Optimal number of boiler tubes.

[Raygers]

tenor thank you for another thought-provoking post. I am/was intending for smokebox coils but you have made me rethink this. One point though, from your photo it looks like it has one tube down the middle, would that not defeat the object of using flue gases to heat the coils? There are no coils directly in the path of the gases, the boiler I'm planning is a multi-tube so there will be direct heating of the coils.

 

[Steamchick]

Raygers, I have made/re-worked boilers like this with a central flue, and with a set of smaller flue tubes. Depending on the engine demand (little engine with 15psi max and slow running) or a bigger engine requiring 30psi and a lot of steam... and the rule of thumb I use it to try and get the steam from the dryer/superheater from the hottest place last, coolest place first. If impractical to take the last bit os superheater don the central flue, and around inside the firebox before going "outside" to the engine then even a loop down and back helps heat the steam, especially if it has been a few times around the smoke box zone before it goes down the central flue. But if ONLY little flues, then the steam pipe can't go down at all, so just any turns in the smoke box should help give a few degrees of extra heat to the steam, which means dryer at the engine.
A case of sometimes we just do the best we can (with the boiler we were given to re-commission!).
K2

 

[peterl95124]

hmmm, for model sized boilers, flame tube length divided by 80 is way too small inner diameter to be practical, hence we need to focus on radiant heating rather than conductive heating,
thoughts ?
comments ?
recommendations ?

Peter.

 

[Steamchick]

Thanks Martin for feedback (post #40). Very useful info re: superheater length. I found a "pipe length vs. pressure drop" calculation on't web: looks like 3mm steam pipe a foot long gives 6~10psi drop, but 5mm pipe only 1 or 2 psi drop. - So I'll see what sizes I use and can improve upon.
https://www.pressure-drop.com/Online-Calculator/Of course. any pressure drop means condensation, I lag my steam pipes but they are still losing some heat, and the pressure drop will condense water as well. - Probably the main cause of "wet steam"? - Then if condenses more with pressure drop down little holes in engines! - Hence the need for whatever dryer/superheater is possible!
K2

 

[Steamchick]

Peter, at the beginning of the thread there were alternative calculations (better than the "80" rule I used). Proper rules based on more modern texts than the 1967 book I got the "80" from. - Post #3 etc.
Please use those rules.
K2

 

[user 52078]

tenor thank you for another thought-provoking post. I am/was intending for smokebox coils but you have made me rethink this. One point though, from your photo it looks like it has one tube down the middle, would that not defeat the object of using flue gases to heat the coils? There are no coils directly in the path of the gases, the boiler I'm planning is a multi-tube so there will be direct heating of the coils.

Ah, l failed to explain that hole down the middle is the coal chute, so has a lid on it most of the time. The chimney is off to one side, the fact you cannot see it is a result of the design to get plenty of superheater surface in there.
Martin

 

[HMEL]

Forgot to mention... For "commercial" burners, you would need to design and make a gas-jet/air-intake/Venturi and mixing tube, and a pressure-levelling plenum to go beneath the burner itself. (I can draw a design, based on a 4in design I have made).
I just wonder what Tenor's calculations make of these options in terms of useful Steam production? - Perhaps the coal fire is simply "more powerful"?
K2

You are correct in thinking coal burns differently. A btu is a btu but coal does not form a lot of water of combustion because of the amount of hydrogen available. This hydrogen forms water and contributes to stack loss because the enthalpy of vaporization can not be recovered easily. Maximum coal firing efficiency is usually around 84% while a gas unit is about 78%. Wet coal on the surface can be bad news because now you introduce a lot of water. Like wise some low grade coals are high in entrained water so and you look very carefully at the coal specs when purchasing it.

