Plenum gap

[Silverbull]

Hi everyone, quick question here about the plenum. Is it absolutely necessary to have a gap around it packed with insulation? I'm building a ribbon burner and would rather not if I don't have to. There will be 2 inches of wool insulation between the shell and bricks plus itc100 on the bricks.

Thanks

[Frosty]

Welcome aboard Silverbull, glad to have you. If you'll put your general location in the header you might be surprised how many members live within visiting distance. Lots of information, especially materials, tools and equipment availability and price are very location specific.

No, there aren't any MUSTs where building forges and burners are concerned, some things just work better than others. There's no need to wrap insulation around the plenum, I found it's a bad idea where mine were concerned. It's hard enough keeping the fuel/air mix cool enough it doesn't burn back as it is. My burner blocks are a lot thicker than yours.

Are you making a heat treat forge? Otherwise the extra heat sink of that much hard firebrick in the chamber sucks extra fuel bringing to temperature. Another good reason for that much thermal mass is if you're cycling a lot of pieces through the forge rapidly.

I recommend against ITC-100 for the kiln wash in a propane forge. It doesn't fire hard and rubs off pretty quickly, it's also bonded with Kaolin clay so it's not armor against borax based welding fluxes.

Plistex 900 is a high alumina kiln wash that fires hard like a coffee cup so it's durable at welding temps and the alumina is resistant to borax fluxes. It's also much cheaper.

Frosty The Lucky. 

[Silverbull]

Thank you Frosty for your time. The burner block is 2.5 inches thick by 9 and 4.5. The brick lining was given to me and I jumped ahead and built it this far before discovering that those were a bad idea, it's for forging unfortunately, not heat treating.  I will use the itc100 I ordered on a different build then and look for the plistix 900.

[Silverbull]

Frosty, I didn't mean wrapping the plenum on top of the forge body. I meant if I needed to have a gap between the burner the forge body, then having to insulate in the gap. In the picture, I already fixed the burner in, not gap.

Thanks again 

[Frosty]

Use with what you have now, it'll be okay, ITC and all unless you have somewhere better for it. You can buy Plistex and other forge supplies in the Iforge shop at the top of the page. 

I like having some room around the burner so it can be aligned and adjusted. I pulled a real stupid beginner mistake when I mounted the NARBS, I welded them to the shell. Now I have to cut the shell apart if I want to change something and believe me brother I'd like to. The forge works well enough and if I make more NARBs I have a different forge in mind. I don't spend a lot of time in the shop anymore though. 

If you have the fuel supply lines, reg, etc. I'd fire that baby us as she sits to see how the burner likes the environment.  

What are you going to use for the "inducer" AKA mixer, etc. and how are you going to mount it?

The T burner is my design but when it's screwed into a ribbon burner it isn't a T burner anymore. Make sense? I've been calling them "drivers" and waiting for a better term.

Frosty The Lucky.

 

 

[Silverbull]

I'm a welder by trade, the shell is only tacked and easy for me to get back inside it someday plus the burner isn't even welded to it. I'm using a T-fitting for my mixer and once I have it together, I'll attach a picture. My air/fuel supply will be made of 2" sanitary Stainless tubing, stuff left over from a job. Here's a couple pictures so far, I'll still in the process of covering the exposed insulation 

 

[Silverbull]

[Frosty]

This is a blown AKA, (Gun) burner? 

My test NARBs showed diffusers are detrimental if you mount the fuel air inlet on the side of the plenum so the flow isn't blowing directly in line with the nozzlettes. 

At any rate putting the diffuser directly on the flow inlet is the reason the current blown ribbon burner plans require such high static pressure from the blower AND blow dragon's breath sometimes 2-3' out the openings. 

If you must use a diffuser I had luck welding a piece of solid sheet across the plenum about 1/2 way to the outlets. The plate only covers a little more than 1/2 the plenum with gaps at each end. This worked fairly well with a naturally aspirated T driver and didn't produce undue back pressure.

All the diffuser really needs to do is equalize the pressure and flow in the plenum so all the outlet nozzles receive equal flow. 

No need to do something special for a propane jet, it doesn't need to aim anywhere in particular, just put it as close to the blower as possible, the flow will go around at least two 90* turns and blast into the plenum. That is more than enough turbulence to mix the propane and air. 

The main consideration with supplying propane goes right back to having a diffuser blocking too much of the plenum inlet. Some of the plans you see online requiring high static pressure actually require higher air pressure than a reasonable propane supply psi. To compensate they crank the psi up and burn huge amounts of excess propane OUTSIDE the forge.

Frosty The Lucky. 

[Silverbull]

Hi Frosty, so are you saying that I didn't need to add the perforated stainless baffle to the inside of my plenum? 

Yes, it is forced air. I haven't got around to hook the blower up yet. I'm waiting on a propane needle valve. I hope it works fine, if not, I'll cut it open.

Thanks

[Frosty]

What I'm saying is don't put it THERE. Figure how many sq/in the plenum is looking down on the outlet nozzles and put a solid plate that leaves gaps at each end that equal about 1/2 the area. 

