Thanks Toymaker. I figured Richard was making some pun from the "stress" of these calculations, not a medical judgement... but he will surely explain later.
K2
K2
Monotube Flash Boiler Design
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Richard Hed
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Post Traumatic Stress Disorder. It's the long term feeling or fear you get when a boiler blows up and scalds your face.
ajoeiam
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I think what is meant is - - - - P= (2*T*S/D) - - - - lol (see the P. . . T . . . S . . . D ... .)
Thanks AJ. Makes sense!
The boiler is finally starting to look like a monotube boiler.
Yes, I know my tubes are spaced much closer together than is generally recommended, however I want to try this tight pack to see how well, or not well, it works. Remember, the hot exhaust gases from the burner will be entering the boiler tube coils at the same velocity and force as air from a typical "leaf blower", because that is what I'm using to blow air into the combustion chamber.
A few specs:
All tubing is 5/8" (16mm), total length, roughly 83 ft, ( 25.4 meters). Tube surface area: 1963 sq inches.
Pot (outer shell) dimensions: 30 cm diameter by 32 cm tall.
Pressure tested to 100 psi, which is as high as my shop air compressor will go.
The burner slides into the center hole created by the tube coils.
Yes, I know my tubes are spaced much closer together than is generally recommended, however I want to try this tight pack to see how well, or not well, it works. Remember, the hot exhaust gases from the burner will be entering the boiler tube coils at the same velocity and force as air from a typical "leaf blower", because that is what I'm using to blow air into the combustion chamber.
A few specs:
All tubing is 5/8" (16mm), total length, roughly 83 ft, ( 25.4 meters). Tube surface area: 1963 sq inches.
Pot (outer shell) dimensions: 30 cm diameter by 32 cm tall.
Pressure tested to 100 psi, which is as high as my shop air compressor will go.
The burner slides into the center hole created by the tube coils.
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
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
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Yes, gas flow from the burner will be highest as the hot exhaust gases exit the burner, and will immediately slow as the gases move into the larger volume of the boiler area where they're allowed to expand and cool as they come into contact with the tubes. However, even if the exhaust gases are cooled back to the temperature of the air entering the blower (room temp), which is highly unlikely, those gases would still have roughly the same volume as when they entered the blower; in this theoretical condition, gas flow rate would be controlled by the larger volume of the boiler container as compared to the blower's exit area, and I've taken that into account.
The SES boiler-burner also used tubes placed much closer together than commonly recommended and it used a rather large fan to blow air into the combustion area,....and it worked quite well; I'm hoping for similar results.
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Richard Hed
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Looks just like my still.
Principle similar to the ideal gas law.
Also besides water vapor and CO2, depending on the fuel there are a number of other products of combustion. Burners also need to run with excess air that can vary from 2%-10% residual O2, depending on the fuel. Too little and you get lower flame temperatures and generate CO, too much and you blow the hot gasses through the boiler faster than the heat can be extracted , lowering the overall efficiently.
As an aside, I share many of the potential safety concerns you have expressed. Just for chuckles I ran this thread by an associate who is an ME - one of his specialties is forensic investigation of pressure vessel failures. His comment was that as presented, this design is an accident waiting to happen.
Why? Be specific.
Hi Toymaker, I think you missed my point that the exhaust is made of the incoming air plus combustion exhaust gases from the fuel you are adding... thus I think you have more gas escaping the exhaust vent than air from the blower.... thus the velocity will be higher at the same temperature.... ergo the pressure inside will be higher. If you did not burn fuel, then the air would be at the same pressure at the same temperature, but you are exhausting at maybe 180deg.C or something? (temperature of the hot coils).
But you are burning fuel: The temperature change within the combustion causes the temperature and pressure gradient to maintain the higher pressure at the end of the combustion phase, thereafter the pressure takes a downward gradient through the coils etc, to the exhaust.
All I am saying is that you may need to manage the exhaust passage to be larger than the intake to prevent back pressure within the boiler, which can affect the burner...
