Monotube Flash Boiler Design

[NapierDeltic]

Hi, I could be wrong, but mentioned standard might address to high volume water boilers designed for overheated liquid water- hence the low Max. pressure -below 7bar.
As a side comment, monotube boilers are part of low fluid volume boilers, which are assessed in various documents as low risk boilers. I must stress that this assessment does refer to the effects of a catastrophic failure (instant steam volume, explosion effects) , not at design rules and it is not backed by any standard I know! And low risk does Not mean No risk!
Toymaker has to know all objections and concerns and to address them for his safety mainly.

 

[TimTaylor]

While the concept is interesting, I have to ask if using R123 is practical or realistic?

1. R123 and similar refrigerants are designed for efficient operation at lower temperatures - author is talking about operating at or near it's critical point, which is risky with any fluid.

2. Using R123 would require a completely closed system with as close to 100% recovery as possible to be both cost effective and environmentally responsible. For comparison, R123 costs roughly $40.00 USD per pound in 100 lb bulk quantities, while water cost averages $0.0002 USD per lb. That's a 200,000:1 cost ratio compared to water.

3. While R-123 is not flammable at ambient temperatures and atmospheric pressure, it will become combustible when mixed with air under pressure and exposed to strong ignition sources, posing a significant safety hazard as the boiler could go "boom" if the radiation disk or tubing were to develop a leak.

4. R123 will also chemically decompose at temperatures above 250C. While the author's theoretical design operating specifications are below this, it could easily be exceeded at the radiation disk due to direct flame impingement if fluid flow was compromised or restricted. Even a simple candle flame reaches temps of 1000C.

I also share some of Steamchick's concerns regarding potential mechanical integrity issues.

 

[Steamchick]

Not being able to comment on R123... I was simply suggesting that a silver soldered copper labyrinth is more like a silver soldered copper boiler, and as such I was trying to offer some knowledge of how these are Regulated in the USA. I.E. There are real safe physical limits for the use of such assemblies - everywhere on this planet. It is the properties of copper that are the limits, not a political decision.
The intent is to make a tube flash boiler.... so I suggested using a tube.
Most of the flash tube boilers I know use better materials in seamless tubing - not copper.
Please check your design, particularly as I am not alone identifying some risks.

 

[TimTaylor]

Not being able to comment on R123... I was simply suggesting that a silver soldered copper labyrinth is more like a silver soldered copper boiler, and as such I was trying to offer some knowledge of how these are Regulated in the USA. I.E. There are real safe physical limits for the use of such assemblies - everywhere on this planet. It is the properties of copper that are the limits, not a political decision.
The intent is to make a tube flash boiler.... so I suggested using a tube.
Most of the flash tube boilers I know use better materials in seamless tubing - not copper.
Please check your design, particularly as I am not alone identifying some risks.

Here in the US the regulatory requirements for pressure vessels, including boilers, vary individually by state. Some refer to ANSI (installation/inspection) and/or ASME (materials, construction), others roll their own or use a combination. Most have some form of exemption for small hobby boilers based on volume and operating pressure, for example, less than 1.5 cu ft steam volume and not exceeding 30 psig.

 

[Richard Hed]

The "specific heat" of a phase change, that is, either from ice to water or the reverse, or from water to vapor or the reverse, has several different names for the process. It is a fact, as yuou say, that during these phase changes, extra heat is needed (or needed to subtract it). It is this, that water has a high energy need, so at the boiling point of water, it actually needs MORE heat once it has reached BP to actually vaporize. Each and every chemical has different specific heats, thus, a chemical like R123 will have a higher or lower SH (specific heat) of vaporization.

In picking a working fluid, one has to know if it is combustible, like propane, gasoline, butane, etc, or if it will break down into other chems at high (or sometimes low) temps. No-one would want butane for a working fluid because sometimes a pinhole leak can develop in the boiler. Also, one needs to know if the chem will deteriorate the metal of the boiler. There are all kinds of factors. Well, lucky for us, chemists, physicist, materials experts have all workt out most of these problems for us. All we have to do is follow some rules.

I have built a prototype "water" boiler something like the drawings above for the R123. I am not trying to get any efficiency but rather, looking for cheap (that is, free) fuel to fire up a boiler. I also have a plan for a "heater/boiler" system to heat a house and run an engine for electric supply. But that system is much different than the "cheapo" boiler system.

 

[Toymaker]

While the concept is interesting, I have to ask if using R123 is practical or realistic?

1. R123 and similar refrigerants are designed for efficient operation at lower temperatures - author is talking about operating at or near it's critical point, which is risky with any fluid.

