Monotube Flash Boiler Design

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Toymaker

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First, I’m a retired electronics engineer with almost no formal education in thermodynamics. All the meager knowledge I have on boiler design comes entirely through self-learning. So don’t take anything I state as being 100% accurate. Which means I’m open to advice, critique and suggestions,... which I may, or may not take :cool:

I’ve started designing a Monotube Flash Boiler to be used with the Burner I’ve already built. My boiler design is roughly based on the SES automotive boiler built in the early 1970s which supplied steam at 1000 psi at 1000F, and reportedly provided instantaneous zero-to-full flow. The SES boiler used water as the working fluid, but my boiler will use the Freon R-123, instead of water which will require some major changes as compared to the SES boiler, most notably, a far shorter monotube will be needed.

I copied the below P-h diagram from an internet page which already had existing lines, so please excuse and ignore those lines marked with circled numbers.

Looking at the P-h diagram for water, I’ve drawn a red-orange horizontal line starting at 100C & 7 MPa (1000psi) which roughly intersects the 500C (1000F) line. Moving from left to right along this line represents the enthalpy needed to raise 1 kg of water from roughly 480 kJ on the left up to about 3300 kJ on the right; which means the SES boiler must transfer 2,820 kJ into each kg of water inside the boiler to change it into a useful steam.
1663387109589.png



Now let’s look at the P-h diagram for R-123:
1663387148838.png

Starting at the red “1” on the left and going right to the Blue line near the red “3” represents the amount of Enthalpy needed to raise 1kg of R123 from roughly 228 kJ on the left up to about 485 kJ on the right; which means my boiler only needs to transfer 257 kJ into each kg of R123 inside the boiler to change the R123 into a useful vapor at 3.5MPa (500 psi) and 185C.

So, 2,820 kJ for water vs 257 kJ R123,….that’s eleven times less energy per kilogram needed to be transferred into R123 compared to water to go from liquid to useful vapor. In other words, a monotube boiler using R123 will need far less tube surface area compared to a boiler designed to use water.

Below is my current monotube boiler design; wider blue grid lines are 1” spacing. Green circles represent 5/8" tubing. Numbers inside the green circles indicate tube circumference. Total tube length = 348" (29 ft). Tube surface area + Hollow disc = 700 sq in. I'm always fine tuning the design details so expect changes.

Planned flow rate: 5.21 kg/sec.

1663387197517.png

Hot exhaust gases exit the burner with an annular shape and immediately impact a hollow disc containing flowing liquid R123. The hollow disc is part of the monotubes and will receive both radiant and convective heating.

The photo below is of the burner in operation and is provided here to give readers a visual image of how the exhaust gases exit the burner.
1663387251157.png


More specs and details in the next post.
 
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First, I’m a retired electronics engineer with almost no formal education in thermodynamics. All the meager knowledge I have on boiler design comes entirely through self-learning. So don’t take anything I state as being 100% accurate. Which means I’m open to advice, critique and suggestions,... which I may, or may not take :cool:

I’ve started designing a Monotube Flash Boiler to be used with the Burner I’ve already built. My boiler design is roughly based on the SES automotive boiler built in the early 1970s which supplied steam at 1000 psi at 1000F, and reportedly provided instantaneous zero-to-full flow. The SES boiler used water as the working fluid, but my boiler will use the Freon R-123, instead of water which will require some major changes as compared to the SES boiler, most notably, a far shorter monotube will be needed.
snip
So, 2,820 kJ for water vs 257 kJ R123,….that’s eleven times less energy per kilogram needed to be transferred into R123 compared to water to go from liquid to useful vapor. In other words, a monotube boiler using R123 will need far less tube surface area compared to a boiler designed to use water.

snip

OK - - - don't got no formal training in thermodynamics either but - - - logic tells me that your possibly looking at a perpetual motion device - - - - 11 times less energy - - - is that fully possible?
Are there other possible issues that make this less 'doable'.
Refrigerants work well for moving heat.
Is that all the steam is doing?

