Monotube Flash Boiler Design

[NapierDeltic]

I had a flame pad like this:
https://www.tnsoutdoor.com/campingmoon-fire-pad-burner-pad-twill-high-mesh-us-120-us-150
but for domestic use; those round you can find in general stores, made from stainless steel for kitchen stove.
I was very disappointed because it corroded and broke into pieces, eaten by stove's flame.
Conclusion -not all stainless steels resist high temperature oxidation, they are not refractory and even at normal temp their resistance is average, limited to normal dishware conditions; it is what also metal casting enthusiast say about ad-hoc stainless steel crucibles, although the corrosion agents here are multiple.

In what concerns brazing, Google for Oxigen concentrator /welding. It is an interesting mean to increase temperature and power density (welding-source of oxygen- or brazing); sometimes a bit too much - I have tried it, to inject a stream of oxygen in burner's admission... The speed of flame is much faster/ commercial burners are already built to work to their limit/ a.s.o.
For welding, power is limited by oxygen flow available but the method is much more tested and you can find tutorials. Works for small jobs.

 

[Steamchick]

I have used old electric filaments from old electric fires. I don't think there is any iron in them. Because they were intended to glow red with the electric current, and not deteriorate, they seem to last a long time as radiant elements inside a firebox with a regular camping stove burner. But very hard to find old element wires. I think they are probably nickel/tungsten alloy?
Proprietary wire (woven or random) seems to be a ferro-chrome alloy... I met a trade name "fecralloy" - or something, suggesting fe - cr mix.
K2

 

[aka9950202]

I have used old electric filaments from old electric fires. I don't think there is any iron in them. Because they were intended to glow red with the electric current, and not deteriorate, they seem to last a long time as radiant elements inside a firebox with a regular camping stove burner. But very hard to find old element wires. I think they are probably nickel/tungsten alloy?
Proprietary wire (woven or random) seems to be a ferro-chrome alloy... I met a trade name "fecralloy" - or something, suggesting fe - cr mix.
K2

Nickel-chrome wire is used to make heating elements. It can be purchased online. Another free source - hair dryer s, toaster ovens, electric heaters.... etc

When terminating to regular wire use crimp connectors. If cannibalising the above mentioned sources, look at how they electrical connections are made.

Cheers,

Andrew in Melbourne

 

[The Ignoble Troll]

There's a short expression used among steam automobile enthusiasts I rather like; "Keep the Fluid moving!!" It means that as long as the working fluid is moving through the tubes fast enough to carry away the burner's heat, the boiler tubes will be fine. I'm hoping that keeping fluid velocity on the fast side will also help prevent decomposition, but honestly, that's TBD.



R-290 = Propane;....highly flammable fluids are most definitely NOT on my list of possible working fluids. Also, I prefer a liquid with a boiling point above room temperature.



Yes, both XP30 (aka R-514a) and Opteon MZ (aka HFO-1336mzz-Z) are possible choices. However, I don't know if I can buy them here in Thailand.

I was only 3/8ths joking about R290.

Both over tempurature and over pressure valves are available for it right off the shelf. Arranging safe venting or flaring is possible whereas anything halogenated and decomposing can not be safety vented.

The toxicity of decomposed freons is unbelievable.

 

[The Ignoble Troll]

I have used old electric filaments from old electric fires. I don't think there is any iron in them. Because they were intended to glow red with the electric current, and not deteriorate, they seem to last a long time as radiant elements inside a firebox with a regular camping stove burner. But very hard to find old element wires. I think they are probably nickel/tungsten alloy?
Proprietary wire (woven or random) seems to be a ferro-chrome alloy... I met a trade name "fecralloy" - or something, suggesting fe - cr mix.
K2

Nicrome is designed to aggressively self passivate, which prevents erosion from oxidizing.

Thermal cycling eventually sheds some of the oxide sheath leading to wires thinning and hot spots.

Stain-less is a better name then "stainless". It is designed to also passivate. If you run stainless steel through a ball mill and make an ultra fine hydrocarbon slurry and throw it into a fire, you will find it burns enthusiastically.