 

[Steamchick]

Just in case anyone is interested in making Ceramic burners, or attempting to make wire matrix burners, I am willing to share calculations and methodology. This may not give exact answers, but I find I get close enough first time around that there are few "fine tuning" iterations needed to make a good burner. I have made and developed burners for "customers" - at a cost to cover materials, not labour. - because these things are for sharing and teaching others (Children, interested adults, at shows, etc.).
But I stress, I am happy with ceramic burners, yet am not yet entirely happy with the wire matrix burners I have made... I think there is much work for me to do to get a crude formula that will work "right most times". Even my calculations for gas and air entrainment and mixing can be widely off the mark, as these things appear to work at one set of circumstances, but do not appear to scale as well as I should like. e.g. my biggest venturi entrains maybe twice as much air as I want, so perhaps needs more gas... as it is performing better than expected? But that is generally a good thing, unless the burner is simply oversized for the boiler flue "back-pressure", when a smaller venturi for the selected gas size is best. I currently use a sleeve to block around half the air intake so it works as I want for burner surface development...
I guess tenor, Martin, Charles, et la. can explain I am getting chewed-up by ignoring Reynolds numbers.... or something!
Write /PM me with your design of boiler and requirements, and I'll discuss what I can suggest.
K2

 

[Steamchick]

Martin,
Just re-reading your post #40:
Considering the depth of a coal fire, fully filling the firebox to a "normal" point maybe 10mm below the bottom of the fire-hole door, I can compare a cylinder of glowing coals versus a ceramic burner thus: BUT THIS IS VERY CRUDE! and just the radiant heat output...
Coal radiant cylinder: dia 4 1/2" x 1 3/4" high. @ 1220C.
Ceramic radiant dia 4 3/4" dia x 3/16in thick: @900C.
Wire-mesh burner dia 3 1/2" dia x 3 in high? @1100C.

I use the Stefan-Boltzman equation with S-B constant: 5.67 × 10−8 J/s · m2 · K4.
And consider emitted heat as "t1", and received heat at copper firebox walls as "t2", with the heat transmitted to the boiler as (t1 4th, - t2 4th): Boiler at 100psi ~200deg.C - copper surface emissivity 0.9, area of firebox 5" dia x 4 1/2" high less firehole of 1 1/2" x 2 1/2", and flue tube etc. holes of 55 x 1/4" dia bore (where the radiant heat shines straight through!).

I have a few reservations about these "relative" powers, due to estimation of emissivity, etc. - What emissivity should be used for a white ceramic material? How much surface ash is cooler and blocking "core temperature" of the (black) coals, so the mean emissivity is effectively reduced? Wire mesh is approx 50% wire at the surface and glowing temperature, with inner fibres shining through at higher or lower temperature, so what is the mean emissivity of an actual burner? For all of these, should I assume there is a fraction of surface glowing at "max temp, and the rest at a lower temp? - should stainless steel wire have a very low emissivity? also white ceramic? but "black coal a high emissivity and grey ash a lower emissivity?
The only thing I can suggest for Tenor's calculations is that whatever radiant energy is used, the remainder of the "fuel power" is used as "hot gas" for conduction to copper surfaces.
I guess it needs more study than my garage or photos can estimate?
K2

 

[ajoeiam]

Hmmmmmmmm - - - have been following this thread - - - - fascinating AND interesting.
The base equipment is a 'fire-tube' system.

Wondering if there are any links to similar 'conversations' re: 'water-tube' systems?

TIA

 

[Steamchick]

Ajoeim, the main difference between the fire/flue-tube design and water tube boilers, is simply the huge difference in most when you consider the draught from fire to exhaust. Forcing the draught through flue tubes can be a limiting factor on many boilers. Simply, on model locomotives, the fire can degrade through lack of draught when the engine stops, through lack of air-flow. Hence the need for a decent blower, when in the station, re-fuelling the fire, watering, loading passengers, etc.
Water-tube boilers usually have larger gaps so do not restrict the draught.
But heat flow calculations can be done just the same way in principle, I am sure.... it is a combination of radiant heat - direct line of sight from hot fire to cooler boiler surfaces, plus hot gas cooling by conduction to cooler boiler surfaces.
Do you have a real application to be studied? Or just an interest in the explanations from experts (Martin, Charles, et al) and ramblings of sanatorium (e.g. me!).
K2

 

[ajoeiam]

