Making a gas forge- A few Questions

[rookieironman]

 I am making a gas forge. I am using a water heater tank. The dimensions (outer) of the tank are 24 inches long by 20 in diameter. I have enough cerachem to line it to a three inch thickness. I am going to make a forced air ribbon burner for it. I am going to build the blower so that it  will provide a static pressure of at least 25 inH20. That should be plenty from the stoichiometric calcs. I have  a refractory coating to coat the cerachem and a couple of firebricks (good to 3000 F) for the deck.  My main question at this point is what do I need to do to install the cerachem? Will it stay in place once I coat it with the refractory and then coat it with ITC100?

[Mikey98118]

I am not going to say its too large; that depends on way to many factors you haven't listed.  Also the ceramic blanket you're asking about come in three different denseness and two different recommended use temperatures; we need to know which you are employing to give answer your questions.

What I can state with what you've given us to go on, is that three layers is too much cash layout, considering that the third (outer) layer can consist of Perlite bonded together with sodium silicate, which will also be tougher in this lower heat layer.

[rookieironman]

Cerachem 8 pound density rated to 2700 Degrees. I have 50 sq ft

 

[ede]

If you do a 3 inch lining, that would leave you with a 14 inch diameter.  I've heard of ribbon burners backfiring from the forge being too small for the burner, but haven't heard much about forges being too large for ribbon burners.   Do you have a ribbon burner size picked out?  I would call Pine Ridge to find out what size burner they recommend, or maybe Wayne will chime in.   

This guy part one  used a 10" quick tube  for his ribbon burner forge and went with a pine ridge burner part two .  

Regarding the blanket staying in place, when I built my forge I took inside diameter x pi to calculate how much blanket to cut, then added an inch or so for a tension fit and it stays just fine in place.  If the first layer you cut is too small and falls out, cut one longer and use the first one on the next inner round.  

 

 

[Mikey98118]

That is an excellent, but more expensive choice of ceramic fiber, the density is sufficient for low cost rigidizer to be used to look the fibers in position; thus three layers is fine.

[Frosty]

That comes close to 3,700 cu/in not counting openings. That's large enough you'd probably be better off just buying a used gas fired pottery kiln and tweaking the liner for what you intend to use it for.

Were it my project and I HAD to build it I'd go with a multi port gun burner. If you are shooting for welding heat you're looking at TEN 3/4" burner outlets. I'd manifold them and run it on a suitable blower and natural gas if I could get a large enough line. Propane is fine but you're looking at a commercial size furnace and it will use a LOT of fuel, I'd happily trade a few BTUs per cubic foot for the significantly lower price per cu/ft.

Frosty The Lucky.

[rookieironman]

Actually 2700 Cubic inches. And I'm not aware of commercial size furnaces that small. But thank you for your concern. I will run right out and try and find a 'used gas fired pottery kiln'  and compare that expense with the $10 total I have spent thus far. I suspect the difference will certainly allow me to purchase a fair amount of propane.  

On 11/25/2016 at 4:33 PM, Mikey98118 said:

That is an excellent, but more expensive choice of ceramic fiber, the density is sufficient for low cost rigidizer to be used to look the fibers in position; thus three layers is fine.

Would you suggest an inital layer perlite/sodium silicate in addition to the three layers of cerachem?

On 11/26/2016 at 4:01 PM, Frosty said:

That comes close to 3,700 cu/in not counting openings. That's large enough you'd probably be better off just buying a used gas fired pottery kiln and tweaking the liner for what you intend to use it for.

Were it my project and I HAD to build it I'd go with a multi port gun burner. If you are shooting for welding heat you're looking at TEN 3/4" burner outlets. I'd manifold them and run it on a suitable blower and natural gas if I could get a large enough line. Propane is fine but you're looking at a commercial size furnace and it will use a LOT of fuel, I'd happily trade a few BTUs per cubic foot for the significantly lower price per cu/ft.

Frosty The Lucky.

