Random Thoughts on Steam Injectors

Julian, I am sorry I did not respond to your earlier post on lifting injectors. It may help if I explain why I asked. I recently was allowed on the footplate of a full size showmans traction engine, and was asked to put some water in the boiler. all I needed to do was to open the steam valve, and the injector picked up and did its job. There was no need to open the water valve. I queried this and was told the injector was a lifting injector. I have to open the water valve then the steam to add some water to my miniatures. if I did not shut the water valve when I was finished the water would drain away through the injector. This did not happen on the full size engine. I assumed the description Lifting injector meant there is no need to shut the water valve as the injector was above the water level, but on reflection this is probably not right, so I am missing something I guess, but what?

Chris Gunn

hi chris,

ive sent you a PM

cheers,

julian

Andrew you wrote:

"Some while ago I considering buying a small bench top injection moulding machine just to play around with, but I suspect they would be inadequate for PEEK?"

You are correct, a benchtop prototype inj moulding machine will not give you the precise control and will not likely give you the high heat required for inj moulding PEEK. (they will mould many other common plastics but not the high temp medical ones like PEEK and PSU, etc.)

A better bet would be to go to an inj moulding firm or mouldmaking firm and tell them what you are up to, be sure to mention you plan to mould PEEK, ask them if they have a standard insert holder, get the dims for inserts to fit it, and design or have designed an insert set. Make the inserts then take them back to the moulder and ask them to make a proto run in PEEK, when they can fit it in their production schedule. If you can give them the processing datasheet for your particular inj moulding grade of PEEK at first meeting, and preferably give them a bag of pellets you have purchased when you are ready to mould, they will likely be able to turn it around quickly. JD

" So I would think that a plastic body should survive, at least for experimental purposes?

Andrew"

It might survive, I believe others have done it, I recall seeing pics in ME or EIM of a clear bodied injector test bed. However if you do this is is at your own risk, as by the manufacturer's datasheets for normal clear acrylics and polycarbonate indicate they are not rated for use at high temps or pressures. If you did make a clear plastic injector, I would recommend making a 1/2" thick polycarbonate blast shield to stand behind while running it under pressure, big enough to shield your whole body in case something goes wrong. Even with being behind that shield I would stand a few feet away and let a camera do the up close image recording, and make sure there is no one else nearby. I'm not kidding, it is that dangerous to operate clear plastic enclosed vessels with pressurized steam. JD

andrew cant read this because he blocked me some time ago apparently,

but very little will be gained from observation. observation of concentricity of the jet from the combining cone is interesting as is the same from the delivery cone to check machining. both john baguley and myself have conducted such experiments.

far more will be gained by using modern pressure probes to ascertain how the innards are working throughout the length of the injector. also the temperature of the overflow which is fundamental.

if anyone else cares to relay this to andrew it might save a lot of effort.

cheers,

julian

Edited By julian atkins on 05/01/2016 00:37:20

Edited By julian atkins on 05/01/2016 00:52:20

Julian wrote:

"far more will be gained by using modern pressure probes to ascertain how the innards are working throughout the length of the injector. also the temperature of the overflow which is fundamental."

You have tested these two things, to KNOW these things to be true? if so please share data. If not please present your opinions as opinions.

I have very little knowledge of injectors myself, but I have learned in industry not to trust dogmatic opinion statements like yours above unless they are accompanied by an apparatus list, test procedure, and test data. JD

Jeff,

i know how injectors work. i dont have to experiment or guess these days. i was taught how to make them by one of the best and most experienced of miniature locomotive injectors makers 32 years ago, and have been making them myself and for friends and repairing defective commercial injectors since.

you are welcome to have a drive of any of my miniature locos fitted only with 2 100% reliable injectors with no hand pump or axlepump.

the only issue with andrew's injectors is the higher steam pressure/working pressure and hence greater working range. as many fullsize injectors dont work as well as decent miniature injectors i dont foresee any particular problems. the fullsize injectors could have learnt a lot from miniature injectors in the same way correct loco draughting in miniature was well in advance of fullsize 115 years ago. the trick is very fine machining tolerances and concentricity, correct choice of taper angles (which are all well established both in fullsize and miniature), correct proportions and and a few tricks for a lifting injector, and a very good finish on the cones plus accurate cone throat sizes.

as previously stated, all this was known over 100 years ago.

i have spent 30 years experimenting with these gadgets especially when dealing with repairing odd commercial products.

the proof of a reliable working injector is how it works. you can carry out all sorts of tests to check things, but apart from checking overflow temperature nothing beats a sound understanding and knowledge plus checking whether it works!

cheers,

julian

Edited By julian atkins on 05/01/2016 01:44:58

How do you intend to communicate these valuable lessons you have learnt over 30 years? This is a forum dedicated to learning and open communication rather than trumpet blowing, so there's little benefit if you croak taking all this knowledge with you. Sound understanding and knowledge is what I think is being asked for.

