Hi All, Just back from a couple of weeks holiday and having cleared the mail I now have the correct 0.23mm jet. Many thanks to John "threedots" who offered and sent me this new jet and didn't want any payment! Cheers John! As you can see the issue has now gone completely: And the eyebrow singe shot: You can see how now the flamelets have separation from the burner and no more overheating. This seems to also clarify that a five row burner cap is equivalent to the four row, at least for the No. 4 burner. I'd assume that the four row has bigger holes but my original was too degraded to tell. Thank you to all those who gave the valuable input that resulted in this success. Cheers Colin
Hi, There are a lot of experimentalists in CCS and I carried out the same "over-jetting" experiments as John when I recently fettled a 1931 Primus No.4. I was using a NOS "Con" steel cap, a good inner cap, and brand new Primus/Optimus 0.32mm and 0.23mm aperture jets. These photos were taken at high stove pressure/power, approximately 5 mins into the burn. In the case of operation with the larger jet I would not want to operate for much longer! First we have operation with the 0.32mm aperture jet: You will see in the above photos that the No.4 silent burner is over-powered by the larger-than-specified jet, and the burner cap is getting seriously hot! I do not believe we have under-burn but obviously an excessive amount of heat is being transferred to the outer cap. After having a cup of tea, and allowing the stove to cool of, I replaced the jet with a new 0.23mm aperture model, fired up the stove and took these photos: It can be seen that despite running the stove at full power the burner can comfortably handle the quantity of fuel provided. There is a large powerful flame-pattern but it all looks under control and we do not experience a glowing outer cap! There can be no doubt that the No.4 burner was designed for a 0.23mm aperture jet. Aren't stoves fascinating? Best Regards, Kerophile.
Hello Colin and glad it all worked out for you in the end. Your topic enlightened myself and other members to what can cause the symptoms your burner was showing. "Presscall" was on the button with what was wrong with it. He has since posted some interesting pictures and analysis in another topic in this forum, showing how an over-sized or worn jet would cause a Primus No.4 burner(and other silent burners), to overheat without showing the usual flame yellowing that shows an over rich fuel to air mixture. Cheers, John
Hi, I have been giving some more thought to why the outer cap heats more with the fuel-rich flame than the desired blue flamelets. If you look at the photos I took of the same burner operating with different jet sizes you will see that the shape and location of the little flamelets in similar with both jets: the flamelets are actually "pushed off" the burner and are some distance away from the cap holes. This indicates to me that we do not have combustion under the cap, and certainly no flame directly from the jet as you do in classic under-burn. What is different about the two flame patterns is the colour of the flamelets. The over-size jet produces fuel-rich yellow/orange flamelets, whilst the 0.23mm aperture jet produces the desired blue flamelets. Yellow/red flames emit far more radiant energy that do blue flames...that if why infra-red sources are used in radiant heaters. Think of the asbestos or wire elements of Bowlfires and Tilley heaters. I reckon the reason that the cap get very hot with over-jetted silent stove burners is primarily because of the excess radiant energy from these fuel-rich (yellow-orange) flames. Best Regards, Kerophile.
