Building a Coal Forge Part 2 - the Air Supply

by Albin Drzewianowski

OK, we are halfway there. We have roughly a bowl shaped depression to hold the burning coal. In the bottom of the depression is a tuyere, which allows us to blow air into the fire with out the coal falling down into tuyere. The air can be introduced from the side or from the bottom. A side draft tuyere or side draft forge (not to be confused with a "side draft flue" which is used to remove smoke from the forge) is the older system and is preferred if you are using charcoal. The negative aspect of a side draft is that the air nozzle tends to burn up. Traditionally, this problem was alleviated by having a nozzle within a water jacket to cool the air nozzle and keep it from burning up. For a hobbyist, the solution is to have an easily replaceable nozzle, probably made from stainless steel. The traditional American coal forge is bottom draft. Since the air enters at the bottom and heat travels up, you don't have the same heat issues as in a side draft tuyere.

What are we going to use to deliver the air? You have two basic options: a manual system: hand cranked blower or bellows; or an electric powered blower. A common question is: Can I use an air compressor? The answer really is no. We are looking for volume, not pressure. I suppose, that somewhere there are some smiths using an air compressor for their coal forges, but it really is not a practical solution.

In my opinion, especially for beginners, nothing beats a hand cranked blower or a bellows. You will burn up less steel, consume less coal, and really learn fire management with a manual air supply. Unfortunately, these are not easily obtained by the beginner. It is possible to build a bellows; the plans are available in a number of books. It is basic woodworking. The only really difficult part is applying the leather (or naugahide) and the trick there is to make a paper pattern first and cut the leather based on the paper pattern. The down side of the bellows is that it takes up a lot of floor space. The one in the Historic Forge at the Farm Museum is about 4' by 6', that is 24 square feet of floor space lost. Colonial Williamsburg and Furnace Town have their bellows mounted up in the rafters to save space at floor level. This has the disadvantage that there is a long tube with bends to deliver the air to the forge. The longer the tube and the more bends there are, the more loss of efficiency in air delivery. The Historic forge has the optimum system, the nozzle comes straight out of the bellows into the tuyere, very short run, no bends.

A hand cranked blower is the best option (for the beginner, IMHO). The problem is that they are hard to come by. It seemed to me that somewhere in the last year or so, I had seen/heard of a company again making hand cranked blowers with aluminum housings, instead of the traditional cast iron, but I could not find the reference. So, you are stuck with locating a used blower. There are different size hand crank blowers. Initially I had a small one that was originally made for a rivet forge and I found it undersized for my table-sized forge. I was able to trade up to a Champion 400 size and that works very well.

When looking at one of these hand-cranked blowers, you want one where the handle turns smoothly. Once up to speed and you stop cranking, the handle should go around for a couple of revolutions before coming to a stop. Also, it should be quiet, mine is quite noisy (but then I am wearing earplugs when I work, so noise is not so much of an issue for me). You may find an old one that is frozen up, i.e. the crank does not turn. This may be because of congealed grease and dirt; and it can be made operable by liberal use of solvent to dissolve the grease. However, if the gears are broken or damaged, it is almost impossible to get replacement gears or parts.

You have many options when it comes to an electric air supply. There are many air sources that you can scrounge up: hair dryer, vacuum cleaner, blower from a forced hot air furnace, squirrel cage blowers. Use your imagination. There are various blowers available through mechanical and farm supply companies. Once you have the blower, you need to be able to control the air blast. The best way to do this is with an air gate. This gives you very exact control on the amount of air going into the fire. Air gates are available from wood worker supply companies, as they are used in sawdust collection systems. Many have used rheostats to speed up and slow down the blower's motor. For the most part these do not work as well as an air gate. When you change the speed on a rheostat, it takes 10s of seconds for the fan speed to change. With an air gate the change is immediate. If you do decide to go the rheostat route, try to get one intended for a ceiling fan. It will be more heavy duty than the ones that are used with light fixtures. Also be advised that some motors do not do well with rheostats. The major blacksmith supply companies sell blower assemblies but they tend to be on the expensive side.

Now, regardless of whether your air supply is manual or electric, you need to connect the air supply to the tuyere. I have found the best solution is the aluminum flexible vent pipe as used for clothes dryers (sold in hardware stores). It is available in 3" and 4" diameters. It comes all squished together accordion style and you just pull it out to the length that you need. The plastic dryer vent tubing could be used, but you need to watch out for heat close to the tuyere. You might find yourself melting it. So have a good long pipe come out of the tuyere and then connect the plastic vent pipe to that. The air supply could be connected using steel pipe, but that would be a lot more work. As with the bellows the fewer bends and the fewer acute angles the better.

Finally, you can incorporate the best of both worlds. You can have as forge that has BOTH a hand crank and an electric powered blower. The air supply to the tuyere has a "Y" with a butterfly valve that allows the air to come from one of two sources. This way you can use the electric blower as you are getting your fire started, but then switch to the hand cranked blower for greater control while forging.

-- RebStaup - 25 Jan 2008