Steam & Excursion > Technical question - fire tubes in boilers

I have a question about fire tube design in boilers. I know the tubes being just circular pipes is easiest, but with modern techniques, could this be improved upon? Let's say I was building a clean-sheet modern steam locomotive that only had to look and function externally like the old locomotives. Should the fire tubes in the boiler still just be round? Or should they be finned for greater surface area? Should they be finned on just the outside or the inside? My thinking is heat sinks like in computers are finned for far greater surface area to transfer heat to the air, so would finning the tubes in a boiler produce greater efficiency?

Hmmm, we will have to mull this thought over. Finned on the inside of the tube would create greater soot collectors and harder to keep clean so that would negate the heat transfer capability. Fins on the outside would create a difficulty in installation. 

Bob Krieger
Cheyenne, WY

Having worked in our family's plant with a firetube boiler that had to be retubed, I don't see how they culd roll the firetube if it was a fin shape.  In other words, like Bob said itwould be very difficult on the ouside.

Randy Lundgren
Elgin, TX

Ok I will ask this question: on a oil burning locomotive could you put in turbulators to help with the heat loss?

In large scale railroading, 2.5" scale have heard of those using propane doing something like that on the inside of tubes.

CPRR Wrote:
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> Ok I will ask this question: on a oil burning
> locomotive could you put in turbulators to help
> with the heat loss?

What "heat loss"?

I'll get Wes' ire but the best thing about superheater elements is that they disturb the gas flow through the
tubes and move the heat outward in the tubes. Hot firebox gases roaring through open tubes isn't the best way to
convect heat into the water. The "Turbulator" idea is a very good one though the accumulated cinders or waste
oil would be a real problem to clean.

Out

What about making some flues 2 pass?  

Multi pass (up to 8) are widely used in water/oil exchangers, issue on a steam locomotivewould be the adverse
affects on the draft. A backwards boiler (water tube) would benefit from being multipass.

Out

Turbulators are only useful for clean gasses, i.e. propane, which isn't used in full size practice, with the exception of one or two park trains.

Turbulators are common in hot water heater applications. They work by slowing the gasses down through turning the laminar flow into more turbulent flow.  A fluid flowing through a tube unless in turbulent flow tends to form a film at the tube wall which impedes heat transfer.   A Turbulator breaks this film and makes the flow turbulent raising the effective Reynolds number. 

Any kind of turbulator or fin in an oil fired or coal fired locomotive would make it impossible to clean, unfortunately, and would, as others have said, soot up very quickly. 

I installed turbulators in my propane fired Chloe and they did show a marked improvement in heat transfer efficiency.  I used less fire and thus less fuel after that for the same power.  The exhaust at the stack felt noticably cooler.

This is what home made turbulators look like for live steam boilers fired by propane.   Stainless steel strips twisted into spirals.   There are also commercially made turbulators that are a sort of woven mesh.

As to draft, it does have a small effect on draft, but propane burners need very little so there is no issue in regards to gas firing.

Best,

-Mike

Mike Massee
Tehachapi, CA
Photography, Railroading and more..



Edited 3 time(s). Last edit at 11/27/19 11:15 by Harlock.

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Turbulators in industrial heat exchangers are used commonly on the water side, hardly a "gas".

Out

callum_out Wrote:
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> Turbulators in industrial heat exchangers are used
> commonly on the water side, hardly a "gas".
>
> Out

I should clarify that I was only speaking about gas turbulators in fire tube boilers.

Mike Massee
Tehachapi, CA
Photography, Railroading and more..

That does make a difference!

Out

Guys...
First, Callum is right on his first point.
Probably also true on his other points...

Comparing tiny boilers and drafting to a full size loco boiler is a fool’s errand.
Remember, that as you break up the burning gasses into small streams traveling down the separate flues and tunes, the
surrounding ‘cold’ water quickly sucks the heat out of the flues’ gasses.  By traveling about 24” to 30”,  into the flues and tubes,
the flues’ gasses pretty much reach equilibrium of temperature with the surrounding boiler water.

