Monotube Flash Boiler Design

[Toymaker]

The ph diagram in english units shows critical temperature at 532 abs lbs/ f in2 and temperature of 382 F. Given the high heat flux doubt you can keep it from flashing.

It appears you have a type-o as you've labeled both values as Critical Temperature.
So, to clarify, for R123:
Critical Pressure is 532 psi.
Critical Temperature is 362.63F (183.68C)

High flow rate in a high heat flux environment is the key to preventing liquids from flashing into vapor, which is why I will need to switch over from the plunger feed pump I'm currently using for water, to a centrifugal pump for R123.

Now, just in cast you have insomnia tonight thinking about flash boilers, critical pressures, R123, etc., here's a little reading material that should put you right to sleep; it's Du Pont's Technical Information sheet on R123,....even includes a P-h diagram in imperial units at the end

enjoy

 

[Steamchick]

Thanks Toymaker. Yup. Read it, fell asleep. Not something I understand. But it seems you are trying to work right on the top limit for temperature and pressure. I.E. very close to the saturation line. Am I right in thinking that the liquid state is inside the saturation line and outside, it boils?
I think the whole "engineering" you are undertaking is culturally different to what I am familiar with. NASA, Racing teams, etc. Work right at limits where they have high risks (1/100 failures is accepted?), small safety margins, and get as close as they can to physical limits. Not my understanding.
My 45 year career was based on working in a very safe area, 1/ 10 million failures down to 6ppm failures, depending on whether items were critical or non-critical.... Factors of safety of 6 to 8, etc. Electrical switchgear to last 40 to 60 years, cars with minimal warranty costs in 3 to 15 years and 60, to 100 thousand miles, steel structures in the sea to last over 40 years, etc.
Working on model steam boilers, and having seen serious scalds and the permanent damage that steam can do to people, I prefer to work with at least a factor of safety of 6. Most regulations use FOS of 8. So I recommend on this engineering discussion site that a FOS of 8 it always required.
I just don't know how to calculate how safe your design of boiler will be...
K2

 

[Toymaker]

Thanks Toymaker. Yup. Read it, fell asleep. Not something I understand. But it seems you are trying to work right on the top limit for temperature and pressure. I.E. very close to the saturation line. Am I right in thinking that the liquid state is inside the saturation line and outside, it boils?

You're partially right. The working fluid, water or R123, is a liquid everywhere to the left of the saturation line. Everywhere inside the saturation line the fluid is part liquid & part gas; half way between the saturation lines yields 50% liquid and 50% gas. Everywhere to the right of the saturation line would see only a gas.

I think the whole "engineering" you are undertaking is culturally different to what I am familiar with. NASA, Racing teams, etc. Work right at limits where they have high risks (1/100 failures is accepted?), small safety margins, and get as close as they can to physical limits. Not my understanding.
My 45 year career was based on working in a very safe area, 1/ 10 million failures down to 6ppm failures, depending on whether items were critical or non-critical.... Factors of safety of 6 to 8, etc. Electrical switchgear to last 40 to 60 years, cars with minimal warranty costs in 3 to 15 years and 60, to 100 thousand miles, steel structures in the sea to last over 40 years, etc.
Working on model steam boilers, and having seen serious scalds and the permanent damage that steam can do to people, I prefer to work with at least a factor of safety of 6. Most regulations use FOS of 8. So I recommend on this engineering discussion site that a FOS of 8 it always required.
I just don't know how to calculate how safe your design of boiler will be...
K2

I know that I'm operating right on the edge of stress limits, but I do take safety precautions such as only operating my boiler when it's inside it's double walled housing of stainless steel.

 

[Steamchick]

Toymaker, It would be interesting to compare calculations, as I use this website to learn stuff I didn't experience in 45 years of paid Engineering. Do you feel able to post your design calculations on the strength of the copper flash boiler, so we can all learn?
Thanks,
K2

 

[HMEL]

It appears you have a type-o as you've labeled both values as Critical Temperature.
So, to clarify, for R123:
Critical Pressure is 532 psi.
Critical Temperature is 362.63F (183.68C)

High flow rate in a high heat flux environment is the key to preventing liquids from flashing into vapor, which is why I will need to switch over from the plunger feed pump I'm currently using for water, to a centrifugal pump for R123.

Now, just in cast you have insomnia tonight thinking about flash boilers, critical pressures, R123, etc., here's a little reading material that should put you right to sleep; it's Du Pont's Technical Information sheet on R123,....even includes a P-h diagram in imperial units at the end

enjoy

Depends on which side of the curve you ride. You are operating so close to the critical point it is doubtful you can keep the state parameters and most likely you could have a 10 % change in temperature without batting an eye. Not to mention the ability to control the operating pressure. But you can build it maybe it will work but the odds are against it.