The refractory chrome oxide, which is what prevents AO cutting, and the oxygen blocking nickel oxides passivate the surface of stainless but their ability to remain passivating is very dependant on alloy, contamination, wear and so on.

 

[Steamchick]

Thanks Andrew (post#463).
All I needed to know is that it works heated by butane or propane flames. No need to worry about electrical connections as I don't use it heated electrically on model steam boilers.
I have bought some "stainless" steel wire wool - intended for passing car exhaust silencers, the gases are hot and corrosive. But my application is red hot - maybe 1200 deg.C. - for use as radiant material in radiant wire wool gas burners... I have tried making a couple of experimental burners, but normal steel wire wool burns away quickly! But I have proven a principle on a blowlamp in a fire-tube of a boiler where I gained heat transfer using a tube of wire wool for the periphery of the blow-lamp flame to heat to red-orange - and managed to increase the gas pressure as a result.
A bit of fun.
K2

 

[The Ignoble Troll]

Thanks Andrew (post#463).
All I needed to know is that it works heated by butane or propane flames. No need to worry about electrical connections as I don't use it heated electrically on model steam boilers.
I have bought some "stainless" steel wire wool - intended for passing car exhaust silencers, the gases are hot and corrosive. But my application is red hot - maybe 1200 deg.C. - for use as radiant material in radiant wire wool gas burners... I have tried making a couple of experimental burners, but normal steel wire wool burns away quickly! But I have proven a principle on a blowlamp in a fire-tube of a boiler where I gained heat transfer using a tube of wire wool for the periphery of the blow-lamp flame to heat to red-orange - and managed to increase the gas pressure as a result.
A bit of fun.
K2

I've made a similar sorta burner stacking stainless steel screen. The flame ends up like a Meker burner flame and the mesh keeps the flame, if sized correctly, from getting into the fuel air stream so you can have broad very hot but soft blue flames.

Great for heating large items for silver soldering. Would probably work for steam generators?

 

[Steamchick]

Hi Troll,
Do post a photo...
Would it scale to the size or the burner on this thread?
WOULD the burner already designed and built for this boiler, with its "tube" flame, benefit from having a "tube of wire mesh" to generate some more of the heat as radiant instead of hot gas? There is a large radiant surface from the end plate on the combustion chamber, but that only shines onto the end centre coil part ot the tube aray.
A wire-mesh tube in the tube of the flame would create a cylinder of radiant heater to shine on the surrounding tube coils... probably increasing heat flow in that part of the boiler?
K2

 

[The Ignoble Troll]

Hi Troll,
Do post a photo...
Would it scale to the size or the burner on this thread?
WOULD the burner already designed and built for this boiler, with its "tube" flame, benefit from having a "tube of wire mesh" to generate some more of the heat as radiant instead of hot gas? There is a large radiant surface from the end plate on the combustion chamber, but that only shines onto the end centre coil part ot the tube aray.
A wire-mesh tube in the tube of the flame would create a cylinder of radiant heater to shine on the surrounding tube coils... probably increasing heat flow in that part of the boiler?
K2

I'm sorry but no photos. They all went away when photobucket stopped being free and the last 10 years I was out of the DIY game. Now regretting purging a lot of stuff, but was living somewhere not conducive to a home shop... but the metal bug kept crawling around calling, hence the coming move.

In the burners I just mentioned, the screens stayed cool because of the flow of fuel and air, so there was no real radiant heat from them.

They were cool enough that my prototype was made from a cut up CO2 pellet gun cartridge with only brass screens and a wick carburetor made from film canisters (getting old sneaks up on you eh). The brass screens would only faintly glow red in darkness which explains why they did not crumble.

I can not comment on the lifespan of your idea inside the burner, but the burner is holding up so I'd assume they would last as long as the guts of the burner?

 

[Toymaker]

A closer exam of the impact of blue-flames on metal.

I wanted to touch on this subject again because I find it very interesting and hopefully informative for forum members.
The last two photos show the aluminum disk after being exposed to extended, direct contact with blue flames as seen in the video in post #450. I'm leaving the images full size to better show the details.

