks to the direct fired units, including adding the combustion products to the ventilation air as well as the associated water vapor (believe it or not some companies are more concerned with condensation on the walls of their insufficiently insulated buildings then their employee's potential breathing of combustion byproducts).

Be that as it may, you are correct that the vast quantity of air being introduced by the direct fired makeup air unit far overshadows the relatively minor amount of combustion byproducts added to the space.  You also have (or should have) additional general exhaust in your facility as well as specific vehicle exhaust (that connects right up to the vehicle exhaust pipe). 

As a reference the current International Mechanical Code requires a ventilation system that exhausts and makes up enough air so that the open side of a paint spray booth has a capture velocity of 100 ft/min.  This means if the open side of your booth is 12' x 10' Hi you need 12,000 CFM of exhaust and makeup air during operation.  If it is 0 degrees outside and the makeup air needs to enter the building at a minimum of 65 deg. F for comfort, this means the direct fired unit needs to add 842.4 MBH to the air.  That sounds pretty substantial, but if you consider that an indirect system is likely only 80% efficient, that system will need an input of 1,053 MBH to deliver the same amount of heated air (with the associated ongoing cost).

I'm not going to do the combustion calculations regarding the CO byproducts of burning 842 MBH of natural gas, but have been assured by manufacturers that the dilution factor takes care of it.  Needless to say it is different if you have a (3) burner forge in a 20 x 20 shop with each burner outputting 70 MBH (like a T-rex at 30 psi say) and limited ventilation...

Probably wish you hadn't asked this question now :P

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  • 5 weeks later...

This is a great topic and I'm glad I've read all of it. I have been forging outside for the entirety but have wanted to move my operations inside as many would, but haven't due to the nature of my available structure (detached 1400sq ft mechanics garage). 

 I use Coal & Charcoal and am currently building a single atmospheric burner (Riel style) propane forge the size of a 30 lb. L.P. tank so ALL this awesome info is being absorbed into my noggin right now. 

 I have the ability and space to build a new structure but it will need to be small-ish comparatively @ ~ 15' x 20' x 8' max. It will also have to be viewed as a non-permanent structure since I am maxed out based on my lot size.  I think I could safely erect a post framed, structure with mostly closed walls and "complete" it as time goes by. The township allows the use of those concrete pyramid foundation blocks, so it would be viewed as a shed by them (non permanent) but look like a small closed in pavilion. I can say from experience that those blocks are pretty good for a properly braced structure. I have an 8' x 10' roofed clubhouse/deer blind sitting 6' in the air on 4 of those right now and it's pretty darned sturdy.

 

    Todd

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