Injectors

[Eric CoxParticipant @ericcox50497]

A simple explanation

[Eric CoxParticipant @ericcox50497]

For ignoramuses like myself can you explain in "simple" terms

1) What does an injector do

2) How does it do it

3) What does it mean if you have water and steam coming out of the overflow.

[julian atkinsParticipant @julianatkins58923]

1) it squirts hot water into a boiler

2) a jet of very fast moving steam condenses in water and the resulting very fast moving jet of 'water' is converted to a slower moving jet of sufficient pressure to overcome the boiler pressure.

3) it isnt working.

cheers,

julian

[Stuart BridgerParticipant @stuartbridger82290]

I'm glad someone was brave enough to ask that question

[Michael MallesonParticipant @michaelmalleson22793]

At last, an explanation without any mathematics, none of which helps anyway when the darned things just won't work.

Mike.

[Paul LousickParticipant @paullousick59116]

Injectors are designed to work at different pressure ranges and should be selected to suit your operating pressure. They often do not work when too hot. (turn the water on first, then the steam. Then steam off first and water off last) (unless it is a lifting type injector with only a steam valve). If not working they may need cleaning by dismantling and soaking the cones in vinegar to remove calcium deposits.

[Mark P.Participant @markp]

I thought that there was an imp inside with a little pump.

[Nigel BennettParticipant @nigelbennett69913]

Apart from trawling back through ME over the years for Lawrie Laurence's and others' articles, "Miniature Injectors Inside & Out" by DAG Brown is a mine of information on the subject. He also covers fault-finding and the design of water valves. Even if you don't intend making your own, it's probably worth a punt.

[Neil WyattModerator @neilwyatt]

Want a non-mathematical description of the physics?

Hot steam is squirted out of a nozzle, sucking up cold water. The water has to be cold enough it doesn't boil, but mixes with the steam to form a powerful jet of hot water, collected by another cone, that 'punches' its way into the boiler past a valve.

Neil

[Anonymous]

Posted by Neil Wyatt on 06/10/2015 15:25:45:

Want a non-mathematical description of the physics?

Hot steam is squirted out of a nozzle, sucking up cold water. The water has to be cold enough it doesn't boil, but mixes with the steam to form a powerful jet of hot water, collected by another cone, that 'punches' its way into the boiler past a valve.

Neil

Ah, but how does the non-mathematics explain the following? We have two input streams; a mass of steam at boiler pressure, so it has stored energy due to the pressure, and a greater mass of water at atmospheric pressure, so no stored pressure energy. We end up with the combined mass of steam and water at, or more than, boiler pressure, so lots of stored pressure energy. Where does the extra energy come from?

Andrew

[Neil WyattModerator @neilwyatt]

I know this is a trick question, but I deliberately didn't complicate my original answer with the high efficiency of injectors…

What extra energy?

If you turned the boiler heat off and ran the injector the boiler would cool down, and lose pressure, more quickly than with the injector off.

It's a very efficient process, but some heat will always be lost through the injector pipework and the injector itself.

If you tried to stop this by insulating the injector, it would get so hot that the incoming water would boil and the injector would stop working.

Neil

[MuzzerParticipant @muzzer]

Presumably the "heat of condensation"?

[Anonymous]

I'm sorry I asked now. Never mind, I'll run through the literature and sort it out for myself.

Andrew

[Paul LousickParticipant @paullousick59116]

When it works, I just call it man made magic !

[Stewart HartParticipant @stewarthart90345]

Posted by Andrew Johnston on 06/10/2015 15:54:36:

Posted by Neil Wyatt on 06/10/2015 15:25:45:

Want a non-mathematical description of the physics?

Hot steam is squirted out of a nozzle, sucking up cold water. The water has to be cold enough it doesn't boil, but mixes with the steam to form a powerful jet of hot water, collected by another cone, that 'punches' its way into the boiler past a valve.

Neil

Ah, but how does the non-mathematics explain the following? We have two input streams; a mass of steam at boiler pressure, so it has stored energy due to the pressure, and a greater mass of water at atmospheric pressure, so no stored pressure energy. We end up with the combined mass of steam and water at, or more than, boiler pressure, so lots of stored pressure energy. Where does the extra energy come from?

Andrew

The cold water condenses the steam this causes a vacuum sucking in more water and starting the whole process off, this is why they don't work very well with hot water:- hotter the water the less steam it can condense the lower the vacuum, also this is why its important that none of your joints leak as it will just sucks in air.

I was lucky enough to have been tutored by a very knowledgeable and skill-full model engineer in their manufacture, with his help I made a batch of five that all worked I was chuffed to death, making your own injectors certainly give you bragging rights on the track.