 

[Toymaker]

Toymaker, It would be interesting to compare calculations, as I use this website to learn stuff I didn't experience in 45 years of paid Engineering. Do you feel able to post your design calculations on the strength of the copper flash boiler, so we can all learn?
Thanks,
K2

K2, Haven't we already beat this topic to death? A word search of this thread for "Barlow's formula" found 17 posts; "Tensile" shows up in 24 posts, and "yield" took up 3 pages of posts. But OK, just for you K2, I'll show my logic, reasoning, and numbers one more time.

Use Barlow’s formula to find max tube internal pressures: P = 2*T*S/D

Where:
P = Pressure (Note: my max pressure for R123 = 500 psi -- max pressure for steam = 400 psi)
T = wall Thickness: smallest measured: 0.024"
S = Yield: 11,000 psi ---- Tensile: 35,000 psi (From: Copper Tube)
D = Outside Diameter as measured: 0.625"

Note: Copper tube sold for AC use is not pure copper, it's an alloy. Therefore, the Yield and Ultimate Tensile strength numbers taken from copper tube suppliers are likely more accurate than using values for pure copper.

Yield Pressure = 844 psi || Ultimate Tensile Pressure = 2688 psi

Using 85% strength at 200C:
Yield Pressure = 718 psi || Ultimate Tensile Pressure = 2284 psi

Using 80% strength at 250C:
Yield Pressure = 675 psi || Ultimate Tensile Pressure = 2150 psi

While using water as the working fluid, boiler operating max limits will be 232C (450F) at 400 psi: these values have been used on multiple copper tube boilers of varying design in DIY steam autos and have proven to be safe.

I am 100% comfortable knowing that throughout initial testing using water/steam, my copper tube boiler has a safety margin of 275 psi just to exceed the yield strength,...and 1750 psi safety margin before the copper goes, "pop!".

 

[Steamchick]

Thanks,
I'm learning all the time. - Just missed the Engineering education by doing a Physics degree, that taught me loads about stuff that doesn't make things work (like "Model Engineering" does).
The simple thing is that I am experienced with products having factors of safety of 8 (or more), but you are working a bit closer to the limit so it makes me nervous.
At 200psi I would be using a tensile stress limit of 3500psi (ASME based) but you are using 35000psi. 10 times more than me. I think I understand it now.
But I think you are in control so wish you success. As NASA have been miss quoted "Failure is not an option" where boilers are concerned.
Enough said.
K2

 

[JLaning427]

Failure is ALWAYS an option!

It often has catastrophic consequences, but it is always possible, regardless of what NASA' Apollo 13 folks had to say.

J

 

[Steamchick]

I hate to think what may happen if Toymaker's flash boiler fails.. a Hot cloud of R123? I dont know if it would be more or less dangerous than steam. Steam is only a problem when you can't see it. As it expands and cools it becomes a visible white cloud. No idea what R123 would do. But Ftoymaker appears to be well in control of that...? Hopefully his controls and session will ensure no failures. Even with cars, and domestic appliances, we don't plan for failure. Just cry when it happens. Motorcyclists and rock climbers expect it to happen and spend all their time avoiding accidents.That's why they are usually smiling. Fun, a risk of danger and survival.
K2

 

[wce4]

I hate to think what may happen if Toymaker's flash boiler fails.. a Hot cloud of R123? I dont know if it would be more or less dangerous than steam. Steam is only a problem when you can't see it. As it expands and cools it becomes a visible white cloud. No idea what R123 would do. But Ftoymaker appears to be well in control of that...? Hopefully his controls and session will ensure no failures. Even with cars, and domestic appliances, we don't plan for failure. Just cry when it happens. Motorcyclists and rock climbers expect it to happen and spend all their time avoiding accidents.That's why they are usually smiling. Fun, a risk of danger and survival.
K2

"Steam is only a problem when you can't see it."
Ain't that the truth. Back in the early 80s in the engine room of The USS Austin main boiler feed pump blew. Had to stand fast and wait for the BT (boiler technician) to secure the steam to the pump, less get cut in half by the steam.
Luckily no one did.

 

[Toymaker]

"Steam is only a problem when you can't see it."
Ain't that the truth. Back in the early 80s in the engine room of The USS Austin main boiler feed pump blew. Had to stand fast and wait for the BT (boiler technician) to secure the steam to the pump, less get cut in half by the steam.
Luckily no one did.

