Glenn

Forge CFM - how much is needed?

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From the email:

I have a question about forge motors and CFM. My twenty year old hand made forge used an underpowered bathroom vent fan (50 CFM) and I need to replace it. I have a pancake fan (75 CFM) or another old computer fan that might generate 300 CPM. What do you guys think? I pipe the air from the fan box through 3" ducting that has a damper installed in it and then it transitions into a 2 inch pipe tee and flange that bolts to the the top of an old inverted cut down hot water heater. Really, it's not much more than a rivet forge, but I've fashioned a lot of tools and hardware with it over the years and I'm anxious to get back to using it.

Nett

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Solid fuel requires pressure and some amount of CFM. The 400 Champion was one of the largest hand cranked blowers of its time so it's logical that 250 to 400 cfm is a good range for a normal shop fire. Electric fans running at higher speed may be somewhat smaller in size but should still put out about this much volume.

However, it is more important to have a level of static pressure to push the air through the fire. For this reason, you should use a centrifugal type blower (or a bellows) because both produce pressure - not just high levels of cfm. On the other hand, a typical squirrel cage blower produces little pressure due to its design and is ill-suited to burning solid fuel properly. They are more suited to gas forges where it takes very little pressure to mix the gas and conduct it to the forge body. Some people will tell you how great their squirrel cage blower works with coal but they have likely never used a forge with the proper air supply.

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You need very little CFM for a forge, what you need however is a consistant air pressure. Squirrel cage blowers and the like are not good at operating with a pressure diferential between the input and the output. A type of electric blower that does work well for pressure however is a plain old hair dryer. Most often you will still need to choke down the hair dryer a whole lot still or you'll have way too much air.

I have a 60CFM hydroponics blower which can blow the fire right out in my firepot.

A champion 400 hand cranked blower only puts out like 20 cfm or so UNDER PRESSURE, and that's key. A normal blower that you buy at the store today only lists the free flow CFM, not the CFM under inches of water column

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A typical fan may actually be rated like this.

Static Pressure;
Inches of Water - 0.0 - 0.1 - 0.125 - 0.250 - 0.375
CFM------------- 98 -- 94 --- 93 --- 84 ---- 52

I'm not sure what the equivalent inches of water column a typical forge fire is, but there is considerable static back pressure, thus why on different types of fans you may need a large CFM rating like 200 or 300.

I have not seen any research into what the static pressure of a forge fire of various sizes is however, nor how to determine that effectivly. But this just goes to show that CFM is not the actual number you want to be looking at, it's CFM under pressure. It takes very little CFM for a roaring forge fire, no more than a light breeze really, but it takes a lot to get that amount of air under pressure.

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I read an article in an old Anvil's Ring where a German shop had a huge air compressor driving most of the tools in the shop (like the hammers) and they had a 3" line going along the wall and branching off to each forge, which were all burning coke. They used this compressed air with the pressure choked down considerably to supply each fire.

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Many industrial places use an air compressor to drive forges. What is done is the compressed air is shot into a larger chamber to expand before it goes into the bottom of the forge to provide the right amount of air at the right pressure. This works very well, but with the noise and power consumption associated with an air compressor it's not (usually) that great for a home forge / small shop.

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An air compressor will work but burns a lot of electricity as it compresses the air to 100PSI +/- and then the air is used at the forge at well under 1 PSI.

Plus I would think many small shop air compressors would not put up with required duty cycle. If I remember correctly my "3 HP" (yeah right... Probably actually 1.5 hp) has a duty cycle of 35%, that means the motor running 35% of the time.

It is very easy to measure "inches of water pressure" as literally that is what it is .

Place a 1/4" or 1/8" pipe tape between the blower and the tuyre. Install clear vinyl hose on the pipe tap. Yes it would be better to install the vinyl hose as far form the tuyre as possible.

Pour in a few tablespoons of water in the tube. Operate the forge in normal operating conditions and measure the difference in the two water levels.

For example if the water in the tube pushes up 1" in one tube and down 1" in the other tube that would be 2" differential and hence 2" water pressure.

