But for good or for ill?
Monotube Flash Boiler Design
- Thread starter Toymaker
- Start date
Help Support Home Model Engine Machinist Forum:
The graph does not represent what will happen. For instance to calculate the pressure with the base hypothesis of four active cylinders you would have each area times the force total force of the pistons to yield one pressure output. This should be a constant. The pressure will dip when one cylinder is removed and increase when its replaced by another. So the pulse frequency will be when the pistons unload and load. The more active cylinders the less the pulse will be, but still depends on the volume each cylinder produces.
Smaller pistons require a higher rpm thus a higher pulse frequency. Magnitude is a function of the volume each piston contributes. Volume is a fuction of piston diameters, rpm and horsepower delivered.
However, the purpose of the pump is to supply water at the steaming rate and enough pressure to overcome the piping and head losses. For traditional designs a feedwater valve is used and often in conjunction with a variable speed drive. To prevent cavitation a recirculation valve is used.
I guess the question is if the pump will cavitate due to the operation of the valves at the ports. If it does the pulses could be quite severe. With oil I would expect the risk to be low with water not so sure. Just because its a piston pump does not mean it will not require a minimum suction head or will not cavitate.
The design looks like it will minimize the pulses but the other factors such as valving and port size may cause other problems.
I suspect that pressure pulses from the input point (pump) will trvel at the speed of sound inside the liquid until the critical temperature/pressure point where steam is created. The oscillation of pressure from the sound wave will cause and oscillation of the phase change and this energy exchange may be helpful, or destructive.
There is a similar condition in gas where it is flowing - naturally through a tube and heats a "membrane" of heater input. It is called Rijke's tube resonance.
https://en.wikipedia.org/wiki/Rijke...e tube is a,an excellent example of resonance.
A very clever guy (Joan Lluch) I met on another thread developed this with the type of radiant heat burner he introduced into his boiler for a steam loco - designed for such a gas burner. https://www.mylargescale.com/threads/gas-burner-making-high-pitch-flute-like-noise.81946/
He resolved the issue by changing the air-fuel mix, such that the right conditions did not occur for the Rijke's tube resonance to occur. I think we made the flame front thicker? - I.E. When the oscillating gas can transit from hot-zone to cold-zone and back again across the heating element, it sounds - and resonates very loudly. When the cold to hot interface is too thick for the gas to travel across and back it cannot sound. I.E. the zone is thicker than the wavelength of the resonant frequency at the temperature and pressure of gas.
In a flash boiler, there is a zone where the temperature does not rise as the water is converting to steam. If this zone is very short, compared to the tube length, the Rijke's tube resonance could occur as the pressure pulsations (at the SAME frequency) stimulate the oscillation of this zone - if the zone is much shorter than the wavelength of the resonant frequency in the gaseous steam. (I think?).
The analogy being, that if the system's resonant frequency is stimulated by pump pulsation frequency, then a noise will sound - easily picked-up by the simplest sensor - the ear. As long as a resonant sound does not occur, then all is OK.
"SOUNDS SIMPLE" - - or am I being too simplistic?
When resonance occurs, (a mechanic once told me) it is the machine shouting "Help! Turn me off - before I explode!" And as long as we heed such warnings life should be good. Think of it this way - hit a bell and the sound is resonant - a single note. As a Tympanist I used to pick up a note from the band/orchestra and sing it close to the drum-head. when re-tuning to a different note. When truly in-tune the drum-head would sing back. - So loud I once had a conductor tell me to stop tuning at that quiet point in the music as he could hear it! - It was Out-of-tune to the key being played at that time. So we agreed where I should "sing quietly" to the drum, so it's "loud reply" didn't affect the music. Resonance is always LOUD as it absorbs virtually all the energy available as re-broadcasts it on a single frequency.
If not relevant or useful, just ignore this.
I can also explain why running two pumps at a SAME rotational speed - one with 2 pistons and one with a multiple of 2 times the pistons gives a higher frequency, but it still resonates at a numerically equivalent frequency. NORMAL practice in industry is to either add a damper or something to change the resonant frequency of the system, not necessarily change the driver, I.E. the pump. You plan on using variable speed to vary the flow, so if the resonant frequency of the BOILER is unchanged, you'll still hit a resonant frequency some way when the pulsations from the pump match it. A 2 piston pump may hit resonance of 16000Hz at 8000rpm, but a 9-piston pump will hit the same 16000Hz resonance at ~1778rpm. A different pump speed, but the same resonance. Change the resonance to 16Hz, and you won't run a pump slow enough to hit that resonant frequency. Similarly if the resonance of the boiler is too high for pump stimulation of resonance.
