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Forge CFM - how much is needed?


Glenn

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my bottom blast forge use a side channel blower of the same type as john B uses but a bit smaller, have run up to 6 forges on a 1hp.

the tue is quite small and I like to have 1 to 3 psi behind it so I can get a fire from 2" to 12" diameter depending on fuel depth and valve setting

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I use an gutted-out old clothes dryer as a base and blower for an 18" Rivet Forge.

Generally speaking, a clothes dryer blower puts out about 150 cfm.

This blower is more than adequate, for the relatively shallow fire in a Rivet type Forge.

However, there is a noticeable reduction in heat, as ash accumulates in the Tuyere.

A 30 second "clean out" solves that problem so easily, that I've never seriously considered increasing the blower capacity.

The motorized Champion 400 on my "Table Forge" easily blows a 10" deep Coal fire, ... but I have no idea what cfm it produces.

 

.

 

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  • 2 years later...
  • 1 month later...

having read though this page, i am still wondering if larger air flow is better to have in place at the beginning of the design stage, and reduce it with a splitter box (if that is the right word). the other option i though of was the hair dryer method. 

i found this on line, fairly cheap, already has a hose attachment and it produces 39cfm with a modest presuure

Commercial link removed per TOS.

i am planning on using it in a home made box that i made, https://photos.app.goo.gl/C5wuNsK1JvIAwm0C3 ( i have since taken the sand out and just lined it with fire brick)

the other option i thought about was using a manual bike pump. it would great more pressure but way less cfm and i would have to pump it my self

 

thoughts? sorry about my crummy set up so far lol

 

Edited by Mod34
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Hello, and welcome to IFI. If you haven't yet, please READ THIS FIRST!

Looking at your forge photo, I have to say STOP RIGHT NOW!!! By the looks of it, that pipe may be chrome plated, and you would be putting yourself at high risk of serious health issues if you put it anywhere near a fire. Look up the health effects of hexavalent chromium exposure if you don't believe me.

That said, your forge is a step in the right direction. Look at the threads about JABOD forges in the Solid Fuels section for some ways you could make that into a really top performer.

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1 hour ago, JHCC said:

Looking at your forge photo, I have to say STOP RIGHT NOW!!! By the looks of it, that pipe may be chrome plated, and you would be putting yourself at high risk of serious health issues if you put it anywhere near a fire. Look up the health effects of hexavalent chromium exposure if you don't believe me.

that is a very good catch. thank you sir. i took it from a old closet rod, so yes most likely is chrome plated. i will replace with some black pipe. thank you!

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Black pipe is a good option. Also, consider a side-blast rather than a bottom blast; the latter really needs a better clinker-management system than you'll achieve in a JABOD-type forge.

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this is going to be charcoal fuel, sort of base off the box of dirt forge example in the front page. i hope i made it large enough not to burn the wood lol. i was thinking of puting some refractory cement on all exposed wood parts?

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Read these two threads, and if they don't answer your questions, we'll talk further:

 

Oh, and when you have read those threads, start a new thread with the headline "Asking for advice on a JABOD-type forge" or something like that. Makes it easier for others to find if it's not buried in a thread on CFM recommendations.

 

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2 hours ago, JHCC said:

 

Oh, and when you have read those threads, start a new thread with the headline "Asking for advice on a JABOD-type forge" or something like that. Makes it easier for others to find if it's not buried in a thread on CFM recommendations.

 

i was at the start asking for cfm question haha, but the automod took out the external link to amazon. i was basically just asking if an air pump for using to inflate beds/toys and what not would work? it was roughly 35 bucks and produced 35 cfm

23 minutes ago, ThomasPowers said:

Why pay for refractory cement when free/cheap clay will work?

that is a good question, i am not a very experienced person with nature in that sense, call me a desk worker if you like, but how would i go about find clay? keep digging in the ground? i live on what once was a marsh low lands sort of place, since been filled in and developed on.

 

is their a disadvantage to using refractory cement to cover the exposed wood sections?

 

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Nobody is dissing you for lack of experience. Thomas asks question this way to get folks to think. (Platonic method I think?)