I still recall watching a safety video in boot camp demonstrating the proper way to find steam leaks in overhead pipes; sweep the pipe with a broom and when the bristles are sliced off the broom and fall to the floor, you've found the leak

 

[JLaning427]

I hate to think what may happen if Toymaker's flash boiler fails.. a Hot cloud of R123? I dont know if it would be more or less dangerous than steam. Steam is only a problem when you can't see it. As it expands and cools it becomes a visible white cloud. No idea what R123 would do. But Ftoymaker appears to be well in control of that...? Hopefully his controls and session will ensure no failures. Even with cars, and domestic appliances, we don't plan for failure. Just cry when it happens. Motorcyclists and rock climbers expect it to happen and spend all their time avoiding accidents.That's why they are usually smiling. Fun, a risk of danger and survival.
K2

Out of curiosity, I went and looked at the SDS for R123. One I found was from Chemours. R123 is usually non-flammable, but it appeared that under certain circumstances (mixed with air and at some pressure) it is flammable. And if you overheat it (I want to say the threshold was around 300 degrees) it can break down into hydrochloric and hydrofluoric acids (and HF is NO joke, look it up).

Toymaker, good luck with your plan!


J

 

[Toymaker]

Out of curiosity, I went and looked at the SDS for R123. One I found was from Chemours. R123 is usually non-flammable, but it appeared that under certain circumstances (mixed with air and at some pressure) it is flammable. And if you overheat it (I want to say the threshold was around 300 degrees) it can break down into hydrochloric and hydrofluoric acids (and HF is NO joke, look it up).

Toymaker, good luck with your plan!


J

Yep, chemical decomposition of R123 begins at 250C, which is why I've set a max operating temperature of 184C, which is also the critical temperature. R123 can also affect your nervous system in high enough doses. These are all reasons why I'm using water for all my initial testing; I expect to have many hours of run time using steam before I switch over to R123.

 

[Steamchick]

Hi Toymaker, Please excuse the "stupid" question.... but if you are doing initial development with steam, I guess when you change to R123 there will be a performance increase? (different pressure, temperature and energy transfer?). How much more performance will you expect from the R123? (A bit like comparing petrol engines versus diesel engines? It's not just about raw power, but how it is delivered.).
I remember the turbine is quite simple for R123, and steam should need a more complex turbine/rotor to get best performance, but I understand steam is perhaps the safer option for initial shakedown of such a (new?) innovative system.
Working in car manufacturing I saw between 1986 and 2018 how emissions laws in Europe changed about every 10 years, so there were times when Petrol engines gave the better performance, then Diesel engines, etc. as the performance went up and down as regulations and technology changed a bit out of phase.. (and the picture is now more confusing with various Hybrids and Battery Electric cars!). But that was with different engines with the different fuels. I am assuming you'll be testing the R123 engine with steam initially? - So absolute performance will not be the target when using steam, but it should be OK for proving your controls and other features?
Hope this makes sense?
K2

 

[Toymaker]

Hi Toymaker, Please excuse the "stupid" question.... but if you are doing initial development with steam, I guess when you change to R123 there will be a performance increase?

Yes

(different pressure, temperature and energy transfer?).

Yes. Since I don't want to exceed 200C by too much, I will keep the steam temp at around 232C (450F) which should give me close to 400 psi. R123 at the slightly lower temp of 184C (363F) will give me 533 psi. Higher pressure in steam turbines always equate to greater power out.

How much more performance will you expect from the R123? (A bit like comparing petrol engines versus diesel engines? It's not just about raw power, but how it is delivered.).

Honestly, I haven't bothered to do those calculations

I remember the turbine is quite simple for R123, and steam should need a more complex turbine/rotor to get best performance, but I understand steam is perhaps the safer option for initial shakedown of such a (new?) innovative system.
Working in car manufacturing I saw between 1986 and 2018 how emissions laws in Europe changed about every 10 years, so there were times when Petrol engines gave the better performance, then Diesel engines, etc. as the performance went up and down as regulations and technology changed a bit out of phase.. (and the picture is now more confusing with various Hybrids and Battery Electric cars!). But that was with different engines with the different fuels. I am assuming you'll be testing the R123 engine with steam initially? - So absolute performance will not be the target when using steam, but it should be OK for proving your controls and other features?
Hope this makes sense?
K2

Starting out with steam instead of R123 allows me to find and fix any leaks using a non-toxic working fluid. And, of equal importance, steam allows me to make sure my software, pressure sensors, temperature sensors, and RPM sensors are all working.

I wont be too concerned with performance from the turbine using steam, as a 3 stage turbine is simply not capable of extracting all the energy in steam. R123 will have fully expanded by the time it exits the 3rd turbine stage and will be close to room temperature.

 

[Steamchick]

Thanks. That was more or less what I guessed. I guessed at leak testing and an instrumentation and control shakedown. I also guessed power would be serous down due to turbine configuration when using steam. But didn't know what the gas medium change and pressure change would do. Not important.
Thanks,
K2

 

[Toymaker]

I made a few necessary changes over the past few weeks as testing revealed the motor driving the wobble-plate air compressor and the motor driving the blower to be too small as they were both becoming excessively hot when pushed to their upper limits.

