I need to build a chimney for my new coal forge. Because of the joist spacing and foam insulation in my ceiling, I can't go straight up with my chimney and instead, I would have to go up about 8 feet, then turn 90 deg and head out the wall after about 6 feet of horizontal run. I have read that such a bend would cause poor draft performance.

 

At some of the schools I have attended, they use a mega ventilator (1000's of cfm) mounted inside the pipe just outside the building. All the forges are ganged together and exhaust via this ventilator. I have found these ventilators on Grainger with a CFM range of 750-1400. The concern I have is the max operating temp of these ventilators which is 300 deg F.

 

So, my question is:

 

-what temp is the typical coal forge chimney exhaust gas. Is 300 deg F a sufficient spec for a ventilator? I realize that the forge itself is well above this temp, but with the mixing of ambient air with the heat of the forge, just what is the temp of the mixture as it exits the building?

 

Thanks for your help.

 

Probably 400°F or so. Mix in some additional air to cool it down (i.e. gang in a open pipe).

Can you angle the pipe at a gentler angle, say 45 deg. to make the flow better than if it exhausts at 90 deg.?

Eric - thanks for your thoughts on temp. Bringing in some cool air may be a good option.

 

Arkie - I would be able to do that but I still would have 180 deg of bending to deal with: 90 deg inside and 90 deg outside to rise the chimney. Do you think the 45 deg approach would still yield an adequate draw?

Two 45's total 90 deg.  Gotta be less restriction or turbulence than two 90's.  Someone else on the forum must have had the same problem and probably will chime in with some suggestions or actual experience.  I know that some wood stove mfg. don't recommend 90 deg. bends if avoidable to insure proper draw where vertical stacks can't be used and sometimes recommend 45's.

With my 12" square flue I can hold my hand inside the top of the chimney when the fire is going good - hence the fresh air dilution of the fire due to the drawing of fresh taking place. My fire and my forge. No "ventilator",  just straight up and out of the shop.

I would suggest it depends on where you place the thermometer. Closer to the intake will be much hotter than closer to the exhaust end of the chimney. 

 

The 45* elbow at the top of the hood and another at the wall exit would be much better than a 90* elbow.  Again 2 of the 45* elbows outside would be better than a 90* elbow. Think flow design for fluids or air. I would be careful of any internal chimney fan due to the conditions in which it would be working. There are ways to work around that problem but why not just make a chimney that works.

 

Consider BP1048 the through the wall chimney.  

That is a very interesting approach to side wall exhaust. Thanks for the link. 

Thanks also to the others who chimed in.

If anybody else has a side wall exhaust, I would appreciate hearing from you to see how well it works.

Thanks

Did you happen to solve the issue? I'm in the middle of trying to fix mine... same issue, 90 degrees straight out the wall with a 5' rise to get to it.

Glenn and I talked on the phone a little last night and I realized there are couple common misconceptions about smoke and stacks. 

First intuitive mistake is thinking smoke is "sucked" up the stack, it's not. Warm air does NOT rise. Cool air sinks and pushes it up. It's exactly the same thing that keeps a boat afloat or soda coming up your straw. The smoke from your fire is warm and has expanded so it's less dense, lighter than the cool air it displaces. The denser air sinks pushing the lighter smoke out of it's way, upwards. 

I know it sounds like more semantics but it's not, it's the reality of how it works and understanding why smoke goes up a stack lets us help it along.

When smoke enters a cold stack it runs into a contained column of dense air and one of two things has to happen to get it moving up, either means the cold air has to go. The hot air can be just so large a volume and hot the surrounding cold air eventually forces it up and out. This is demonstrated by fire places, stoves, forges, etc. that smoke till they start drawing. Make sense?

The other and preferred method is to allow the cold air an easy path out. A smoke shelf in a masonry chimney does this by forcing the warm smoke to one side of the chimney, the cold air column is then pushed to the other side and with no resistance from below flows down into the fire place where it's warmed and joins the smoke. Once the entire chimney is full of smoke the air pressure outside the fireplace keeps it moving up and out. Make sense?

That brings us to smoke management in a horizontal run. Clearing the cold air column is harder because there isn't much if any fall to get that initial exchange moving. Sloping the "horizontal" works okay but there's a better way. The T I described earlier is industry standard, you can buy Ts with the little flap valve that prevents cold drafts when the furnace or stove isn't burning.

How the T works is by providing a clear path for the cold air column in the vertical run to fall out. The smoke having a path of little resistance flows but there is cold air present and they segregate, cold on the bottom of the horizontal run smoke on top. When it gets to the T it flows up on the close side of the stack and the cold air is displaced to the other side. Same action a smoke shelf initiates. The open T has ambient pressure on the bottom arm so there's no differential to make cold air want to rise. Once the stack is warmed virtually ZERO air enters the bottom of the T and the little counter balanced flap closes. Once the stack is warmed the smoke only has one place it CAN go, up and out. Make sense?

I don't know why but smoke shelves don't seem to work in stacks, I'm sure there's a reason but I never looked into it. Once I knew how to make my wood stove draw well I was good. 

I wish I could explain this more concisely but I'm a talky guy and terse isn't really my regular writing style. I'll try to be less windy if you guys have questions.

Frosty The Lucky.

Frosty, thanks for the reply and the information. I've never thought about air movements like this before, but it does make a lot of sense, even if its completely backwards from how we learn about it in school.

With the "T", would I face the T opening downwards to allow the air to drop? I know that may sound like a dumb question but Im very much a hands on learner. Doing my best with all the text and pictures Im reading, but my brain doesnt really work well that way.

Also, what about adding a draw out fan at the junction point in the wall?

The letter T has a leg pointing downwards and two horizontal arms on top. Yes? 

Letting smoke stacks work more easily the horizontal run of stove pipe connects to the T's LEG and it's ARMS point up and down outside the wall. The upwards pointing ARM connects to the stack running above the roof. The downwards pointing ARM is the cold air relief and may or may not have the flapper valve. 

Is that more clear?

If you add a booster fan do NOT use the T as described! The fan WILL blow smoke out both arms. All you want is an elbow, the cold air column can't stop the fan driven smoke or air.

Frosty The Lucky.

Thanks Frosty! That was very clear.

I'll be doing some measuring this evening, then getting to work on it this weekend.

After reviewing everything from the other post (and looking around my shop) I realized I still have quite a bit of work ahead of me and that my current smoke stack will not function as intended without a serious overhaul .

I've no clue as to the temp of the exhaust gasses. But today after forging for about four hours, the hood, and flue were surprizingly cool. While warm, I could hold my bare hand there indefinately.  But it's a rather cool day today.

The bottom section of my woodstove flue is stainless and has achieved a deep purple temper colour after about 2 years of winter use.  I don't even try to touch it!

The flue on my coal forge is a nice hand warmer just a couple of feet up from the bottom.  

Funny that most people seem to expect the opposite.

After initially reading this thread a few weeks ago, I started keeping a mental record of activity and flue temps at my forge.

My forge is coal/coke fueled, bottom draft. Chimney arrangement is a stainless steel 'super-sucker' side draft feeding a galvanized 10 inch HVAC duct going vertical 20 feet. Several years ago I installed a BBQ thermometer about 14 inches above the inlet. I made a SWAG that thermometer accuracy is +/_ 50 F. When working small stock (up to 5/8 inch) indication is between 100 and 150 F. For forge welding with me cranking the blower steadily I can hit around 400 F indicated.