Gas Jet Size

Hi all, I have purchased on ebay a Stuart HB4 Boiler to run my Stuart Victoria.The boiler did not come with a gas jet so I contacted Stuart Models who were very helpful and told me I need a No 8 Jet. After this I thought there must be a way of working out witch jet is required for witch boiler or burner, after spending an hour on the Net I have drawn a blank, I then thought I need the an Expert so hear I am, I have read the post by mick H on LPG Gas jet sizing but he seems unable to get anywhere. There must be some one who builds model boilers and reads this magazine that can explain how this is done or why we should not !!

Also dose the orifice get larger or smaller the higher number ?

Many thanks Keith.

PS mick if you have found out any information please come back

By doing a search for "LPG gas jet sizes" (Using duckduckgo.com) I found this information on the first page which might help. I think the size is in inches as some jet sizes are given as a fraction rather than a number and the decimal size in inches matches the fraction. I have never seen metric dimensions given as a fraction.

Les.

Edited By Les Jones 1 on 10/01/2017 13:04:31

Many thanks Les looking at the chart I would presume the size of the Burner is what determines the size of the Jet. I also have a vertical boiler of similar size but with much larger burner and a No 6 Jet it would be nice to know how to calculate it so as to arrive at the correct jet.

Keith

Hi Keith,
I thought that you just needed to know the diameter that coresponded to a No.8 jet. I have just looked at the chart again and realise that it does not say what units the numbers are in. I think the top line is the gas pressure and I would make a guess that the units are inches of water. I think the numbers under these columns are the heat output of each jet size for that gas pressure. As BTUs are shown at the top I would guess that the heat output would be BTUs per hour. (I have never heard ovf anyone using BTU per second or BTU per year for example) You would need to confirm these gusses before using the data for calculations. You could convert BTU per hour to horse power and from the power of your engine and its efficiency you could work out how many BTU per hour are required.

Les.

Many thanks Les, My Mathis is not that good but I know someone whose Mathis is so I shall ask her to have a look. It's not that I am stuck because Stuart models told me witch one to use but I would like to understand how they arrived at it.

Keith

I have not got much further on this subject than previously outlined but the following may be useful. After a lot of messing about experimenting with different jets I finally settled on a number 8 for my Gauge 1 V2 loco. The boiler on this loco is about 11 inches long and approx 2 inches diameter. The ceramic burner is approx 2 inches x 2 1/2 inches.

The following notes may also be of interest. As designed, the boiler is of the sealed system type where all the air required for combustion (primary air) is sucked in by the gas jet. A number 8 and a number 10 jet failed miserably and from the fumes coming out of the chimney it was quite evident that nowhere near complete combustion was taking place. I then fitted a number 5 jet which although smaller, sucked in more air as the gas velocity through the smaller orifice was greater. This improved things somewhat but performance was not as I expected. I then decided that I was not happy with the sealed system and remade the ceramic burner to allow for some secondary air to enter the firebox. The smokebox was also altered to give a Stephenson effect. Now there was not enough gas for the air available and so the number 8 was refitted and the whole thing has balanced out quite nicely with a nice cherry red glow to the ceramic burner. The gas used is butane with a liquid feed from the bottom of the gas tank fed to the burner jet via a preheater loop silver soldered to the firebox. This is so much easier to manage than taking gas from the top of the gas tank with the consequent drop in pressure as the tank cools.

In short I would say that a number 8 is capable of delivering a lot more gas than my loco will take and I would certainly not argue with Stuart Models.

If you do come up with any information enabling a calculation to be made matching jet to application I would love to see it. I got rather bogged down trying to research this subject and finally decided that as I had found a solution to my immediate problem my time would be better spent on other workshop matters.

Mick