A thought on making silent caps

Discussion in 'Fettling Forum' started by davidcolter, Mar 6, 2011.

  1. dikman

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    This is all good stuff, particularly the photos of existing slotted caps. I made a cut in some brass with a standard hacksaw blade, and although I couldn't measure it accurately, it's a bit less than 1mm, so would probably work. A Junior hacksaw blade is about 0.6mm, so will probably be a bit thin. As the slot will have an effective open area somewhat larger than an equivalent (vertical) row of holes, I'm wondering whether using Gary's existing 2 x 2 mm template would work - glue it on but cut down every second row, which would give 4mm spacing. Not sure how far to cut down, maybe three holes deep? Wouldn't be too hard using a fine-tooth hacksaw blade. The cap could then be soldered on, and if needed the slots could be extended into the cap slightly afterwards.

    However, Gary's idea of using a lathe is an interesting one. Fit the cutoff wheel into the chuck and make some sort of a clamp so the compound rest could be used to feed the tube into the cutter.

    At least I'd stop breaking drill bits!!
     
  2. idahostoveguy

    idahostoveguy R.I.P.

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    The thing about the PW stove and similar is that there's a mixing chamber/manifold that thoroughly mixes air/fuel before hitting the burner so many configurations can be realized on those burner heads. Another case in point is the Coleman HGP that has long narrow slots in the burner, but again there is a massive mixing chamber so that when the mixture arrives the burner will burn properly.

    So my point is that mixture before burn is critical to making the burner work properly in just about any burner head configuration.

    Just a thought...

    Here's the HGP burner from above:

    1278913275-HandyGasPlant457G-30.jpg



    Here's the M2A burner with slots:

    1254019042-m2a-lightshow0.jpg


    And one more weird one:

    1259469450-mini-212-10.jpg


    sam
     
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  3. pysen78

    pysen78 Subscriber

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    Yeah my first thought was with razor saws too. I used them a lot in my teens when building model airplanes. Should be pretty easy if you build a jig for it, but may well generate a blister or two!

    Those dremels seem to have pretty thin cutting disks. I don't own one, but it's a thought..
     
  4. dikman

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    I thought I'd give it a try (curiosity, and all that). Basically I used Gary's midi cap specs, but instead of drilling the holes I cut down on each alternate vertical row. I figured that without the webbing between each hole that should effectively double the open area, so cut down 4 rows (approx. 8mm). Using a standard hacksaw blade, the slots came out 1mm wide, pretty well the same size as when I drill the holes.

    1300587682-Slotted_burner1.JPG

    I had a minor mishap while soldering it - my neighbour came over to see what I was doing and distracted me slightly, enough that the heat stayed a little bit too long in one place and nearly melted a small bit between the slots (I won't show you that bit!). I also rushed it a little and didn't clean it thoroughly enough in places, but it was still enough to try the concept.
    The finished cap in my TK800.
    1300587696-Slotted_burner4.JPG

    Burning was interesting. It looked ok at moderate pressure, but when I opened the valve to max the flames started blowing away from the cap. (It simmered fine, btw). I swapped over to a stock cap, with holes, as a comparison, and that didn't experience the "blowing away effect, so it was definitely the cap.
    1300587720-Slotted_burner7.JPG

    Sorry about the slightly blurry image, I still haven't mastered the art of photographing burners in the dark!

    Next, I extended the slots along the top, about 4mm, to see what effect that would have.

    This burned a bit better, but still tended to "blow away" under higher pressure

    1300587741-Slotted_burner11.JPG


    1300587730-Slotted_burner12.JPG

    At one stage, when I turned the pressure right down, it almost went out so I opened up the valve and it turned into a roarer! Within about 10 seconds the cap was glowing red!! Impressive, but too hot and uncontrollable!

    In short, it will work, but may need more work to get the optimum construction method. I'm not sure whether I can be bothered persevering any further.
     
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  5. theyellowdog

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    Looks great, It may not blow out on a stove that allows the cap to sit a little deeper. Something with a larger bell or without fins. I would love to try this on my nova with the ground down fins.

    The glowing and roaring you discribe sounds like underburn, either flame inside of below the cap.


