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Forge CFM - how much is needed?


Glenn

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I use a speed controlled furnace blower at the moment. The speed controller doesn't give me full control on the lower end so I also have some air dumps I can use if need be. There's really no need to calculate it all out if you give yourself some control over the amount of air that actually gets all the way to the fire. Especially since it will vary depending on what you're doing on any given day.

I used a hair dryer (with the heating elements disabled) for a long time, but got tired of the constant noise.. I used to fall asleep with the sound of it still in my head. Therefore, I would choose something that's reasonably quiet. I also agree with TP, even a cheap hairdryer puts out too much air and those are pretty small motors.

I would just use whatever you have on hand or can get for relatively cheap and then adjust from there after you use it for a while and can get a better grasp on what you need. The main problem people run into is back pressure, but given your tendency to overthink -- I don't mean that as an insult, just an observation -- I'm going to assume this is something you have already considered. 

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Last fall I was experimenting with the end of a railroad fishplate as an anvil, a firepit with wood as a forge, and a handheld AC heatgun as the blower.  I could get to some kind of orange, but the whole setup was pretty awkward.

Even though that DC fan specs 213CFM, with only 22.8w power input, its output flow at forging pressures might be something like 1/4 of what a Champion 400 hand-crank blower might push.

I have battery and an AC air mattress inflators and they both whine so loud I can't imagine listening to them for longer than a mattress fill.

Last year I was using an old bathroom blower for drying out a waterlogged sunfish hull, but it died. I thought about a marine blower like one of these below, but I found that the ~1CFM natural flow from a black ABS corrugated drainpipe as a chimney worked pretty well for the humidity-extracting purpose.

The $50 12V marine blower from Amazon looks like it might be 60W, but that still might deliver only a fraction of what a hand crank puts out.    Then I thought a junkyard automotive heater fan might be a good GTTS solution for a quiet 12V blower, but it's hard to see the specs.

We've been living like hermits under COVID, so I haven't been able to go scrounging for my hobbies.

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There are more DC mattress inflators than AC, I pick them up at yard sales for a couple bucks or get them tossed in with a bundle of other stuff. Well, used to, I have a small pile of them now. If you'd caught the season end close out sales you could've gotten one at WalMart for probably under $5, new in season they range around $13. They're available online, like everything these days. 

Frosty The Lucky.

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Frazer you are probably right.  I didn’t realize Dave had a battery and thought he was trying to run dc direct from solar panel to fan.  With a large enough panel 200 watts this could work.  The fan I suggested looks like computer fan and probably would only produce 75cfm.  The centrifugal fan Dave suggested looks nice; since same type as hand cranks.  Is 100 true cfm the target cfm?

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Depending on the size of your forge a manual dual action mattress pump might work. It's what I started with but soon got an AC version and was using a rechargable jump starter/power station to power it. I agree that the whine can be annoying. I put mine in a styrofoam cooler that is used for shipping and cut a hole for the air hose and a slit for the cord to pass through. It deadened the noise considerably. You can see the cooler and jump starter sitting to the right of the forge. FB_IMG_1607942621371.jpg.95410242be44eb22cd6005408da59f7b.jpg

Pnut

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Cool setup pnut--It's nice to see systems that look like a practical, achievable step up from mine.  On the stump is that a vise, bending jig, guillotine tool, or something else?   

Don't laugh, but here is a picture of my improvised picnic-table, firepit, hot-air gun, RR fishplate-stuck-in-the-dirt setup.  The soft fishplate works a little better when I grind it flat, but I really wanted more space. JBOD is next.

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My whiney Coleman AC Mattress blower is 170W.  That looks like overkill relative to a 1/6HP furnace blower or Champion 400.  I'll try it out.

With the air-horsepower link removed above, the conversions between PSI, CFM, and power is this constant:

1 HP = 745.7W  =  100psi * 2.296 CFM   (or 1 CFM = 745.7W / 229.6psi or 1 CFM = 3.248 W/psi )

So for my 170W blower at 0.4psi back pressure and 100% efficiency, maximum ideal CFM is:

170 W * 1 CFM/(3.248 W/psi) / 0.4psi = 130 CFM 

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Out of curiosity, how are you calculating your back pressure? Are you only considering the pipe or are you also considering that the tuyere will have fuel piled up on top of it, which will slowly turn into ash and clinker and clog up your grate. Ergo, your back pressure is not a constant.

Going a little overkill on air (up to a point) isn't necessarily bad. As has been noted, excess air can be vented with a valve, or leaving a gap between the blower and the pipe leading to the forge, or any other means you can devise. However, the only solution to having too little air is to buy/find another blower. 
 

1 hour ago, Dave F said:

My whiney Coleman AC Mattress blower is 170W. ... a 1/6HP furnace blower or Champion 400.

I think any of these would be fine. 

If you're looking for practical forge builds, the JABOD is a great option. I will also link to this one, just to show there are many ways to skin a cat. 

I'm linking to my build (before I switched to the furnace blower) but that's really just to bring you to the end of the thread, there are plenty of other ideas in there that are better and easier than mine.

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That's the stump I was using to set the end of my work on when I was punching it. That's just a brush sitting on top of it and a hammer handle sticking up that's sitting on a shorter stump in front of it:D. That stump is what I use now for my post vise. I've also gotten rid of the repurposed night stand forge. The pressed sawdust boards only lasted about a year. I use a kettle grill or a plywood box that I fill with Dry kitty litter and build a firepot from bricks similar to the MARK III JABOD thread. 

Pnut

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Frazer, I'm not measuring or computing backpressure, I'm choosing the top end of the 0.1-0.4psi range of forge backpressures from the old Machinery's Handbook pages, where it looks like they are talking of the junction into the forge, then I over-optimistically assume 100% efficiency in converting the electrical power into air power.  Once you de-rate the blower down into it's actual operating envelope, then the pressure vs. flow tradeoff is pretty much P1 * V1 = P2*V2  -- If the clinkers double the pressure, they'll halve the flow.

I'm planning on my 25yo "Aussie Folding Grill" as the B for a JABOD.

 

pnut, a year outside is pretty good for particle board.  My family was in the particle board industry since the 40s.  We used it for everything and turned lots of it back into sawdust.

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I have been thinking of upgrading my tuyere fan with more cfm.  The hvac fan I am using now is only 100cfm.  It is quiet and works to get the work bright orange with rare hues of yellow.  I cant forge weld, and the hammering is more labor intensive with typical orange color.  I am leaning toward a centrifugal design with larger diameter like the hand cranks but electric with variable speed.  The extra 100cfm should help with back pressure as the ash builds up and get me to yellow and maybe the rare white.  This one is 200cfm with 10" diameter centrifugal and around $50 bucks with good ratings for continual use and quiet.   VIVOSUN 4 Inch 203 CFM Inline Duct Ventilation Fan with Variable Speed Controller

719d7M38ZAL._AC_SL1500_.jpg

 

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