Actually 2700 Cubic inches. And I'm not aware of commercial size furnaces that small. But thank you for your concern. I will run right out and try and find a 'used gas fired pottery kiln'  and compare that expense with the $10 total I have spent thus far. I suspect the difference will certainly allow me to purchase a fair amount of propane. 

[Latticino]

On ‎11‎/‎22‎/‎2016 at 6:37 PM, rookieironman said:

 I am making a gas forge. I am using a water heater tank. The dimensions (outer) of the tank are 24 inches long by 20 in diameter. I have enough cerachem to line it to a three inch thickness. I am going to make a forced air ribbon burner for it. I am going to build the blower so that it  will provide a static pressure of at least 25 inH20. That should be plenty from the stoichiometric calcs. I have  a refractory coating to coat the cerachem and a couple of firebricks (good to 3000 F) for the deck.  My main question at this point is what do I need to do to install the cerachem? Will it stay in place once I coat it with the refractory and then coat it with ITC100?

Word to the wise: sarcasm aimed at those trying to help you will likely get you ignored by folks who could save you a lot of grief.

Incidentally I get approximately 2,770 cu inches, assuming you are going to line the ends with fiber blanket as well.  If you factor in some openings in those ends, also assuming that you are planning on having doors that fit over those openings instead of inside same.  Including those openings, assuming they will be around 6" diameter x 3" long, you have closer to a total interior volume of 2,940 cubic inches.  In any event a rather huge forge for an individual.  As Frosty noted if you are going to run at forge welding temperatures you will likely use up a lot of propane.  Unless you have a power hammer or a good striking team you will be hard pressed to hammer a length of stock heated to an 18" length.

Stoichiometric calculations are all well and good, but those calculations should be taking into account the required airflow at the heating rate you need, not the blower static pressure, per se.  Of course if you have done the static pressure loss calculations for the required airflow for perfect combustion at the heating rate you have determined (presumably from the combination of radiant, conductive and convective losses from the forge interior, and factored in the losses due to exhaust of heated combustion products, not to mention choosing the correct skin friction value for the assumed airflow characteristics in your shop) you should be fine, as that will set the characteristic system curve for you to coordinate with your blower's fan curve. 

By the way, what loss coefficient did you project for the custom forced air ribbon burner?  Any idea about the optimal multi port opening quantity and sizes to coordinate with the anticipated flame velocity for the variety of heating rates you are going to want for your forge?  Frosty has done empirical tests that worked for designing his ribbon burner.  Unless you get lucky, just winging it won't work.  Of course you could do the computational fluid dynamics calculations for the fuel air mixture, but if you miss something it is going to be a lot of effort for an unexpected result.

And you are going to build your own blower as well.  I'm really impressed that you have been able to anticipate the external static pressure it will put out at the airflows you are expecting.  I'm pretty sure industrial blower manufacturer's have to empirically test their new designs to ensure they meet the required fan curves (check out the AMCA standard testing for example).  Perhaps you would be better off sourcing a used blower from the same type of place you got the high temperature insulation and refractory for only $10.  Hey, I once picked up a high pressure blower from a liquidator for $25, but that was quite some time ago.  In my limited experience of building forced air gas fired burners 25" WG is more than should be required, unless you plan on including a lot of equivalent duct length and a control valve or two.

 

[ede]

Rookieironman,

I did the calculation here: http://www.online-calculators.co.uk/volumetric/cylindervolume.php   

for burner volume and think Frosty is correct at around 3,700 cu/in. 

24" long at 20" diameter, subtract 6" to arrive at 14" diameter. 

 

 

[Latticino]

You need to allow for the reduction in length that the insulation at the ends makes (24-6=18).  Then you need to add in door openings, of some sort.

[WayneCoeArtistBlacksmith]

One of the biggest mistakes that new to gas forges make is to build to large.  Check out the attachments on the Forge Supplies page  for Build a Gas Forge and Ribbon Burners.