    I like it!
     
  6. dikman

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    Thanks. I'll be interested to have it dissected, so to speak, by others with more experience. I have very little idea what I'm doing (not that it's ever stopped me before!).

    If you lived a bit closer (!) we could probably work something out (any excuse to go for a bike ride :) ).
     
  7. davidcolter

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    Very nice work!
     
  8. Wim

    Wim Subscriber

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    Hi dikman, I think the "roaring" of your burner was caused by underburn. But a very nice attempt on making a cap that (in my view) makes the nicest flames, I very much like the burning pattern made by slotted caps! Somewhere in the back of my head, a little voice says one needs more but thinner slots to make it work ok.
    You mentioned a bike ride, would that be pedal or mobike?

    Best regards,

    Wim
     
  9. davidcolter

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    Something my dad has done is grind down the sides of a hacksaw blade on a coarse stone to take off the waviness of the edge. This cuts a narrower slot which he uses for making ukulele fretboards.
     
  10. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi dikman, nice experiment.

    OK, here is some flame theory:

    1. Every combustible gas or vapour mixture has a characteristic 'flame-speed'. You can measure this in a laboratory by timing the passage of a flame front down a clear pyrex tube filled with the combustible mixture.

    2. If you blow the mixture, at the correct velocity, into the pyrex tube you can maintain the combustion front within the tube. By slightly varying this mixture velocity you can move this flame front up or down the tube.

    3. If you apply too much pressure you will blow the flame-front clean out of the tube and may extinguish it.

    4. If you reduce the mixture velocity the flame-front will travel back towards the inlet side of the tube, and could vanish into the feed tube supplying the mixture.

    5. From this you can see that designing a burner, to operate over a range of outputs is quite difficult. You have a mixture of essentially fixed flame-speed but you are varying the velocity of the mixture exiting the slots or holes. You do this by either varying tank pressure, or using a regulator valve.

    6. At low feed-rates it is possible that the gas velocity gets below the flame-speed of the mixture and the flame dives through the apertures and gets under the cap, causing the dreaded under-burner, which can melt your burner cap.

    7. At high velocity you can exceed the flame-speed and push the flamelets off the burner, and possibly extinguish them.

    8. The clever burner designer has to chose the correct size and area of slots or holes, such that the burner can accommodate the full range of mixture pressures that the stove can achieve.

    9. If the slots/holes are too big or too numerous the stove may work OK at higher power, but as soon as you throttle back the mixture velocity exiting the apertures falls below the flame-speed and the flame will dive under the burner cap and cause under-burn.

    10 Make the slots/holes too small or too few, and at high pressures the velocity of mixture coming out of your slots/holes will be so much higher than the flame-speed that you will blow the flames off the burner and extinguish them.

    Here endeth the basic lesson on flame physics.
    Hope it helps.

    Best Regards,
    Kerophile.
     
  11. BernieDawg Banned

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    Big congratulations, Dikman!! :D/ I was really hoping you would give this a go. And, you did, with great success I would say. :clap: :clap: 8)

    Out of curiosity, I tried one, too.

    1300635364-IMGP7395.jpg 1300635375-IMGP7411.jpg 1300635383-IMGP7412.jpg 1300635401-IMGP7413.jpg 1300635410-IMGP7415.jpg

    In the pictures above the cap is in place on a Primus 96 with kerosene. I found that it worked well at all flame down to what I'd describe as a "high simmer" shown in the second photo. Below this point the flame would fall into the cap and go into underburn. I think perhaps a thinner slit is in order. I, too, used a hacksaw with a blade that was marked 0.8mm. Due to the clumsiness of this operator, the slots came out to about 1mm in width. I think I went about 10mm deep with my slits.

    For folks that don't fancy the jigging and drudgery of drilling a lot of holes, this is, I feel, a very accessible entry point to cap building. I hope others will try it, too.

    Cheers,
    Gary
     
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  12. davidcolter

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    The size of the hole/gap has another effect, in that the material quenches the flame and stops it passing through if it is small enough. The mesh screen of the Davy safety lantern for use in mines relies on this.

    A hole of a given diameter, say 1mm, will have a greater quenching effect than a slot of 1mm because a flame trying to pass through a slot is only quenched at the sides, not all the way round.

