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Forge Hoods Explained

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A funnel is a device that takes a large opening and reduces it down, usually so you can pour liquids into a small mouth or opening in a container.

You choose the funnel to match the task at hand, that is to say a small funnel in the kitchen for moving spices from a large (quart) container into the little bitty containers that fit in the cabinets. The large end of the funnel if maybe 2 inches in diameter and the small ind about the size of a wooden pencil or less. If you pour too fast you over fill the funnel and get a mess all over the counter.

If you are putting oil in the motor of the car, you have a funnel with a 4 to 5 inch opening on the large end and about the size of a quarter on the small end. Again if you pour too fast you will overfill the funnel and oil will run out over the top and make a mess.

There are even larger funnels for farm use such as moving grain from a container to a transport belt and into a larger bin, again sized for the job at hand.

Think of a hood as an inverted funnel, channeling the smoke toward the chimney. First off you need the small end to be of the appropriate size to handle the smoke produced by the fire. 10 inch diameter is suggested with 12 inch being better. Anything less will not handle the volume of smoke to move it up and out of the work area. The large end of the hood should be as close to the smoke source as is practical. If the hood is placed up next to the ceiling, it has little chance of collecting the smoke, which will fill the room in a short time. If the hood is placed as close to the fire and smoke source as practical the smoke is encouraged to go into the hood and with a proper size chimney, to go up and out.

The hood should be such that the working access to the fire is as small as possible and still have good access to the fire. This keeps excess room air entering the hood to a minimum, again think of the hood on the ceiling example. All this will create draft which will suck smoke up and out and will draw in room air through the small(ish) working access opening reducing any opportunity for smoke to escape.

Bends in the chimney reduce the draft so use two 45* bends rather than a 90* bend. The taller the chimney the more draft. The exact configuration and he exact dimensions for YOUR hood in YOUR shop is something you will need to play with and adjust as needed. Guidelines are just that, a place to start.

 

The original 55 forge with hood was a fire in the center of the bottom of the drum with a window cut as an working access port. The window was 2 inches above the bottom of the drum, 4 inches below the 2nd ring, and 16 inches wide with an arched top to the window. All fire and metal being heated was done INSIDE the drum. This worked well for most projects. A small hole can be cut in the back of the drum for pass through projects.  The chimney was another 55 gallon drum placed on top of the first one making a 24 inch diameter hood and a 24 inch diameter chimney. Yes it was a big chimney and was therefore called the 55 Forge with a Supercharger. It would suck the fleas off the dogs back when he went past. (well almost anyway).

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You were quite clear about "Your mileage may vary" so I'll try and ask this in a generic way--

How badly do horizontal/near horizontal portions of the run diminish draft in real world applications?  I have an existing hole in the sidewall of the barn from an old wood stove that I'd prefer to expand and use rather than going through the huge hassle of going straight through the 12' high ceiling of the shop and then penetrating the roof of the barn (bad place to access).  That would require a horizontal or sloped run that would probably add up to about 50% of the total flu length.  I can experiment (and probably will) but if horizontals significantly choke off draw, it might be just throwing away time and money to do that.

Since I've not run a chimney before, it'd be nice to hear an opinion/example of whether 4 to 6 feet of horizontal is a losing proposition or little problem at all.

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The problem comes in getting The pipe above the peek of the roof if the existing hole is not in the center of the end wall. A gust coming over the peek and down the pipe blows... 

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Yes, that's a cost I'll have to pay in my case.  The enclosed shop is to one side of the barn (12 foot sidewalls) and the barn peak sits at about 20 feet so there is no way to get a chimney in a good position without a loong run. Everything will be a compromise.  Doesn't help me that the side I am dealing with is the primarily windward side of the barn in a fairly breezy area either.  

Mostly curious about how significantly a horizontal (or probably more like 15 degree slope from horizontal) tends to diminish draw in a chimney.  Obviously it's better to avoid such things but the question has come up generically several times without much of an answer.  Curious if anyone had good or bad results when they did their own "experiment" that incorporated a significant horizontal run.

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Horizontal run? The youtube videos by Joey Van der Steeg. I have read time and time again on this forum that horizontal runs slow the draft. If that were actually the case, why does his horizontal draft flue pipe operate so well? I mean, you really need to watch the video. I guess what I'm saying to the folks who say no 90° bends; you are obviously more than wrong.I'm not, in anyway picking a fight. When you watch his (many many videos) you will now be perplexed cuz you read somewhere no 90° bends and now you be....."oh, i guess what I read is incorrect"

Joey will be more than happy to point out that the law of physics is the same anywhere on this globe.

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Being cheap; as is known in these parts.... The side wall of my smithy is covered by the hail damaged tin roof of a colleague at work and when I put it on I arranged it so the large hole in the roof for his woodstove chimney was right where I wanted to put the coal forge...So I just have a 10" diameter 10' long section of spiral seamed ductwork going up at around  75 deg .  The end rests on my forge table.

