Injector water temperature

[nigel jones 5Participant @nigeljones5]

[nigel jones 5Participant @nigeljones5]

My pannier tanks are next to an unclad boiler, so the water in them is bound to get warm. Is this likely to stop my injector working altogether?

[BazyleParticipant @bazyle]

The injector on my Pansy doesn't work and I;ve been told not to expect it to. Been meaning to set up a seperate supply but not really necessary with the axle pump is more than adequate so not enough incentive to rise up the round to it list.

[Martin CooperParticipant @martincooper52042]

I agree small injectors are fickle things, and mine (3 1/2" loco) doesn't work either. But just a small word of caution – my boiler inspector likes to see two methods of filling the boiler. In my case, there's a good axle pump and a hand pump in one of the side tanks.

When I get some spare time, I intend to take the feed to the injector directly from a tank in the driving trolley.

Martin

[nigel jones 5Participant @nigeljones5]

thanks…but is this down to temperature of the water?

[Nick MoodyParticipant @nickmoody91191]

Definitely. The injector on my Polly V works perfectly with cold water but is very reluctant when it gets warm. I have got side tanks with a balance pipe so I get round it by topping up the right hand tank that feeds the injector with cold water before using the injector and hey presto it works!

I did run in France with it once in summer where the water supply from the tap was about the temperature of a warm bath – the injector was having none of it! Thank heavens for the hand pump!

NickM

[Martin CooperParticipant @martincooper52042]

Yes, I believe the water temperature is critical, along with the layout of the pipe runs. But how cold is cold enough? I don't know the answer to this.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Fizzy ,

(1) The boiling point of water varies with its pressure . Roughly the lower the pressure the lower the boiling point . In the working of many common types of injector and especially lifting injectors a partial vacuum is caused by the condensing steam and this lowers the pressure in the water flowing in to the injector .

With cold water no problem but as the water temperature rises the combination of warm water and low pressure can cause the incoming water to boil and so stop the injector action dead .

Boiling is usually only partial – causing bubbles in the flow – like cavitation in effect .

Actually problems begin before actual boiling occurs with surface vapour being lifted off the water – like fog .

There is no absolute temperature of water that causes problems but anything above 40 deg C is bound to be a disaster and anything above 25 deg C could cause hit and miss .working .

Best plan is always to have stone cold water .

(2) The action of an injector is not the same as a pump in the way it generates delivery presuure of water into the boiler .

A pump which sees any restriction will just generate more and more pressure until it either breaks through the restriction , stalls or falls apart .

An injector has a maximum delivery pressure which only depends on its design and on the steam pressure – there is a top limit and not a very high one .

So an injector seeing the same sort of flow restriction as the pump will probably just stop working .

To this end all pipe and clack designs for an injector must be much better thought out than for a pump .

Regards ,

Michael Williams .

[John BaguleyParticipant @johnbaguley78655]

Hi Nigel,

Tank engines will always have problems with injectors as the water temperature soon reaches a level where the injector refuses to work. Anything above 30°C and you will be struggling, probably less with a poorly designed one. The best bet is to top the tanks up with cold water on a regular basis or better still, feed the injector from a separate tank on the driving trolley.

The reason the injector stops working is that the incoming water becomes too warm to fully condense the steam when they are mixed in the combining cone. As the temperatire of the water rises from cold, the injector action becomes less efficient until it finally fails altogether and you just get water and steam out of the overflow.

It is possible to mitigate this problem to a certain extent by shimming the steam cone further out from the body to increase the water to steam ratio but the effect is limited. The problem then is that the injector becomes less reliable at lower temperatures. Jim Ewins reckoned he could design an injector to work with hot water but I've yet to find his secret!

A decent injector will easily pump 200 psi and more (from practical tests) so will overcome minor restrictions from the following pipe work and clacks. Too much is made of 'easy bends' etc. although it is best to give delivery clacks more lift than those intended for hand/axle pumps.The problem is that a lot of the commercial injectors are poorly designed/made and will only work if everything is 'spot on'.

It's interesting to note that an injector will still work even if the outlet is blocked – the water just comes out of the overflow rather than out of the delivery pipe as it has nowhere else to go.

John

Edited By John Baguley on 14/02/2013 00:58:36

Edited By John Baguley on 14/02/2013 01:10:24

[Ady1Participant @ady1]

At the top of Mount Everest water boils at 71 degrees centigrade

This is because the air pressure is so low

http://en.wikipedia.org/wiki/Boiling_point

Edited By Ady1 on 14/02/2013 09:56:57

[julian atkinsParticipant @julianatkins58923]

hi fizzy,

to quote from D.A.G. Brown's excellent book 'Miniature Injectors' p.87

"feed water temperature of 40 degrees C results in a delivery temperature of around 95 degrees C, so you can start to see the reason why conventional injectors cannot cope with high feed water temperatures; boiling starts to take place at the point where the liquid stream should be going into the delivery cone"

there are various ways to avoid this:

1. try and insulate as much as possible the tanks

2. fit a quick acting large drain valve so the tanks can be emptied and re-filled with cold water from a stand point tap

3. fit a well tank or similar with connections to a wagon fitted with a water tank, or just couple up the injectors to such a wagon with say an isolating valve on the loco tanks

4. leave the injector water valve open. this has a similar effect to 2. in that you will be topping up the tanks far more regularly with cold water from a stand point tap.

5. fill the tanks with cold water from the mains rather than a decorative 'above ground' water tank on hot sunny days, not that we seem to get many of these!

i use a combination of all or some of the above depending on which of my tank locos im driving.

it should also be noted that as the feed water approaches 40 degrees the top pressure operating range of the injector reduces. i always fit both a low pressure and a high pressure injector to my locos, not necessarily for this reason, but the physics of the way injectors work is worth noting i hope.

cheers,

julian

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Who is D.A.G. Brown ?

[julian atkinsParticipant @julianatkins58923]

hi Michael,

D.A.G.Brown is Derek Brown, miniature injector expert and author of the above book published by TEE 2 years ago. he often gives talks and demonstrations at the shows and to club meetings. the only person publically to explain why the late Ted Linden's commercial injectors worked so well.

cheers,

julian

[Dennis WAParticipant @denniswa]

D.A.G. Brown : Derek Brown is a member of the SMEE and has spent many hours demonstrating on the SMEE stand at model engineering shows.

An engineer with 2 books and many articles to his credit, his book Miniature Injectors Inside and Out published in 2011 is the best reference on the subject that I have read – and yes, I have read the other references too (D.E. "Laurie" Lawrence, Jim Ewins, B. Palmer, LBSC, CM Keiller, …).

Dennis

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Julian and Dennis ,

Thank you for your replies .

Regards ,

Michael Williams .

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