You say "the trick is very fine machining tolerances and concentricity, correct choice of taper angles (which are all well established both in fullsize and miniature), correct proportions and and a few tricks for a lifting injector, and a very good finish on the cones plus accurate cone throat sizes." but are you able to explain to design a nozzle from scratch for a particular application or can you only say "this is one that works, copy it exactly and you will be lucky"? That is the difference between an engineer with profound knowledge and a technician who simply copies with no understanding.

The words pompous and Mount Olympus spring to mind.

i have not the slightest intention of sharing on here 'these valuable lessons over 30 years' if i am only a 'technician who simply copies' (thanks, Muzzer)

i am quite the contrary.

there is no point in me spending ages typing stuff for Andrew's benefit, much as i would like to help him, if he has blocked me over 12 months ago.

i have shared quite a few 'valuable lessons' on the modeleng proboards site last year re injector making. anyone interested can do a search on there and PM me either there or here, and i will try and do my best to pass on all information and experience for those wishing to make their own miniature injectors.

my post bag usually contains something along the lines of the following - 'here's a duff injector i bought - please can you fix it for me'!

most of these examples are anything but technician copy jobs - which in the case of injectors is quite something in it self if a copy job. it is very often a case of working everything out from basic principles and applying same to achieve a working injector of non-standard dimensions and proportions.

ive done 14 in 2015 for friends which in each case has held up progress on my own current miniature loco project, but i do it gratis because i hate to see injectors that dont work on our miniature locos.

anyway,

nuff sed!

cheers,

julian

I'm still looking at injectors, although in general not bothering to post. However, I have recently bought a reproduction of a book first published in 1893. As expected it contains a fair amount of maths, including calculus. But the interesting thing that struck me is that a lot of drawings of early injectors, and the analysis, use a steam cone that is convergent only. There is no, or only a very short, divergent cone. So that limits the steam at the output to about 0.6 of the input pressure with a maximum velocity equal to the speed of sound in steam at that pressure. Clearly these injectors work, but I've never seen a model injector without a convergent/divergent steam cone.

Since the output pressure of the steam cone is well above atmospheric pressure these injectors cannot be lifting. But that may well not be a problem for a traction engine where the injector tends to be low down on the tender side.

Fascinating stuff.

Andrew

It's good to see that you are persevering with this, Andrew

... Not my area of interest, but I admire your 'need to understand'

MichaelG.

That's because I'm an engineer, not a modeller. While it is satisfying to make parts, generally that's the easy bit. What really interests me is how and why the parts are designed the way they are. Plus the drawings are inadequate, and just plain wrong, in some areas. So I am modelling the whole engine in 3D CAD and re-designing along the way as needed, or for interest.

There's no way I could be described as a model engineer, in at least one sense, and probably both senses.

Andrew

Hi Andrew,

I have only recently found this thread on your musings about injectors and applaud your inquisitive approach. It might already be known, but the only way you will really understand is to question things and work it out.

Way back you asked about the reason for a particular divergent angle on the steam cone. The answer is to be found in the simple truth:

"Accelerating fluid flow is easy, Decelerating fluid flow is damned difficult."

I have spent a working life in fluid dynamics finding that one out.

More specifically, in a conical diffuser once you reach a certain cone angle you will get flow separation - the boundary layer start to detach at some point downstream of the throat. This is precisely the same effect that causes a wing to stall at too steep an incidence. Initially you will get instability (buffeting) followed by complete BL detachment (stall). Obviously, in a cone of fixed angle you don't have the option of pushing the stick forward to get out of it. What you can do is keep the cone angle below about 10 - 12 degree included angle. Less than this angle and you will get a reasonably controlled (and hence efficient) reduction in flow velocity, turning the velocity energy back into pressure energy. There are charts published of loss coefficients Vs. geometry which show the detail of this effect.

I quite liked your explanation of why a steam nozzle must be convergent - divergent to extend the choking point. You are actually using Bernoulli's theorem to drop the exit static pressure at the throat, hence increasing the p1/p2 ratio which extends the sub-sonic flow regime. Not my specialism, but I hope this provides an alternative way of looking at it.