The shop's construction being mostly wood free: steel walls, purlins, roofing, etc. I don't have much to be fussy about and the weather here is generally one where more ventilation is a good thing!

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SReynolds:

Here are some laws of physics for you:

  1. Unless you have a mechanical assist (fan) for your hood exhaust duct in the horizontal, the horizontal run only contributes duct friction to the passage of that exhaust.  Duct friction slows down the passage of forge exhaust fumes in your exhaust and reduces the impact of the "chimney effect" forces that drive the exhaust in the vent.  The fume exhaust speed is directly proportional to the amount of fumes that are captured at the face of the hood, so slower velocity in the stack means less combustion products capture.
  2. Empirical testing has determined that 90 degree elbows can be modeled as equivalent lengths of straight duct for the purposes of determining the duct friction in your system.  A good rule of thumb for a long radius R/D greater than or equal to 1.5), round duct elbow is that it is equal to approximately 10' of straight duct length.  Gored and short radius elbows are worse (see ACCA Friction rate reference charts).
  3. Duct friction is your enemy for effective venting of a forge hood.  The chimney effect only gives limited force to pull the fumes from a hood.  The more resistance to that force you have, the less effective your hood will be.  It is highly recommended to reduce the horizontal runs, abrupt changes in diameter and offset elbows in your stack.
  4. Another thing that can lead to duct friction is rough duct interiors (i.e. flexible duct).  Try to duct with smooth, hard duct surfaces whenever possible.

Joey Van Der Steeg is a talented smith, but that does not make him a good hood designer.  In the latest videos I've seen, his shop appears to be quite tall and pretty large.  He may not have significant concerns about hood capture efficiency.  I also haven't seen any illustration of the entire duct run from his overhead hood.  In one video I saw he had a (what appears to be overly small) flex duct connected to the top of his hood with a very nominal horizontal run, that appeared to turn up afterwards.  It is possible that this duct connects to a fume exhaust fan just upstream.  I haven't seen a video which covers this completely, perhaps you can tell us which specific video he has that illustrates his exhaust ducting and indicates how well it works.

Actually I just viewed one of his videos where he discusses his quite large overhead hood (October 29, 2011).  He specifically mentions that he didn't design it, that it does not always capture well because the duct leading from it is too small, and that he will be adding a fan for improved mechanical exhaust!  OF course you can design a suboptimal stack if you add mechanical exhaust.

Bottom line is that horizontal runs are not helpful, other than to relocate the building penetration, as required.  Provided you have a sufficient diameter exhaust duct, and you have enough vertical exhaust run to use barometric pressure to drive the exhaust (or use a vent fan), some horizontal ducting is certainly acceptable, but should be minimized.  How much horizontal run and/or duct elbows are reasonable depends on a lot of factors.  These include the outdoor temperature your vertical stack experiences, the relative size of your hood, the height and diameter of your vertical stack and the characteristics of your exhaust fan, if any.

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I don't see a mechanical fan in any of his. Possible there had been one. I don't know of one. I'd have to ash him. He has horizontal run and then a vertical run. Just pipe.

I DO have a mechanical fan and mine won't work (draw) well at all. It is totally suboptimal. He suggested a horizontal  extension to place the flue pipe CLOSER to the fire pot. So I made one. Have yet to install that.if it does work, all the other suggestions I have received over the years will go out the window such as; open a window. Raise the height of the chimney. Remove the rain cap. Build a larger capacity chimney. Not one suggestion to move the flue pipe closer to the forge fire. It may be just that simple....................

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In my opinion a side draft hood should always be as close to the coals as possible. A good design feature is also an inclined  "cinder shelf" to catch same before they enter chimney. Velocities in a well designed stack can easily entrain cinders and ash which can coat your stack or start a fire.

With a properly sized and installed fan, your setup should be effective.  Rain cap is a little obstructive,  but fan could take care of that if done correctly. Can't tell type or stack configuration from the photos though. Does ceramic liner go all the way up, or is brick fascia the stack/chimney? 

Are there more than one opening into said chimney? If so the upper one should probably be closed down a bit.  

This design should be able to be tuned to work well even  without a fan.

I don't see anywhere to put a fan in this design.  Where is it?

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The top pic has a light switch and ladder.  The fan is inside chimney about 7' up from floor where the ladder is. Around the corner a bit from that light (fan motor) switch.

Only one opening. Right at the firepot.  I don't believe the flue tile is all the way up the chimney. But I don't know. The opening in the top at the bird screen is rather large so I guess not.

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If it doesn't, it should (otherwise it is just a big fume collector that conveniently dumps right at head height).  Just that the collector attachment to the stack for the metal hood should be variable so it can be balanced to favor inlet at the sidedraft hood.

