Arch formula

[cp4449]

Old age is kicking in....I have a LE 1" 0-6-0 with a propane burner. It is a keyhole style firebox. Need to add a arch. Could some one give me the formula for a arch pattern I can put it? And would a curved arch be better than an angle made one?

Thanks in advance

[Marty_Knox]

I don't think that arches work that well with propane. For a coal burner you need to figure a minimum of 125% of the net area of the tubes over the arch.

[Kimball McGinley]

I recall an article in LS or Modeltec where the author made "turbolater" strips to go inside the tubes instead of an arch. He cut strips of SS slightly narrower than the tube ID, then twisted them numerous times. The idea was to disturb the laminar flow and boundary layer in the tube so that the hot gases impinged on the tube walls. Claimed it worked well.

[Bill Shields]

I have arches in two of my propane burner locos and they work well.

HOWEVER...I designed the boiler / firebox and burner layout specifically to use an arch.

Turbolater helixes help a lot with fire tube heat exchange but don't do a lot in the firebox end where a lot of the heat is normally transferred.

[ccvstmr]

cp...an arch by itself will contribute to the improved "heating" efficiency of a propane fired loco. I will add, there are other elements that come into play. Wrote something and posted such details on Chaski many years ago. An arch...turbulators...close off any air gap between the ash pan and mud ring, distributed bed off flames, a baffle to restrict secondary air at the burners and more will all help improve the operation. Any one by itself may not make a great contribution...but put them all together and you should notice much improved operation.

As to your original question, don't know if there are any formulas per se. In my experience...you want the opening between the arch and the crown sheet to be approx. 10% of the grate area. The arch should extend back 2/3 of the way in the firebox...and should have several bends instead of a right angle where the exhaust gases come across the top of the arch and "fan out" to all the flues. Make sure the arch doesn't close off your lower rows of flues. A keyhole firebox will present a challenge as the arch will need "side wings".

Here's a photo from years ago when I installed my arch...the "baffle plates" have since been replaced by perforated screen material...as I found the draft would suck the flames off the burners.

xburner+arch plate2a.jpg (68.38 KiB) Viewed 3115 times

Hope that gives a place to start with the arch fabrication and installation. Carl B.

[cp4449]

Thank you all. The first step is to make turbulators....

[Bill Shields]

MAKE THEM SMALL(er) than you would think than the tube actual ID...you do NOT want them to stick during insertion / removal...

[ccvstmr]

cp...I'll 2nd Bill's comment about making the turbulator stock smaller than the flue inside diameter. You may want your strip stock narrower as much as .030 to .040". I'll add...round the edges of your strip material...use a file, sand paper, anything. Last thing you want is sharp edges vibrating, scraping, cutting or other the inside of your flues.

Made my flues by locking down one end of the turbulator strip material in a 3 jaw lathe chuck. The other end was clamped to the carriage tool post holder. The carriage "lock" was snugged down, but not tight. As you turn the lathe chuck by hand, the material will twist and either try to pull the carriage or pull the strip material out of the clamp. You'll have to experiment how many twists you can complete before the strip material tries to kink.

For my Allen 10 wheeler...the flues are 22" long (IIRC). The strip material was at least 24" long to start. I got 2.5 or 3 complete twists in that length.

As mentioned, you're simply trying to break up the laminar exhaust flow in the flues and induce turbulence to increase heat transfer. And when you're done, you should have something that looks like this.

Carl B.

[cp4449]

Thanks a lot guys on the comments of the turbulators.