Monotube Flash Boiler Design

[HMEL]

I'm sure you really meant to say the opposite; that pH values below 7 are acid and above 7 are bases.



Stanley Steam cars would simply open a valve and vent the boiler water every few hundred miles,...no reason I cannot do the same with my boiler. Then refill with a few gallons of distilled water. Cheap and easy.

Yes, meant to say the other way around.

 

[HMEL]

Are you sure about these two statements?

No, thinking about vertical sale its reversed numerically so I wrote that wrong. Should be number less then 7 acidic greater then 7 basic.

 

[HMEL]

I'm sure you really meant to say the opposite; that pH values below 7 are acid and above 7 are bases.



Stanley Steam cars would simply open a valve and vent the boiler water every few hundred miles,...no reason I cannot do the same with my boiler. Then refill with a few gallons of distilled water. Cheap and easy.

As I understand the Stanley steamer it was a fire tube boiler with a steam generating system space. You are making a water tube boiler and so far I have not seen a drum or vessel to release the steam, The steam simply exits the piping. Steam will be formed along the walls and flashed out the nozzle. It is in these locations solids will be deposited. In a boiler with a drum the solids will accumulate and will be removed by the blowdown based on cycles of concentration. Practically a TDS test will make sure they are under control. The steam will exit above the waterline and is often scrubbed with screens and a cyclone to prevent carryover. A fire tube boiler will also have a similar situation except the heating surface goes through the vessel but it also has a water level. Because of this I see no way to remove the solids because once they are deposited they are staying there. You can calculate how fast this will occur based on the water quality and steaming rate.

 

[Toymaker]

As I understand the Stanley steamer it was a fire tube boiler with a steam generating system space. You are making a water tube boiler and so far I have not seen a drum or vessel to release the steam, The steam simply exits the piping. Steam will be formed along the walls and flashed out the nozzle. It is in these locations solids will be deposited. In a boiler with a drum the solids will accumulate and will be removed by the blowdown based on cycles of concentration. Practically a TDS test will make sure they are under control. The steam will exit above the waterline and is often scrubbed with screens and a cyclone to prevent carryover. A fire tube boiler will also have a similar situation except the heating surface goes through the vessel but it also has a water level. Because of this I see no way to remove the solids because once they are deposited they are staying there. You can calculate how fast this will occur based on the water quality and steaming rate.

You are of course correct about the Stanley using a fire tube boiler; I should have used the Doble or the SES as my example, as both used monotube boilers. I don't believe either of these cars used a drum either, but I'm not absolutely certain of that.

Perhaps it's my lack of experience with steam generators, but I'm having a difficult time understanding how the water in a relatively small, closed, steam system becomes contaminated. If I start by using distilled water, where do the minerals or other contaminates, which potentially coat boiler-tube walls, come from?

 

[HMEL]

You are of course correct about the Stanley using a fire tube boiler; I should have used the Doble or the SES as my example, as both used monotube boilers. I don't believe either of these cars used a drum either, but I'm not absolutely certain of that.

Perhaps it's my lack of experience with steam generators, but I'm having a difficult time understanding how the water in a relatively small, closed, steam system becomes contaminated. If I start by using distilled water, where do the minerals or other contaminates, which potentially coat boiler-tube walls, come from?

It comes in with the distilled water. Even though its distilled it will carry with it some dissolved solids and some entrained gases. Once in the boiler if not removed it will deposit somewhere. Now the boiler is steaming and it brings in more water and it gets another round of solids although small to be deposited on the wall. And this will keep happening. Eventually over time these small concentrations add the volume left behind begins to grow. The time it takes depends on the steaming rate and the makeup for any losses you have. So it now depends on the quality of the water how long this takes. Now there is a belief that de-ionized water is pure but this is not the case. It depends on how the water is made because if its pure it will rapidly dissolve almost anything thus the name aggressive water. Water is a very good solvent because of the molecular polarity of the H2O molecule. Now for a small hobby boiler with very limited run times such as you find on model boats it is not an issue. But when you are building something for power over a sustained time its a different story. And even if you add distilled water and want to raise the ph you will need to add a buffering salt which adds to the difficulty. But there is always the option is run until failures occur in the tube and then replace them. By the way based on the power you hope to develop I would not consider it a small system.

 

[Toymaker]

It comes in with the distilled water. Even though its distilled it will carry with it some dissolved solids and some entrained gases. Once in the boiler if not removed it will deposit somewhere. Now the boiler is steaming and it brings in more water and it gets another round of solids although small to be deposited on the wall. And this will keep happening. Eventually over time these small concentrations add the volume left behind begins to grow. The time it takes depends on the steaming rate and the makeup for any losses you have. So it now depends on the quality of the water how long this takes. Now there is a belief that de-ionized water is pure but this is not the case. It depends on how the water is made because if its pure it will rapidly dissolve almost anything thus the name aggressive water. Water is a very good solvent because of the molecular polarity of the H2O molecule. Now for a small hobby boiler with very limited run times such as you find on model boats it is not an issue. But when you are building something for power over a sustained time its a different story. And even if you add distilled water and want to raise the ph you will need to add a buffering salt which adds to the difficulty. But there is always the option is run until failures occur in the tube and then replace them. By the way based on the power you hope to develop I would not consider it a small system.

