Injectors - A General Guide to the Model Automatic Injector

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AnthonyDuarte
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Injectors - A General Guide to the Model Automatic Injector

Post by AnthonyDuarte » Thu May 06, 2021 2:08 pm

Hi Everyone,

I've put together some information on injectors into a "general guide." Includes information on how they work, best practices for maintenance & installation, how to size them to your engine, etc... Eventually I'd like to add more to it and make it into a printed booklet, but it's not of much use sitting in my documents folder. (it's been sitting there for about a year).

I've added it to the "documentation" section of my website.
This hideous link will take you right to it:
https://static1.squarespace.com/static/ ... c+Injector

Or you can go to the main documentation page and find it there:
https://www.eccentricengineer.com/documentation

Anthony

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BAdams
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by BAdams » Thu May 06, 2021 4:04 pm

Thanks!

Brook

James Powell
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by James Powell » Thu May 06, 2021 6:50 pm

That is well written, and to the point.
Thank you Anthony for writing that up,

James

piedmontg
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by piedmontg » Thu May 06, 2021 7:33 pm

Your write-up is very informative and on target. I have not seen anything like that information compiled in one place.

Thank You

Bob

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Builder01
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by Builder01 » Fri May 07, 2021 6:06 pm

What makes these injectors "automatic"?

AnthonyDuarte
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by AnthonyDuarte » Fri May 07, 2021 6:51 pm

Builder01,

An "Automatic" injector refers to two things. One is the nozzle design (the type of injector that has a washer), and secondly the operation. They self-prime without assistance from gravity or a secondary lifting/priming nozzle, and will automatically restart if the injector's operation is interrupted.

Penberthy, Nathan, Metropolitan, and a number of other companies advertised this type of injector as "automatic," and the injectors from EE, SuperScale, and Ohlenkamp are all miniature versions of this design.

Anthony

Bill C
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by Bill C » Sat May 08, 2021 11:12 am

Thank you, Anthony!

Very good of you to contribute to the hobby and knowledge base this way!

Bill C.

Asteamhead
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by Asteamhead » Sun May 09, 2021 4:55 pm

Anthony,
Thank you for this fine paper :!: Nothing to be added. Should be handed to each beginner to answer any of their questions :) .

Asteamhead

trainriders
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by trainriders » Wed May 12, 2021 2:45 pm

Thanks for the info, the document is very helpful. How are the British model injectors different? Do you know of a similar article for that style. I've inherited a small injector that appears to be of the British model style w/o any documentation and I'd like to learn more.

AnthonyDuarte
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by AnthonyDuarte » Wed May 12, 2021 5:40 pm

Trainriders,

There's a book out there somewhere about that style of injector. If I remember correctly it's more about how to make one, but I can't remember the title. I saw the book at GGLS. I believe Jerry Kimberlin brought it in, perhaps he can chime in on it?

In my opinion, there are a few fundamental flaws with that design. My biggest gripes are the knife edges around the steam cone (where it's most prone to erosion), and the practice of limiting the high end of the operating pressure by restricting the combining gap. That combination is definitely "designed to fail," though I'm sure that's not the intention.

Here is a side by side comparison of the cross section of an EE injector and an English model injector:

EE:
EE Cross section.jpg

English Model Injector:
INJECTOR_cut-a-way_no8.jpg
INJECTOR_cut-a-way_no8.jpg (11.73 KiB) Viewed 2035 times

There are 3 primary differences between these.

1) The combining gap (between the steam and combining cone) on the automatic injector is generous. This allows for a range of adjustment up to the highest end of the operating pressure range. It also allows more room for the venturi at the tip of the steam nozzle to do its job. By contrast, the steam cone on the English injector design is shoved way into the combining cone. This is done to "set" the high end of the pressure range (typically to 100psi). This is why the English injectors tend to have a hard cutoff at a certain pressure, whereas an automatic injector will only start to drip, and with some adjustment from the steam valve, it can be made to work higher than its rated operating pressure, albeit with some loss of efficiency.

2) The angles of the English injectors are quite large, at least in the examples I have seen. Significantly larger than full sized injectors. In order for them to still work with these wider angles, the cones have to be shorter so that the volumes are still proportionally the same. I'm not 100% sure about this, but I believe this makes them more sensitive. I could be completely wrong about that, so any fluid dynamics engineers out there feel free to chastise me on that point. But all evidence suggests that the only injectors in the world that have angles that large are the English model injectors.

3) The check washer and cross drilled holes are unique to the automatic injector. I am aware of a few other injectors that accomplish the same thing in a slightly different way, but it is universal on the automatic design. The function of these features are what makes the operation seem "automatic." These features give the steam somewhere to go when the injector is first starting up. Without all those gaps in the nozzles for steam to vent, the initial blast of steam would simply blow back into the water line. Since it can freely vent out of all the holes and gaps, the injector is able to create a constant uninterrupted vacuum on the water line. When the injector is operating properly, the washer will lift up and seal against the combining cone due to the drop in pressure inside the nozzles, and the high velocity water will simply pass over the holes. Any water that does escape out of those holes will get sucked back in through those same holes. When operation is interrupted, the check washer will be pushed open and allow steam and water to vent out to make sure the vacuum at the water inlet is not interrupted. This is what allows the injector to lift and automatically restart.

This turned out to be a much longer response that I anticipated, but I think this is useful information. Perhaps it should make its way into the article in some form.

Anthony

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Re: Injectors - A General Guide to the Model Automatic Injector

Post by jkimberln » Wed May 12, 2021 8:06 pm

As Anthony mentioned, there is a book about the English injector. ISBN: 1 85761 134 9 . It is called "Miniature Injectors, inside and out" by D.A.G. Brown, and published by TEE Publishing Ltd. It is a nice little book and does go through the theory and practice of injectors, and how to make them in several sizes. The angles used in these injectors are mostly 13, 9, and 6 degrees in various diameters depending on delivery rate. It is also interesting to note that none of Brown's cones are longer than 0.530" and so in the text he mentions that none of the reamers need be longer than 5/8". He also insists on accuracy and mentions that even 0.002" can be critical in an injector's operation. The thing I find interesting is that if you listen to Brown and his admonitions you may get the idea that injectors are strange and mysterious and you may never attempt to make one.

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Builder01
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Re: Injectors - A General Guide to the Model Automatic Injector

Post by Builder01 » Wed May 12, 2021 8:38 pm

Is there a check ball missing on the cross section of the "English" injector?

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