If the plenum is square maybe put the diffuser plate kitty corner leaving the gaps in the corners. 

The important part is do NOT weld it close to the inlet port! You don't want to constrict the flow you only want to disperse it evenly in the plenum. Before I stopped using diffusers completely I found putting them about 1/2 way to the outlet nozzles worked best.

If the plenum has 2" clear between the inlet port and the burner block containing outlet nozzles. (I call this the plenum depth.) I placed the diffuser 1" up from the outlet nozzles. 

If the plenum is 6" long I would've made the diffuser 3" long and welded it in with 1 1/2" gap at the ends. 

That basic ratio worked with a T burner for a driver and believe me they don't develop much static pressure, they'll float a business card but have trouble blowing a piece of corrugated cardboard off. They're very sensitive to backpressure, much will just stop them. The mixing tube is also very short, a 3/4" T burner has a 6" mixing tube if built properly. You see all sorts of bad versions on the internet, if it has a choke plate it's a bad design. 

All a diffuser does is equalize the flow rate to all the outlet nozzles. A perforated plate still passes more flow through the holes under the inlet port. You want to deflect the flow and the longer distance it travels before reaching the burner block the more equalized it is and will disperse relatively evenly in the space above the burner block before taking the path of slightly  more resistance and going through all those little holes.

Make sense? (I hope)

Frosty The Lucky.

[Silverbull]

That makes sense, thank you sir

[Frosty]

You're welcome, it's my pleasure.

Frosty The Lucky.

[Latticino]

Loving the orbital welded SS tubing and tri-clamp fittings.

[Silverbull]

Got it completed except for the retractable tool rest, works well. 

[Sigmund]

Here is a +1 to doing your research on this site. I have been a member here since 2019 and have always what I needed by scouring these forums before asking a question.  Every single time my question has been answered somewhere. I searched the forums for a few hours last night and was seconds from asking a new question and I found this gem of thread.  Ill try to do a better job of posting to verify a post that worked. As this is my first post,  Its great to meet all of you and I appreciate your collective wisdom and sharing.  I think my location is in my bio, but we are going to find out soon  

Not to hijack this post, but rather clarify the point of ribbon burner mounting gap and procedure for those looking to answer the question.  I am building a my second forge as a propane bottle ribbon burner forge following all of the great advice on this site via the design on Wayne Coe's site. I tried casting in place over my freon bottle forge and it was a GONG show... Thing is  like an hourglass now that forge is fairly useless. I went with a 30lb propane bottle as I have many that I have acquired over the years that are unused, vs the one 20lb bottle I have.  Maybe that was penny wise and pound foolish, but its already cut/shaped and it's the middle of winter. 

Here comes the verification/clarification.  On my ribbon burner install, my plan is to wrap my burner in a layer of cardboard and plastic wrap slathered in crisco, then cast around the burner.  

1. Should I cast  Kast-o-Lite to wrap around my rigidized Kaowool to cover the 2" that are exposed in the burner gap/opening? 
2. I am not planning on stuffing any raw kaowool between my ribbon burner and where it sits inside the forge. I don't want loose wool floating around and in to the air so am just going to leave that cardboard sized gap open to the world.
3. Any general reference on sizing the opening vs size of burner.
4. If there is a post available that answers all of these questions, I couldn't find it, but would sure love to read it!

Silverbull - Awesome build, I hope its trucking along like you want it to. 

 

Thanks in advance!

[Frosty]

Welcome aboard Sigmund, glad you delurked!

I've run into the gap problem and what worked pretty well in the 2 NARB forge was to weld the plenum to the forge shell. Unfortunately that eliminates the ability to change the aim point of the NARBs. I won't do that again. 

However, when experimenting I simply set them over an opening on a 1" angle iron flange around the plenums. The Kastolite 20 burner blocks extend beyond the plenums far enough to reach a bit past the forge liner's flame face. I lifted the NARBs until the burner blocks ended inside the liner and they run much cooler.

When the blocks get too hot the air prop mix pre-ignites inside the burner outlets and then back fires into the plenum. This is B A D. 

Back to your question. They ran just fine just resting on the flanges and didn't leak flame around them when the doorways got mostly closed. The flanges are continuous around the plenum.

Frosty The Lucky.

[Irondragon Forge ClayWorks]

6 hours ago, Sigmund said:

searched the forums for a few hours last night

One of the best ways to search the forum is to use your favorite search engine like google and add    site:iforgeiron.com   to the end of the string. You might be amazed at the number of hits returned.

I can't control the wind, all I can do is adjust my sail’s.
Semper Paratus

 

[Sigmund]

Frosty!  Thank you! I am going to go with the flange plan!  The setback of burner - critical pro tip!

Ill try and pull together a build photo montage to continue the sharing and delurkification.

Irondragon! Thank You! I did a bunch of google searches but didn't include site:iforgeiron.com  That is the way to go. I think I was also using the wrong keywords due to a critical error between the chair and the keyboard.