So, considering the amount of contaminants brought in via distilled water will be miniscule, and considering that my boiler will be operating only a few hours each day, max, seems likely that even without blow-downs, I can look forward to many, many years of trouble-free operation before any appreciable amounts of scale build up. This is essentially what TimTaylor said back in post #303.

I'm not building a boiler that will operate 24-7 for weeks or months at a time,...more of a recreational vehicle that will be taken out for pleasure trips

 

[ajoeiam]

No, thinking about vertical sale its reversed numerically so I wrote that wrong. Should be number less then 7 acidic greater then 7 basic.

Wiping brow - - - relief - - - - I was hoping that I hadn't mis-remembered my very many days ago chemistry!

 

[Toymaker]

For those interested, Jay Leno demo's his fully-restored Doble E-20 steam car in this video: Doble E-20 Steam Car
Jay demo's how the blow down is performed 26 minutes into the video; a valve is opened and steam vents onto the ground, under the car.

 

[HMEL]

So, considering the amount of contaminants brought in via distilled water will be miniscule, and considering that my boiler will be operating only a few hours each day, max, seems likely that even without blow-downs, I can look forward to many, many years of trouble-free operation before any appreciable amounts of scale build up. This is essentially what TimTaylor said back in post #303.

I'm not building a boiler that will operate 24-7 for weeks or months at a time,...more of a recreational vehicle that will be taken out for pleasure trips

I admire your confidence.
And with a ph of 7, oxygen , copper tubes, high heat flux, and scaling
I will use the quote from Jimmy's World on youtube
What could possibly go wrong!
Have fun
Tale Care HMEL

 

[Toymaker]

I admire your confidence.
And with a ph of 7, oxygen , copper tubes, high heat flux, and scaling
I will use the quote from Jimmy's World on youtube
What could possibly go wrong!
Have fun
Tale Care HMEL

HMEL, with the exception of copper (Doble used steel tubes) you're describing Doble's monotube boiler

Doble cars used water from a garden hose to fill the monotube boiler and for make-up water, so lots of oxygen, minerals, and a pH of something close to 7; scaling was pretty much certain. The burner generated in excess of 1 million BTU, so, very high heat flux.

Doble's cars were engineering master pieces. I'm honored you would place me in that same category

 

[Toymaker]

Boiler input & output temperature & pressure sensors partially installed and calibrated.

I modified two AN-10 male to female adapters with Fuel Gauge Port by adding a PT100 temperature sensor (RTD) and then calibrating them in the software to display temperature in Centigrade and Pressure in PSI. A pressure sensor is screwed into the fuel gauge port. The physical mods to the AN-10 adapter are shown below. One adapter will be placed between the feed pump output and the boiler's input, while the other adapter will placed on the boiler's steam output just before the boiler's control valve.

These sensors allow the DDC (ECU) to read feed pump output pressure, feed water temperature, and steam output pressure & temperature.

 

[Toymaker]

Feed Pump using Pressure Washer

I finally found a feed pump solution I mostly like. It's much heavier than I wanted, but It's capable of much more pressure (400 Bar, 5800 psi) and higher flow rate than my boiler needs, but since the output is controllable from 0 to 3000 psi, in this case, over-capacity is not necessarily bad.

Using an adapter plate and spindle attachment, I was able to mount the pump section from the pressure washer shown below, onto a 36 VDC motor rated at 1 kW. As a test, (and to make my wife happy), I spent about an hour pressure washing a concrete patio using the 36 VDC motor powering the 3-plunger pump to 20 MPa (2900 psi). The motor is powered by a PWM speed control board and a 36 VDC switching power supply. Now that my pressure washer cleaning duties are complete, I have replaced the 60 MPa gauge shown in the pics, with a 0 to 700 psi gauge, which is the pressure range I expect to see on the boiler. My next choir will be attaching the needed high pressure tubes and fittings between pump and boiler.

 

[ajoeiam]

Feed Pump using Pressure Washer

I finally found a feed pump solution I mostly like. It's much heavier than I wanted, but It's capable of much more pressure (400 Bar, 5800 psi) and higher flow rate than my boiler needs, but since the output is controllable from 0 to 3000 psi, in this case, over-capacity is not necessarily bad.

Using an adapter plate and spindle attachment, I was able to mount the pump section from the pressure washer shown below, onto a 36 VDC motor rated at 1 kW. As a test, (and to make my wife happy), I spent about an hour pressure washing a concrete patio using the 36 VDC motor powering the 3-plunger pump to 20 MPa (2900 psi). The motor is powered by a PWM speed control board and a 36 VDC switching power supply. Now that my pressure washer cleaning duties are complete, I have replaced the 60 MPa gauge shown in the pics, with a 0 to 700 psi gauge, which is the pressure range I expect to see on the boiler. My next choir will be attaching the needed high pressure tubes and fittings between pump and boiler.