Not sure what kind of fan you have wholly interior to the chimney.  Most motors I'm aware of don't like that kind of heat.  I'd try adjusting your sidewall inlet closer to the fire and possibly making it smaller.  A small local opening will suck in fumes at a high velocity and pull the flame towards it, once the stack is drawing properly.  They typically aren't self starting though.  ON the other hand you shouldn't need the typical lit bundle of newsprint to start the draw provided the fan actually works.  I assume you have tested that it is pulling exhaust?

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Yep. That fan pulls in a decent 'mount of draft. The flames bend into it somewhat when on. But the tile flue pipe is so high and far from the fire pot only half the smoke and flame makes the bend. The flyash and smoke goes straight up and swirls about inside that large hood. At head level.

The hood is 100% decorative.  It serves only to catch the fly ash and deposit it onto my head, neck and shoulders.  Thus I wear a large (4") brim hat and have installed a fly ash catcher direct above the flue tile in the form of a sheet metal shroud.  It functions perfectly.  No more fly ash. But still smoke at times when green coal is coaking or a student dumps coal into the coke fire etc. Normally there is minimal smoke when I'm working alone. But you want zero especially when shop is clised up in cool weather. 

Yes that seven inch flue tile is sort of a nozzle effect. I see that in other designs where the flames pull into a small opening and into a large box or large flue pipe. This must be same as the chimney is quite large .

 

The fan motor isn't inside the flue. It sets outboard of flue  on a shelf enclosed. 

Thanks for all the help. Will install the extension after the holidays.  The historic village is currently closed and I don't work there in January /February. Good time to experiment further. 

Picture Joey drew me for the design of extension.  The extension I fabricated.  And the students inside shop with the recently installed fly ash hood.

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I bought this hopper/ stand combo recently. My plans are to turn it into my new forge by putting a 3/8" sheet top on the stand and put in a firepot.

I have been trying to decide what is best way to take advantage of the shape of the hood. I was thinking of flipping it upside down

and mounting the flange to the table top. I am considering putting a 12" x 12" opening on the front.

The hood will take a 12" pipe when finished. My flue pipe will go up 10 feet.

Can anyone weigh in on what may be a good style or shape or amount of opening in the hood sides so I can get to the firepot?

Sorry about the bad picture...

Thanks guys

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Maybe this will simplify my question. I'm trying to decide which opening may be better on my forge project

Or, hopefully someone else has a much better idea!

It will be located in a drafty environment.

Thanks again guys!

 

 

project.jpg

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Since you will be doing mainly large scroll work for railings it had better be the wide one; oh wait; since you will be doing mainly bottle openers and knives the small opening will work better for you.  Or why not make the front hinged with the smaller section built in and then you can run it either way depending on what you are doing?

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My set up starts with an 8" pipe with a slight bend, goes to a 90 degree and reduces down to a 6" pipe. (It's a 3-foot length of double wall metalbestos which passes through my shed wall.) It expands to an 8"  90-degree elbow and extends up to a 4-foot pipe which is 3 feet over the peak of the shed.

Crazy right! But it works because I built an enclosure around my tiny rivet forge and made a small opening. Less cool ambient air mixes with the hot air for the coals and I get a fantastic draft.

 

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Edited by mutant
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On 1/24/2018 at 11:14 AM, ThomasPowers said:

Since you will be doing mainly large scroll work for railings it had better be the wide one; oh wait; since you will be doing mainly bottle openers and knives the small opening will work better for you.  Or why not make the front hinged with the smaller section built in and then you can run it either way depending on what you are doing?

I like that idea! Thank you Thomas!

On 1/24/2018 at 11:45 AM, mutant said:

My set up starts with an 8" pipe with a slight bend, goes to a 90 degree and reduces down to a 6" pipe. (It's a 3-foot length of double wall metalbestos which passes through my shed wall.) It expands to an 8"  90-degree elbow and extends up to a 4-foot pipe which is 3 feet over the peak of the shed.

Crazy right! But it works because I built an enclosure around my tiny rivet forge and made a small opening. Less cool ambient air mixes with the hot air for the coals and I get a fantastic draft.

 

That is a nice set up! I see your logic!

 

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My hood (2/3rds of a 55 drum) hangs from the rafters on chains and goes to 10" pip going straight up. I have sheet steel inside the hood that I can slide up or down for what I need to do. Once the forge is going it has great draw and if needed I can slide the sheet steel up and have full access all around the forge.  It also makes it simple to move the forge for cleanup and maintenance. I really like that hopper as a good idea and think it will work great with 12" pipe. 

 

 

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Naw that's an older picture from October 2016. Don't have any recent ones on my phone. :)   My shop is up and down on somewhat cleaned up and complete wreck. Think this spring it will get a major overhaul. 

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Greetings all, 

         Lots of ways to skin a cat.  My main Forge hood is an old recycled free standing fire place I got out of the junk,  It has served me well for years. The pipe is 12 inches and goes straight through the roof.I can remove some fire bricks for big or long stock. 

 

        Forge on and make beautiful things     

        Jim

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