By the time the gasses reach the smokebox and stack ....they’re pretty ‘cold’ compared to the 3,000 F. firebox temps.
The number of exposed return-bends of the superheater tubes at the firebox-end determines the temperatures of superheat that the steam reaches.

Again, the effective heat-transfer length of flues and tubes was ‘thermal-efficiency’’ measured and is between 18’ and 21’ in length.
So, with the heat being sucked-out of the incandescent flue gasses—- in the first  3-feet, or so, from the firebox—-  adding “improved turbulence” seems only to add troubles at ‘flue-blowing’’ time.  Coal-burners are famous for getting problems of plugged flues . between boiler-washes.  ( 10% or less, in percentages of plugged flues). 

95% of the boiling of the water occurs at the firebox and over the crown sheet.
 The primary purpose of the flues is to provide a space for the superheaters and getting flue-gasses to the smokebox.
In actuality, very little heat is available for being transferred into the water during the length of the flues.

The BEST boiler efficiency is during a white-hot firebox temps and low draft, low-firing rates, together with low steam flow
through the units.Increasing the “dwell-time” of the steam in the units ( increasing the amount of superheat),
is accomplished by skillful operation of the loco.

I’ve seen Doyle operate the 4449 with a  partially closed-off throttle ( trapping the steam in the units — for longer amounts
of time) and a medium cut-off on the valvegear.  The superheated steam was probably near the 800-deg temps, he was
using ‘small-sips’ of stream with each stroke of the pistons —- using the steam expansively.  
With low steam flows, firing the engine is way easier since the low draft-rates contribute to white-hot fires.  

Doyle was very attuned to the smoothness with which 4449 rolled down the tracks .... being very careful to operate
the engine ( valve gear settings and using the best throttle combinations)  as smoothly as possible —- for varying demands
as to speeds, hills and signals—- as they influence the “loading” on the engine.

‘Forcing’ the fireboxes with lots of drama, noise, sturm-und-drang , and pounding may make pretty pictures;
but, are ineffective, and are hard on the boiler, the firebox side-sheets and the entire loco...

Look to efficient loco operation, for the best & easiest economies of fuel and water.

​W.

(i’ll clean-up the appearances of the chopped-up sentences...later...)



Edited 9 time(s). Last edit at 11/27/19 18:27 by wcamp1472.

You might be interested in some of the turn-of-the-last-century experiments by Baldwin (in particular) with various tweaks on the fire tube boiler, the Santa Fe experiment in jointed boilers (discussed on TO, I didn't find the link quickly), and application of water tube boilers on locomotives.

Generally the problem is bounded by two factors: You've got reduced control of the boiler water, as it's not conditioned and recycled, so you've got major contaminant/maintenance issues to design for that preclude things like cracks & crevices.

The other issue, and the one that really led to feedwater heaters, is that to superheat steam to 750F that means the exhaust gasses need to be leaving the superheating surface at something approaching that temperature, otherwise you're using your steam superheat to heat your exhaust gas! Oops, wrong way! Ideally you'd have the superheat in the firebox, (think steam tube?) and the boiler (steam generator) after the superheat is done. Now you've got a really awkward collection of vessels that doesn't easily package into a mobile piece of equipment. Steam turbine powerplants address the same issue by using closed circuit water recovery and pushing the energy back into the feedwater, and using "steam generators" where the water tube water is allowed to flash to superheated steam. The actual water tubes are running very high pressure to make sure they're always liquid, not gas. (I know there are professionals on this board - I'm certain I've got that description overly simplified. Sorry!)

For locomotives, the rough equivalent is either direct injection of exhaust steam into the feedwater (Worthington and Coffin style among others), or heat exchanger (Elesco). Feedwater heating is solving the same problem you're looking at, but on the "other end" of the energy cycle, making the fire tube flu/superheater unit into something that can be compromised a little for boiling water and that compromise used for improving the superheat.

