Forrest also sell the jets

Yours are metric ones hence the M6 x 0.75 thread, the ones from Forrest etc are the older imperial, though some will say BA is metric

I've not found them but if an ANSI one also exists I expect that will be different again and have UNF or similar threads

Hi Michael, Thanks for the posting, I did some more digging and decided to go metric, I only use imperial when necessary such as working on Stuart models, even then as my machines are metric I have to translate which is not a problem as i had to do a lot of that in my early engineering career when the uK was entering the modern world . I found an 0.35mm jet which he produces and according to information I can glean from the mysterious world of gas jets it equates to a #16 which seems to be recommended size for the ceramic burner I'm constructing. They're not too bad a price either especially as delivery costs are included. I can afford to experiment somewhat with different jets, I know I could look and learn more about them but life's too short, I'd raather be making than learning too much theory these days.

I'm not really sure that the jet size matters that much as long as they are not too small as the gas flow can be controlled by the regulator valve surely? I don't really understand being a tyro in this part of the hobby, I've generally used spirit (meths) burners or gas poker burners in the past.

Regards

TerryD

Welcome to the wonderful world of Gauge Size numbers. They're deeply confusing. They were conceived in the first half of the Industrial Revolution before the need for full standardisation was accepted. Almost every trade and many individual companies created their own system of gauge measure and there isn't much consistency between them. Old engineering reference books are full of different Gauge tables: back in the day it was a huge mess, and despite much tidying up Gauges still have to be approached with caution today. Electric motors can be rewound with a choice of American Wire Gauge, Standard Wire Gauge or Metric magnet wire. Metric is simpler because it's the actual wire diameter in millimeters, but AWG and SWG are less rational. Gas jets suffer the same problem – they're an old technology, identified using ye olde gauge numbers, or metric.

The jet size does matter, usually. The ratio of air to gas is important so the size of the jet depends on the type of gas being burnt, the size of the combustion chamber, and the size of the air-input. If too much gas is in the mix, the flame either won't ignite at all, or burns smokey yellow, producing low heat and poisonous Carbon Monoxide. If too much air is in the mix, either the flame won't ignite, or a large proportion of it's energy is wasted heating excess air which then disappears up the chimney. To get reasonable efficiency the mix, combustion chamber and heating surface have to be balanced, and the mix is also a balance.

However, a poor design that would be completely unacceptable on a central heating boiler, might well be 'good enough' for a small model steam boiler. If inefficiently burnt gas heats faster than meths the owner will probably be happy!

One thing to watch is that gas flames are considerably hotter than spirit burners and if a concentrated gas flame is played on a small part of the boiler, it might put more heat into the metal than the water is able to to remove by boiling. This could cause the boiler to fail by softening the metal, or maybe melting a nearby soldered joint. (Spirit fired boilers don't expect gas flames and might use soldered joints. Gas and coal boilers are stayed, brazed and dimensioned to take the heat.)

I would guess a small combustion chamber would need a small jet, or several small jets, rather than one big one – for mixing.

Dave

Edited By SillyOldDuffer on 28/09/2022 09:54:58

Hi Dave,

Thanks for that explanation it starts to reduce the fog to a mist. I can see the simplified theory of gas to air ratio but if you have a gas jet that is a bit oversize surely you can regulate that by varying the gas flow and the Bunsen style air control on a burner?

At the moment I'm considering a ceramic style burner using the standard gas fire radiant which should with the right distribution of gas create a less focussed heat source. As for the boiler construction the Stuart 504 it is intended for seems to be well constructed and there are many types of gas burner on sale including Stuart ones to replace the original meths burners so it should not be a problem. My own boilers which I have constructed so far are basically pot boilers with no external boiler tubes (although I might get a bit more adventurous) silver soldered with Easy Flo or equivalent so I think that they should be ok.

I also thought that perhaps gas jet sizes were related to an older standard such as number drills or wire gauges but wire gauges get smaller as the number increases while the opposite is true of gas jets i.e. from memory in SWG 10g is 1/8th " while 16g is 1/16th" and 21g is 1/32nd". not sure about AWG or BWG but I think that they approximate having used AWG electrical wire in the past.   In the case of number drills they do not seem to relate to older jet sizes, again diameter is inverse to number. I find it all quite confusing hence my decision to stick to metric where there is direct correlation between size and number – my simple mind can understand that much at least

Regards

TerryD

Edited By Terryd on 28/09/2022 13:52:54

The jet size and gas pressure not only govern the volume of gas ! Another criteria is the speed of the gas and it's abillity to entrain/mix with air and burn. This is an interesting balancing act ! A book on the subject is "gas burners for forges,furnaces and kilns" Though for much larger burners than you have in mind it gives and insight into the design. Also Chris Chastain s book on gas fired smelting furnaces. Noel.

Well whilst the metal under the flame does cool through the edge, I think the effect is small compared with cooling by water. Say a flame heats a circle of metal 25mm in diameter, and the metal is 1mm thick. Then the edge area of metal available to conduct heat away into boiler's metal work is about 78 sq mm compared with the 490 sq mm surface covered by water. Also, water absorbs a great deal of extra heat (latent heat) when it changes state from a liquid to a gas. The cooling effect of water is much greater than the cooling effect of the boiler's metalwork, which is why radiators are often water filled.

Examples of concentrated heat causing the temperature of a small area of a metal sheet to rise well above that of adjacent metal include gas-axe cutting, spot welding and laser beams. Severe damage is likely whenever a boiler runs dry, but it will also occur if a flame puts energy into the metal of a boiler faster than the energy can be removed by cooling. This graph shows the yield strength of copper falling dramatically with rising temperature:

So quite safe to operate a copper boiler between 100 and 150°C, but it's lost about half it's normal strength at 300°C, and falls to zero at about 700°C. As that's much less than gas flame temperature, close to 2000°C, it's important the metal of the boiler be kept cool. Absorbing heat by having it boil water is a good way of doing it.

Dave

Hi Terry, I have had a quick flick through all 3 of S Chastains books they would not really be much help as whilst they deal with combustion of fuels it is on a scale way bigger than what your looking for. The first book I mentioned is the best but very expensive – in part because it is one of the few that covers the subject in some detail and in practical terms but again is on a scale much larger than you need. If you can get it from a library that would be the way to go. One clever and cheap idea is the use of MIG welder contact tips as a jet,( 0.6mm is the smallest ) ! Smooth flow and the need for ( I think )a 15 times jet diameter straight run to allow the gas to accelerate before exitting the jet as a true cone and smoothly entraining air. Good luck and best wishes Noel.