Stew

[julian atkinsParticipant @julianatkins58923]

hi Stew,

in the interests of accuracy may i be permitted to correct you on a few points?

it is not the case that 'the cold water condenses the steam this causes a vacuum'. when the steam leaves the end of the steam cone nozzle it is already below atmospheric pressure. the steam nozzle plus first half of the combining cone acts as a sort of ejector. if the proportions and details of these parts and the check valve are correctly designed and made, the injector will be of the lifting type. it will also then be 'automatic' and self-restarting.

it is not the case that 'the hotter the water the less steam it can condense the lower the vacuum'. hot water will be lifted as much as cold water. what does happen with hot water (above 46 degrees celcius) is that in the combining cone the steam's considerable latent heat causes the water to boil, and so the injector wont work.

cheers,

julian

[Cornish JackParticipant @cornishjack]

Practically all modern aircraft use the same principle for fuel tank 'draining' (i.e. getting the dregs out) known as jet pumps. I used to explain the operation as being similar to egg blowing but that only worked with the 'wrinklies'. The young F/Os just looked blank, so, much dry marker expenditure and hand-wagging ensued. How do they work?? Yes, jolly good question, well formulated, nicely presented, thank you!!! The parallel is asking an Italian to describe Sophia Loren with his hands tied behind his back.

rgds

Bill

[MuzzerParticipant @muzzer]

Never been convinced I understood exactly how these are connected up. I understand the use of venturis to draw water in with the steam and also for slowing the stream down again to allow the pressure to overcome the boiler pressure. Obviously the water (and overflow) connections must be at low (atmospheric) pressure otherwise the steam wouldn't come out in the first place.

My question is – is there a significant fraction of the steam lost to the overflow connection? I always imagined there must be but haven't ever heard it explained. When using the injector, is a fair bit of steam dumped up the stack?

Murray

[Neil WyattModerator @neilwyatt]

My question is – is there a significant fraction of the steam lost to the overflow connection? I always imagined there must be but haven't ever heard it explained. When using the injector, is a fair bit of steam dumped up the stack?

Murray

No.

[jason udallParticipant @jasonudall57142]

All seems odd to me.
“Slow down the steam to overcome the boiler pressure”…???

That would seem to , when taken to extreme.allow a simple pipe loop . steam to steam. to induce a circulation …
Mmm stick a turbine in the way and you have free energy…can’t be right…
Yes I know these things work but always smelt abit fishy on the thermodynamic front.

[jason udallParticipant @jasonudall57142]

And if this seems odd take a look at vortex nozzles. ..very cold air from compressed air..

[julian atkinsParticipant @julianatkins58923]

hi jason,

it is all very simple really.

the speed of the steam issuing from the steam cone nozzle is used to impart pretty much the same speed to the water it hits in the annular gap between end of steam cone nozzle and entrance of the combining cone. the latent heat in the steam imparts extra energy to what becomes a sort of emulsified mix of steam and water and then a jet of water as the combining cone decreases in size. the condensation of the steam in the water takes place in the first half of the combining cone. the speed is converted to pressure in the diverging delivery cone. nothing should emit from the overflow once the injector picks up unless very low pressure (30-20psi) when regulation of the water is required.

yes, it is all very clever!

to make a successful injector in miniature you dont have to know 'why' but 'how', and appreciate the nuances and pitfalls of what makes these things work. ive spent 30 plus years experimenting with these gadgets. all my locos have only 2 injectors that are utterly 100% reliable, and no hand pump or axle pump.

the design parameters are so well known that anyone ought to be able to make their own except that certain tolerances are crucial plus machining proceedures catch out the unwary. making the tapered reamers is also a big draw back. i am often sent 'problem' injectors to sort out due to damage to the cones or bad commercial examples. i have rectified a dozen or so for friends so far this year and have 6 waiting to attend to – one of which is a 'new' commercial product supplied by a well known ME supplier that simply will not work due to the cones being the wrong size proportionately and very badly made.

i have noticed a sharp increase in queries since John Cashmore and Gordon Chiverton died. Len Steel has taken over Gordon's injectors and are available via Pavier Steam. usual disclaimer.

cheers,

julian

[duncan webster 1Participant @duncanwebster1]

Julian'sexplanation is on the right track. The steam is expanded through the steam cone and converts its pressure into momentum. The water then mixes with the steam, and condenses in the combining cone. Mix of water and steam then has lower velocity than the steam jet, but higher density and so has the same momentum. The delivery cone then exchanges the momentum back into pressure to push the water back into the boiler. If the water is too hot it won't condense, so it won't work. The latent heat of the steam doesn't contribute, in fact if it were lower we should be able to use hotter water. This is simplistic as it doesn't allow for the inefficiencies of the nozzle.

Edited By duncan webster on 08/10/2015 01:13:41

[John OlsenParticipant @johnolsen79199]

Perhaps we could note that the injector is one of the earliest inventions to come out of the aerospace industry, having been invented by Henri Giffard as a lightweight means of feeding water into the boiler of his airship in 1852.

Bear in mind that although all the heat energy that was in the steam does end up back in the boiler (neglecting any dribbles from the overflow) it is at a much lower temperature, having done work. For those who have studied thermodynamics, we can say that the entropy has been increased. If we used the injector to pump water to a height instead on into the boiler, we would find that we have to burn fuel to keep doing it, so work is being done and fuel is being burned, and the efficiency will not be 100% or anything like it. But still a very useful device.

John

[Neil WyattModerator @neilwyatt]

For anyone not scared of the maths (or willing like me to glaze over and skip to the end) here's some work that calculates efficiency of about 85%. (it is in English):

uni-obuda.hu/journal/Bukurov_Bikic_Prica_37.pdf

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