Trouble with overhead forge hood

[Morten311]

Hello everyone.

I am seeking some help with my forge hood. it's an overhead hood with the following dimensions : bottom = 840mm, top = 330mm, depth = 700mm. smokeStack is 200mm in diameter and about 5.9 meters tall sticking about 2-3 meters above the tip of my roof and it has two 90(ish) degree bends. the hood is 540mm above my forge. 

The problem is that the hood is not drawing very well and i am aware that side drafts are better but i thought that now that i have more space i could use an overhead hood with great success, but no, it does not work at all. not when it's cold nor when it's hot.

if someone could help me, that would be great.

 

thanks

[Latticino]

Stack diameter optimally should be larger (10 or 12 inch is recommended).  90 degree bends are a major friction source and will both reduce flow and lead to problems with draft initiation.  Is there also a substantial horizontal run of vent duct?  How about the transition between the hood and the stack? If it an abrupt connection that can also cause trouble with friction.

When you light your forge, do you ensure hot exhaust is flowing up the stack by burning something directly under the stack inlet to start the draw?  You have to get rid of that plug of cold air before the vent will work.

Also, you need to have new, clean air entering your shop to makeup for the exhaust.  Do you have a vent or window opening?  Any other sources of powered exhaust in your shop?

I'm not a fan of power assisted exhaust for forges, but that is an alternative if you can't solve your issue in another fashion.  You need to be careful to keep the fan motor out of the exhaust stream.

[George N. M.]

It may depend on how large a fire you have in your forge versus the size of the hood and chimney.  There has to be enough heat coming off the fire so that the gases/smoke keep rising until they exit the top of the chimney.  If they cool too much they will stop rising and plug the chimney with stagnant smoke.  If this is the case you may have to replace your chimney with a smaller diameter one or start working with a larger fire.

Try building a larger wood fire on your forge and see how that draws.  It may because it is putting out a larger volume of hot gases.

Also, do you have sides on your hood?  If so, the stream of hot gases may be drawing in cool air as it rises between the forge and the hood which will dilute and cool the gas stream.  If that is the case, covering some or all of the sides will minimize mixing  may keep the gas stream hot enough to rise up the chimney.  Free hanging hoods work with large forges where there is a lot of hot gas rising but not so well with small units.

All the things Latticino mentioned are important considerations too.  I was assuming a straight vertical chimney with no bends in it.

"By hammer and hand all arts do stand."

[Irondragon Forge ClayWorks]

Pictures of your setup will help. If your hood is like the one I installed above the forge, it will never draw a strong enough draft to keep all of the smoke out of the shop. I had to add extra pipe and a hood closer to the forge.

[Frosty]

200mm. is less than 8" George I think his stack should be at least 250mm.  

Making the stack turn corners is a bad thing, it almost always disrupts draw. 

Unless your hood is close to the fire it's not likely to draw well either. 

We need pictures of your set up.

Frosty The Lucky.

[Morten311]

so here are the pictures of the stack and the hood. it is fairly big but i designed it that way because of future projects. the T-bend on the outside is for rainwater to pass through so it doesn't enter my forge ( past problem ) and it is so low to the ground it should not have an impact on the smoke. I have to point out that this size of smoke stack has worked before in my previous workshop, but i used the super sucker at that point. however the super sucker had to be quite close to the firepot restricting the sizes of my work pieces, which is the reason for the overhead hood. And the new workshop is quite big so there should be enough air to replace the exhaust. The new workshop is 18x9x3.6 meters

[Glenn]

Extend 3 side panels of the hood to within a foot or so of the forge. Extend the front panel down so you can see the back of the forge from where you are standing.  This will encourage the smoke to go up to the hood and then up the chimney.  200 mm is 7.87 inches which is a bit on the small size for forge chimneys.  10 inch (254 mm) or 12 inch (304 mm) is suggested. The height of the chimney outside is working in your favor and should provide a good draft.

Be sure to block off the bottom of the T on the chimney outside.  You can open it for cleaning etc.

[ThomasPowers]

Put those panels Glenn suggested on hinges so you can flip them up out of the way when you need the space.  Also is the bottom of the rain T open?  If so you are losing some of your draft.  It doesn't take much of a hole to let the water out, you should be able to close off at least 90% of the bottom opening.  Perhaps a large funnel?

[Morten311]

Hi 

I have been using my forge with some changes made to the hood and smokestack. I put on the sidepanels as some of you suggested and i have reduced the opening in the bottom of my smokestack so the hole is about 80 mm instead of 200 mm. My hood still does not draw very well until the fire has been burning for an hour or two and if i put a little too much fresh coal on the fire ( a handful extra ) it smokes up my shop. Would it help if i reduced the height of my hood by 50% but keeping the width and depth intact, therefore decreasing the max volume of smoke that can be gathered in the hood?

[ThomasPowers]

Is there still an open end on the ground side of the T?  If so it's messing up the draw.  If it's just for rain a capped end with a few drain holes should work for rain.

[Morten311]

It is mostly closed off execpt for a small hole in the bottom

 

[Morten311]

so, would it help if i cut the hood in half in height? you know, so its only half as tall or would it impact the performance in a negative way?

everything else that you guys suggested has been apllied to the hood and it made a minor difference. if there is anything else then i would likelytry that as well.

[Irondragon Forge ClayWorks]

Moving the hood down closer to the fire would help more.

[ThomasPowers]

Decreasing the size of the opening will improve the draw; lowering it is one method of doing so.

[Morten311]

if it gets too low there is not going to be enough room for bigger workpieces. that is the whole reason for the overhead hood., but i have moved it a bit closer than on the pictures. from where i´m standing the front edge of the hood lines up with the back of the forge.

my one concern is, maybe the hood has too much volume for traping the smoke instead of getting it out? could that be another issue?

[Deimos]

The top plate of your chimney, is it flat or does is have another shape?

[Morten311]

 it looks like this

[Deimos]

And on the inside, is it flat above that mesh piece? or does it follow the shape of the outside profile?

[Morten311]

it´s flat just under the mesh

[Glenn]

Crush up and burn 2-4 sheets of newspaper before you start your fire. This will preheat the chimney and start it to drawing.  Cut a strip of cardboard box 2-3 inches wide and 24 inches long, and roll it into a cylinder.  Put a crushed up sheet of newspaper in the bottom of the fire put and then the roll of cardboard on top of that.  Then put a double hand full of your fuel on top of the cardboard.  Another way to to put small sticks of kindling on top of the cardboard, get a fire going and then add your fuel a double hand full at a time.  This should get a good fire going and the heat and smoke should be drawn up the chimney.

Keep a hole about 1 inch diameter in the  top of the fuel for fire to escape and burn the smoke.

Each forge and each fire is different. You will have to play with your fire at your location using your fuel to get the right combination.

What is the diameter of your smoke stack?  How long and what size is the horizontal run from the fire to the smoke stack ?

[ThomasPowers]

You know people have made hoods where the pipe slides up into another pipe.  With counter weights you can adjust it to the height needed for the project at hand.

[bluerooster]

Where the pipe goes through the wall, use two 45 degree bends as opposed to one 90 degree bend. Do same on the outside.  Cap off the bottom of the outside flue, drill a 2mm hole in the cap to let water out.  My hood is a 55 gallon drum, attached to the forge, with an 8 inch flue running straight up through the roof.  I still get a little smoke from time to time with green coal at start up, and when the wind is not in my favor.  My flue is small at 8 inches.