I'll add a rider to that if I may, George. The greater thermal conductivity of a brass cap as opposed to steel (such as you used in your experiment) better absorbs the excess energy from overfuelling and the flames remain blue, but the cap glows red, as with my over-jetted No.4 stove and Arachnocampa's brass-capped example before he installed the 0.23mm jet. Here was my stove in that state John
John and George, I am glad you fellows are researching this overheating issue. I must say that what I thought I had begun to understand about silent burner operation has been completely thrown askew by what you are discovering. Here are some results of my own posted not to be argumentative, but, instead, in hopes that you can explain what it is that I'm experiencing and perhaps where I've gone wrong on my thoughts on silents. I have only one R132 burner that is resident on a 1921 Primus 34. The stove was mistreated by it's previous owner having been over-pickled to pinkness, sandblasted and brushed with polyurethane varnish. This is the best I could do to bring it back. It is good these pictures are small, I reckon, since it looks much better from a goodly distance. The burner has been cleaned of all carbon deposits (heat and air). The inner is, I suspect, a modern Primus metal inner as it does not have the steep sides of the R675 shown in the 1922 catalog in the Reference Galley. The jet is snug with a 0.23 pricker. The lid is a Chickenthief reproduction which fits not so good, but ok enough to get the job done. Burning shots at moderate, high and up-skirt (to demonstrate the glowing skirt). Swapping the jet out with my handy bench stove (a tatty Opti #1 Traveler), I get these shots: Moderate flame: Full out (wet paper towel on the tank to mitigate heat on this little 1/2 liter stove on a long burn): Half a second after releasing the pressure from the "full-out" burn: Inside that yellow haze of vaporized but overrich kero, I'm not seeing any overheating of the perfed ring. When I shut the stove down, I'm not seeing any overheating of the perfed ring. This would be consistent with my "theory" that an oversized jet will be pushing both more fuel and more air through the ring than a standard jet. The additional fuel/air (at a much lower temperature than the combustion zone) should act to cool the perfed ring rather than bring it into incandescence. The higher pressure and velocity of the fuel/air should also propel the flamelets farther from the perfed ring, and thus, remove the heat away from that area. This seems to be the case in my example. But, it does not seem to be the case in Colins, Georges or Johns. Admittedly, my results are the "off" ones and I'm certainly willing to concede that I'm incorrect in my theories or doing something wrong in the operation of my stove. Please help me understand how I've goofed up here. I am struggling to rearrange my thinking on this and could use some help. I am very confused. Thanks! I have a brass Primus cap for the #5 size stove in answer to John's question posted somewhere, so I reckon they made them. Cheers, Gary
Yes, found one myself, Gary! This from my latest post on the 'underburning' thread Underburning in a silent burner cap It's very interesting what you've just said and demonstrated, Gary, but I think there's a chance it's not contradictory evidence. Let me have a go at getting this right. Running the oversized jet on that burner with the Chickenthief cap (I've one of those on this Optimus 5J that you kindly contributed the inner cap to you'll remember Opti 5J with Chickenthief outer and BernieDawg inner caps ) you've shown that at moderate tank pressure/burner setting the extra fuel energy makes the cap glow red and give out blue flames. On high tank pressure/burner setting with greater fuel energy the flames become yellower (fuel rich) and the cap cools down to below red heat, so the excess combustion energy has found a new emission path in which the flames become luminous instead of the cap glowing. Why it does that is because at a certain optimum pressure it must be easier for it to go that way rather than the other, rather like a trickle of hose-pipe water down a sloping path will take one route, then if the tap's turned on a bit more it becomes more of a stream and takes another route. As you and George have often pointed out and expounded so eloquently, the characteristics of combustion and fluid (gas) flow in a silent burner are complex and sensitive to variation from 'blueprint'. The pressurised, burning fuel vapour and air mixture has such a tortuous route to take from jet to atmosphere in a silent burner, as opposed to a roarer or lipstick burner. Something like a rust patch or pitting on the roof of a silent burner cap can sabotage the ideal case and yellow flames and often failure to continue to vapourise can result. I believe the failure to vapourise can happen because the excess fuel energy is diverted into producing the luminous flames which don't conduct the heat back to the burner tubes as well as blue flames and the right fuel/air mixture for combustion do. Why does your burner with the Chickenthief cap shift a gear and put out luminous flames after going through the 'red cap' phase, whereas my and Arachnocampa's No.4's don't? Henrik's (Chickenthief's) cap was a masterly production, but it's less 'blueprint' than my No.4 outer cap or Arachnocampa's brilliant adaptation of a stock outer cap. That might well be the reason, is my view. Ok, I've reached a point where my head's glowing red and luminous flames are about to sprout, so that's enough from me for now! John
Yup. These silents will do that. I'm at the same point having just (barely) made it through your well-thought out explanation before my head exploded. I'll concede the point for now and give it some more thought as time passes so I don't overload my brain cell. If I come up with anything that sounds reasonable, I'll let you know. Thanks muchly for the indulgence. And, good on ya for continuing to post these fascinating topics. Cheers, Gary
Excellent test John....and right on the money. I had this exact same problem with the exact same results with the Primus 4. The cap was getting far too hot. Your jet/nipple solution was correct. I went through a box of parts and found a little bag with misc. used jets, I salvaged from other stoves. One by one, I went through each one until I found the smallest opening...possibly from an old 8R or something. In any event, it worked like a dream!