Damping, other than by addition of a tuned resonator (Helmholtz resonator), is different, as it absorbs the energy between each input pulse, so there is nothing left when the next pulse arrives. A Helmholtz resonator - tuned to prevent pump pulsations at the boiler resonant frequency from reaching the boiler, would be placed on a Tee-connection, located between the pump and boiler, such that the energy of pulsations - at resonant frequency - all goes into the resonator - and out again - such that the pulsations on the main-line do not go into the boiler. Like adding a large or suitably sized Capacitor on the output of a noisy electrical feed to eliminate the noise the is seen by equipment further along the line.
Tap the boiler (at pressure and filled with water) and listen to the note it "rings". Use your phone with software to tell you the frequency of that note - or compare to a piano - and then you'll know when resonance will occur, unless the phase change to steam is sufficient damping to eliminate of change the resonance. (The opposite of the Rijke's tube condition).
If it doesn't "ring" then the water, stress from pressure or something is either damping the noise, or raising the resonant frequency above the range of your hearing. (Speed of sound in water and metal is much higher than in air, so very high resonant frequencies). In this case, the pulsations from the pump (in the audible frequency range) are too slow to reach the boiler resonant frequency.
Also, if you start with a COPPER boiler that is annealed, it is likely to dampen the resonances in the metal, but after some use the copper work hardens, and the hard copper "rings" much better, as the internal material structure does not dampen the resonance. It then becomes prone to resonance fatigue failures. Tin is used for Organ pipes, so the pipe-metal resonance (very low, well damped) does not interfere with the air-column resonance (Audible range). But Tubular Bells are hard metal to "ring" at the metal-length resonance (Undamped, Audible range oscillation), tuned to the tube of air column length, so that resonates and amplifies the sound created by the walls of the ringing tube.
Some of the above may be useful, some inaccurate, but it is "what I know", so please correct me if I am wrong and ignore what does not affect this problem. (Is there really a resonance problem? - Or just a "measuring instrument resolution" issue?).
K2
There is a similar condition in gas where it is flowing - naturally through a tube and heats a "membrane" of heater input. It is called Rijke's tube resonance.
https://en.wikipedia.org/wiki/Rijke...e tube is a,an excellent example of resonance.
A very clever guy (Joan Lluch) I met on another thread developed this with the type of radiant heat burner he introduced into his boiler for a steam loco - designed for such a gas burner. https://www.mylargescale.com/threads/gas-burner-making-high-pitch-flute-like-noise.81946/
He resolved the issue by changing the air-fuel mix, such that the right conditions did not occur for the Rijke's tube resonance to occur. I think we made the flame front thicker? - I.E. When the oscillating gas can transit from hot-zone to cold-zone and back again across the heating element, it sounds - and resonates very loudly. When the cold to hot interface is too thick for the gas to travel across and back it cannot sound. I.E. the zone is thicker than the wavelength of the resonant frequency at the temperature and pressure of gas.
In a flash boiler, there is a zone where the temperature does not rise as the water is converting to steam. If this zone is very short, compared to the tube length, the Rijke's tube resonance could occur as the pressure pulsations (at the SAME frequency) stimulate the oscillation of this zone - if the zone is much shorter than the wavelength of the resonant frequency in the gaseous steam. (I think?).
The analogy being, that if the system's resonant frequency is stimulated by pump pulsation frequency, then a noise will sound - easily picked-up by the simplest sensor - the ear. As long as a resonant sound does not occur, then all is OK.
"SOUNDS SIMPLE" - - or am I being too simplistic?
When resonance occurs, (a mechanic once told me) it is the machine shouting "Help! Turn me off - before I explode!" And as long as we heed such warnings life should be good. Think of it this way - hit a bell and the sound is resonant - a single note. As a Tympanist I used to pick up a note from the band/orchestra and sing it close to the drum-head. when re-tuning to a different note. When truly in-tune the drum-head would sing back. - So loud I once had a conductor tell me to stop tuning at that quiet point in the music as he could hear it! - It was Out-of-tune to the key being played at that time. So we agreed where I should "sing quietly" to the drum, so it's "loud reply" didn't affect the music. Resonance is always LOUD as it absorbs virtually all the energy available as re-broadcasts it on a single frequency.
If not relevant or useful, just ignore this.
I can also explain why running two pumps at a SAME rotational speed - one with 2 pistons and one with a multiple of 2 times the pistons gives a higher frequency, but it still resonates at a numerically equivalent frequency. NORMAL practice in industry is to either add a damper or something to change the resonant frequency of the system, not necessarily change the driver, I.E. the pump. You plan on using variable speed to vary the flow, so if the resonant frequency of the BOILER is unchanged, you'll still hit a resonant frequency some way when the pulsations from the pump match it. A 2 piston pump may hit resonance of 16000Hz at 8000rpm, but a 9-piston pump will hit the same 16000Hz resonance at ~1778rpm. A different pump speed, but the same resonance. Change the resonance to 16Hz, and you won't run a pump slow enough to hit that resonant frequency. Similarly if the resonance of the boiler is too high for pump stimulation of resonance.