This is "bread and butter" to jet engine designers... But I have seen "Wrongly sized" exhausts on many steam boilers because they assume "air in = air/exhaust out"... without considering "air + fuel in = Exhaust gas out".
I'll let you work it out, or "try it and see".
K2
But you are burning fuel: The temperature change within the combustion causes the temperature and pressure gradient to maintain the higher pressure at the end of the combustion phase, thereafter the pressure takes a downward gradient through the coils etc, to the exhaust.
All I am saying is that you may need to manage the exhaust passage to be larger than the intake to prevent back pressure within the boiler, which can affect the burner...
This is "bread and butter" to jet engine designers... But I have seen "Wrongly sized" exhausts on many steam boilers because they assume "air in = air/exhaust out"... without considering "air + fuel in = Exhaust gas out".
I'll let you work it out, or "try it and see".
K2
Richard Hed
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Do you mention somehwere how thick the walls are? I must have missed that but that will tell how much strength there is.
Hi Toymaker,
PLEASE do an hydraulic test on your boiler coils/assembly.
AS A MINIMUM at twice your proposed working pressure of 500psi.. = 1000psi.... ("Twice NWP" is a common standard).
ASME (that you choose to ignore) require a room temperature hydraulic test at > 2.4 x NWP because of the weakening of copper at your proposed maximum working temperature for the copper... I.E. 2.4 x 500 = 1200psi.
Actually, the design should be capable of an hydraulic test "WITHOUT DAMAGE or permanent change of shape" at 8 times the NWP. - If you are sure you have a Factor Of Safety that high?
All the boilermakers and testers I know ensure the hydraulic tests are thoroughly examined and "Good" before ever applying any working fluid and heat to a boiler.
I am sure you can rig a water or oil pump to the boiler and do an Hydraulic test and check for distortion and leaks.
Then you NEED to prove your Safety Relief valve will work at not exceeding 6% over the Normal Working Pressure.... = at less than 530psi for NWP = 500psi.
Your 100psi air test is a good leak test if you have done it under water to check for leaks in any joints, but does not do anything to prove the strength of the boiler. At least 15 minutes submersed, and NO BUBBLES formed anywhere on the surface of any joint is a pass. The air should pressurise the boiler, then the valve closed while monitoring the pressure, and the pressure should stabilise with temperature and then NOT DROP further.
I have witnessed tests where a 3mm air bubble has taken >10 minutes to form. That is a FAIL. (It was used to prove a different test could NOT identify a failure).
Only after all that would anyone I know consider applying heat to the boiler.
Stay safe and continue your development.
K2
PLEASE do an hydraulic test on your boiler coils/assembly.
AS A MINIMUM at twice your proposed working pressure of 500psi.. = 1000psi.... ("Twice NWP" is a common standard).
ASME (that you choose to ignore) require a room temperature hydraulic test at > 2.4 x NWP because of the weakening of copper at your proposed maximum working temperature for the copper... I.E. 2.4 x 500 = 1200psi.
Actually, the design should be capable of an hydraulic test "WITHOUT DAMAGE or permanent change of shape" at 8 times the NWP. - If you are sure you have a Factor Of Safety that high?
All the boilermakers and testers I know ensure the hydraulic tests are thoroughly examined and "Good" before ever applying any working fluid and heat to a boiler.
I am sure you can rig a water or oil pump to the boiler and do an Hydraulic test and check for distortion and leaks.
Then you NEED to prove your Safety Relief valve will work at not exceeding 6% over the Normal Working Pressure.... = at less than 530psi for NWP = 500psi.
Your 100psi air test is a good leak test if you have done it under water to check for leaks in any joints, but does not do anything to prove the strength of the boiler. At least 15 minutes submersed, and NO BUBBLES formed anywhere on the surface of any joint is a pass. The air should pressurise the boiler, then the valve closed while monitoring the pressure, and the pressure should stabilise with temperature and then NOT DROP further.
