How to size an Injector?

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Kimball McGinley
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How to size an Injector?

Post by Kimball McGinley » Thu Jul 12, 2018 2:13 pm

I am very slowly building a Don Young "Marie Estelle" 2-1/2" scale 7-1/2' gauge 0-4-0 Porter. This has a 6" diameter copper boiler.
I am ready to plumb it up and expect to run at 100 psi.

How do I get the correct sized Injector? I have a red "English" injector from Coles, I think it might not like the 100 psi pressure, and may be too small. Some time ago, when they were closing, I sort of panicked and bought a Super-Scale Economy Injector, but I am almost certain it is too big?

Is there a formula? Maybe something like fill 1/2 the glass in 30 seconds or such?

I am very interested in the new Eccentric Engineer injectors too.

Any help appreciated.
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AnthonyDuarte
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Re: How to size an Injector?

Post by AnthonyDuarte » Thu Jul 12, 2018 4:05 pm

Hi Kimball,

I’m glad you’ve brought up this topic! I feel it’s very important to have an appropriately sized injector for our models. It’s awfully harsh on a boiler to fill it in 5 seconds, and it can be risky having an injector that can’t keep up with steam demand.

I’m no physicist or expert on this, so if there are errors in my calculations/conversions/assumptions, I would gladly invite discussion on the matter.
What it comes down to is calculating your maximum possible water consumption, and getting an injector that delivers at least as much, or ideally a little bit more. You want to be able to fill your boiler at a reasonable rate while your engine is working its hardest.

The variables you need to know are the following:
  • Operating PSI (let’s assume 125psi)
  • Cylinder bore x stroke (let’s assume 2”x3”)
  • Cutoff in full forward (let’s assume 70%)
  • Driver diameter (let’s assume 7”)
  • Max operating speed (let’s assume 7mph, a common maximum)
In this example, the volume of the cylinders is 9.424in^3 (1"^2× π × 3”), at 4 strokes per revolution, that’s 37.7 in^3 per revolution, and at 70% cutoff, that’s effectively 28.39 in^3 per revolution.

Okay, now we need to know the locomotives RPM at max operating speed. 7 miles per hour in inches per hour is 443,520 inches/hr, which is 7,392 inches/min. Our driver diameter is 7”, which gives a circumference of 21.99 inches (7×π). 7,392in/min ÷ 21.99in = 336.15rpm. Of course we never exceed the speed limit of 7mph!!! But let’s be generous and call it 350rpm.

We know the volume of our cylinders at 70% cutoff is 28.39in^3 per revolution. 28.39 in^3 x 350rpm = 9,936.5 in^3, which (thanks to google conversions) is 344.12 pints per minute.

So now we know the volume of steam (in pints) used at max RPM on our locomotive. We need to translate that to water consumed. Water at atmospheric pressure and room temperature is 997kg/m^3, and steam at 125psi is 4.41kg/m^3. So, (4.41 kg/m^3 ÷ 997 kg/m^3) x 344.12pints/min = 1.522 pints per minute.

This is the theoretical consumption of JUST your cylinders. You also have a blower running, an atomizer if you’re burning oil, possibly steam pumps, steam lost from leaks and safeties (generous fireman!), etc. I would recommend a factor of safety of at least 2x over what your cylinders would theoretically consume. In this example, that would translate to 3.044 pints per minute. When you’re charging up a 2.5% grade at 7mph with your blower blasting and safeties popping, you want your injector to be able to add water to the boiler, and not just maintain water level. Certainly don’t want it to come up short!

In the near future I hope to add a page to my website where one can plug in some known variables and calculate a recommended injector capacity.

-Anthony (EE)

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JTolan
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Re: How to size an Injector?

Post by JTolan » Thu Jul 12, 2018 4:27 pm

Anthony's small scale injector would be your best bet. Your engine is smaller than my Chloe, and a superscale is too big for it.

Kimball McGinley
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Location: Laguna Niguel CA

Re: How to size an Injector?

Post by Kimball McGinley » Fri Jul 13, 2018 2:13 pm

Thanks Anthony and Jeff. I think Anthony's calculations are a bit big for my Porter, if so it just increases the safety margin. ( I can't off the top of my head remember bore and stroke)

The little red guy is labeled 22 oz., so that is about 1/2 of the EE Small.

Anthony - saw your video - picked up at 35 psi - and after steam heating - WOW - any of your small injectors for 1/4" lines in stock? Please PM me with an answer and include cost for a set of recommended repair/consumable parts.

Is there any interest in my brand-new Super Scale Economy injector from 2012? Brand-new, never used, connected, or even disassemble. If so, I will post on for sale board.

AnthonyDuarte
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Re: How to size an Injector?

Post by AnthonyDuarte » Sat Jul 14, 2018 9:14 am

PM sent

Asteamhead
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Re: How to size an Injector?

Post by Asteamhead » Sat Jul 14, 2018 5:59 pm

Hello, just my 2 cents:

For our engines are running all about the same speed (as Anthony pointed out already), steam consumption depends mostly on the effective drawbar pull. This is about 20 % of the friction weight (max) of a typical engine. Specific steam consumtion per hp - hour is about 50 pounds per hour with our pretty small steam engines with all their losses.
These astimations may be gathered in just one number :idea: :

You need about 1 pound of steam (or water) per hour per pound of effective tractive force (or per 5 pounds of friction weight of your locomotive).

This is nearly independent of size, cyl. dimensions, wheel diameter and so on :!:
Normal quality standard of the locomotive's condition estimated, of course.

Asteamhead

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Fred_V
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Re: How to size an Injector?

Post by Fred_V » Sun Jul 15, 2018 8:08 am

I have a water consumption calculator spread sheet. you can plug in your engine size and wheel dia and it will calculate water needed. PM me with email address and I'll send it to you.
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Fred V
Pensacola, Fl.

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