Damping, other than by addition of a tuned resonator (Helmholtz resonator), is different, as it absorbs the energy between each input pulse, so there is nothing left when the next pulse arrives. A Helmholtz resonator - tuned to prevent pump pulsations at the boiler resonant frequency from reaching the boiler, would be placed on a Tee-connection, located between the pump and boiler, such that the energy of pulsations - at resonant frequency - all goes into the resonator - and out again - such that the pulsations on the main-line do not go into the boiler. Like adding a large or suitably sized Capacitor on the output of a noisy electrical feed to eliminate the noise the is seen by equipment further along the line.
Tap the boiler (at pressure and filled with water) and listen to the note it "rings". Use your phone with software to tell you the frequency of that note - or compare to a piano - and then you'll know when resonance will occur, unless the phase change to steam is sufficient damping to eliminate of change the resonance. (The opposite of the Rijke's tube condition).
If it doesn't "ring" then the water, stress from pressure or something is either damping the noise, or raising the resonant frequency above the range of your hearing. (Speed of sound in water and metal is much higher than in air, so very high resonant frequencies). In this case, the pulsations from the pump (in the audible frequency range) are too slow to reach the boiler resonant frequency.
Also, if you start with a COPPER boiler that is annealed, it is likely to dampen the resonances in the metal, but after some use the copper work hardens, and the hard copper "rings" much better, as the internal material structure does not dampen the resonance. It then becomes prone to resonance fatigue failures. Tin is used for Organ pipes, so the pipe-metal resonance (very low, well damped) does not interfere with the air-column resonance (Audible range). But Tubular Bells are hard metal to "ring" at the metal-length resonance (Undamped, Audible range oscillation), tuned to the tube of air column length, so that resonates and amplifies the sound created by the walls of the ringing tube.
Some of the above may be useful, some inaccurate, but it is "what I know", so please correct me if I am wrong and ignore what does not affect this problem. (Is there really a resonance problem? - Or just a "measuring instrument resolution" issue?).
K2
Yes, adding accelerometers to sense & measure vibration is a good idea, especially during the development phase.
And you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam.
Oh boy I can't wait for this move to be over. Then you guys can dog pile my projects
Move ?? What move ??
I can only get to the critical point with R123, or some other Organic working fluid. The critical point of water is 374 C
and 3,200 psi,...my copper tube boiler would most certainly rupture long before I reached those numbers. The critical point for R123 is 184 C at 533 psi, which is well within the capabilities of my boiler.
That's a interesting paper you linked,... the primary focus is on how pump pulses cause a sort of feedback problem, where the pump pulses induce small changes in the angle of the swash plate in a variable plate system, which then causes more pump pulses. This all occurs because the swash plate angle controls use the hydraulic output from the pump.
Just read this: "you're right about the boiler making a fair bit of vibrations as the water first begins to boil, but it quiets down once the boiler is putting out a steady flow of steam." - Excellent! What you are demonstrating is the slow vibration of initial boiling, maybe hitting a low resonant frequency, but when the boiler develops pressure and more steam the boiling is with smaller bubbles, and a higher frequency of "input" above the low resonant frequency of the boiler so it doesn't shake rattle and roll!. You can use that to determine a resonant frequency from the low note of those vibrations (use a phone with some software if it is a better calibrated ear? - it may be sub-sonic for the ear?) and compare that to frequency of pulsations from the pump. A pump that does not reach the resonant frequency is good.
K2
K2
I very much appreciate everyone's inputs, suggestions, analysis, and pointing out what you believe are my errors; you've all helped me reach the conclusion that the best technical solution is to use a different pump, one with more than two pistons.
I'm opening a new thread under "A Work In Progress" where we can all discuss (and ridicule if you must) my DIY feed pump. Hot Link: 9 Piston Feed Pump
I'm opening a new thread under "A Work In Progress" where we can all discuss (and ridicule if you must) my DIY feed pump. Hot Link: 9 Piston Feed Pump
I'm in the middle of a move and thus between houses with no functioning work shop, so I can't start anything. Only dream lol.
Sorry I got the criticality mixed up.
I can relate,....when I moved from San Francisco to Thailand I waited for 3 long months for all my household goods, including all my hand tools and benchtop CNC mill, to arrive, then several more weeks for my crates to make it through Thai customs.
Well happily it's not that rough, but the new place had a lot of hidden work. I was supposed to be happily nesting in a pile of swarf months ago!