If the reason you are planning to build so large is to be able to get large, odd shaped items into the forge the instructions in the attachment will cure that problem.  Just because you have the tank is not a good reason to use it to build a forge.  You should be able to find a 20# Propane bottle or a Freon tank for free.

Let me know if I can help you.

Wayne

[rookieironman]

On December 13, 2016 at 11:54 PM, ede said:

Rookieironman,

I did the calculation here: http://www.online-calculators.co.uk/volumetric/cylindervolume.php   

for burner volume and think Frosty is correct at around 3,700 cu/in. 

24" long at 20" diameter, subtract 6" to arrive at 14" diameter. 

And subtract six for a length of 18"

According to pine ridge a burner with approx 30 sq inches of burner should work for my 1.6 cubic feet of heated space.  3 x 10 burner should suffice.  

   I happened upon a dust collector/ blower for 75$ that provides 660 scfm and    a static pressure of 25. So I won't have to make my own blower anymore.  

Help is appreciated. Advice to scrap all my work thus far because it will never work and should go out and buy something is accepted in the spirit with which it is offered. 

Computational fluid dynamics investigation of the flows ( including anticipated losses due to boundary effects and friction) involved indicate a high probability of success. Thanks for suggesting that.  

[Buzzkill]

Sounds like you have it figured out.  I have a couple questions though.  Would you be so kind as to post some pictures, either WIP or when you get finished?  I'd like to see what you end up with here.  I'm also curious about what you intend to create with that forge.  Again, we love pics so don't be shy about sharing the forge and/or what comes out of it.

BTW, I don't think anyone was saying it wouldn't/couldn't work.  I believe the comments were directed more at fuel consumption and the long term costs of it. If you have way more forge than you need you effectively waste fuel which costs you a lot of money over the life of the forge.  In that vein I guess I do have one more question.  How are you going to feed this beast?  I hope you have a couple 100 pound or larger propane tanks you can link together, or that you can use the natural gas from your house.  I can tell you that one or two BBQ size propane tanks will not be sufficient for that. One or two small tanks like that will freeze up in a short period of use.

One way you can kind of have your cake and eat it too is to create baffles out of refractory material that you can slide into place to decrease the volume of your forge when/if  you desire, but remove them when you need the full volume.  Of course your burner(s) placement will affect where these barriers can be placed inside the forge.

[Latticino]

I have worked in furnaces designed by Charley in the past and used some of his concepts for the configuration of my glass furnace I built back in the late 80's.  It ran flawlessly for over 10 years.  He wasn't making multiport burners at the time (so I used a Giberson burner tip), but I have always respected his design and engineering chops.  You most likely won't go wrong in buying one of his burners and following his guidelines for chamber size.  I'm glad to see that he is still in business.  All bets are off if you just use the general size of the burner block though, as number and size of ports, as well as baffle design, has a large influence.

We still are gently suggesting that your forging chamber is rather large for a small shop.  Of course it is small for a glory hole or glass furnace (which is where both Mr. Correll and I have the bulk of our experience.

As regards your question on the fiber blanket installation, I have used two methods to date.  If the blanket is put in in layers, and the density is correct, slight compression will hold it in place until rigidized.  You are planning on using a rigidizer prior to your refractory coating I hope.  This rigidizer can be applied to the surface of each layer as installed.  The other method is to lightly dampen the blanket and accordion pleat it around the chamber curve.  The advantage to that method is that it both forms a natural arch and can provide a nice thick lining, rather than using multiple layers of thinner material.  I would still use rigidizer.

[Mikey98118]

Also, accordion construction would allow him to compress the sandwiched layers a whole lot, to help against shrinkage...but really thick insulation--no matter how he does it-- is going to need an additional SAFETY step, in the form of an oversize exhaust port tube around the burner, to keep heat buildup in the ceramic blanket from over heating incoming air in the burner's mixing tube from building up too high during long thermal cycles.

And yes, a thick Perlite outer layer of insulation is a good idea, but only with the special port tube.