View attachment 152770 View attachment 152771

View attachment 152772

View attachment 152773

Ideas - - - man - - - more ideas - - - wonder what the longevity of that girl would be if it were running 24/7 - - - hmmmmm?

 

[Toymaker]

Ideas - - - man - - - more ideas - - - wonder what the longevity of that girl would be if it were running 24/7 - - - hmmmmm?

The end use for my steam engine is to place it into a recreational vehicle, meaning it will never be run 24/7. All modern automotive monotube boilers use plunger pumps; I'm simply following their proven track record. Finally, from Google, typical pressure washer lifespan is between 60 to 500 hours, depending on build quality; and then, it's only O-rings and seals that need replaced.

Do you have any suggestions for a better feed pump ?

 

[Toymaker]

Hydraulic Power Equation: Power = (P x Q)/600

Where: Power is in kW
P = Pressure in Bars
Q = Flow in Liters per minute

I stumbled upon the above formula a few weeks ago,...would have saved me a lot of time and a bit of money had I found it a few months ago,...but, better late than never.

The design max output pressure of my boiler is set at 550psi (38 Bar). Given that feed pump pressure should be greater than boiler steam output pressure, and allowing for a generous increase of pump pressure, I used 55 Bar for the feed pump. From earlier calculations, the max expected flow rate for the boiler is 8 LPM (liters per minute).

Therefore: (55 Bar x 8 LPM) / 600 = 0.73 kW

Ignoring friction and other efficiency losses, the absolute max power needed from the motor powering the hydraulic pump is 730 Watts. As luck would have it, I had a 1 kW, 36 VDC motor laying around my workshop, which is the motor shown in post #332.

 

[ajoeiam]

The end use for my steam engine is to place it into a recreational vehicle, meaning it will never be run 24/7. All modern automotive monotube boilers use plunger pumps; I'm simply following their proven track record. Finally, from Google, typical pressure washer lifespan is between 60 to 500 hours, depending on build quality; and then, it's only O-rings and seals that need replaced.

Do you have any suggestions for a better feed pump ?

Not really - - - - just wondering what is used by the big boys - - anyone out there know of a way of finding out?

 

[Toymaker]

Not really - - - - just wondering what is used by the big boys - - anyone out there know of a way of finding out?

Not sure what you mean by "big boys", but the closest comparable commercially produced monotube boiler to mine was made by the SES company for their mid-1970s steam car. You can view and download the official government assessment of this project, which gives fairly complete details on the feed pump; SES Final Report. Discussion of the feedwater pump starts on page 18.

Stanley Steamers also used positive displacement piston pumps,...Stanley Steamer Feed Pumps.

As for my pump's life expectancy: in my application, the pump is driven by a variable speed DC motor which will be running at far lower RPMs than in it's designed pressure washer capacity where it runs at 3000 RPM whenever the AC motor is turned on. When used as a pressure washer, the pump delivers 400 Bar (5800 psi); my application as a feed pump will see the pump delivering a max pressure of 55 Bar (800 psi),...and for most of the run-time the pump will be asked to deliver a nominal 34 Bar (500 psi), resulting in far less wear on the pump, and extending the pump's maintenance period.

 

[user 52078]

I think the pressure washer pump is an excellent solution. The range of flows tends to be fairly limited, and generally higher than needed for a boiler, but if you don't mind gearing down or running a DC drive that is not great bother. Here in UK many of them come from Italy, there are some other models here:
Interpump pumps
It might give you inspiration for alternatives if needed.
I agree that running it much slower and at reduced pressure from rated should give it a good life. And when it wears out, they tend to be cheap as chips because they are manufactured by the squillion. What's not to like. I doubt you could make something from raw material for a similar cost.
Martin

 

[peterl95124]

Feed Pump using Pressure Washer
Now that my pressure washer cleaning duties are complete, I have replaced the 60 MPa gauge shown in the pics, with a 0 to 700 psi gauge, which is the pressure range I expect to see on the boiler.

I hope you do remember why the oxygen tank on Apollo-13 exploded :-( !!!

 

[Toymaker]

I hope you do remember why the oxygen tank on Apollo-13 exploded :-( !!!

I'm not sure what point you're trying to make, as final reports of the Apollo-13 incident determined that a routine stir of an oxygen tank ignited damaged wire insulation inside the tank, causing tank pressure to rapidly increase resulting in an explosion.

IF, you're drawing attention to the possibility that my feed pump is quite capable of over-pressuring the boiler tubes, thereby rupturing them, I've addressed that possibility by installing pressure and temperature sensors on the feedwater input to the boiler and the steam output as shown in post #331 above. The ECU monitors these pressures and temperatures and prevents feed pump induced over-pressure conditions by reducing feed pump RPM whenever pressures reach the max limit. As an additional safety precaution, a purely mechanical pressure release poppet valve is installed on the steam output side of the boiler.