Sunday, November 01, 2009

On forges, chimneys - and blowers.

I often get direct questions related to specific problems in blacksmithing. I often will turn those responses into blog postings (to share the information, after the work of writing them!)
...I saw your videos on youtube and in the one explaining your shop ''set-up'', I noticed you are using an updraft hood..
I've heard alot about side-drafts and how they do the job way better then the other one.
I'm halfway done with building an updraft for my forge, and I was planning to use a power ventilator or something like it, to help it suck up the smoke.
- Samuel
In case you have not seen it, the reference is to this YouTube segment:
Sam Asks: Does your hood work well? ...what kind of ''fan'' do you use?
I have a chimney set up adapted from an 1880s book on smithing.

There is a roughly 12 x 12 square pipe that extends from the ceiling with a second piece about 11 1/2 x 11 1/2 , maybe four foot long, that fits inside this. The smaller section ends in the more or less chopped off pyramid that is the hood. Thats maybe a foot top to bottom and extends to about 2 x 2 feet on the lower edge. Originally this inner section was counter weighted. The cable eventually eroded in the coal smoke, so now it takes two hands to lift the hood. (The hood piece is not heavy, it just binds if you try to lift it with one hand.) A simple pin through a hole holds it in place.
Now I can actually drop the hood tight on to the top of the forge table. That would pretty much get every last bit of the smoke contained. In actual practice, I lift the hood all the way up to give me about three feet of clearance for cleaning and setting a new fire. Once lit, I drop the hood down to about 18 inches above the forge table. This is quite effective for gathering the smoke during normal forging operations.

The use of a blower to extract smoke is important in this set up.
In the ceiling space above the enclosed forge room, I had installed a large furnace squirrel cage blower. (These prove pretty easy to gather at the dump - at least around here.) That type of blower has two circular inputs - one on either side, and one large output in the middle. The output is roughly 12 x 10 inches. This is fixed to another section of rectangular duct that runs at right angles to the line of the chimney. In my case this piece sticks out through the end wall of the building.
As the blower has two input sides, I boxed in the side opposite to the one attached to the forge hood. This ends in a simple trap door that opens down into the forge room on the ceiling. In warm weather I open this, thus pulling room air out at ceiling height. This not only grabs any smoke the forge hood might miss, it also pulls a lot of the hot air out of the room as well. Needless to say, come winter I need to work with that trap shut.
Exterior View:
On the lower left (partially obscured by the tree) you can see the output vent from the blower. The straight line passive stack shows on the upper right. The wedge shaped piece of metal diverts our heavy snow fall around the base of the chimney.
Interior View:
The retractable hood in the full up position. To the upper right can be seen the open 'hatch' that leads to the blower to extract room air.

Although this set up on its own does certainly extract all the smoke from the forge, I have also extended the line of the forge chimney straight up through the roof using a length of 10 inch diameter stove pipe. This ends in a standard fire place style cap (to keep the rain out). This second vent point actually will actually extract most the smoke during the forging sequence - passively.

I don't know exactly how much air the blower pulls - but it is a HUGE amount. Enough that you can not keep much heat in the shop come winter. This presents is own problems here in Grey County.
Sam Asks : Do all the side-draft (users) exaggerate the weak abilities of the updraft hood?
The problem with any side draft chimney set up is getting it built correctly. There is no doubt that a properly designed side draft will preform like a vacuum cleaner. I have seen some that will actually suck smoke back down towards it and out!
The main thing with a side draft is getting the correct proportions of smoke hole to shaft size. To get that magic draw, the proportions of side opening to chimney square area needs to be in the range of 1:3 to 1:4. (So smaller opening than the size of the chimney) If there is not a large enough difference, the chimney stack will just not draw correctly. The exact details of the construction can effect the potential draw as well (like too much rough mortar left on the inside surfaces when using brick. All that being said, I have seen extremely good results using a straight piece of 10 inch diameter pipe with a brick sized hole cut in side. There is sure to be some relationship between the placement of the inlet opening above the forge table. Most I have seen are located with the bottom edge of the cut out about 4 - 6 inches above the table surface.
A passive chimney set up of any kind is also effected by roof layout, local setting, variations in wind and weather. The exact placement of the chimney stack above the roof line is important, as is the orientation of the roof line to prevailing winds. Placement of near by trees can effect function of a chimney. Rain or dry, hot or cold, direction of wind a given day, all can combine to sometimes render even the best passive draw chimney ineffective.

One thing to consider (??) is that with a side blast, the chimney stack has to sit to one side of the forge fire, and fairly close to it. This may or may not interfere with the placement of a working piece into the fire as desired. Not a problem with smaller objects, or generally for linear ones. May be a consideration with larger 3-D pieces. (This is another reason I set up the hanging hood arrangement - I can work pieces potentially from all four sides of the forge.) Bear in mind that most likely the weight of your chimney stack will bear down on that side of the forge table as well. In the classic 'traditional' forge, the heavy brick construction is not because of the forge itself, but required to bare the weight of the brick chimney above. Of course there are ways to limit that, but now construction is becoming more complex overall. (Balance that against the light sheet metal construction of the galvanized sheet I chose for my set up.)

I think the comments about the weakness of the straight line, passive chimney are fair - when compared to a correctly designed (!) side draw chimney. However, my system is essentially a powered blower extractor. No real comparison in terms of raw volume of air moved between it and even the best designed passive system.
Sam Asks : Does your shop end up full of smoke?

First off, remember that there is only a significant volume of smoke generated during the initial 'coking up' phase of working with a coal fire. I can easily lower that hood close to the top of the forge table, grabbing 95 % of the smoke generated for those first few minutes and the blower ramming it out through the side vent. That extra hatch in the forge room ceiling pulls out a good volume of room air - and any of the remaining smoke with it. After coking, working a fresh fire does not produce that much smoke. At that point I could actually turn off the blower and allow the straight line stack to passively vent the fire. (In actual fact, I just leave the blower on all the time I'm working.)

I had suffered lung damage from working in a living history museum (unnamed for liability issues - but its the large one in Toronto). The health standards there were absolutely Victorian. The management at the time acted as if they thought Ontario Health and Workplace Standards did not apply to them! I quit that position, the longest season and highest paid artisan interpreter job in Ontario, largely over these horrible safety conditions.
The problem there was two fold:
First, the chimney itself had been built like a fire place - NOT correctly like a forge. The chimney actually tapered in smaller for the first three feet or so. This resulted in hot air being piled up as it tried to enter and rise, filling to overflowing with smoke. To make matters worse the proportion of bottom opening to stack cross section was almost exactly backwards. The straight stack section (above the constricting flare) was about 10 x 12 inches. The opening at the bottom was about 16 wide by 18 inches high (so a 2.4 : 1 ratio - against the optimal 1:4!)
Second, the chimney top was :
- located on the east side of a building with prevailing winds from the west
- ended lower than the building ridge line
- a tree on the west side had grown up to almost 1/3 higher than the building peak.
The net effect of all that was that nine days out of ten, the wind blew over the top of the tree, slanted down on to the building ridge, then effectively blew back DOWN into the chimney. This was such a problem (compounded by the management's insistence that it was all due to my lack of skill!) that I ended up contacting over 30 other living history museum blacksmiths from all over North America, I got replies from most of them describing their own forge and chimney layouts for comparison.

Anyway, the short (after the long) of this was that when I established the Wareham Forge as a full time business and constructed my own workshop, I was painfully aware of the dual problems of coal smoke and chimney construction. The use of what is basically a simple powered negative pressure system for my forge is a result of all this.

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