How to size an Injector?
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 Posts: 719
 Joined: Wed May 31, 2006 2:13 pm
 Location: Laguna Niguel CA
How to size an Injector?
I am very slowly building a Don Young "Marie Estelle" 21/2" scale 71/2' gauge 040 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 SuperScale 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.
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 SuperScale 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.

 Posts: 230
 Joined: Tue Jan 19, 2010 10:46 am
 Location: Mountain View, CA
 Contact:
Re: How to size an Injector?
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:
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)
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)
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)
Re: How to size an Injector?
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.

 Posts: 719
 Joined: Wed May 31, 2006 2:13 pm
 Location: Laguna Niguel CA
Re: How to size an Injector?
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 brandnew Super Scale Economy injector from 2012? Brandnew, never used, connected, or even disassemble. If so, I will post on for sale board.
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 brandnew Super Scale Economy injector from 2012? Brandnew, never used, connected, or even disassemble. If so, I will post on for sale board.

 Posts: 230
 Joined: Tue Jan 19, 2010 10:46 am
 Location: Mountain View, CA
 Contact:
Re: How to size an Injector?
PM sent

 Posts: 382
 Joined: Tue Jul 31, 2012 1:59 pm
 Location: Germany, Duesseldorf
Re: How to size an Injector?
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 :
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
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 :
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
Re: How to size an Injector?
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.
Fred V
Pensacola, Fl.
Pensacola, Fl.