(So I'm trying to get verification that your huge energy consumption difference is 'real' rather than purely theoretical - - - if it is true then why aren't any of the large boilers working this way? - - - One would think the big engineering outfits would love to have this added boiler efficiency.)
 
OK - - - don't got no formal training in thermodynamics either but - - - logic tells me that your possibly looking at a perpetual motion device - - - - 11 times less energy - - - is that fully possible?
Are there other possible issues that make this less 'doable'.
Refrigerants work well for moving heat.
Is that all the steam is doing?

Perpetual motion ?? Nope, not at all, just normal physics and chemistry. R-123 simply takes far less enthalpy (heat) to make it boil compared to water. There are many gases that boil at temperatures far below the freezing point of water, like propane and butane. Boiling points and heat capacities are just properties of all chemicals,...no magic, just science.

(So I'm trying to get verification that your huge energy consumption difference is 'real' rather than purely theoretical - - - if it is true then why aren't any of the large boilers working this way? - - - One would think the big engineering outfits would love to have this added boiler efficiency.)

Google "ORC" or "Organic Rankine Cycle" and you'll discover that hundreds of "steam" engines have been built using various Freons, some using R123, and are currently working all around the world. Because most Freons have lower boiling points than water, most ORC generators are designed to use waste heat from any industrial process that throws away a lot of heat, such as glass manufacturers, and food processors. ORC electric generators are even used on some large diesel trucks where the truck's hot exhaust gases are used as the heat source to the boiler.

Have a look at the two above P-h charts and focus on the right-most nearly vertical lines, from 1 to 2 in the chart for water and 3 to 4 on the R-123 chart. That falling line depicts the pressure drop of the working fluid as it goes through the turbine, which turns the enthalpy (ie heat energy) into the mechanical work of spinning the turbine. The numbers on the bottom of the diagram show how much enthalpy can be put into the working fluid or taken out of it. Dropping straight down from "1" on the water diagram shows us that at 15 MPa and 600 C, the steam contains about 3600 kJ/kg of energy. At this point the steam enters the turbine where the pressure, temperature, and enthalpy all drop to "2" on the diagram. Again, dropping straight down from "2" shows us the enthalpy of the steam is now roughly 2100 kJ/kg, meaning that a 100% efficient turbine would have changed 1500 kJ/kg into mechanical work.

Now working through this same process using the P-h diagram for R123, shows us the maximum energy attainable from R123 is only 85 kJ/kg, or 17.6 times LESS energy from R123 vs Water, and that's why power plants don't use Freons.
 
For what purpose?

If you're inquiring about the purpose of the SES boiler, it was used to power a car back in the 1970's.

If you're asking what purpose my boiler will serve, it will supply "steam" to a 3 stage axial flow turbine which should deliver about 200 HP (150 kW). The thread on this project is located here: Ambitious ORC Turbine and includes some pics of the disassembled turbine.
 
As promised, here's a few more details about the boiler (and the turbine it will power):

As mentioned in my first post, the maximum flow rate of R123 pumped through the boiler will be 5.2 kg/sec. (Max flow rate is limited by the max flow through the turbine's nozzle). As mentioned in post #3, the max energy which can be extracted from the R123 as it passes through the turbine is 85 kJ/kg, so, 5.2 kg/sec x 85 kJ/kg = 442 kJ/sec (or 442 kW). That's at 100% efficiency which we all know is not going to happen.

Calculated max power output of the turbine is 144 kW, (193 HP).
 
A few words about safety: R-123 will decompose at temperatures above 250 C. Decomposition products are said to be pungent, irritating, toxic gases, which are quickly noticeable and avoidable. However, a temperature sensor will be installed in the boiler output pipes and monitored by a computer which will keep R-123 temperatures below 200 C at all times by limiting burner output, and increasing feed pump flow if needed.

