Pre-heating boiler water

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Bill Shields
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Re: Pre-heating boiler water

Post by Bill Shields » Wed Apr 07, 2021 8:37 pm

No argument on life expectancy.

Your understanding of basic thermodynamics is however a bit lacking.
Too many things going on to bother listing them.

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NP317
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Re: Pre-heating boiler water

Post by NP317 » Thu Apr 08, 2021 1:00 am

For perspective on my following comments, my working experience included >25 years rebuilding and operating full sized steam locomotives. And these locomotives were all logging locos up to 120 tons weight. My comments do not include SuperPower steam boilers, as I have no experience with those. Someone else can educate us on those.

Looking at full-sized locomotive boiler structures you will find flexible stay bolts around the firebox areas, to accommodate differential metal expansion and contraction, especially of the firebox. But no flexible stays are at the smokebox end of the boilers.
And injectors generally introduced water at the front end where the temperature is lowest, decreasing the likelihood of thermal shock.
I have actually HEARD stay bolts popping (breaking) during a cold hydrostatic test, and have replaced my share of broken stay bolts.
i am an advocate of warm hydro tests in full sized boilers, and learned that from USA State and Federal inspectors.

Our Hobby boilers present a different situation. Over the length of our small boilers - average 2 to 8 feet - the thermal and pressure expansion rates are small enough to not require flexible stay bolts. My two 1/8th scale locomotive boilers exhibit a length expansion of less than 1/8". Please tell us otherwise if your boiler is different.
And the likelihood of thermal shock from cold/cooler water injection at the smokebox is similarly less likely. I am not aware of front end damage from water injection in our small boilers. Possibly scale build up though, as in full sized boilers. Again, please share if your experience is different.

That leaves me to believe that pre-heating the boiler water for our Hobby Boilers primarily slightly improves the overall heating efficiency, and does not really effect thermal shock in any meaningful or measurable way. And my belief is based on familiarity with the lifetimes I see for our Hobby Boilers.

I am always open to learning of other situations you have experienced.
Craftspeople like Marty Knox can surely add more knowledge to my comments.
RussN

michaellynn2
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Re: Pre-heating boiler water

Post by michaellynn2 » Thu Apr 08, 2021 4:40 am

Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different parts of an object expand by different amounts.

I do not intend to try and sway others beliefs and experiences on how to operate their boilers. I did extensive research into the pros and cons of adding water to a boiler at various temperatures. I have read materials on the failure of boiler tubes at areas where water was introduced to a boiler. If one has no concern of mechanical loads on their boiler then continue to operate as usual. I can only speak for my boiler and its operation.

I have at various club meets talked to fellow live steamers as to why they love their injectors. The subject of preheating always comes up. ALWAYS!! Do you have an injector on your locomotive? I understand that in a fix it is fast at filling a boiler. Is that the only reason? Again, I do not intend to try and sway others beliefs and experiences on how to operate your boilers. It is, your boiler. I go to the extreme a little. After all, my locomotive is in my dining room.
michael george

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Bill Shields
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Re: Pre-heating boiler water

Post by Bill Shields » Thu Apr 08, 2021 7:15 am

If I may ask.. what are general dimensions and material of construction of your boiler?
Too many things going on to bother listing them.

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Greg_Lewis
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Re: Pre-heating boiler water

Post by Greg_Lewis » Thu Apr 08, 2021 11:13 am

Setting aside for a moment the debate on whether preheating is necessary or effective, my mentor, the late Cal Tinkham, had a maintenance-free feedwater "warmer." Since his axle pump piping had to come back to the cab for the bypass valve anyway, he piped it up to a check valve on the backhead. Inside, there was a tube from that foward over the top of the crown sheet up to the point where the injector checks were on the sides of the boiler. Thus the feedwater was warmed on it's trip through the boiler. While not as hot as a smokebox coil might produce, it wasn't cold either. Such a setup is easy to do and about as fail safe as you can get. His boiler produced enough steam for two locomotives and that engine has run for literally several thousand miles without a hitch.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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NP317
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Re: Pre-heating boiler water

Post by NP317 » Thu Apr 08, 2021 7:27 pm

That sounds like a good simple idea!
RussN

Andrew Pugh
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Re: Pre-heating boiler water

Post by Andrew Pugh » Sat Apr 10, 2021 2:21 am

Bill Shields wrote:
Wed Apr 07, 2021 8:37 pm
No argument on life expectancy.