Please explain why you believe operating a boiler at the critical point of the working fluid is risky. It's my understanding that operation at the critical point yields the greatest efficiency, and is in fact the goal of any commercial boiler used for driving turbo machinery.

2. Using R123 would require a completely closed system with as close to 100% recovery as possible to be both cost effective and environmentally responsible. For comparison, R123 costs roughly $40.00 USD per pound in 100 lb bulk quantities, while water cost averages $0.0002 USD per lb. That's a 200,000:1 cost ratio compared to water.

My overall design is a closed system with only two possible leak points; 1) rotary seal on the turbine shaft connecting to the driven air blower, and 2) flow regulation valve seals.

Valve seal leakage should be negligible and is not a concern, however, leakage around the 60,000 rpm rotary seal is of course a concern. This seal is located inside the turbine housing at the "steam" exhaust end where temperature and pressure will be near ambient, so again, I'm not overly concerned with leakage. I'm am concerned about shortened life-span of the seal due to high rpm, so I will keep close watch on this seal.

3. While R-123 is not flammable at ambient temperatures and atmospheric pressure, it will become combustible when mixed with air under pressure and exposed to strong ignition sources, posing a significant safety hazard as the boiler could go "boom" if the radiation disk or tubing were to develop a leak.

You're expressing a common miss-conception concerning pressurized fuel leaks. Even if the working fluid were propane or gasoline, the leak you propose would only result in an additional flame (assuming there were enough oxygen within the combustion gases), and not an explosion. Consider how a hand-held propane or butane torch works; the tank contains a pressurized highly combustible vapor, and when you open the torch valve, you've created a "leak", which you then ignite with a match or a spark. There's no explosion,...no boom,... just a flame.

4. R123 will also chemically decompose at temperatures above 250C. While the author's theoretical design operating specifications are below this, it could easily be exceeded at the radiation disk due to direct flame impingement if fluid flow was compromised or restricted. Even a simple candle flame reaches temps of 1000C.

I also share some of Steamchick's concerns regarding potential mechanical integrity issues.

Various Freons, including R123, have been, and still are, being used as working fluids all around the world for over 30 years. I'm not breaking new ground by using a Freon as a working fluid. Please take a little time and at least skim this research paper on selecting the right working fluid for a given application: Working fluids. Or Google, "Organic Working Fluids" or "ORC Turbines" and read through the papers of your choosing.

Chemical decomposition is my biggest concern in using R123; the primary precaution I've taken to minimize this risk is to place the boiler under computer control which will monitor (via thermocouples) temperatures inside the radiation disk as well as the boiler input and exit temperatures. The computer will also keep the working fluid in constant motion to avoid any local heat build-ups.

Hopefully I've addressed most, if not all, of your concerns.

 

[Toymaker]

Not being able to comment on R123... I was simply suggesting that a silver soldered copper labyrinth is more like a silver soldered copper boiler,

I'm not a model steam engine hobbyist so I'm not familiar will all the various boilers that have been built and used over the years, but I've never seen a boiler that uses as much internal webbing support structure, and in such a small cross-section, as my spiral labyrinth disk uses. Maybe some dimensions would be helpful: each channel is only 0.6" X 0.6", which means the disk is essentially a spiral of 5/8" round tubing which has been pressed into a square shape. So, even though the overall disk has a 6" outside diameter, each internal channel is only 0.6" wide, and the entire disk is only 0.6" thick. Hopefully this new info alleviates your structural concerns.

<snip>
The intent is to make a tube flash boiler.... so I suggested using a tube.

I too would much prefer using tubing to make my spiral disk, but all the literature on tube bending that I could find told me that it's ill advised to make such tight bends as the outside radius wall becomes way too thin. If you know of a technique to safely bend tubing into the tightly wound spiral I need, please share that info.

Most of the flash tube boilers I know use better materials in seamless tubing - not copper.
Please check your design, particularly as I am not alone identifying some risks.

Google "monotube boiler design" and you'll find lots and lots of tube boilers using copper tube, including many used in steam cars which use boilers as large and larger than my boiler; here are two that popped up with my search:

BTW, quite a few steam automobile DIY builders construct what they call 400 x 400 monotube boilers using copper tube; i.e. 400 psi at 400F. At least one builder used 3/8" copper tube with a 0.020" wall thickness, which is way thinner than I thought possible. Many of these builders have been running their boilers for years without problems. You'll find a wealth of experience and knowledge concerning monotube boilers on this forum: SteamAutomobile
One of my personal key take-aways from their years of experience with monotube boilers,...keep the fluid moving!