Dirt out of the garden, ditch or on old mud hole works just fine. Add some sand to help it dry without shrink checking (cracking) maybe horse manure, straw, grass clippings old leaves, etc. It's all been used over the millennia. It's not rocket science it's a box of dirt. Sift out the rocks, if they're been wet for very long they can explode if steam pressure builds up. I don't even try to guess if they've been wet long enough to soak up water I just toss them aside. There's a process for drying rocks for fire rings, BBQs, etc. but rocks don't have a place in a JBOD forge. You can sculpt mud or compacted clay for the fire face or forge back or whatever it's called. I don't remember. :unsure:

Don't sweat it, we all over think and design things, can't help it. What we do on Iforge is offer up our experience so folk don't have to make the same mistakes over and over.

Think of the JBOD as another type of portable hole. Holes are handy things. 

Frosty The Lucky.

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1 minute ago, JHCC said:

Socratic. The Platonic method is all talk and no action. ;) 

 

Thanks John, maybe I should just PM you and ask first. I can never keep them straight. :rolleyes:

Frosty The Lucky.

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Training folks how to research their own answers and not depend on just a few loud guys who might be WRONG!

(not to mention the weight gain involved with Platonic Solids...but perhaps that's better than a hemlock smoothie at the end of the day...)

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51 minutes ago, Iguy said:

is their a disadvantage to using refractory cement to cover the exposed wood sections?

Yes: it's a waste of money. Refractory cement is basically good for one thing: sticking together other materials that have to withstand heat. It's not a good flame face, and it's not a good insulator. In a JABOD, the dirt/mud/sand/kitty litter/whatever both forms the firebowl and insulates the box from the heat. If your box is big enough (that is, a bare minimum of 2-3" of dirt between the fire and the wood; more is better), you'll be fine.

 

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48 minutes ago, ThomasPowers said:

Training folks how to research their own answers and not depend on just a few loud guys who might be WRONG!

(not to mention the weight gain involved with Platonic Solids...but perhaps that's better than a hemlock smoothie at the end of the day...)

Depends on who's having the smoothie doesn't it?

Frosty The Lucky.

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  • 2 years later...
On 2/4/2008 at 11:54 AM, Jmercier said:

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.

The 1914 Machinery's Handbook has some design info for the pressures-- 1.5-6 oz per square inch, times  (1lbf/16ozf * 27.7inH2O/psi) = 2.6-10.4 in-H2O.   

image.thumb.png.6e543a45f7fab1d043749afd6bdfa06f.png

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With this

On 5/11/2015 at 2:19 AM, the iron dwarf said:

my bottom blast forge use a side channel blower of the same type as john B uses but a bit smaller, have run up to 6 forges on a 1hp.

the tue is quite small and I like to have 1 to 3 psi behind it so I can get a fire from 2" to 12" diameter depending on fuel depth and valve setting

... and several of the other refs on the thread, a 1/6hp or 125W furnace blower seems adequate. Ignoring efficiency and using the calculator at link removed with the various pressures on the thread, that 125W gives CFMs of:

13 CFM  at 3psi /48 ozf/in2 / 20kPa 

38 CFM at 1psi / 16 ozf/in2 / 6.9kPa

96 CFM at 0.4psi / 6 ozf/in2 / 2.8kPa

384 CFM at 0.1 psi / 1.5 ozf/in2 / 0.690 kPa

 

numbers.jpg

 

I might look for a 125W 12V furnace blower for the solar panel by my shed.

 

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I came to this thread wondering about the sizing of a blower, and it looks like the answers are all is that rather than CFM, you need an adequate pressure vs CFM balance, and psi * CFM converts into watts.

I've got an oversized solar panel for lawn tractor battery maintenance, so I can get some 12V power out of my shed a bit easier than 120V, and I was curious about what sort of blower I could use.  A cheap 24W or 65W motorboat inboard engine blower seems a bit undersized, but maybe there's a 10-15A car heater fan I can get from the junkyard.

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I have a 100cfm ac tuyere fan.  It is a 4” diameter hvac booster fan located 10’ from the firebox.  1” pipe with 2 elbows. I doubt it is 100cfm once it reaches the fire.  I think it would be more powerful  1’ from the fire.  If a solar panel is used for a direct dc fan, running only when sun is out, it will need to be close to firebox with no bends and probably still barely adequate cfm.  A battery and inverter will get stronger fan.  I have several 150watt+ panels but running direct with no battery is tough. I just run long drop cord to shop.  A 200 watt panel and dc direct fan could work if matched correctly.

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