Your understanding of basic thermodynamics is however a bit lacking.
My thermo is definitely rusty.

I do however remember that the conduction through the tube material is but one of the ‘resistances’ involved in the transfer of heat from the hot gas in the smoke box to the water flowing in the tube. The relative thermal conductivity of stainless vs copper may not have as large an effect as one may think. I haven’t run the numbers, but the wall resistance could be small in comparison to the convective resistance between the hot gas and the tube.

Just something to consider. 8)

-Andrew

k36no4862002
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Re: Pre-heating boiler water

Post by k36no4862002 » Sat Apr 10, 2021 1:26 pm

I have been reading this topic with interest. I have a k36 and used 2 injectors for years. I recently made 2 new injectors for my challenger and wanted to check the flow that they put out in an attempt to gain an idea on volume.
I fitted the injector and put the output feed into a plastic watering can.
Started.the injector and I found.quite quickly thqt the watering can was getting.hot, so hot that I thought.it might melt, so it must have been coming out pretty damn hot, so my question I guess.
What temp does the feed water from an injector run at into a boiler and what temp via a pump and heat system would you obtain, I know there is a difference on the fast.quick injector and steady feed from a pump which is another item, I was just.curious on the optimum temp qnd what we currently get
Paul Edmonds,
Surrey, United Kingdom.
2.5" Scale D&RGW K36, 1.6" Scale CHallenger 4-6-6-4 (still ongoing)

Berkman
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Re: Pre-heating boiler water

Post by Berkman » Sat Apr 10, 2021 1:47 pm

often though, on the full sized locos, injectors were somewhat throttled down using the water valve, not the steam valve. So the same amount of steam injecting a lessor amount of water, no doubt this would result in an increase in water temp vs running an injector "full on".

I have heard throttling injectors using the water valve can cause more wear on the internal seats, but not sure on this. Also I was always told to open the water valve back up for 5-10 seconds before turning the injector off, which will allow the injector to cool some, as they can get hotter while throttling the water.

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Bill Shields
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Re: Pre-heating boiler water

Post by Bill Shields » Sat Apr 10, 2021 5:24 pm

thermal conductivity comes into play with everything.

if you don't understand it, then you need to learn
Too many things going on to bother listing them.

Emfinger
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Re: Pre-heating boiler water

Post by Emfinger » Sat Apr 10, 2021 5:48 pm

Bill Shields wrote:
Sat Apr 10, 2021 5:24 pm
thermal conductivity comes into play with everything.

if you don't understand it, then you need to learn
At this point I have no idea what to do....What I'm going to do is put 4 or 5 loops of 1/8" copper in the Smoke Box. I have lots of room there. I have an oil burner so I don't want outside forces impacting the burner flame ! Such as increased draft caused by opening the throttle, So I took the exhaust pipes from the smoke box and dumped them out the sides. The only thing in the smoke box is a blower tube....1/4".

I may put a spark screen in the smoke box....what do you think?

Tom

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Bill Shields
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Re: Pre-heating boiler water

Post by Bill Shields » Sat Apr 10, 2021 8:11 pm

You will probably do better with a bit larger diameter copper...unless you are speaking of 1/8 ID copper.

Suggest you go with thick wall rather than the thin wall stuff commonly available.

Screens in model smokebox are more trouble than worth.
Too many things going on to bother listing them.

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