 

[Toymaker]

The "specific heat" of a phase change, that is, either from ice to water or the reverse, or from water to vapor or the reverse, has several different names for the process. It is a fact, as yuou say, that during these phase changes, extra heat is needed (or needed to subtract it). It is this, that water has a high energy need, so at the boiling point of water, it actually needs MORE heat once it has reached BP to actually vaporize. Each and every chemical has different specific heats, thus, a chemical like R123 will have a higher or lower SH (specific heat) of vaporization.

In picking a working fluid, one has to know if it is combustible, like propane, gasoline, butane, etc, or if it will break down into other chems at high (or sometimes low) temps. No-one would want butane for a working fluid because sometimes a pinhole leak can develop in the boiler. Also, one needs to know if the chem will deteriorate the metal of the boiler. There are all kinds of factors. Well, lucky for us, chemists, physicist, materials experts have all workt out most of these problems for us. All we have to do is follow some rules.

I have built a prototype "water" boiler something like the drawings above for the R123. I am not trying to get any efficiency but rather, looking for cheap (that is, free) fuel to fire up a boiler. I also have a plan for a "heater/boiler" system to heat a house and run an engine for electric supply. But that system is much different than the "cheapo" boiler system.

I'll be interested to see your designs and builds,....hope you post them.

 

[Steamchick]

Hi Toymaker,
I was trying to draw your attention to the accepted limits for silver soldered assemblies used as pressure vessels at temperature and pressure. (At least in part of the world with a bit of Engineering expertise). I do not suggest that Thailand is any different as the ASME Regulation limits are based on experience of what works and what has failed over a couple of hundred years, as well as a lot of very technical theory... As such they make a good benchmark for similar products - because Copper and Silver Solder are the same the world over.
You plan to run R123 at not exceeding 185deg.C. ("R123 into a useful vapor at 3.5MPa (500 psi) and 185C."). That's OK (~365deg.F.) compared to the ASME regulation's 400F max. for Silver soldered copper boilers.... But looking at the simple structure you have made it appeared that 500psi would need very thick copper plates either side, and very good joints holding it all together.... Your schematic drawings do not show the details of the actual design of metal thicknesses, joints, etc. but I was just expressing a few words of caution.
Yours is a fascinating project, and I am sure you are writing about it here to gain alternative ideas and extra information from the forum. If you wish, I shall refrain from offering such comment.
Just tell me what you wish?
K2

 

[Steamchick]

Toymaker, I have just done a quick sum that suggests a copper tube at 0.625in OD for 500psi would need a wall 0.042in thick = just below 0.54in bore... = the passage for the R123! But my sums may be WRONG somewhere? - I have used:
t = D x NWP/ (2 x max permitted stress) with a tensile stress limit of 3800psi for the safe stress in the copper at 365F. This includes the ASME factor of safety for tubes used in water boilers. (Extrapolated from ASME limits).... What do you use in your boiler coils?
How about telling us what your material thicknesses are for the Radiation shield, and how you calculated them? (My best estimates show at least 0.14in thick?).
Then we can all learn a bit more (I don't know how you have worked out your design, material thicknesses, etc..., or the stress limits you have applied? - I can learn a lot from you if you can help?).
K2

 

[Toymaker]

Hi Toymaker,
I was trying to draw your attention to the accepted limits for silver soldered assemblies used as pressure vessels at temperature and pressure. (At least in part of the world with a bit of Engineering expertise). I do not suggest that Thailand is any different as the ASME Regulation limits are based on experience of what works and what has failed over a couple of hundred years, as well as a lot of very technical theory... As such they make a good benchmark for similar products - because Copper and Silver Solder are the same the world over.
You plan to run R123 at not exceeding 185deg.C. ("R123 into a useful vapor at 3.5MPa (500 psi) and 185C."). That's OK (~365deg.F.) compared to the ASME regulation's 400F max. for Silver soldered copper boilers.... But looking at the simple structure you have made it appeared that 500psi would need very thick copper plates either side, and very good joints holding it all together.... Your schematic drawings do not show the details of the actual design of metal thicknesses, joints, etc. but I was just expressing a few words of caution.
Yours is a fascinating project, and I am sure you are writing about it here to gain alternative ideas and extra information from the forum. If you wish, I shall refrain from offering such comment.
Just tell me what you wish?
K2

I do suspect Thailand uses many of the same guidelines for boiler construction as the US or some European country, but only professional boiler makers over here will know what those guidelines are and even bother to somewhat follow them,...that's what I meant by my earlier statement, "This is Thailand". "This is Thailand" doesn't mean the rules are different here, it means, no one cares what the rules are,... as long as it works, that's good enough.