I have witnessed tests where a 3mm air bubble has taken >10 minutes to form. That is a FAIL. (It was used to prove a different test could NOT identify a failure).
Only after all that would anyone I know consider applying heat to the boiler.
Stay safe and continue your development.
K2
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Max pressure was discussed in post #66; I have a safety factor of 8 at 200C. Tube wall thickness was noted somewhere else, but it's 5/8" Type L copper tube which is defined as 1mm wall.
On pressure relief safety valves.
A misconception sometimes seen with old steam locomotives. These valve are used to prevent over pressure WHEN THE HEATING SOURCE is mis-managed. I.E> a coal fired locomotive boiler, with too much fuel in the firebox, standing in a station, will often be seen to vent steam from a safety valve, because there is too much fire (Usually with a steam blower forcing the fire). It is not the design intent for the safety valve to be used by "careless" firemen to frequently control the steam pressure in the boiler.
The intent of the Safety Pressure relief valve is to prevent ALL and any over pressure in the boiler. So should be checked when first firing a boiler (to be sure it has not stuck or seized), and if defective the "fire" must be stopped immediately.
The SIZE of the Safety Pressure Relief Valve must be large enough so that at maximum fire heat input, with a fully blocked exit passage from the boiler, the safety pressure relief valve prevents any over-pressure above 6% higher than the stated Normal Working Pressure for the boiler. Can you do that test when you first fire the boiler?
K2
A misconception sometimes seen with old steam locomotives. These valve are used to prevent over pressure WHEN THE HEATING SOURCE is mis-managed. I.E> a coal fired locomotive boiler, with too much fuel in the firebox, standing in a station, will often be seen to vent steam from a safety valve, because there is too much fire (Usually with a steam blower forcing the fire). It is not the design intent for the safety valve to be used by "careless" firemen to frequently control the steam pressure in the boiler.
The intent of the Safety Pressure relief valve is to prevent ALL and any over pressure in the boiler. So should be checked when first firing a boiler (to be sure it has not stuck or seized), and if defective the "fire" must be stopped immediately.
The SIZE of the Safety Pressure Relief Valve must be large enough so that at maximum fire heat input, with a fully blocked exit passage from the boiler, the safety pressure relief valve prevents any over-pressure above 6% higher than the stated Normal Working Pressure for the boiler. Can you do that test when you first fire the boiler?
K2
The short answer is: Yes,... easily done in a subroutine. The entire procedure could be part of the start-up routine executed on every start-up.
Living in a warm climate near lots and lots of pleasantly warm water, both fresh and salty, my plan is to rent a scuba tank, connect it to my boiler, which has been submerged in a lake, maybe 2 feet down, then slowly open the scuba tank valve. If there's a leak, I'll see the bubbles and if there's a rupture or worse,....the worst that will happen is that I'll get a little wet. Note that I'll be sitting safely on a dock while cranking up the pressure.
Lots, and lots of development, wiring, programing, etc. before I get to the point where I need to test the relief valve.
The coils were placed inside the pot, which was then filled with enough water to cover the coils.
Sadly, I'm no longer in contact with any of the gas turbine designers I once worked with,...40+ years ago. I wish I were,...they'd likely get a kick from my hobby.
Sounds like an interesting career. Do tell us more.
K2
K2
As I stated back in post #98, I'm just an old electronics engineer, muddling his way through mechanical engineering and trying his best to use what little he learned about metallurgy and thermodynamics form his previous jobs. My work on gas turbines was limited to designing the the electronic controls for those engines, and in order to do that, myself and the rest of the ECU team needed to learn how these engines worked, but not how to design one or calculate things like air mass flow rates through them,...that was left to the ME and Thermodynamics teams. We EEs focused on sensor inputs such as pressure transducers, thermocouples, RTDs (Resistance Temperature Device), and outputs such as IGV (Inlet Guide Vanes) controls, fuel valve control, etc. The first 3 sentences in post #1 are: "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."
