Author Topic: Steam water feed injectors  (Read 26450 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
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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 166°C. 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 90°C. 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
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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,

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.   

Offline Jon Steensen

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Re: Steam water feed injectors
« Reply #27 on: August 26, 2021, 07:41:44 AM »
At first a steam injector seem a little counter intuitive, and in fact it took me some time to understand what is going on.  Where does the energy to pump in the water come from? It seems logical that it should take the same amount of energy to pump the steam back in the boiler as you will gain by letting it out. So you’d think there is no energy left to force the water against the pressure inside the boiler. However this is obviously not true, as an injector has been demonstrated to work many times.

I could go all technical and talk about enthalpy, entropy and adiabatic processes, but to not lose those why are not firmly rooted in thermodynamics, I will try to give a more intuitive explanation.  The thermodynamic stuff is for engineers like me, who would do the design of the injector, and it is a bit complicated, or at least requires a lot of calculations.

The key is that the steam condenses before it re-enters the boiler. The amount of work that is required to force a fluid against a pressure depends on two things: How big is the pressure and how big a volume do you need to force in. This is quite intuitive for everyone who has tried to inflate a bike tyre with hand pump. The higher the pressure in the tire, the harder you have to press the pump handle, and it takes more energy to pump in ten strokes than it does to do one. The key here is that a certain amount of steam has a much larger volume than it did once it was pumped into the boiler as water, and hence you can extract much more energy when letting the steam out than you spend to force it in as water (the 10 strokes out, one stroke in thing). This also explains why a steam engine can drive its own feed pump.
If you imagine a piston with water on one side and steam on the other, the pressure on the steam side is just sufficient to press in the water against the pressure of the same side. However by doing so we put in way more water than is required to compensate for the steam we let out, and the rest we can use to do some other useful work.
The same is true for an injector, where the volume of steam we take out the boiler is way higher than the amount of water it will be turned into, when it is returned into the boiler. But in order to understand how exactly it is possible we need to understand another fact.

Imagine drilling two holes in a pressurised boiler, one above the water line and one below it and observe what happens. You will of course see a stream of water being expelled at one of the holes and a jet of steam coming from the other, but what is more important, you will see that the steam is moving really fast, whereas the water is going a lot slower. This is due to the fact that the steam is much lighter than the water, and hence the force due to the pressure drop can accelerate it to a greater speed. It is like you can make a bike accelerate a lot faster than you can make a car go when you push it, since the bike is so much lighter and hence easier to accelerate
The pressure drops when the fluids exit the boiler and the fluids speeds up, as pressure is converted to speed. This process can also be run in reverse: make a speeding fluid stop, and the pressure will rise. This is why you feel force from a jet of water press against your hand when it hits it and you hand slows the water down, and this is exactly what happens in the delivery cone. Since water is so much heavier than air, it will reach a greater pressure when slowed down. You will experience that blocking a stream of water going only a few meters per second generates a significant force, whereas you can hardly feel a gust of wind at the same speed.
It is just like the flow of water and steam (a light gas like the air) flowing out of the boiler, but in reverse.

In an injector steam is taken out, and accelerated to great speeds in the nozzle, whereafter it is combined with a stream of cold water. In the process the fast steam slams into the water, which will slow down the steam and speed up the water until both streams travels at the same speed.
Since the steam loses speed to the water, the mixture cannot speed up to the same speed as the steam alone had, as its energy now has to drive both the steam and the water. But here comes the important part: It does not have to for it to presss the mixture back into the boiler.
This is due to the fact that once the steam mixes with the water it gets cooled down so it becomes water; and as we experienced with our punctured boiler and the jet of water hitting our hand, water will give a much higher rise in pressure when you slow it down than steam will, and therefore it can run up against the pressure of the boiler going a lot slower than the steam did.

Since the “trick” in an injector is that the steam condenses when meeting the cold water, we also find the explanation for why a stream of warm feed water is problematic  there, and why you need to bypass the injector to get rid of warm water that may be stuck in there. If the feed water is too warm, it cannot absorb enough energy from the steam without the water being heated to its boiling point and becoming steam, which is much harder to pump into the boiler.
An injector cannot be driven on compressed air either, since the air will not reduce in volume by condensing like the steam and hence it requires almost the same amount of energy to pump back into the boiler as it gains by going out of it, leaving no energy to drive in the water.

Since an injector depends on the condensation of the steam, designing an injector is a tricky balancing act. Make the steam nozzle too small and you do not have enough steam to speed up the water to sufficiently  to cause  a big enough pressure rise so it can enter the boiler. Make the nozzle too big and you add too much warm steam to dump the heat energy in the water, and you cannot make it condense and become easy to get back into the boiler.
The combining cone has to be long and narrow in order to allow the fastflowing mixture to stay in it for long enough to give the steam enough time to condense, and the delivery cone has to be long and slender to give a nice gradual convertion of speed to pressure in order to not have step pressure gradient that can cause turbulence. Turbulence will cause an energy loss, and if becomes too bad, too much speed is lost and the pressure cannot be raised sufficiently to get the water in the boiler.

Offline Admiral_dk

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Re: Steam water feed injectors
« Reply #28 on: August 26, 2021, 11:28:28 AM »
Thank you for the explanation   :praise2:

While it seems a bit long during the read - it all makes good sense @ the end - because of the whole explanation ...!... so if any of you are interested in the why - please read it all.

Best wishes

Per

Online Kim

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Re: Steam water feed injectors
« Reply #29 on: August 26, 2021, 06:35:13 PM »
That is a great explanation!  It's the first time I've had a sense of how/why injectors work.

Thanks for taking the time to write that explanation for us, Jon,
Kim