Assuming I don't want the copper to deform, you're probably right about needing very thick plate, but I don't care if the copper bulges under pressure,...as long as the joints don't fail, I'm happy. I fully expect all the channels to deform and bulge out, as the copper sheet is only 1mm thick. Now, before you have a heart attack, first, I plan to hydro-statically test the entire boiler before I try boiling any fluids in it. Second, the folks building their own DIY boilers in the SteamAutomobile forum have successfully used copper tubing at half that thickness.

Am I a little concerned that my disk might spring a leak? Of course I am,...I'ld be a fool to not be. Which is why I have multiple back-up designs should this first idea fail. Feel free to express all the concern you want, just know that I always have at least one back-up plan and that I never put myself or anyone else at risk during testing.

 

[Steamchick]

I rely upon expert advice of "what works and is safe enough for the Insurance/Regulations/Law"! (Sometimes the Regulations, etc. may not be as safe as I want).

NASA etc. do not work to "regulations for pressure vessels" as their rockets would not get off the ground. Occasional "destructive malfunctions" are acceptable to them, it seems? - but not in my workshop. I plan to stay safe.
K2

 

[Steamchick]

Comparing 1mm thick and my calculations for 3.56mm thick...
I suspect that as a square rule operates, your .040" material will be only withstand :-
(0.04/0.6)squared / (0.14/0.6)squared
times the pressure...
You work out the numbers before you even make a thin walled assembly. It may save you much unnecessary work.
Arithmetic is simple when you know what numbers to use. You are clever enough to do all the calculations of gas flow, heat etc. so please try some simple stress calculations, it will help you design the job properly.
Or if you have done them already, please share so we can learn? (I am wrong more often than I like! - Which is why I share these things, so better men than I can correct me.).
K2

 

[Toymaker]

Toymaker, I have just done a quick sum that suggests a copper tube at 0.625in OD for 500psi would need a wall 0.042in thick = just below 0.54in bore... = the passage for the R123! But my sums may be WRONG somewhere? - I have used:
t = D x NWP/ (2 x max permitted stress) with a tensile stress limit of 3800psi for the safe stress in the copper at 365F. This includes the ASME factor of safety for tubes used in water boilers. (Extrapolated from ASME limits).... What do you use in your boiler coils?
How about telling us what your material thicknesses are for the Radiation shield, and how you calculated them? (My best estimates show at least 0.14in thick?).
Then we can all learn a bit more (I don't know how you have worked out your design, material thicknesses, etc..., or the stress limits you have applied? - I can learn a lot from you if you can help?).
K2

First, please define your acronyms,...I don't want to guess what all your symbols might mean.

Second, I forgot to mention in previous posts that I'm not using Silver Solder, I use BCuP-2 instead.
Silver Solder melts at 1145 F while BCuP-2 melts at 1460 F.

 

[Toymaker]

Comparing 1mm thick and my calculations for 3.56mm thick...
I suspect that as a square rule operates, your .040" material will be only withstand :-
(0.04/0.6)squared / (0.14/0.6)squared
times the pressure...
You work out the numbers before you even make a thin walled assembly. It may save you much unnecessary work.
Arithmetic is simple when you know what numbers to use. You are clever enough to do all the calculations of gas flow, heat etc. so please try some simple stress calculations, it will help you design the job properly.
Or if you have done them already, please share so we can learn? (I am wrong more often than I like! - Which is why I share these things, so better men than I can correct me.).
K2

It's a shame that brazing is becoming a lost skill, losing out to the ease and speed of electric arc welding. When done correctly, the bond formed by brazing two pieces of copper together is actually stronger than the base copper alloy. Also, the allowable temperature for brazed joints exceeds that of the copper alloys that it is used to join, therefore the temperature rating of the copper tube and fittings are the controlling factor. Source: Brazing Copper

There's no need to get too deep into math equations, as published charts and graphs provide all the information we need. The first chart to look at is here: Type L Copper Tube

The wall thickness of my 5/8" diameter copper tube is 0.040" which defines it as "Type L". Reading the chart for Type L (which uses 150 F as the working temperature) we see that 5/8" tube has a working pressure of 537 psi for annealed pipe, but the actual burst pressure is close to 3400 psi. That's an enormous difference between working pressure and actual burst pressure!!

