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Steam water feed injectors

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I've just completed making some steam injectors for my 5" gauge locomotive.

Injectors are a very cleaver little device that uses the flow of steam through a series of cones to pick up water and to feed it into the boiler against the boiler pressure, it's all to do with changes of velocity and pressure I won't even start to explain the physics of it as its way beyond me it enough just to know they work.

This is what they look like.

They are notoriously difficult to make to work as they have critical features that are difficult to actually measure, you have to get to these features in a round about way, and as I found out there are other pit falls just waiting to catch you. I did have a great deal of help from a friend and very talented model engineer (Pete) who gave me a great deal of advice as to the best way to go about making them, he also has a injector boiler test rig, that greatly help.

The first part to make is the body:- this is fabricated from some 3/8" thick brass bar.

First thing is to square the bar up and centre for the 5.5mm through hole postion

Then in the four jaw set up on this centre with a wobble bar.

Turn up and thread 5/16" * 32 ME and ream through 5.5mm

Then drill through for the overflow chamber and clack valve and tap again 5/16" * 32 ME.

Drill through for the water feed and the overflow and make some 5/16" * 32 ME. connectors

Silver solder the connectors and overflow to the body and drill through.

Make and fit a cap for the clack valve

And that's the body made.

Next up I'll show you how to make the taper D bits and how to make and fit the cones.



I've been looking at the theory of steam injectors, with a view to possibly designing one for my traction engines.

The isentropic expansion of the steam in the steam nozzle and the pick up of water by the resultant low pressure, but high speed, steam jet seem fairly straightforward. What I haven't completely got my head round yet is the conversion of the high speed jet back to a sufficiently high pressure to feed into the boiler. How can a flow of steam from the boiler, at a given pressure, inject a larger mass flow of water into the boiler at, or slightly above, the same pressure? I think it has to do with the enthalpy of the incoming steam. When the steam jet is condensed by the feed water this heat energy is given up to the water and it is this extra energy that allows the water to be injected into the boiler. So by this argument an injector wouldn't work using compressed air, as the air would not contain much heat energy.



--- Quote from: jadge on June 16, 2015, 11:51:49 AM ---How can a flow of steam from the boiler, at a given pressure, inject a larger mass flow of water into the boiler at, or slightly above, the same pressure?
--- End quote ---

Don't forget that the water is cold, when the hot steam draws in and mixes with the cold water it forms a vacumme, which in turn sucks in more cold water the combination of which are at a higher pressure than the original steam, this higher pressure mixture can then force its way into the boiler  ;).

Which is why they don't work with warm water  :ShakeHead:


Good work there Stew

Made a few in the past all but one worked , I could not solve it so it went in th bin

A good read on the subject by DAG Brown

LBSC though he knew how they worked but a lot of his info was flawed , DAG Brown has it sorted , but you do need a injector rig to test/prove the flow rate

A major fault is faulty soldering ,pin holes and flux inclusions that are ok at first but fail in service

The last set I did was built for my class 4 but built into a scale body with the injector itself pushed in and sealed with o rings very small section ones , sorry I have no pics as they went to a new home but not on the loco when I sold it

Have fun but remember the full sized ones with the crocodile mouth work very well but do not scale , as the size gets smaller they get harder to make at 7 oz a min it's tricky

The other problem is the steam pressure range as you do not always have full steam that's why you need to build a test rig

They do need a clean regularly with very dilute citric acid NO poking the cone with a wire please
And a water filter is a must as is a good clack with a larger lift than used with ram pump
The sound of a injector feeding is music to my ears ( if they still worked)

Got the popcorn out for the long haul


Hi Stu,
Back when I was into steam and built my traction engine I studied the operation and making of injectors. At that time I didn't know enough to even try and with the intricacies of the jets I don't know if I'd still want to try. I'll be following along to see your procedures.


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