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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 6:07 am 
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How much back pressure is acceptable is unclear from google. There is a discussion here from 117 years ago: https://books.google.com/books?id=vf5LA ... or&f=false

:)

- Bart

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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 6:38 am 
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barts wrote:
How much back pressure is acceptable is unclear from google. There is a discussion here from 117 years ago: https://books.google.com/books?id=vf5LA ... or&f=false

:)

- Bart

There are some Locomotive designs that take the idea of putting the output as close as possible to the limit by bolting top checkvalves directly into the boiler, injecting into the steam space.

Bringing this up might just be confusing the issue because they did that for some interesting but valid reasons that don't exclusively relate to back pressure. But it is related...

Attachment:
check.jpg
check.jpg [ 127.87 KiB | Viewed 43 times ]

"Here is a picture of a duplex check valve on top of Canadian National locomotive no. 1531. They are mounted just ahead of the bell."

Discussion of Top Checks - actually looks like there are a lot of pro ported benefits.
http://www.rypn.org/forums/viewtopic.ph ... ck&start=0

Clear as mud ?
-CB

P.S. What I expect to happen to the injector is this, adding more tubing, and every fitting is going to decrease the maximum pressure that the injector is capable of handling. If a moderate amount, maybe no performance change will be evident - but according to the penberthy spec I'm already right at the edge of what the penberthy is rated to handle. *However* there is some amount of slop that penberthy builds into the design that is not listed in the specs. I know this to be true because the pressure rating says 50-200 psi, and mine will start working around 35psi.

I don't want to run near the edge, or I'm just going to box my self in a corner where the injector is so efficient with the economizer but can't operate right at the mawp of 200psi....


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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 7:04 am 
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If I were you I'd fit a mechanical pump, but that's just me. I'd then plumb the injector right after the feed water heater and economizer (yes, there would be a check valve needed) to avoid the drop - but the injector would be for emergency use.

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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 7:24 am 
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You guys never give up about your mechanical pumps. :)

-CB


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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 9:36 am 
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cyberbadger wrote:
You guys never give up about your mechanical pumps. :)

-CB


Possibly not, and with good reason in the environment of a steam launch.

Mechanical pumps are not best suited to locomotives, because the engine steam usage for a given speed depends enormously on the load being pulled, and the gradient, and therefore injectors are almost universal. Conversely, in a launch, average propeller shaft loads vary little for a given shaft speed, so a mechanical pump can do a good job of keeping the boiler level constant without attention, as mine does, though of course some form of automatic bypass makes the control even better. With fire-tube boiler this isn't particularly important, but with a low water content water tube type, it most certainly is.

Add to that the fact that launch engines are relatively small, and in some cases downright tiny, compared with locomotives, and we enter a regime where injectors become much more sensitive to mechanical precision, feed temperature, build-up of any dirt or scale within the injector etc. Some boat injectors do work nicely, and do so reliably, but if you experience a significant number of different steamboats, you will find injectors that need a bucket of cold water emptied over them before they'll work, some that sing nicely only when hit with a heavy mallet, some that are downright temperamental, and some that just don't work. I don't attend model steam locomotive meets, but I gather at them the subject of injectors is treated more like a religion than a science (which no doubt is part of the fun).

If yours work reliably, then fine, and carry on enjoying using them. Personally I feel happier if there's an engine driven pump available at least as a backup to an injector, and it would seem that I'm far from alone in that.


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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 10:21 am 
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"If I were you I'd fit a mechanical pump, but that's just me. I'd then plumb the injector right after the feed water heater and economizer (yes, there would be a check valve needed) to avoid the drop - but the injector would be for emergency use."

Bart is exactly right, and if you want the economy that comes with an "economizer", then the first step is to use a mechanical feed pump. Here's why:

The injector uses a relatively large amount of steam to operate, with the benefit that all the energy of that steam use shows up as heat into the feedwater. That is fine and OK, provided you have no other method of pre-heating the feedwater.

If you have a mechanical pump driven from the main engine, you consume a trifling amount of extra steam to drive this pump, about 1%-2% of the main engine's output is needed to drive the feed pump. Because the mechanical pump is so efficient, the feedwater is not heated in this mechanical pump.

Downstream of the feed pump, lots of available heat can be found in two obvious places: The main engine exhaust steam can heat the feedwater considerably in a conventional heat exchanger, followed by an economizer, getting heat from the boiler flue gas that would otherwise exit the stack at temperatures well above the boiler steam temperature. These two sources are available with much less steam consumption than any injector would use.

If you use an injector, the feedwater is already heated before the feedwater heat exchanger, so a feedwater heat exchanger is almost useless if you use an injector.

Using an injector to force feedwater into the boiler, considering the extra pressure drop associated with an economizer, will hurt the already low efficiency (considered as a pump) of the injector, indeed the injector may not function at all with this extra pumping duty. If it does work, it will use more steam than it normally does, pre-heating the feedwater more than it does in normal service, again largely negating the benefit of the ecomomizer.


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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 10:57 am 
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I'm no expert on injectors and certainly not in boats, but I do know there are a few basic rules which if flouted will definitely give you nothing but unreliability. First and foremost they must be fed with unrestricted COLD water, next there must be no air leaks into the water feed. They should be mounted where they are away from heat, but keep pipe runs as short as practical (especially to the clack) and they need a good sized steam feed pipe. After that, each one seems to have it's own characteristics and minor foibles, water full on to start then cut back, steam on slowly or cracked full on etc.

If an injector fails to pick up then turn the steam off and leave the water on to cool it, before trying again.

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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 4:53 pm 
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fredrosse wrote:
If you use an injector, the feedwater is already heated before the feedwater heat exchanger, so a feedwater heat exchanger is almost useless if you use an injector.

Using an injector to force feedwater into the boiler, considering the extra pressure drop associated with an economizer, will hurt the already low efficiency (considered as a pump) of the injector, indeed the injector may not function at all with this extra pumping duty. If it does work, it will use more steam than it normally does, pre-heating the feedwater more than it does in normal service, again largely negating the benefit of the ecomomizer.

This is a well thought out explanation, but then why is this configuration then mentioned in the penberthy literature?

Shouldn't the penberthy literature say, don't use an ecomizer it's uselss?

Image
Image

To clarify I am considering 10-25ft of economizer after the injector. Only the second image from US Injectors is talking specifically about after the injector. I'm pretty sure preheating the water will just make the injector fail. The table on page 1 of this thread says I'm near the performance boundary.

-CB


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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 5:51 pm 
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Hey! I bought an iron once that included advice to not iron your clothes while you were wearing them. Who knows what people will do with something?

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 Post subject: Re: Economizer tube material
PostPosted: Thu Dec 07, 2017 5:58 pm 
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Lopez Mike wrote:
Hey! I bought an iron once that included advice to not iron your clothes while you were wearing them.

I think hands down climbing equipment has the best documentation:
http://www.storrick.cnc.net/VerticalDev ... Icons.html
:lol:
-CB


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