Ajoeim, the main difference between the fire/flue-tube design and water tube boilers, is simply the huge difference in most when you consider the draught from fire to exhaust. Forcing the draught through flue tubes can be a limiting factor on many boilers. Simply, on model locomotives, the fire can degrade through lack of draught when the engine stops, through lack of air-flow. Hence the need for a decent blower, when in the station, re-fuelling the fire, watering, loading passengers, etc.
Water-tube boilers usually have larger gaps so do not restrict the draught.
But heat flow calculations can be done just the same way in principle, I am sure.... it is a combination of radiant heat - direct line of sight from hot fire to cooler boiler surfaces, plus hot gas cooling by conduction to cooler boiler surfaces.
Do you have a real application to be studied? Or just an interest in the explanations from experts (Martin, Charles, et al) and ramblings of sanatorium (e.g. me!).
K2

Grin - - - of course I have a real application - - - but its not in the 3" copper tubing size category.
At least a couple steps larger.
I'm looking for independence from the powers that be - - - they seem to think that they can legislate mass stupidity and I'm still in the resisting stage.

What's a not stupidly costly redundancy model - - - 3 of 1/2 total size needed for the system?
(That would be to provide the opportunity for working on one system while still being able to drive the complete system.)

Dunno - - - am I allowed to work on this here - - - it is steam engine related (I am inspired by the mill pumping engines that ran 50 to even over 100 years (I think I read of some) in constant operation)?

 

[Steamchick]

Ajoeiam. The sums are the same, the numbers bigger. Martin is the wizard I think? What engine do you want to run, so how much steam do you think you need?
K2

 

[user 52078]

HMEL
Yes, coal and gas firing are different in a lot of ways, and I agree that gas tends to be a lot richer in hydrogen hence more moisture and wasted energy up the stack. To offset that, coal suffers unburnt fuel losses, typically 10 - 20% on large locomotives working hard. It is also really difficult to get tight control of the air:fuel ratio on coal, less so on gas / oil.
Here in UK we have been trialling coal alternatives including rape oil cake, 50/50 coal / olive stones and various mixes of pulverised coal moulded into brickettes. The results for materials of broadly similar calorific value are showing a 100% spread in fuel consumption, suggesting a lot of other factors are at play. So I fully agree with your points.
Steamchick,
I also use the Stefan Boltzmann equation for radiation heat transfer. It needs modifying to account for emissivity as you state. Choosing those emissivity values for a given application is not easy. I assume a coal fire surface e = 1, but accept that may not be right. I would expect your ceramic burners to be near 1, otherwise they would not be widely used. But as for precise values?????

When setting up my program I found that some of these fudge factors had to be massaged to give the right answers, but if you have calibrated against test results, that is as near as you can reasonably get.

Ajoeiam,
Fundamentally water tube boilers are similar in terms of calculation as for fire tube. Have a search around a website called Thermopedia.com (Thermopedia link) and you will find formulae for water tubes. They tend to be a bit less accurate than the simple firetube set up, but useable.
One thing I should say is that my calculations assume the water side is at constant (saturated temperature) which is not so. One of the advantages of water tube boilers is that the water side circulation is excellent, whereas it can be a bit sluggish in fire tube boilers. I don't even attempt to calculate natural circulation on the water side of boilers - those are the sort of sums that the nuclear industry do to make sure reactors don't go pop (well, KERBOOM actually), so they are well beyond my ametuer pay grade.
What you might find useful if you live in colder climate is to have a boiler supplying a steam engine / turbine for electric generation and then using the exhaust steam for building heating. It is often done in industry with a need for process heating by steam (e.g. sugar industry, paper making etc.) and gives good overall efficiency at least in winter. I shan't bore you with the thermodynamics here.