I have recently been looking at draughting, which is another application of the jet pump (and another area that everybody knows everything about because Kank and Snart said so in 1834). Anyway, one of the more reliable papers indicated that for good cavitation performance the area for the entrained fluid inlet was critical, which translates to radial gap between steam cone and mixing cone, plus axial standoff between same. This makes sense because the lowest pressure area is exactly there. You are trying to entrain water into a jet of steam moving at silly velocity and if you drop the static pressure below vapour pressure, cavitation (and hence loss of performance) will follow. This also explains why lifting injectors or hot water injectors are a *** to design because these areas will be critical.

By the way, I think you need to uplift your design water flow. You have not allowed for cylinder condensation which is likely to put up the water demand by another 50% or so, even allowing for steam jacketed cylinders. See the paper by Bill Hall "Measuring Steam Engine Performance"

Keep up the good work and the analytical approach. I copped out on injector building, there is a unit by Terry Baxter's old firm (He of "Injector Man" fame in another place) which is the vicar's knickers. Works beautifully and is encased in a lost wax casting which is a dead ringer for a Gresham & Craven vertical injector as used on tractions.

Best Wishes,

Martin

Martin: Thanks for the detailed explanations. I'm on holiday next week, but the flying weather isn't looking great, so I may well have a lot of time to revisit injectors. Just for completeness I might just finish off this thread by looking at the combining cones and delivery cone.

Your notes on the divergent cone angle are helpful; they've clarified what I sort of understood, but not well enough to expound on it. Regarding aircraft wings in the 1990s there were a number of gliders that didn't exhibit pre-stall buffet. In the UK they couldn't be flown without an external stall warning device. That's a small flap on the leading edge of the wing that flips as the stagnation point moves.

I hope you're planning to write up your notes on draughting and other areas in due course, presumably for ME.

The injector size was based on what TerryB told me. He kindly said I could use his bending rolls for my rear wheel rims. While I was there we naturally talked injectors. He asked about my engine and immediately gave a size. That got me wondering how he knew that, and is what started me on the calculations.

I'm still not sure if I'll ever actually build an injector to my own design. But I remain cautious about commercial items. A friend, who is building the same engine as me, bought one from a well known supplier of boiler fittings. Apart from being the wrong hand and other issues, it looked quite rough, not what I was expecting to see for a premium product. To be fair it was never actually used, so we don't know if it worked.

What I really need to do is retire and then I'd have time to look at this stuff properly.

Andrew

MJ -"This is precisely the same effect that causes a wing to stall at too steep an incidence. Initially you will get instability (buffeting) followed by complete BL detachment (stall). Obviously, in a cone of fixed angle you don't have the option of pushing the stick forward to get out of it."

I have a feeling that there is a confusion between 'incidence angle' (wing to body) and 'angle of attack' (wing to airflow). Push the stick forward as hard as you like, but you won't alter the incidence angle until you strike terra firma!!

Interesting correlation with flying since modern aircraft fuel systems commonly use 'jet pumps' as tank scavengers and the principles appear to be quite similar

rgds

Bill

How do you calculate the appropriate sized injector to boiler ratio? I want to build a boiler and purchase an injector from a manufacturer as they seem too difficult for me to build.

I have found an equation: W = (xHv + (Ts - Twd))/Twd - Tws

Here: W = weight of water

x = dryness of steam

Ts = Temperature of steam at absolute pressure

Twd = Temperature of water discharged at injector

Tws = Temperature of water suppled to injector

Hv = Enthalpy of steam at absolute pressure

This has given me some clarity in how to calculate the flow rate of the injector. I understand that the boiler must be appropriate to fit this flow rate. I don't want the boiler to lose pressure or temperature because the flow rate is too fast for the boiler. Is this the correct path to try and understand all of this?

Suggest you find a copy of Mr. D.A.G. Brown's book "miniature injectors inside and out" which has a guide to injector sizing as well as all constructional details needed.

"find a copy of Mr. D.A.G. Brown's book"

A PDF is also findable on the NET

No idea where your equation came from, but I'm not convinced it's appropriate. The size of the injector is determined by the amount of steam used by the engine. See the first post in this thread for calculations. If the boiler can't provide that steam, or can't accept the calculated injector flow without losing pressure, then the boiler is too small. It is normal to size the injector slightly above the nominal flow rate needed, so it can be used intermittently.

Andrew

The equation is an approximation of flowrate of the injector. It shows (approximately) the "feed" to the boiler.

I found an injector that I want that claims:

"Hobby Boiler Injector by D & S of Manchester UK. Feed lines are 3/16" and the Steam line is 5/32". Operating range is 75 to 100 Psig and can deliver up to 22 ounces a minute."

If I construct a boiler that is a cube (128 in^3 volume) that is holding roughly 2 lbs of water:

Would I have to ensure that the outlet (supply steam to do work) is greater than the inlet (from injector)? I assume I must... Is there some sort of typical ratio?