I'm focusing on the tiny "flash" structures which are left-overs from where the end mill failed to remove all the aluminum material. Looking closely at the small triangle-shaped fins you can see these left-overs attached to the tips of the triangles. The photo below was taken before the disk was subjected to any flames.

The photo below was taken after all testing was completed. Notice the tiny flash structures shown by the red arrows, as well as the soot deposits.

Below is a magnified look at the flash which measures roughly 0.050" long x 0.010" thick x 0.165" deep.

I'm truly surprised that these tiny structures survived being repeatedly immersed in blue flames for several minuets at a time without either melting or burning off. Although these structures are coated with soot, they are indeed aluminum metal underneath.

 

[NapierDeltic]

Mostly I am concerned by the radiation plate/disk of the burner.
What is "cooled" - even by lower temperature gases on their return, is more protected, both chemically and from mechanical strength point of view.
The burner has to take mechanical shocks of the car and, maybe, misfiring blow. Here is where a jet engine engineer's advice could be of use.

 

[Steamchick]

I am sure that the radiation plate of the burner will deteriorate with time, but it really is just speculation as to how long it can last. IMHO.
I think the aluminium disc example just shows how some expectations can be wrong. But there is always a fine line between success and failure in these games.
Good design plus good testing usually leads to good products.
K2

 

[The Ignoble Troll]

Awesome machining.

 

[Steamchick]

Hi Trill,
I think the whole project is "Awesome"!
And I have learnt LOADS from Toymaker... His stuff is not what many call "toys"! A couple of orders of magnitude more (in many ways) than I attempt!
(But whisper that quietly, in case he is shy of praise).
K2

 

[The Ignoble Troll]

His whole design is fascinating and his turbines are simply magnificent.


Edit: @Toymaker, have you considered a duel fluid system? Use rapidly cycling water in the burner and a plate heat exchanger to heat the refrigerant?


Some pros I see are;

Potential hotspots won't produce bhopal like toxic clouds, just a little steam and cavitation

You get to take advantage of the greater heat capacity of water AND the properties of a refrigerant for the turbines working fluid -as talked about earlier in the thread where you explained the design of the turbines needed a different fluid then steam, even if steam carried more energy

Over pressure valves and over tempurature valves are on the shelf of your hardware store for hot water tanks.

With the heated fluid separated by the burner you can actually store heat in a water tank, then letting the flow increase at a fixed burner setting would give you a power boost

Plate exchangers that can take these pressures are cheaper everyday

With water as the primary loop, you can dial in each half of your system without having to adjust the whole, basically you get a power plant thermal transmission.

They hot water won't ever overheat the refrigerant to the point of decomposition if you are using recirculated water/steam that's at atmospheric pressure.

And lastly, a dual working fluid system gives you the chance to dump excess water heat in a radiator, thus giving you the ability to automate a single 3 way valve and either dump/store or transfer the heat to keep the whole system balanced in, say, traffic where you might want to keep full power ready, but decoupled.


Like how M1 crews keep the tanks turbines spun up a bit in sketch situations so that they can just drop the clutch and scoot. A tank of almost boiling water would be like having you engine revved up and ready but without wasting power. A thermal battery.



VEVOR Heat Exchanger 3"x7.5" 40 Plates Brazed Plate Heat Exchanger 316L 3/4" MPT Heat Exchanger B3-12A Beer Wort Chiller for Hydronic Heating https://a.co/d/cgl45Wd

The linked exchanger is rated to 3MPa. Just need to size it correctly.


The dual fluid loop is standard in all sorts of applications from hvac building heating to power generation.

 

[Toymaker]

I'm guessing what you are all calling the "radiation plate/disk of the burner" is what I call the "restrictor plate"; ie, the stainless plate in this photo ?? I consider this plate to be "sacrificial". As K2 mentioned, it will deteriorate with time,...how much life I get out of it is still TBD. I've considered replacing it with a ceramic plate (Zirconium or silicone nitride) but I cant think of a way to attach a ceramic to the stainless shell. I don't have the tools needed to drill attachment holes in a ceramic such as Zirconium.



Troll, I like your idea of a thermal battery, but that will have to wait until after I get a bare-bones engine thrown together and spinning.