Just like today's modern car engines, all aspects of this project will be controlled by a micro controller which will monitor R123 temps and pressures at several locations as well as the RPM of the turbine. Boiler feed pump output pressure, burner fuel and air flows, and throttle position will all be under computer control.
 
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A few changes to the boiler:

I caught and corrected a math error affecting and increasing overall tube length, which necessitated a larger boiler pot, and reconfiguring fluid flow and tubing layout.

Finished designing the boiler tube output connections to allow for easy insert and extraction of the tube assembly.

Still designing a stainless steel skeletal structure to hold the tubes in place.

1663571547421.png
 
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The burner shown in the first post is mostly finished. I've had it running numerous times testing min and max burn rates, which are roughly 1 L/Hr up to 20+ L/Hr; I say "20+" because I stopped increasing the fuel flow rate at 20 L/Hr knowing that I only need 14 L/Hr max for the boiler and turbine.

The burner uses diesel so 10 L/Hr should produce 106 kWh and 14 L/Hr should produce 148 kWh which is equivalent to nearly 200 HP-Hours. Having the ability to go much higher will allow for quite a large boiler inefficiency.

So how much boiler tubing surface area is needed? I started by looking at a known working boiler, the SES unit, and scaling it down for use with R123 instead of water. As discussed in the first post, R123 needs 1/11 the enthalpy (heat) transfer as compared to water; which should also mean an R123 boiler will need less than1/10 the boiler tube surface area, but I will use a little more just to be on the safe side. The SES uses 10,843 sq-in, so 1/10th = 1084 sq-in and my boiler has 1260 sq-in. I'ld love to determine the needed surface area using a math expression, but I've not quite mastered those equations.
 
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The quest for a blue flame from my forced air burner was partially successful after making a few tweaks. I increased the diameter of the restrictor plate which decreased the exhaust exit area; below pics show the different size plates.

93163-Short-Burner-End-Plate-sml.jpg
07Dec22 Burner Restrictor Plate.jpg


The result was at high fuel-burn rates (diesel), the exhaust gases begin to turn blue as shown in the below photo.

07Dec22 Burner test f.jpg


The video also shows a hint of blue flame, and the burner sounds a bit like a jet engine :cool:

 
Excellent improvement, Toymaker. What you need now is a CO meter to check how much CO is in the exhaust gas. That is the real measure of full combustion of the fuel. You will probably never reach zero, but if you can get closer to car exhaust figures that is a real limit. Even 100 times more is better than 1000 times more!
I only have a cheap CO alarm which tells me when I have too much CO on the shelf above my working area/bench. Some blow-lamps for brazing set it off, but I tune my gas burners for boilers so that they do not set it off. My 1979 motorcycle sets it off at 5m range!
Stay safe,
K2
 
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Hi Toymaker, I just had a thought... from the red colour of the choke plate, you can estimate the temperature of the exhaust gas from the burner. - Useful in doing calculations of heat flow in your boiler. Rememeber that below 900deg.C. you do not make an appreciable amount of Nitrous oxides, above 900deg. C. you do. So I suggest you keep to that limit, to avoid polluting your lungs with very nasty fumes.
I suspect the CO content of the exhaust gases will be quite small... Comparable to a diesel car at the same fuel combustion rate, without exhaust post treatment.
Have you tried positioning a cold clean plate maybe 6 inches from the exhaust - for a few seconds - to see if it collects any carbon particulate? - I don't think you will find anything significant with your latest burner mods.
Seeing how the outer can is relatively "cold" compared to the combustion zone (temper colouring compared to red heat), you have made a burner design that manages heat quite well.
I look forward to further instalments.
Enjoy!
K2
 
Have you tried positioning a cold clean plate maybe 6 inches from the exhaust - for a few seconds - to see if it collects any carbon particulate?

Thanks for reminding me that I still need to do a carbon deposit test; it is an important indicator of complete combustion.

you have made a burner design that manages heat quite well.