I think we might have drifted away from the original post a bit!
Martin

 

[HMEL]

HMEL
Yes, coal and gas firing are different in a lot of ways, and I agree that gas tends to be a lot richer in hydrogen hence more moisture and wasted energy up the stack. To offset that, coal suffers unburnt fuel losses, typically 10 - 20% on large locomotives working hard. It is also really difficult to get tight control of the air:fuel ratio on coal, less so on gas / oil.
Here in UK we have been trialling coal alternatives including rape oil cake, 50/50 coal / olive stones and various mixes of pulverised coal moulded into brickettes. The results for materials of broadly similar calorific value are showing a 100% spread in fuel consumption, suggesting a lot of other factors are at play. So I fully agree with your points.
Steamchick,
I also use the Stefan Boltzmann equation for radiation heat transfer. It needs modifying to account for emissivity as you state. Choosing those emissivity values for a given application is not easy. I assume a coal fire surface e = 1, but accept that may not be right. I would expect your ceramic burners to be near 1, otherwise they would not be widely used. But as for precise values?????

When setting up my program I found that some of these fudge factors had to be massaged to give the right answers, but if you have calibrated against test results, that is as near as you can reasonably get.

Ajoeiam,
Fundamentally water tube boilers are similar in terms of calculation as for fire tube. Have a search around a website called Thermopedia.com (Thermopedia link) and you will find formulae for water tubes. They tend to be a bit less accurate than the simple firetube set up, but useable.
One thing I should say is that my calculations assume the water side is at constant (saturated temperature) which is not so. One of the advantages of water tube boilers is that the water side circulation is excellent, whereas it can be a bit sluggish in fire tube boilers. I don't even attempt to calculate natural circulation on the water side of boilers - those are the sort of sums that the nuclear industry do to make sure reactors don't go pop (well, KERBOOM actually), so they are well beyond my ametuer pay grade.
What you might find useful if you live in colder climate is to have a boiler supplying a steam engine / turbine for electric generation and then using the exhaust steam for building heating. It is often done in industry with a need for process heating by steam (e.g. sugar industry, paper making etc.) and gives good overall efficiency at least in winter. I shan't bore you with the thermodynamics here.

I think we might have drifted away from the original post a bit!
Martin

I have constructed several computer programs to determine the firing efficiency of coal, gas, oil and MDF(waste) With gas boilers I found the set up of air fuel ratios to be more difficult because of the tendency of gas to put you in a position of explosive condition. Pulverized coal can also do that because it acts like gas in many ways. Fuel not burned is part of the efficiency equation as well as side wall losses. As far as air fuel control you can in fact get quite close but you have know the tricks of getting good air flow measurements. And there are lot of ways to do that. But most of the hobby boilers I see do not spend a good amount of time in instrumentation. And I am not sure at the smaller scales its worth it. When you get into the issue of boiler circulation thought pattern has to change. The reason is the furnace volume now has to be constructed for radiant heat as well as conduction and if its a coal boiler grate size and type is now important considerations. As for the live steamer guys I would agree gas or oil is far more controllable and much simpler to design as the mechanics of ash removal are not limiting. Most of the issues I see with hobby boilers and some of the larger sizes is in the construction considerations of how they were built and were they done with the right materials. Have long since let my ASME certification as an inspector drop but when I see something that can burn or explode I try to let them know what is wrong and why. By the way when I took the exam for that nearly four hours was devoted to old locomotive inspection and repair and it was the most difficult part to sit for. When I complained after the exam the inspector said that was put in because of all the restoration work they were seeing. I was lucky to pass.

 

[Steamchick]

Well done on getting that pass HMEL:
Martin, I think this thread has become "quite useful".
Prompted by a potentially "Simple" initial question it has brought to the fore many aspects of boiler design that are usually "hidden" in the drawings of boilers that people use. I am learning a lot as it progresses.
This is the reason I enjoy such discussions, as many "cleverer than I" contributors explain in plain language the pitfalls and traps that make "amateur" design work - e.g. mine! - look to be wrong. I have found - by trial and error - that making a boiler design is not only difficult, but even after applying "rules from books" it doesn't always work as planned. Particularly when it comes to "getting the flames to heat the water".
After the boiler has been built, it is usually impossible to modify it when only "half" the fuel expected will burn successfully. Been there, done if a few times! And I have made/modified burners for people with boilers who wanted and expected their boilers to develop the steam to run a loco, or whatever, but find it simply won't develop enough steam to power the train, or even the free loco!
So Rayger's question BEFORE making the boiler is the most intelligent question in a while...
Similarly, the experts who have contributed have produced valuable answers to help Raygers and others (I am VERY appreciative of your teachings!). So I look forward to more discussions about these matters.
THANKYOU ALL.
K2

