Monotube Flash Boiler Design

[NapierDeltic]

Hi Toymaker.
Sorry to disagree with you about volume pressure and velocity of exhaust from the burner.
The air blower provides a certain volume of AIR at near atmospheric res sure at a known rate - Cubic feet per minute. But you are adding hydrocarbon fuel, which burns to provide water vapour (steam) and CO2. This is added to the nitrogen and oxygen of the air. So the mass is increased. The temperature is raised from room 20deg. C. To maybec1500deg. C.
So do some sums to determine a mass flow rate, gas volume and pressure accordingly.
It is how jet engines work.
Then you are extracting heat in the cooling coils so the final temperature has dropped - and pressure, and volume of gas - but the mass flow rate is constant.
Then you can explain how your boiler will work, or not?
Sorry to put a damper on your enthusiasm,
But keep on.
I am sure a few of us are excited by your project.
K2

Hi Steamchick, I do not have any reason to deny what you are saying, but this means Carnot approach - taken as master when comes to evaluate IC engines efficiency - is wrong. Because Carnot is a pure thermal cycle conserving mass and state of fluid (gas) in cycle; not to mention chemical reactions with different phase for reaction products. But how much is the difference?

 

[Steamchick]

Thanks Toymaker. With a Physics background, Engineering is just solving problems of everything. But I flunked Electronics and Computer software courses, so sick to everything else... My respect to your EE career! All the stuff I could not comprehend.
Although I have no formal training in any of the Engineering, I do have 5 years part-time (Apprenticeship) in compressor and car and truck engine refurbishment - from working for my Dad's mate in his workshop all my out-of-school hours.
And have held jobs in Electrical, Civil and Mechanical design, Manufacturing Quality, Installation, Commissioning of new plant and Safety officer, Workforce and Project supervision, planning coordination, etc., etc. Managed Regulations and associated Approvals and Audits for the Company, details such as Car Emissions testing, etc. Just lots of stuff where I have become the "Expert" - when there was no-one else for me to pass-on the job to.... until years later when I trained 3 Engineers to take on my workload so I could retire. And I never received any promotion in 40 years. So I obviously wasn't much good at all that! C'est la Guerre?
My only little steps into "combustion" - apart from being the Engineer "expert" in vehicle emissions for the factory - is to have made dozens of gas burners (mostly ceramic) for model steam boilers. Many problems that arise because the combustion chamber and exhaust pipe gas dynamics are not what the burners were made to see. The whole pressure gradient - and passageway ratios - are not what you may expect from "Simple" calculations. To the extent, that one guy went 2 years trying to resolve his "resonant noise" from his boiler... until I spotted how the pressure from "within the combustion space" was limiting his air supply due to back pressure, so offered him a better design of air intake... which worked! So that was a bit of a challenge and fun!
Enjoy your hobby - it is a fantastic project to finished.
K2

 

[Steamchick]

Hi Steamchick, I do not have any reason to deny what you are saying, but this means Carnot approach - taken as master when comes to evaluate IC engines efficiency - is wrong. Because Carnot is a pure thermal cycle conserving mass and state of fluid (gas) in cycle; not to mention chemical reactions with different phase for reaction products. But how much is the difference?

Sorry, I am out of my depth here....
My "simple" view... not trying to be clever, just trying to understand the gas dynamics from a beginner's perspective:
Toymaker was saying the Air in volume, pressure, temperature would simply be the same as Exhaust out.
I challenged that because the Air-in (a gas) plus LIQUID fuel (he is using diesel fuel in his burner I think?) creates a liguid (condensed water vapour) plus mixed gases from combustion and the nitrogen from the air.... so this exhaust will be different from the inlet. The mass will be constant - (no gain or loss of molecules) but the states of molecules (liquid or gas, combined chemically in different ways) will mean the volume of gas, temperature and pressure, will not directly follow the gas laws. A bit like Cake mixture into the oven, cake out?
I am willing to be taught the RIGHT answer, but I can't tell anyone what it is...
K2

 

[NapierDeltic]

You describe it well: cake mixture....
For instance nitroglycerin mechanical output is both an effect of phase change "Detonation of nitroglycerin generates gases that would occupy more than 1,200 times the original volume at ordinary room temperature and pressure;" Nitroglycerin | Description, Properties, & Uses and exothermic reaction "The heat liberated during detonation raises the temperature of the gaseous byproducts to about 5,000 °C (9,000 °F).[19] With a standard enthalpy of explosive decomposition of −1414 kJ/mol and a molecular weight of 227.0865 g/mol,[21] nitroglycerin has a specific explosive energy density of 1.488 kilocalories per gram, or 6.23 kJ/g," Nitroglycerin - Wikipedia.
I might agree that comparing IC with a pure thermodynamic engine makes it easy to approach from efficiency point of view, but it is definitely not entirely correct.