You could add a few drops of food coloring to make the water levels more visable.

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Older editions of "Machinery's Handbook" contain a section on forging and related information. There is a description of CFM relating to the air requirements of forges and recomendations. I think I have the 22nd edition.Dan:)

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Solid fuel requires pressure and some amount of CFM. The 400 Champion was one of the largest hand cranked blowers of its time so it's logical that 250 to 400 cfm is a good range for a normal shop fire. Electric fans running at higher speed may be somewhat smaller in size but should still put out about this much volume.

However, it is more important to have a level of static pressure to push the air through the fire. For this reason, you should use a centrifugal type blower (or a bellows) because both produce pressure - not just high levels of cfm. On the other hand, a typical squirrel cage blower produces little pressure due to its design and is ill-suited to burning solid fuel properly. They are more suited to gas forges where it takes very little pressure to mix the gas and conduct it to the forge body. Some people will tell you how great their squirrel cage blower works with coal but they have likely never used a forge with the proper air supply.


I hope everyone reads the above. This should be a sticky or chiseled in stone
around here somewhere. This is why when newer smiths ask "where can I find a blower?" the responses always include vacuum cleaners and clothes drier blowers in addition to commercial models. These blowers will provide the pressure necessary to have and maintain a good fire. The conditions in a fire you just started will immediately begin to change as ash, clinker, gooey green coal and coke are formed. Your blower needs to be capable of blowing through 8" of that mess, not just a freshly made fire. You need the pressure provided by a veined (like a vacuum cleaner) or a paddle type fan and you need the excess capacity in that and cfm to make your forge perform.

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Convection Heat Treating (1) must read
(Multiblade forward curve, Type MI fans, Radial blade, paddle wheel type PI fans, Axial blade type F and AX fans)

The most commonly used fan laws in simplified form are:

CFM (cubic feet per minute) varies Directly with RPM
(CFM 1 / CFM 2 = RPM 1 / RPM 2)

SP (static pressure) varies with the SQUARE of the RPM
(SP 1 / SP 2) = (RPM 1 / RPM 2)2

HP (horse power) varies with the CUBE of the RPM
(HP 1 / HP 2 = (RPM 1 / RPM 2)3



Air Flow Vs. Pressure Characteristics
Parallel & Series Operation
Stall of Axial Flow Fans
Basic Fan Laws

(stuffing air as a cooling medium through a packed 1U blade server or as the oxidizing medium through coalcoke have a lot in common :P )


illustration public domain, work of the United States Federal Government under the terms of Title 17, Chapter 1, Section 105 of the US Code.

5177.attach

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I remember a bob patrick demo where he stated you only need 2 ounces of pressure to fire a coal forge. home natural gas lines run at about 4 oz or so into your home also. I have seen 400 blowers with everything from an 8 inch to a 16 inch impeller on them.

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HWooldridge and Steve White (Skunkriv) in their posts of 2/4/08 are right on the money. For serious forge work a blower with adequate pressure is the primary factor. I've seen far too many forges with inadequate blowers.

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I don't know about the HVLP compresser working or not. I would like to add that different type fans have diferent characteristics. I don't remember the ins and outs, but some types only develop a specified CFM at a given static pressure. Others will not move air and will start to cavitate is the pressure is varied too much. Also the suction head will make a big difference if you put some type filter ahead of it to keep it clean.