Here are the before and after photos showing the much larger motor driving the wobble-plate air compressor:

Before: Small motor using 3:1 speed reduction.


After: Larger motor and direct drive.

The larger motor is strong enough to directly drive the air compressor which resulted in the max air pressure increasing from 15 psi with the old motor, to 23 psi with the new, larger motor and direct drive. The new motor becomes only slightly warm when run continuously.

The new blower motor is rated for 18,000 rpm at 36vdc, and as shown in the below left photo, is somewhat larger than the motor it replaced.


The two new motors also meant modifying the software to provide correct air and fuel flows, and also necessitated pulling the burner assembly out of the boiler housing so that exhaust flames could be observed to adjust fuel-air combustion to maximize the amount of "blue" flame. All went smoothly and I will soon place the burner assembly back inside the boiler housing, but first, I want to measure the fuel burn rate 100% power setting.

Note: in the following two photos the stated percent of fuel and air refers to PWM percent used to drive the DC motors; therefore, 37% Fuel does NOT refer to an amount of fuel by weight or volume.

Below photo shows start-up settings: 37% Fuel & 15% Air


Below Photo shows 90% Fuel & 90% Air

 

[ajoeiam]

I made a few necessary changes over the past few weeks as testing revealed the motor driving the wobble-plate air compressor and the motor driving the blower to be too small as they were both becoming excessively hot when pushed to their upper limits.

Here are the before and after photos showing the much larger motor driving the wobble-plate air compressor:

Before: Small motor using 3:1 speed reduction.
View attachment 154301

After: Larger motor and direct drive.
View attachment 154302
The larger motor is strong enough to directly drive the air compressor which resulted in the max air pressure increasing from 15 psi with the old motor, to 23 psi with the new, larger motor and direct drive. The new motor becomes only slightly warm when run continuously.

The new blower motor is rated for 22,000 rpm at 36vdc, and as shown in the below left photo, is somewhat larger than the motor it replaced.
View attachment 154303 View attachment 154304

The two new motors also meant modifying the software to provide correct air and fuel flows, and also necessitated pulling the burner assembly out of the boiler housing so that exhaust flames could be observed to adjust fuel-air combustion to maximize the amount of "blue" flame. All went smoothly and I will soon place the burner assembly back inside the boiler housing, but first, I want to measure the fuel burn rate 100% power setting.

Note: in the following two photos the stated percent of fuel and air refers to PWM percent used to drive the DC motors; therefore, 37% Fuel does NOT refer to an amount of fuel by weight or volume.

Below photo shows start-up settings: 37% Fuel & 15% Air
View attachment 154315

Below Photo shows 90% Fuel & 90% Air
View attachment 154316

Fascinating work there!!! - - - - So what 'does' the 37% fuel refer to?
(I'm guess that 90% air means that its 90% of maximum air flow or ????????)

Did you print the centrifugal fan yourself?

TIA

 

[Toymaker]

Fascinating work there!!! - - - - So what 'does' the 37% fuel refer to?
(I'm guess that 90% air means that its 90% of maximum air flow or ????????)

Correct !! The stated percent of fuel or air refers to PWM (Pulse Width Modulation) percent used to drive the DC motors; therefore, 37% Fuel does NOT refer to an amount of fuel by weight or volume, but instead is a measure of how much power the fuel pump or air blower DC motors are being given from their power supplies. So, yes, 90% air means air flow is at 90% of the maximum possible air flow,...same with fuel flow.

Did you print the centrifugal fan yourself?

TIA

`
The entire blower assembly, encasement and fan, was re-purposed from a cheap cordless leaf blower, like the one shown below.

 

[ajoeiam]

Correct !! The stated percent of fuel or air refers to PWM (Pulse Width Modulation) percent used to drive the DC motors; therefore, 37% Fuel does NOT refer to an amount of fuel by weight or volume, but instead is a measure of how much power the fuel pump or air blower DC motors are being given from their power supplies. So, yes, 90% air means air flow is at 90% of the maximum possible air flow,...same with fuel flow.


`
The entire blower assembly, encasement and fan, was re-purposed from a cheap cordless leaf blower, like the one shown below.
View attachment 154319

Great idea - - - - thanks for confirming your technical terms!!!

Hmmmmm - - - - wonder how that blower would last on a serious furnace boiler?
(I wonder if replacing the bearings and perhaps stabilizing the impellor re: the housing would be enough to transform the blower from an occasion use thing to a serious long term use capable item - - - hmmmmmmmmmmm.)