SRK

MattW Wrote:
-------------------------------------------------------
> I have a question about fire tube design in
> boilers. I know the tubes being just circular
> pipes is easiest, but with modern techniques,
> could this be improved upon? Let's say I was
> building a clean-sheet modern steam locomotive
> that only had to look and function externally like
> the old locomotives. Should the fire tubes in the
> boiler still just be round? Or should they be
> finned for greater surface area? Should they be
> finned on just the outside or the inside? My
> thinking is heat sinks like in computers are
> finned for far greater surface area to transfer
> heat to the air, so would finning the tubes in a
> boiler produce greater efficiency?

Steve Krause
Chillicothe, IL

All of the mechanical possibilities and previous attempts have been well covered. There has been a revolution in the water side performance. It has been largely ignored or treated with skepticism. In some shameful corners, individuals with a vested interest or axe to grind have done a disservice to the steam community. Herewith are the facts for enlightenment:

A relatively new technology in boiler water treatment is available. It has been in use and ongoing improvement, with roots going back to the 1930's in Germany and Switzerland. It came into larger use in power generation and large manufacturing, as a commercial product in the mid 1960's. It came into the steam preservation community as Terlyn LSB in 1995. Through the years, many operations have experienced great success with it. Bill Pettitjean, of Green Velvet, described it as, " A new Paradigm in water treatment. " It is a new standard.

Previous treatments used alkali to get the water pH to at he 10 - 11 region. The problem with alkalis is that they can become caustic in regions of low flow withi a boiler. They form expanding crystals which get into crevices and micro cracks in stressed metal. The crystals grow and push the cracks open, forming weeps, leaks and structural failure. To prevent this, or at least minimize and control it, a simple 1 - 2 - 3 wet chemistry " P " alkalinity test of boiler water is needed. P stands for Phenolphthalein, a common pH indicator. From the results of this test, the treatment can be adjusted to be in the safe range, where metal will be protected, but not at risk of Caustic Stress Corrosion Cracking.

Terlyn LSB ( standing for Locomotive Steam Boiler ) cannot exceed pH 10.4 and does not form expansive crystals. It does not need to be monitored for excessive alkalinity or pH. It is a Solubilizing, rather than Precipitating treatment. No sludge - forming material is added. Minerals which would have caused heat-insulating scale are held in solution until they are either blown down as part of operations or drained and washed-out as part of regular maintenance.

Because of it's cleaning power and ability to hold minerals in solution, dryer steam is produced by the boiler. Foaming and carryover are reduced dramatically. Superheaters are not being grit blasted or subjected to chemical deposits and corrosion product burning. A dirty boiler spits grit, rust and crud which gouges and wrecks valves and engine parts. A clean boiler produces clean steam, and more of it per unit of fuel. So, there is a big improvement on reduced overall costs with better performance and longevity of steam boilers ...



Edited 1 time(s). Last edit at 11/28/19 05:52 by BoilerWater.

“A clean boiler produces clean
> steam, and more of it per unit of fuel. So, there
> is a big improvement on reduced overall costs with
> better performance and longevity of steam boilers”

The production of “clean steam” is simply the results of common boiling of water..... that process produces “clean steam”..leaving all the other ‘stuff’ in the remaining water of the boiler.  

“Clean steam” happens regardless of the chemicals added to the feed water. 
I’m glad that there are improvements in the ‘industry’....

We need the progress ....but, let’s stick to,the facts ....steam is ‘clean’ as a matter of FACT....not the chemicals that are added.
Boilers are concentrators of impurities...by definition.
 The more water that’s boiled, the greater the concentrations of said contaminates. 

​W.

So we and I never ran on anything you could call a mainline but our ph levels were more down below 8 and
we didn't have "impurities" problems. 10.8 is basically rocks somewhat dissolved in water.

Out