Next, lets examine a graph showing strength vs temperature of various metals: Strength vs Temp. My design max temperature is under 200 C. From the graph we find that copper retains 85% of it's strength at 200 C. Doing a tiny bit of math, 85% of 537psi = 456 psi. This means, my design max of 500 psi is only 43 psi above the recommended pressure, but far more important to me,... my max pressure of 500 psi is nearly 2,400 psi BELOW the actual burst pressure at 200 C. These numbers are extremely comforting to me, and I see no glaring reason to be overly concerned that my design wont work.

 

[ajoeiam]

snip

The wall thickness of my 5/8" diameter copper tube is 0.040" which defines it as "Type L". Reading the chart for Type L (which uses 150 F as the working temperature) we see that 5/8" tube has a working pressure of 537 psi for annealed pipe, but the actual burst pressure is close to 3400 psi. That's an enormous difference between working pressure and actual burst pressure!!

snip

You are terming the difference between working and burst pressure in a ration of just over 1:6 as enormous.
That suggests that you are only looking at the numbers!!!!!!
Some steel lines - - - - well you start looking at 10k psi - - - - that's 1:18 somewhat larger.

5:1 is a not uncommon safety factor
10:1 is a common factor when human beings are around and considered

For me an enormous safety factor might start at 25:1 - - - - maybe higher - - - - depending upon the situation.

So yes - - you have a safety factor - - - - its NOT enormous!!

HTH

 

[Steamchick]

... And in the UK, USA, and other countries with a little experience of boilers - in whatever form - the regulations use a minimum FOS of 8... between the calculated stress limit pressure and the Normal Working Pressure. A friend nearly lost an arm from a scald when a gasket failed near where he was working on a ship's diesel engine cooling system. The muscles were not wholly damaged but the remainder painfully healed over the next year, and he is now left handed instead of right handed...
Our concern is to keep you safe.
K2

 

[Steamchick]

Incidentally, ASME (the American Society of Mechanical Engineers) rate copper at 200C with a Maximum Allowable Stress value at 400degrees F. of 3000psi, compared to at 100deg.F. of 6700psi. - with all their knowledge and expertise. So that's what I use... I simply have no better guide than that erstwhile body of gentlemen. Perhaps that is why I have different results for the design elements to your extrapolations?
I am not about playing "top Trumps", just explaining why I have reservations about your design, from the snippets you tell us about.
A simplified formula from ASME regulations for tube strength (hoop stress) is:
t (wall thickness) = P (Working Pressure) x D (outside diameter) / (2 S (stress limit) + 0.8 P):
If you have better information then I should like to know to improve my design ability.
I shall let you design your boiler as you wish.
K2

 

[peterl95124]

one problem with the flat plate spiral "radiation plate" boiler is its small surface area from which to absorb heat, you'll be much better off with a spiral tube, in fact lots of spiral tubes. Thats the game with all boilers and heat exchangers, surface area, surface area, and last but not least surface area.

 

[HMEL]

Wow!
Remind me, what temperature and pressure will this "radiation disc" be operating at?
ASME regulations (USA) for pressure vessels in "Boilers" limits silver soldered copper boilers to 100psi (used with water), and 400 degrees Fahrenheit, = 204deg.C. (Limiting permissible design stress of 3000psi shall be used in calculations). I am not sure how you calculate the maximum stress developed - presumably at a mid-point between the spiral supports?
Exceeding either of these limit values is beyond their remit.
I suggest that your "Radiation plate" is exactly within these design limits as a "boiler".
NASA and other "special" organisations work outside those limits, but have extensive and special arrangements to do so.
Many other countries follow the same or very similar design limits, and insurance companies may choose not to support any claims if these limits are exceeded, as they are based on safe limits for the use of silver soldered copper pressure vessels.
What could happen if the "radiation plate" should exceed these limits, or rupture? Perhaps a close wound coil of tubing would be better? - The tubing can be thick walled and be a better cross section of wall to withstand the internal pressure than your rectangular passage?
K2

I believe steamchick is correct. Even ignoring the thermal expansion issue the flat plate design internal side pressure is going to be very high. Say a 8 inch diameter plate will have 50 square inches or 25,000 lbs of force on the side walls (given a 500 psi pressure )and this coupled with the lower yield strength at the higher temperatures is more then likely to come apart. Now the question is just how fast will it come apart. By the way that is not including the force on both sides of the disk so you are looking at twice the generated stress of value of one side. More likely 50,000 lbs of linear stress. You can no longer use the hoop stress equations of tubes because of the flat walls.