[rookieironman]

On 12/14/2016 at 10:43 AM, Buzzkill said:

Sounds like you have it figured out.  I have a couple questions though.  Would you be so kind as to post some pictures, either WIP or when you get finished?  I'd like to see what you end up with here.  I'm also curious about what you intend to create with that forge.  Again, we love pics so don't be shy about sharing the forge and/or what comes out of it.

BTW, I don't think anyone was saying it wouldn't/couldn't work.  I believe the comments were directed more at fuel consumption and the long term costs of it. If you have way more forge than you need you effectively waste fuel which costs you a lot of money over the life of the forge.  In that vein I guess I do have one more question.  How are you going to feed this beast?  I hope you have a couple 100 pound or larger propane tanks you can link together, or that you can use the natural gas from your house.  I can tell you that one or two BBQ size propane tanks will not be sufficient for that. One or two small tanks like that will freeze up in a short period of use.

One way you can kind of have your cake and eat it too is to create baffles out of refractory material that you can slide into place to decrease the volume of your forge when/if  you desire, but remove them when you need the full volume.  Of course your burner(s) placement will affect where these barriers can be placed inside the forge.

Thanks. Movable baffles are part of the plan.  It  is my understanding that forced air burners use far less fuel than Venturi burners and are more forgiving in terms of design.  The preference in the community, from what I see, is for Venturi burners.   

    And indeed most comments I have received on another site regarding this plan assume massive fuel consumption based on Venturi burner fuel rates. 

I have 16 gauge Kanthol wire I intend to use to hold the blanket in position.  

 

Question: should I use a layer of refractory against the tank wall before I put down the blanket? I am going to coat the blanket with refractory also.

 

[ThomasPowers]

Guess what:  BTUs in = BTUs in no matter what pushes them.  

[Buzzkill]

No, you don't need a layer of refractory against the shell; just in the forge chamber itself.

To follow up on what Thomas was saying, there is a common misconception that blown burners use less propane to produce the same heat.  This is not true.  Blown burners normally operate at less pressure than naturally aspirated burners, but the volume of propane used is identical (or very close to it).  There are some potential efficiency differences between a ribbon burner and a single larger burner port regarding flame front speeds, exhaust, and the time the heat spends in the forge before exiting, but Frosty or Mike can tell you more about those.  Regardless, you will use about the same amount of propane or natural gas to heat that forge whether you are going to use blown burners or naturally aspirated burners even though the pressures involved may be significantly different.  At full volume that forge is going to require a lot of fuel, especially if you plan to reach forge welding temperatures.

 

Oh, and thanks for the pics.  We love pictures.

[rookieironman]

54 minutes ago, Buzzkill said:

No, you don't need a layer of refractory against the shell; just in the forge chamber itself.

To follow up on what Thomas was saying, there is a common misconception that blown burners use less propane to produce the same heat.  This is not true.  Blown burners normally operate at less pressure than naturally aspirated burners, but the volume of propane used is identical (or very close to it).  There are some potential efficiency differences between a ribbon burner and a single larger burner port regarding flame front speeds, exhaust, and the time the heat spends in the forge before exiting, but Frosty or Mike can tell you more about those.  Regardless, you will use about the same amount of propane or natural gas to heat that forge whether you are going to use blown burners or naturally aspirated burners even though the pressures involved may be significantly different.  At full volume that forge is going to require a lot of fuel, especially if you plan to reach forge welding temperatures.

 

Oh, and thanks for the pics.  We love pictures.

My understanding was that there was less fuel wasted with forced air.  No?

[ThomasPowers]

Yes, No, Maybe depending on the details.

Thanks Buzzkill for mentioning dwell time too.

Remember efficiency also depends on use: I often run reducing to avoid decarburization when doing blade work.

[Buzzkill]

"My understanding was that there was less fuel wasted with forced air.  No?"