 

[Steamchick]

Martin, you commented:
I also use the Stefan Boltzmann equation for radiation heat transfer. It needs modifying to account for emissivity as you state. Choosing those emissivity values for a given application is not easy. I assume a coal fire surface e = 1, but accept that may not be right. I would expect your ceramic burners to be near 1, otherwise they would not be widely used. But as for precise values?????
I think I must do some more work to get "the right answer" here.
I had to do "cooling" calculations on Power station electrical equipment being installed in Dubai, where the SUN heating could potentially heat the equipment above the maximum temperature where we wanted to run the plant. And at night, the air temperature could drop below freezing... So to stop the Sun we fitted sun-shades... but that meant we had a screen with sun (radiant) heating on one side, Hot air all around, and equipment trying to radiate heat as the air temperature was so high.... Emissivity factors were key to managing this problem.
So the table:
e.g. Emissivity

Material Emissivity:

• Aluminum (anodized) 0.77
• Aluminum (polished) 0.05
• Concrete 0.92
• Copper (polished) 0.05
• Copper(oxidized) 0.65
• Glass 0.92
• Gypsum 0.08
• Ice 0.97
• Sand 0.9
• Snow 0.8S
• oil (Dry) 0.92
• Soil (Saturated) 0.95
• Stainless Steel 0.59
• Water 0.95
• And from my memory...
• White painted (Aluminium) ~0.2,
• Grey painted ~0.5
• Matt black painted ~0.95

The "simple" analysis - as far as I can remember - is that a body cannot change its emissivity... -
SO (e.g.)

• a White ceramic emissivity of "0.2" - if considered as a "white" surface - could really act like concrete at "0.92" which is close to the "Matt black" 0.95.
• But Coal ash on the surface of coal (cheap household coal or charcoal, "coal substitutes", etc.) may act more like Gypsum at 0.08 than Matt black at 0.95!

And once agreed how the material is performing you can do proper sums, but I can only GUESS as I have no other information.
My GUESS of emissivity:

• For coal = Part hot matt black coals burning at e = 0.95 and part Ash-coated (like Gypsum) at e = 0.1, insulating the coals behind and at a much lower temperature because adjacent to the Cooler gases adjacent to the copper inner surfaces of the boiler where heat is being conducted from the gases to the copper at ~200~250C - so maybe really around e = 0.8 for a loco fire?
• I used e = 0.8 for emissivity of WHITE ceramic: The surface is made of small cones, with gas burning alongside heating the cones, therefore a part of the surface is white at the gas/air temperature at the fuel inlet holes, part is probably over 900C, and the rest of the cone is somewhere between the two? - Maybe averaging 900C? (book max. value... I assume measured as a mean temp. of a larger area?).
• Similarly, I considered the copper in a boiler to be tarnished, so used e = 0.8 (a guess! - poor memory) but should perhaps have used 0.65? - For the fire-hole door, the heat assumed to shine on the inside, 80% absorbed by the door (Lost outside), and 20% reflected "for absorption elsewhere" to become heated water...
• For "stainless steel mesh" I used e = 0.95... (a guess?) but considered inner and outer wires to be heated by burning gas so the whole surface area at ~1220C. - Even though in photos the corners, and odd places are not even red hot.