    This will account for the very thin slots seen in production slotted caps.

    I suspect the thickness of the material also plays a part in the quenching process, with a deeper slot having more quenching effect.

    I believe narrow slots in thick walled caps are the way to success.
     
  13. davidcolter

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    For something like an XGK that only runs at mid to high outputs, a hacksaw job would probably be sufficient. There are rather a lot of XGKs out there in the wild....
     
  14. dikman

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    Kerophile, thanks for the excellent lesson. I could visualise exactly what you were describing, and it helps me understand what my burner is doing. Whilst it's obviously not hard to build a burner cap that will work, getting it to work at it's optimum would appear to be a little trickier! Personally, I'm not looking at out-and-out burning power, but rather something with good simmerability (I think I just invented a new word) and reasonable boilability (another one). I don't think I'm too far off the mark with what I made.

    David, interesting idea with grinding down the hacksaw blade. It's obviously fine for cutting wooden frets, but I'm not sure how it would last on metal. Nevertheless, it's worth a try, I think. (Hacksaw blades are cheap enough).

    Gary, well done, mate. As usual, your finished product looks better than mine! Unlike you, I cut the slots from the top down, because I wanted to be able to extend the slots into the top of the cap if needed (and as mentioned, the burning pattern is really quite pretty with slotted caps). Mine is also slightly lower than yours, necessary to fit under the heating tube on my stove.

    One other thought I just had - you were using kero, I'm using Shellite (Coleman fuel). I know a slightly different jet size is needed for kero, but the Shellite has a higher volatility than kero and I wonder how that would affect the burning (and flame blowout)?
     
  15. anlrolfe

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    Who is brave enough to open the slots up bigger? Wrap stainless screen around the inside to keep the flame from jumping in. I really think it will work.

    AR
     
  16. dikman

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    @Wim, sorry mate, I forgot to reply to your post. Mo'sickle, pushbikes are hard work :whistle: .
    To be more specific, a couple of Triumphs - 2005 Bonneville T100 and a 2008 Tiger 1050. :D .
     
  17. Wim

    Wim Subscriber

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    Hi dikman, very nice bikes! I like the Bonneville a lot but will stay with my '71 BSA A65T ;) as I hardly find myself on the seat of my iron horse the last couple of years :oops: (It has a '69 L engine now as the original one is lying on my workbech for a couple of years now, waiting for the funds to fit an SRM convertion to its crank). There is a lot of activity re old British bikes in and around Adelaide as far as I know, any contacts with them? In early '96 I was on a day run with the local BSA owners, using a friends' A10. Great day! I an still hoping to be able to re-visit South Oz one day (maybe after my retirement ;) )

    Best regards,

    Wim
     
  18. Henry

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    Blimey George that was heavy ;) very informative though.

    I have a silent burner that has only 4 flame outlets :)
    Oh no sorry :doh: it's a roarer ;)

    Funny really, there's you lot trying to make silents and I've been messing around making a roarer from a silent :shock: :shock:
    Don't ya just love this hobby :D/
     
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  19. BernieDawg Banned

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    Thanks. I've had a lot 'o practice, I reckon. Yours looks great in my opinion. :thumbup:

    This is a good point. Kerosene may be contributing to a better burn. I'll have to throw the cap on some other stoves and see what happens on Coleman fuel. Busy, busy at the 'mo. Maybe later this week. I'll give a shout with some results when it happens.

    Cheers,
    Gary
     
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  20. redspeedster

    redspeedster United Kingdom Subscriber

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    I have a French gassie that has what at first glance looks like a standard silent burner cap. But on closer inspection the cap is lined with mesh.

    1300741215-DSCF1683_resize.JPG

    I think this is a measure to help control underburn and may indeed help in this very scenario.

    I have been thinking about silent burners for a while now and my mind keeeps coming back to a design similar to this style of spark arrestor.

    1300741832-strap1.jpg
    I've got some shim washers that are about the right size.
    Just got to get off my A&$3 and make one.
    But then again what about a mini coleman style burner with crinkle washers.
    So much to do so little time. ](*,)
     
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