Yes, I'm familiar with dual fluid systems as they're commonly used in the solar trough power generator systems. A synthetic oil is circulated through the solar troughs and heated to between 300 C and 400 C. The oil is then used to make steam, via a heat exchanger, for a standard steam turbine. It's a great system that would add a lot of extra weight and volume to my engine,...so likely wont use it.

 

[The Ignoble Troll]

Do you have access to a kiln or melting furnace?

Do you have any local pottery supply shops?

Diamond drills are pretty cheap now, a kaowool bushing around stainless bolts would probably cushion a ceramic plate well enough.


If you have access to the above resources, I can set you on the road to a refractory ceramic that is tough and has such a high thermal resistance that I used it to grow rubies.

Otherwise, cutting up a corderite pizza stone might work.

 

[NapierDeltic]

I'm guessing what you are all calling the "radiation plate/disk of the burner" is what I call the "restrictor plate"; ie, the stainless plate in this photo ?? I consider this plate to be "sacrificial". As K2 mentioned, it will deteriorate with time,...how much life I get out of it is still TBD. I've considered replacing it with a ceramic plate (Zirconium or silicone nitride) but I cant think of a way to attach a ceramic to the stainless shell. I don't have the tools needed to drill attachment holes in a ceramic such as Zirconium.

View attachment 155193

Troll, I like your idea of a thermal battery, but that will have to wait until after I get a bare-bones engine thrown together and spinning.

Yes, I'm familiar with dual fluid systems as they're commonly used in the solar trough power generator systems. A synthetic oil is circulated through the solar troughs and heated to between 300 C and 400 C. The oil is then used to make steam, via a heat exchanger, for a standard steam turbine. It's a great system that would add a lot of extra weight and volume to my engine,...so likely wont use it.

I might suggest for the next replacement of restrictor plate a sacrificial design with progressive fail but usually devil is in details. What - at one moment - might seem a solution, could be also the origin of next failure.
For restrictor plate I would go for a relatively light design (mass is not affected by temperature but strength is) made of refractory material/metal-based (can be composite, can be celular, might be even solid plate) but with relative high toughness. K2 said something about nichrome or alike...I don't know the commercial useful shape of such metal and how to improve its qualities through component design.
This is intriguing in your project (not only this, I like the subject entirely); you are a Pioneer!

 

[Toymaker]

Do you have access to a kiln or melting furnace?

Do you have any local pottery supply shops?

Diamond drills are pretty cheap now, a kaowool bushing around stainless bolts would probably cushion a ceramic plate well enough.


If you have access to the above resources, I can set you on the road to a refractory ceramic that is tough and has such a high thermal resistance that I used it to grow rubies.

Otherwise, cutting up a corderite pizza stone might work.

I did a little more internet searching:

I can buy Alumina Ceramic discs in the size I need (75mm OD, 2mm thick) on eBay where they're sold as Alumina Ceramic Gasket Disc. $15.46 for two discs.

3mm diamond drill bits are less than $1 each. After watching a few YouTube videos on how to drill holes in Alumina, I'm confident that I can drill the needed 3 holes.

I can buy 3mm Zirconia Allen head screws & nuts from AliExpress.

But, the three stainless attachment members that hold the disc in place are not something I will be able find off-the-shelf, meaning I'm stuck using stainless parts. These three small pieces sit directly in the path of the combustion gases, and will likely be the first pieces to fail. I suppose this might be one solution to NapierDeltic suggestion that I employ a progressive failure scheme.

However, at this point in my project I think I'm better off waiting to see how long the current stainless parts last.

 

[NapierDeltic]

Yes. Personally I'm a bit scared of ceramics for mobile applications after incidents in NASA's space shuttle program. Though 2mm is not that much. There is also a thermal shock at cold start which is hard to evaluate in time. Though not equal, I had ceramic crucibles that cracked due to unequal and too fast heating.
Trying to clarify my thoughts to myself, i would rather see it as a variable thickness restrictor plate , or better a reinforcement wide ring of stainless steel sheet. When center fails, there is still enough meat on the rim and link to brackets.
We are just discussing opinions and options...