FOUND my fag-packet notes of my sums so I can do them properly for you if you need them? I have never actually verified any of these sums, but work simply on Gas power - with complete combustion - going "IN", and smoke-box gas temperature going "Out". Part radiant heating and part gas conduction (surface area) based to "text book" values...
Hope that clarifies my amateur ponderings?
K2

 

[Steamchick]

I should write a proper paper with proper sums, if anyone wants it, just ask.
"To share the truth, knowledge, and teach good morals are just three good things, of the many we should teach our children".
K2

 

[ajoeiam]

Martin, you commented:
I also use the Stefan Boltzmann equation for radiation heat transfer. It needs modifying to account for emissivity as you state. Choosing those emissivity values for a given application is not easy. I assume a coal fire surface e = 1, but accept that may not be right. I would expect your ceramic burners to be near 1, otherwise they would not be widely used. But as for precise values?????
I think I must do some more work to get "the right answer" here.
I had to do "cooling" calculations on Power station electrical equipment being installed in Dubai, where the SUN heating could potentially heat the equipment above the maximum temperature where we wanted to run the plant. And at night, the air temperature could drop below freezing... So to stop the Sun we fitted sun-shades... but that meant we had a screen with sun (radiant) heating on one side, Hot air all around, and equipment trying to radiate heat as the air temperature was so high.... Emissivity factors were key to managing this problem.
So the table:
e.g. Emissivity

Material Emissivity:

• Aluminum (anodized) 0.77
• Aluminum (polished) 0.05
• Concrete 0.92
• Copper (polished) 0.05
• Copper(oxidized) 0.65
• Glass 0.92
• Gypsum 0.08
• Ice 0.97
• Sand 0.9
• Snow 0.8S
• oil (Dry) 0.92
• Soil (Saturated) 0.95
• Stainless Steel 0.59
• Water 0.95
• And from my memory...
• White painted (Aluminium) ~0.2,
• Grey painted ~0.5
• Matt black painted ~0.95

The "simple" analysis - as far as I can remember - is that a body cannot change its emissivity... -
SO (e.g.)

• a White ceramic emissivity of "0.2" - if considered as a "white" surface - could really act like concrete at "0.92" which is close to the "Matt black" 0.95.
• But Coal ash on the surface of coal (cheap household coal or charcoal, "coal substitutes", etc.) may act more like Gypsum at 0.08 than Matt black at 0.95!

And once agreed how the material is performing you can do proper sums, but I can only GUESS as I have no other information.
My GUESS of emissivity:

• For coal = Part hot matt black coals burning at e = 0.95 and part Ash-coated (like Gypsum) at e = 0.1, insulating the coals behind and at a much lower temperature because adjacent to the Cooler gases adjacent to the copper inner surfaces of the boiler where heat is being conducted from the gases to the copper at ~200~250C - so maybe really around e = 0.8 for a loco fire?
• I used e = 0.8 for emissivity of WHITE ceramic: The surface is made of small cones, with gas burning alongside heating the cones, therefore a part of the surface is white at the gas/air temperature at the fuel inlet holes, part is probably over 900C, and the rest of the cone is somewhere between the two? - Maybe averaging 900C? (book max. value... I assume measured as a mean temp. of a larger area?).
• Similarly, I considered the copper in a boiler to be tarnished, so used e = 0.8 (a guess! - poor memory) but should perhaps have used 0.65? - For the fire-hole door, the heat assumed to shine on the inside, 80% absorbed by the door (Lost outside), and 20% reflected "for absorption elsewhere" to become heated water...
• For "stainless steel mesh" I used e = 0.95... (a guess?) but considered inner and outer wires to be heated by burning gas so the whole surface area at ~1220C. - Even though in photos the corners, and odd places are not even red hot.

FOUND my fag-packet notes of my sums so I can do them properly for you if you need them? I have never actually verified any of these sums, but work simply on Gas power - with complete combustion - going "IN", and smoke-box gas temperature going "Out". Part radiant heating and part gas conduction (surface area) based to "text book" values...
Hope that clarifies my amateur ponderings?
K2

Useful for straight forward type substances.
Now what about for bio-mass - - - and that would likely also depend upon the type of (I would bet!)?
TIA

 

[ajoeiam]

I should write a proper paper with proper sums, if anyone wants it, just ask.
"To share the truth, knowledge, and teach good morals are just three good things, of the many we should teach our children".
K2

Well - - - - I - - for one would like to read such.

TIA