I'm in the process of setting up my shop. Everything I've read talks about how hard it is to get a good draft to exhaust coal smoke. I've got a working powered vent hood that I salvaged. Can I use it & if so how hight above & behind my tuyere would you mount it? Anyone ever done this? Help please!
the key to any chimney that has a good draw is a little thing called a smoke shelf. located about 2 feet up past the opening it is kind of an inclined plane that pinches down the smoke (air flow) then abruptly stops,( chimney again at full size ) this forms a venturi effect that makes the chimney draw like magic. have fun, mark
Should the smoke shelf be on the front, or the back?
My 2 cents:
1.Google: count rumford2.It's my opinion that the smoke shelf is needed to prevent downdrafts. If the chimney is heating up some and it isn't getting a downdraft from above, you don't need one.
Most guys that I know don't use one, although some do have them in side draft hoods that they have made.
The most common successful solution that I know of is the have a straight, vertical chimney on 10 inches MINIMUM ID. The top of the chimney needed to be higher than 2 feet above the peak of the roof that it emerges from. A simple horizontal flat cap on top, about 4 or so inches above the opening keeps the rain out, for the most part. The hood over the tuyere needs to be far enough above the forge table that it does not obstruct the blacksmith frome seeing or manipulating the fire and the work. I'd think that 18 inches or so would be a minimum. My own forge's hood only has the volume about 3 5 gallon pails. It works fine to contain even the start-up smoke if I start the fire correctly. But I have seen hoods that have 5 or 10 times that volume that can't contain the start-up smoke if the draft is poor (due to bends in the chimney, not long enough chimney, etc.. Once the fire is going and the chimney has heated up by 50 °F or so above ambient, the chimney will draw pretty well, if designed as above, but the first few moments of the fire may not create a good draft. Some people stuff a lit sheet of newspaper up the flue as they light forge up to start the draft. Knowing HOW to start the fire can also limit the amount of smoke that is released in starting the fire. There shouldn't be any smoke in the shop if its all done properly. For instance, carefully sorting all the green coal out of yesterday's coke will give you a much cleaner start.
I know several people who were sold on side draft forges that had smoke shelves for a while, but have since switched to conventional overhead hoods, which don't have soke shelves and are happier campers.
Powered hoods: If you are considering a powered hood, make sure that the fan motor is outside of the pipe, otherwise, it will probably overheat and fail.
Pete Stanaitis
Thanks for the answers. Pete, you had a good point about the motor location & now I need to check mine. Does anybody know anyone who has used a powered hood? I'm not sure I want to be the first.
Google "draft inducer" it will get things moving in a cold chimney to take care of the start up smoke and then you can shut it off. A downside of fan based ventilation is if the power goes out your shop fills with products of combustion. A properly sized & built chimney works with no fuss 99% of the time.
Andrew
I agree with EISmith. Natural draft is the way to go. I do have a friend who used a powered exhaust extractor, although I wouldn't quite call it a "powered vent hood". In his system he uses a side draft "hood" that is an opening that is about 18" square at the left side of the forge. This "tube" leads to a fan blade on a bearing and the fan blade is belt driven to a motor outside the tube. This whole tunnel goes through the wall and into a chimney made of 55 gallon drums with their ends cut out. He burns coke exclusively, so he does not have much smoke upon start-up anyway. He has had to replace the fan bearings once or twice over many years of operation and I am don't know about belt replacement intervals.
even if you get a draft going in a powered hood, the fan blades are still in the way if you shut the motor off once the forge is hot. The blades may take up half the space in the pipe or more and this is not a good thing for creating the draft you need.
Pete Stanaitis
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EIsmith wrote:
Interesting comments. I agree. One could consider a venturi concept that has the high pressure small volume of air 'fired' up the chimney from a side angle. No fan needed or it is external.
Once running well with heated air - the side flow can be turned off.
This is after all just another impedance matching circuit. Low impedance at the burner having a wide mouth. High impedance at the pipe out the roof. Transition impedance to match the two - something like a funnel but not as steep as a pouring one.
Mart> I agree with EISmith. Natural draft is the way to go.
Thanks everyone. Info has been most helpful. I have a salvaged stainless stove Ventahood that I was going to suspend over my first forge so that 1) I could use a smaller dia. pipe out the building & reduce its height since its exiting the gable-end wall of 20' tall-ridge on shop bldg. and 2) create some additional air movement through that room in our Texas heat. It seems like the easy way around any draft issues, too. Yes - I had planned on leaving it on while I work as the amp draw doesn't seem that high. I'll install it over the next several months & let everyone know how or if it works. Thanks!
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