My DIY 9 Piston Feed Pump is finally finished and I've begun using it to produce pressure in the boiler (without firing up the burner) which allows me to adjust the software that reads the feed pump pressure and boiler pressure output sensors, which are both displayed on the LCD. Below are two videos showing test pressures of 100 and 200 psi; I pan from the dial guage to the LCD display and back to the dial guage.
I'm pleased with the smooth flow as indicated by the pressure guage.
I'm pleased with the smooth flow as indicated by the pressure guage.
Very nice setup.
Lets make a little steam !! It's been 4 months since I last fired-up the burner and had the boiler making steam, and this is the first time I'm using my DIY feed pump, so I'm taking baby steps as I ease back into running my monotube steam generator. These first tests are primarily to check the pressure and temperature sensors, and as can be seen in the videos,...at least at this idle power setting, they clearly need some adjustment. For these test runs, I'm manually controlling feed pump speed (ie flow rate & pressure), with the goal being to collect enough data to enable me to program the ECU to eventually take over feed pump control, having the ECU setting the feed pump based on a single power input which I select.
The first video shows open flow at about 0.3 LPM (40 lbs/Hr) making very wet steam.
The second video shows wet steam at 70 PSI and 0.5 LPM (66 lbs/Hr), which is also approximately 2 Boiler Horsepower (BHP) or about 20 kW.
The first video shows open flow at about 0.3 LPM (40 lbs/Hr) making very wet steam.
The second video shows wet steam at 70 PSI and 0.5 LPM (66 lbs/Hr), which is also approximately 2 Boiler Horsepower (BHP) or about 20 kW.
Last edited:
Ooops,...I just had an epiphany !!
Back in May of this year I was brainstorming ideas of how to test power output of both my boiler and my Tesla-Impulse turbine. Thinking what I believed to be conservatively at that time, I decided that using my 1/2 KW spindle motor as a generator coupled to the turbine would be a good place to start. I would be able to measure the DC volts across a load to determine the power output. Sadly, I had not considered at that time that 0.5 kW would be well under the lowest operating conditions of the boiler.
What I failed to say in the previous post is that the 0.5 LPM flow rate at 70 psi is the lowest feedwater rate the boiler can be safely operated at,...any slower and the DC motor turning the feed pump is at risk of stalling and stopping. Meaning that 20 kW steam output is the bottom end of boiler operation,...which is 40 times higher than what the 1/2 kW generator could use.
I will need a MUCH bigger alternator coupled to the turbine to perform useful testing.
Back in May of this year I was brainstorming ideas of how to test power output of both my boiler and my Tesla-Impulse turbine. Thinking what I believed to be conservatively at that time, I decided that using my 1/2 KW spindle motor as a generator coupled to the turbine would be a good place to start. I would be able to measure the DC volts across a load to determine the power output. Sadly, I had not considered at that time that 0.5 kW would be well under the lowest operating conditions of the boiler.
What I failed to say in the previous post is that the 0.5 LPM flow rate at 70 psi is the lowest feedwater rate the boiler can be safely operated at,...any slower and the DC motor turning the feed pump is at risk of stalling and stopping. Meaning that 20 kW steam output is the bottom end of boiler operation,...which is 40 times higher than what the 1/2 kW generator could use.
I will need a MUCH bigger alternator coupled to the turbine to perform useful testing.
Out of curiosity, any updates?
After weighing which loading method would be better, electrical generator or water pump, I decided to first try the electrical generator. Even though the load which the generator presents the Tesla Turbine will likely be way too small, the test stand arrangement allows me to read the RPMs, which I cannot do using the water pump, and I really, really want to know the RPM.
After rummaging through several boxes full of old electronic components I've saved over the years, I failed to find any appropriate load resistors,...so I ordered these 3 from China. All 3 are 10 ohm and 300 watts, and they're adjustable. Two of these, wired in parallel and using the adjustment ring, will allow me to accurately measure the voltage and current from the 48 volt generator, at the 500 Watt expected power.
ETA for the resistors is another few days
Can't wait for them to arrive!
More test parts arrived and I'm ready to connect them to the boiler,....now, if it would just stop raining :-(
My boiler test stand,... under the very wet, black rain cover.
My boiler test stand,... under the very wet, black rain cover.
Richard Hed
Well-Known Member
- Joined
- Nov 23, 2018
- Messages
- 2,624
- Reaction score
- 676
Yup, it's the rainy season
Just when I was ready to finally run more tests,...my High Voltage ignitor coil sprung a leak,....and I don't have a spare! I've ordered a replacement (and a spare) but this means another week to 10 days delay.
The red arrow in the pic below points to the high voltage arcing inside the coil winding,...it's not supposed to do that.
The red arrow in the pic below points to the high voltage arcing inside the coil winding,...it's not supposed to do that.
Glad it did not zap you.
Would a fly back work?
Would a fly back work?
