Author Topic: Steam water feed injectors  (Read 20754 times)

Offline mike mott

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Re: Steam water feed injectors
« Reply #15 on: June 19, 2015, 07:09:25 PM »
Thanks for the detailed step by step Stew, I have filed this away for when I am confident enough to have a go at making one down the road.

Mike
If you can imagine it you can build it

Offline Steamer5

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Re: Steam water feed injectors
« Reply #16 on: June 20, 2015, 05:10:32 AM »
Hi Stew,
Only  :popcorn: harmed during the following of this thread! Oh an the occasional :DrinkPint:
Nice "how to" article.

BTW I recently came across what I think is a one of the best article on injectors.....I've been collecting articles on these little beasties for some time......the article is written by Bob Bramson it runs to some 26 pages and will answer most if not all the questions that have been asked. For those that may be interested check out The Whistle magazine for the British Columbia Society of ME.... www.Bcsme.org starts in the April 2012 issue, September, October January 2013, February March April May, to save you hunting around.
Bob has also written a very good article on braking for those of us into locomotives, which is how I found the above, if anybody has a design for a vacuum brake valve I would love to here from you!

Cheers Kerrin
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Offline jadge

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Re: Steam water feed injectors
« Reply #17 on: June 20, 2015, 10:34:05 AM »
Kerrin: Thanks for the link; at first glance the articles look to be a good mix of theory and practical. In due course I'll sit down and read through them in detail. I don't envisage a problem in building an injector, designing one is a whole different ball game, but I'm keen to have a go, at least on paper.

Stew: I'm afraid that steam doesn't generally follow the ideal gas law. Steam is normally considered to be a vapour. Having said that experimentally the expansion of steam in a cylinder is generally taken to be close to constant PV, aka hyperbolic expansion.

Andrew

Offline jadge

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Re: Steam water feed injectors
« Reply #18 on: June 21, 2015, 09:44:04 AM »

I'm not sure I follow that. As far as I can see from my calculations the jet of steam from the steam cone is already below atmospheric pressure.

And it is HOT for Stew's boiler at 90Psi on the gauge, taking into account atmospheric pressure, that would be about 166 degrees C.


I agree that dry saturated steam at 90pisg will be around 166C. However, I do not think that the steam at the output of the steam will be at that temperature. If it was, and given that the pressure is at, or below, atmospheric that would imply the steam had a significant degree of superheat. I think that the expansion of steam through a nozzle is taken as being isentropic, ie, the entropy doesn't change. If we assume that the output steam is at 10psia, then the temperature will be at about 90C. That implies that the enthalpy has changed. Change of entropy is defined as change of energy divided by the average temperature. So if the heat energy in the steam has changed, but the overall energy in the steam hasn't changed, where is the missing energy? It's been converted into kinetic energy, as the steam issuing from the nozzle is moving much faster than the input steam.

From a practical point of view the expansion is not perfectly isentropic as some heat is lost to friction and to radiation from the nozzle body. I wonder if there is any advantage in making an injector from an insulating material, or at least the cones?

Andrew

Offline steamer

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Re: Steam water feed injectors
« Reply #19 on: June 21, 2015, 04:12:54 PM »
Hi Andrew!

Interesting points!    I do know that a hot injector is very unlikely to function, and will often not work at all!

This gets worse in the small scale ones, and you have to turn the steam on quickly so as to pick up water and flood them, and then back off on the steam till the over flow stops....

Don't know about an insulating material....something to think about though....

About as close to perpetual motion as it gets these things.    98% efficient as a feedwater heater....about 2% efficient as a pump.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline robmort

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Re: Steam water feed injectors
« Reply #20 on: August 09, 2015, 07:26:41 PM »
Stew,

thanks for the detailed and very useful blog.

A couple of questions:
1) You seemed to be working from a book or printed instruction for the design; is it yours or which other, and is it available?
2) the delivery cone is usually recommended to have a 6deg  cone angle, but you seem to use 9deg. Why is that?