Thank you :) I borrowed heavily from the design of combustion chambers found inside jet engines, as they too use an inner burner can that is cooled by the input air stream. The main difference which I incorporated was to force the input air stream to swirl around the outer surface of the inner burner can to cool it, and swirl the air inside the burner can to create a sort of fire tornado.
 
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"Based on a jet engine combustion chamber" is what I already figured, so thanks for that. No surprise it is starting to sound like a jet engine - with the rapid combustion, and consequential White Noise of flame fronts at sonic speeds all mixed randomly inside the total flame volume.... I think? I am probably wrong, but I think each micro-flame-front collapses when the combustion pressure drops as fuel+air are consumed, and the rapidly collapsing pressure causes a step in the generated sound wave,,,, these collectively cause the "roar" of a roaring burner. But don't quote me, as I am just an amateur figuring out what makes the noise.
The improved combustion I think is a development of the increased pressure inside the burner itself - having restricted the exhaust annulus. At higher pressure, the gas + air combustion has higher flame-front speeds (Sonic) therefore completes the combustion quicker. The higher temperature generated is also beneficial to the overall efficiency of the engine, although the "absolute" energy available is dependent on fuel flow. - Of course you know all that better than I do?
Good Engineering anyway!
K2
 
Hi Toymaker, I very much enjoy your design and attempt. I agree with your plan to finish first the boiler.
If you accept my opinion, as a non-specialist, I believe the most problematic part in your design is the radiation disk. both on gas side because of high temperatures /radiation energy it receives leading to (more or less alternating) thermal stress; and on steam (R123) side, because you have to avoid running completely dry, overheating (over 200 C) and degrading refrigerant.
If I would build this I would try the following: Temperature sensor going through radiation disk up to its front face; injector to spray liquid R123 only in emergency cases when radiation disk runs too hot; discharge valve with fine control/fine flow that bypasses the last 2 coils; this would act both as safety to evacuate surplus steam from cooling radiation disk and as an additional mean to regulate output of boiler by mixing a small fraction of low temperature steam from radiation disk into boiler's output. Maybe an additional, pure safety discharge valve (electronically controlled also) directly to condenser.
Or maybe you have estimated that radiation disk will be continuously wet, in which case all I have said above is meaningless. Have you evaluated the average transition point liquid-steam along your steam circuit - that is where boiling will take place most likely?
I believe keeping all portions of steam circuit below 200C at such high thermal loads/mass flows is a tough task -that ultimately can be achieved.
Anyway, these are -as I have said - just non-specialist thoughts. Take what you want...
I will follow this thread closely.
Good luck!
 
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The purpose of the radiation disk is to soak up as much radiant heat from the exhaust flames as possible. It's name is a bit misleading as it's not a simple hollow disk, but rather a disk containing channels which direct the working fluid into spirals moving inward towards the center where the working fluid exits and continues on to the remaining boiler tubing. It was constructed from coper sheet, brazed together into one piece; below is a 2D drawing.
Because the disk is located mid-way thru the total tubing length, the working fluid should still be in the liquid state during it's transition through the spirals.
1686292621207.png
 
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
 
Wow!
Remind me, what temperature and pressure will this "radiation disc" be operating at?
Reading that information from the first post in this thread on the included P-h chart (Enthalpy graph), shows my max temperature at 185C at 500 psi.

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.

This is Thailand,... :cool:

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

Perhaps you have the highly specialized tools needed to bend heavy wall tubing (likely a hydraulic system), but I do not. But the real problem with your suggestion is the heat soak time, that is, it will take much longer for the radiant heat to travel through a thick wall as compared to a thin wall. Make the wall too thick and one risks melting the wall surface in direct contact with exhaust gasses while the inner surfaces are effectively cooled by the working fluid.

Finally, if the current design fails, I'll make an identical disk, but instead of brazing, I will spot weld the parts in place using closely spaced welds, and seal any small leaks with brazing. :cool:
 
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