 

[Steamchick]

I am not sure of the comparison with an IC engine... did I suggest that somehow?
As I understand Toymaker's boiler, for mathematical modelling purposes, there are a series of connected chambers, with continuous flow of air, fuel, exhaust gas through the system. Each chamber can be defined by the input mass (flow) of material, temperature, pressure and output temperature and pressure.. Each connecton between chambers is defined by the cross sectional area.
Toymaker's has done a lot of iterative development to make a burner that works well simply venting to the atmosphere. Now he will be adding a chamber containing the coils, with an exhaust connection of undefined cross sectional area , but within the chamber there is a change of temperature and pressure, as the boiler coils extract heat, and limitations of cross-sectional area as he explained due to small gaps between tubes. This will change pressures and speed of combustion in the burner.
Finally the exhaust will exit - presumably into a chimney or exhaust pipe? - With a planned gas temperature, etc.
So not the same as the IC engine you mention.
K2

 

[NapierDeltic]

Sorry, it is off topic for Toymaker's boiler thread. I was just expressing my wonder that a not-exactly-true-to-facts model is generally used for judging IC engines. And you have pointed out one difference.

 

[Steamchick]

I hope I am right, if there is something you think is useful in my rambljng.
K2

 

[NapierDeltic]

I was just trying to add to my previous post that I consider your analitic approach and its premises (mentioned above) in Toymaker's boiler design very positive.
Sorry Toymaker for interfering your thread!

 

[Toymaker]

<snip>
Toymaker's has done a lot of iterative development to make a burner that works well simply venting to the atmosphere. Now he will be adding a chamber containing the coils, with an exhaust connection of undefined cross sectional area , but within the chamber there is a change of temperature and pressure, as the boiler coils extract heat, and limitations of cross-sectional area as he explained due to small gaps between tubes. This will change pressures and speed of combustion in the burner.
Finally the exhaust will exit - presumably into a chimney or exhaust pipe?
<snip>
K2

My experience working with jet engines taught me that increasing the pressure inside a forced air burner (or combustion chamber) is always beneficial, as it always allows more fuel to be burned, resulting in increased heat energy output from the burner. One of the reasons I built the boiler with closely spaced tubing was to increase airflow resistance, which will cause the hot gases to remain inside the boiler, and in contact with the tubes for as long as possible, thereby providing more time for the hot gases' heat to soak through the boiler tubes and into the water.
How my burner functions is much closer to how an jet engine's combustion chamber functions than a typical steam boiler' burner.

 

[NapierDeltic]

Increased turbulence has at least one beneficial result - improves heat transfer to pipes inside and guarantees less hot gas escapes boiler without transferring heat to freon. In fact gases from burner leave the boiler in a second or something like that; so heat exchange should be pushed to maximum.
About the right balance - heat transfer/pressure loss, it is above my current knowledge.
You are right, Toymaker. Jet engine's combustion chamber should be a close match from many points of view.

 

[Steamchick]

Thanks NapierD and Toymaker, this bit of discussion has helped me to understand better what Toymaker is doing to achieve his design goal. I think I learn more from discussing things I don't understand that the straightforward presentation... The difference between my gas burners and the Toymaker forced air burner is simply the forced air, and target of containing the combustion in the combustion chamber. Some back-pressure is beneficial to the forced-air burner, and this heat exchanger, but detrimental to my gas burners (as it changes the mixture ratio by affecting the induced air supply).
I live and learn...
Thanks!
K2

 

[Steamchick]

Incidentally, my previous success with someone's burner and boiler system that generated Rijke's tube resonance was cured initially by a forced air supply (controlled by an Arduino I think?).