Rob

 

Offline sbwhart

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Re: Steam water feed injectors
« Reply #21 on: August 10, 2015, 07:16:32 AM »
Hi Rob

Thanks for your interest to answer your ?


A couple of questions:
1) You seemed to be working from a book or printed instruction for the design; is it yours or which other, and is it available?
2) the delivery cone is usually recommended to have a 6deg  cone angle, but you seem to use 9deg. Why is that?

 

The instructions I'm working from are a step by step guide provided by a friend of mine, if you wish I can scan them and send you a copy just send me a PM with your email address.

From Martin Evans Book " Manual of Model Steam Locomotive Construction" he does indeed have a 6deg delivery cone, The design I work to was derived by my friend after much trial and error he did say that he found the cone angles were not very critical for the injector to work correctly, the critical part is positioning the cones and the only reason for making accurate reamers is that it aids the positioning of the cones easier.

Hope this helps

Stew
A little bit of clearance never got in the way

Offline robmort

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Re: Steam water feed injectors
« Reply #22 on: August 10, 2015, 10:03:27 AM »
Stew,

thanks, i've sent a PM.

Regarding the delivery cone angle, it will be interesting to see how well it works at 9 so hope you report the results.
In fact several sources recommend 6 e.g. DAG Brown, and the experiments reported at http://www.modeng.johnbaguley.info/injectors/injectors4.htm ".............I made a new delivery cone with a 6 angle. This was a big improvement......Obviously the problem was due to the incorrect angle".

Rob





Offline sbwhart

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Re: Steam water feed injectors
« Reply #23 on: August 10, 2015, 10:21:56 AM »
Hi Rob

I made a batch of 5 and they all work fine


Stew
A little bit of clearance never got in the way

Offline jadge

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Re: Steam water feed injectors
« Reply #24 on: August 10, 2015, 12:06:35 PM »
I agree with Stew, I don't think that the actual cone angle is particularly critical. From what I have read regarding full size injectors the most important dimension for the delivery tube is the minimum diameter, as this determines the mass flow of water. This of course is the important number. The delivery tube needs to generate a pressure greater than boiler pressure, but if the mass flow is too small having the correct pressure is not much use.

Full size design notes indicate that the length of the delivery tube is important, as well as it needing to be divergent. If the angle is too steep, with a short tube, the flow of water breaks away into turbulent flow, with resultant reduced pressure. The design notes I have use a curved delivery tube, ie, not a cone, but in fact parabolic, I think? Whether this is of any importance in a small scale injector is a moot point. If I ever get as far as designing my own injector I will try a curved delivery tube. I would make a template, using the CNC mill, and then use the template with the hydraulic copy unit on my lathe to make a curved 'reamer'. Then making the delivery tube is as per standard.

I think that one of the problems with warm feed water is the time taken to condense the steam and hence the length of the combining cones. In full size experiments were done with movable combining cones to widen the range over which the injector would operate. I don't know whether this system was ever used on 'production' injectors.

Andrew

Offline Greg t

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Re: Steam water feed injectors
« Reply #25 on: September 02, 2017, 06:53:02 AM »
Hi I'm new here,I am looking at making my own injector,your step by step discription is awesome,I have one question to start with,how many oz per min does your injector inject,I'm looking at making one to do about 40oz per min what dimensions will I have to change to achieve this?
It's better to try and fail than not to try at all,

Offline ChuckKey

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Re: Steam water feed injectors
« Reply #26 on: September 02, 2017, 09:39:31 AM »
There are several good books on model injector design and construction, incuding 'The Model Injector' by Ted Crawford, published by Australian Model Engineering, and 'Miniature Injectors Inside and Out' by DAG Brown, published by TEE, which I would recommend. For the full-size history, 'Practice and Theory of the Injector' by Strickland Kneass, published in 1894, and reprinted by Lindsay Publications is also very interesting.