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It is pretty well known that any forge that can burn iron or create iron sparks is hot enough to forge weld, if all other requirements for a forge weld are in place. Even if that forge has a very small and weak blower such as a small squirrel cage blower. It may not be fast or efficient, but you can say it works. Nevertheless, here is an experience that the Prairie Blacksmith Assn. of Nebraska had a few years ago: We had Tom Ryan of New York as a demonstrator. Tom was trained in England and also had experience working in France. His project was a multi-scrolled sign hanger that was mostly made of 1/2" by 3/4" A36 bar stock. It consisted of several forge welds and Tom did not use flux. The PBA had a Blacksmith Journal designed side draft forge built as a Blacksmith Association of Missouri workshop project, and it was set up for Tom to use. The forge had a squirrel cage blower that BAM had purchased for the project from Granger. It was 175 -200cfm. It did not take Tom long to ask for another forge as the BAM forge was not able to heat the iron to forge welding temperatures fast enough for him. We switched him to a medium sized Champion 400 hand crank railroad forge which worked fine as long as someone was turning the crank at a relatively brisk pace. After the demo I began forge welding without flux also. Since the Blacksmith Journal designed forge was my personal forge, it didn't take me long to purchase a much more powerful radial fan blower which I did from Centaur Forge. Even though I was making good forge welds with the old squirrel cage blower forge, the new blower made a world of difference in the speed and efficiency of my forgings and forge welds. It also allowed the forging and welding of much larger iron. There is my story for whatever it is worth.

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blksmth,
Was the new blower an electric one or hand cranked? Do you see an advantage to the electric with a control knob or do you think a hand crank is so good or better? I am in the process of building my first forge and would not like to do it twice before I get started.

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Once I figured out the airgate, I stopped using crank blowers ( in the home shop). My first use of electric was a shop vac. On/off. I then brought the dayton speed control from the basement shop ( used for dremel etc) out to the smithy. Still on/off but had better control. This lasted till the vac toasted ( I guess it was a year and was a used vac when I started ). I bought another shop vac. Sometimes availability of other blowers is not good. I don't get out much.

I then built the airgate. Scrapped 3" cyl tube. I found the ( current) blower attached from an HVAC fella. From the pic you can see what it is. Probobly 6-7 years ago and it was (blower) was on a wrecked furnace. I run the blower many days for 5-6 hours. Wide open all the time. The airgate gives me the variance I need. Yes I have burned up a few things over the years. Yes that is a Canedy Otto blower in one of the pics. It stood there until I built the gassers. I do a substantial amount of production work sometimes and the airgate gives me the freedom to go to the other side of the shop ( or around the back of the forge to welding station ). When this squirrel cage toasts it is possible that I will just get a replacement motor from the farm store. I really haven't decided. I now have another blower or 2 that could be put before the airgate. They are belt driven and would require a brush motor to hook up the speed control. I have one or 2 of them also.

My point is, the blower I have in the shop functions great. I can either put half dozen tripod legs in the solid fuel or gasser. 5/8 stock. I can burn the tripod legs in the solid fuel and if I'm not careful I'll have them stuck together. I have no clue about the cfms. No I do not weld billets. From time to time I will do some forge welding. The fire you see in the pot is charcoal and the edge of the sack in the lower left of that pic. The plumbing from the blower to the pot is a ( somewhat) copy of Juniors shop. The pot is a Roger Lorrance and shop built from there on back to the blower. I have no wish to do anything except convey information. I make mistakes most days. This is my setup and some of my experience.

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Edited by Ten Hammers

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Steve, why are you apologising? There's no need to.

All you're doing is showing what you have, what you work with, and how you find it works


Good on you for doing it, more should

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Machineries Handbook 12th ed.

Blacksmith forges require air pressures varying from 1-1/2 to 6 ounces per square inch. Small forges with the blower close to them are adequately supplied with 1-1/2 ounce of pressure. If the blower is some distance away and a long discharge pipe with many bends leads to the forge, even though the latter be small, it may be necessary to carry 3 ounces pressure or more, to overcome the friction in the air ducts. Large forges usually require 3 to 6 ounces of pressure.
This was written in 1945 based on forge sizes and forging standards at that time. The point is made apparent by listing the size of the discharge main at the blower for 1 to 10 forges in use, each forge with a tuyere sizes from 1 to 4 inches in diameter. For instance, a set up using 4 forges each with a 1 inch diameter tuyere would require a 3 inch discharge main at the blower. The 3 in duct has just over 7 inches in area, so 10-1/2 to 42 ounces of air pressure would be required.