I don't think Toymaker will have that problem because the burner turbulence will not form a specific plane of temperature change, but a confused zone - the length of the burner - of temperature change. This will prevent resonance occurring.
On the problem boiler, the burner (about 27kW?) was a radiant design where all the combustion occurred at a plane, with cold unburned gas one side and hot burned gas on the other, in a relatively long passage. Curiously, the first venturi for drawing-in and mixing the gas with air caused a mixture that burned "nicely" but caused the Rilke's tube resonance to initiate, but I offered a different shape of venturi that induced more air, and with the correct size of venturi the Rijke's tube resonance did not occur.
That was an interesting problem that I also previously experienced on a steam boiler with a ceramic burner, and had cured by changing the air intake design.
That's engineering I suppose?
Toymaker,
I hope I have not drifted too far off topic (again) for your boiler project?
Please keep us posted for your latest status.
K2

 

[NapierDeltic]

Steamchick, it is engineering, I say!
I have never heard about Rijke's tube resonance (saw your link , but I think it is a very interesting effect, Is it important? In my mind I can link it to a certain high speed operating mode of engine on SR71 as a ramjet. (bypassing turbine/compressor). All dynamic and resonance compression without aid from moving parts. Kadenacy effect should be from the same area.
But I guess there has to be a specific shape proportion at which effect occurs, which might not be applicable to Toymaker's design. That doesn't mean his design won't be affected by resonance related effects.
I figure it as a stationary rogue wave: if you generate it, it's a huge gain in energy density; if you don't, you can live without it. Resonance happens only in very limited circumstances so could be suited for a stationary motor; but its adaptability to variable operation is not good; unless your Venturi tube design is exactly devised to create an adaptable system...which could be!

 

[Charles Lamont]

I have heard the Puffing Devil fire resonating in the flue and chimney while steam was being raised.

 

[Richard Hed]

I have heard the Puffing Devil fire resonating in the flue and chimney while steam was being raised.

??, Can you tell us more about this?

 

[Steamchick]

Napper D. The Rijkes tube resonance cannot occur in Toymaker's design because the turbulent volume of combusting in a confined space does not describe the plane of heat (mimicking a membrane) required for generating a resonance. But in a 5 inch Gauge railway locomotive it was generating over 105dB at a single note. The only way to stop it was to turn off the gas and stop the heat. An air blower changed the condition adequately so it didn't happen, but needed to be variable so the combustion mix was good as steam demand varied and the gas was adjusted to suit.
My Improved venturi caused more air to be drawn-in to the burner, increasing the speed of combustion, making a very stable flame front that didn't succumb to generating the resonance.
Because the pressure inside the burner was a bit higher from the use of the better venturi, or with the air blower, the flame actually progressed just beyond the surface of the wire element, and we thought this extra flame turbulence damped the resonance before it could generate.
Interesting to note on early jet engines (made by Frank Whittle!) They had a curious and random flame-out then re-ignition that was a major problem for the US test pilots... until FW found a solution. Baffles inside the combustion chamber to change a steady flow to something more turbulent. This could have been some sort of resonance effect? Negated by the increased turbulence?
I do not understand it at all!
K2

 

[Charles Lamont]

??, Can you tell us more about this?

Not much. It happened 15 years ago so it is not easy to recall or describe the sound. On approaching the engine I was intrigued by the noise. A weird hollow thrumming or booming pulsation. Altering the conditions, eg opening the fire door, put a stop to it.

 

[Steamchick]

Your description reminds me of travelling at speed in a car with a single window open - when a sub-sonic thrumming occurs. Opening a second window stops it.
Note the length of the pipe from fire grate to top of chimney., and the short distance of cold air beneath the relatively wide and thin fire. Probably Rijkes tube resonance!
K2

 

[Richard Hed]

Your description reminds me of travelling at speed in a car with a single window open - when a sub-sonic thrumming occurs. Opening a second window stops it.
Note the length of the pipe from fire grate to top of chimney., and the short distance of cold air beneath the relatively wide and thin fire. Probably Rijkes tube resonance!
K2

Well, that's exactly how woodwinds work.

 

[NapierDeltic]

Thank you, Steamchick! Very interesting this phenomenon (Rijkes tube resonance) which appears to be not only acoustic but also thermodynamic. Do you have any ideea about material of the tube in the video? I'm very curious to replicate it as it looks much like magic (I know it is real...). It even seems to show a diode effect.