I'm not sure how that would be possible.  Did anyone explain the mechanism by which this would happen? You indicated before that you had done stoichiometric calculations.  Does the fuel/oxygen ratio change if the air is forced in with a blower compared to being induced by the fuel stream?  FWIW I have used both in a much smaller forge using a single 3/4 inch burner.  Tuning the flame to the desired type of atmosphere was quicker/easier for me with a blown burner, but if there was a difference in fuel consumption I couldn't detect it.  In either case the goal is to have the correct amount of fuel and air entering the combustion chamber in order to maximize the heat output.  Do the molecules in the reaction behave differently in one scenario versus the other?

[Latticino]

Well that sure is a large chamber.  Still not sure what you plan on heating in such a huge enclosure, but follow your dream in any case.  Hope you aren't planning on using that current door system as the only way to get stock in and out of the forge.  When it is swung open you are not only going to lose a tremendous amount of heat, it will be radiating right at your hands as you attempt to pull your stock out of the forge.  I expect you are planning on making a smaller opening in that door and only swinging it open very rarely, if at all.  You do know that you will need a port in the chamber for combustion byproducts to exit, right?

Have you ever actually done any smithing in a gas forge?  Before spending a lot of time constructing your own it might be prudent to find a shop that will let you rent time or get lessons so you can see how more conventional systems work.  It is great that you are trying an innovative design, but there is also something to be said for iterative evolution of design leading to more refinement.  I assume you are open to constructive criticism, otherwise you wouldn't be posting your progress.

Oh, and I agree with the other posters regarding the relatively small disparity between NA and blown burner BTUH output.  I feel that all things being equal blown burners a  bit easier to construct and tune, but that may just be because I have more experience with them.  I've heard a lot of folks discuss use of internal baffles for their forges, but never seen one successfully used.  Theoretically it does appear promising, I just haven't seen a practical installation.  Part of the problem is going to be the turn down ratio for the ribbon burner.  Pine ridge indicates that they recommend running them "full out".

[rookieironman]

7 hours ago, Buzzkill said:

"My understanding was that there was less fuel wasted with forced air.  No?"

I'm not sure how that would be possible.  Did anyone explain the mechanism by which this would happen? You indicated before that you had done stoichiometric calculations.  Does the fuel/oxygen ratio change if the air is forced in with a blower compared to being induced by the fuel stream?  FWIW I have used both in a much smaller forge using a single 3/4 inch burner.  Tuning the flame to the desired type of atmosphere was quicker/easier for me with a blown burner, but if there was a difference in fuel consumption I couldn't detect it.  In either case the goal is to have the correct amount of fuel and air entering the combustion chamber in order to maximize the heat output.  Do the molecules in the reaction behave differently in one scenario versus the other?

The stoichiometric requirements are more readily met with an excess of air as the ratio is nearly 30 :1 air. If you don't have enough air to burn the gas then the gas does not burn. The velocities of the gas required in a Venturi burner require high pressure and usually depend on the excess gas burning in the chamber not the flame wall.

[Latticino]

Theory sounds good, but you need to get some practical experience. 

Frosty alluded to the fact that most of us like to have at least a slightly reducing atmosphere inside the forge to attempt to limit the scaling up of the stock during the heating process when it appears to be most vulnerable to same.  To get that reducing atmosphere you need less than perfect stoichiometric combustion.  The high velocities of gas needed to entrain the air for the mixture are an issue, but with a well tuned NA burner design you can achieve anything from close-to-perfect combustion to a mixture where some of the combustion takes place in the chamber and even at the doorway to the forge, just by adjusting the choke and gas regulator.  I still find it easier to do with a blown burner, but that is just where I have more experience.  In any event the key issue is that perfect combustion is not always preferable.

[Buzzkill]

In addition to that, air that does not participate in the combustion process will produce a cooling effect to some extent since it effectively becomes a continual heat sink, so while excess air may help drive a more complete combustion, it does not necessarily mean maximum temperature inside the forge.  Either too much air or too much fuel will reduce the temperature inside the forge somewhat.