This does NOT suggest what size forge is appropriate for that time period, how large a fire us used, or how large a piece of stock is being forged. I would suggest that in 1945 they were not forging s hooks from 1/4 inch round stock for sell at a ren fair, but that it was a working forge and required to produce what was needed for the client.

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Machineries Handbook 12th ed.

This was written in 1945 based on forge sizes and forging standards at that time. The point is made apparent by listing the size of the discharge main at the blower for 1 to 10 forges in use, each forge with a tuyere sizes from 1 to 4 inches in diameter. For instance, a set up using 4 forges each with a 1 inch diameter tuyere would require a 3 inch discharge main at the blower. The 3 in duct has just over 7 inches in area, so 10-1/2 to 42 ounces of air pressure would be required.


Now THAT is really good info. Some engineer sat down and figured out what actually worked then consigned that data to MH. I have both a 22nd and 18th edition MH but they had removed the references to forging by then. I'll have to try and find one of the older publications.

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Paul B, as you can see every blacksmith has a system that works for them and that they are usually pleased as punch with it. In my case I replaced the squirrel cage with a radial electric blower. The blower has a 10 inch diameter blade and the motor is only 1/5 HP. The blower is the PB150 from Centaur Forge. I have several crank blowers, but early in my blacksmithing I began using an electric blower and that is what I like. I can keep more than one iron in the fire w/out having to have the crank cranked. I don't have to crank like crazy to heat big iron. I have both a speed control which came with the new electric blower and a sliding gate. I usually set the speed control to the lowest speed I think I'll use for awhile, and in order to have the lower noise level. I then control the blower with the sliding gate valve. I made my own sliding gate on my main brick forge, but purchased one for my portable forge. The purchased 3" sliding gate cost me around $10 and if time is worth anything, it is worth purchasing. The blower I purchased uses a 3450 RPM motor so is noisier than I like, but still reasonable. I talked to Centaur about building a blower with a less noisy 1750 RPM motor and they said they had so many shops that need the high speed blower that they were not going to change. I now have 2 forges and have the PB150 on both of them. I will be building a 3rd forge and already have another PB150 for it. There are several ways to get a radial blower without spending the bucks for a PB150. One is a hand crank blower which to me would be better than a squirrel cage electric blower. Another way is to use a vacuum cleaner blower (as has been mentioned on IForgeIron), although I have not tried this. Make a wood or steel radial blower, either hand crank or electric (shouldn't be too hard and there are plans available). Use an old clothes dryer blower (IForgeIron Blueprint). Convert an old hand crank blower to electric although I'm not sure if the paddles can withstand a 3450 RPM motor, but you could try a 1750 rpm motor and see if that would work for you. Sometimes old organ blowers can be found which work with a little modification. Or, if time and energy permits one can make a large and quality bellows, but they are usually not inexpensive to make either. Those are my ideas for today. Hope it helps, Dick

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The Westpoint Forge uses one blower to service 12 forges, its housed in a seperate building and so noise level in forge is low. Each forge has its own slide valve arrangement and tue iron diametes vary from 11/4" down to 7/8"

There has never been a problem with lack of air even when all 12 are being worked hard as when on the damascus courses.

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forges are like the height of an anvil, everybody uses something different, so therefore there is no correct Rule of Thumb, if it works for you, its right, some people use wet coal to bank a fire to keep it small and conserve fuel, others use a whole bucket or more of dry green coal and have a humongous fire to accomplish a minimum of work, everybody is different. Its good tho that different people show what they have that works for them. One can review them all and decide for themselves what will work best for their type of work.

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I found some additional information on the subject. Remember that this applies to forges of that time period, mid 1940s or so. My note says side blown tuyere.

side blown twyere size.jpg

Edited by Glenn

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IForgeIron is set up so the discussion continues when new material is added. The first post was 4 Feb 2008, the last post was Jan 2009, and now in May 2015 after a 6 year dry spell, there is additional information. There are 2-3 other posts on the site on the same topic so you may want to use the search engine to locate more information.

For instance under bellows section ===> Ideal amount of CFM for a forge blower Nov 2014

The archive is your friend, visit when you get a chance.

 

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