Monotube Flash Boiler Design

[The Ignoble Troll]

Making Steam Again Video: I uploaded a short video to YouTube showing the steps I used to find the correct power level to drive the feed pump on my monotube boiler while holding the burner output steady at it's lowest setting. Adjusting Feed Pump flow rate
Next, I'll step through the same process, but with increasingly higher burner settings. This will allow me to collect the data I need to program the ECU.

Thank you for video documenting this. Your electronics work is awesome.

 

[Toymaker]

123 HP (91.7 KW) !!! (calculated)

The second test run was started at idle power level. From idle, I increased the feed pump from 5 to 10, which resulted in a mixture of water and steam exiting the boiler. I slowly increased the power until I could see only steam output from the boiler; that setting turned out to be 72% . The output steam didn't condense into a visible cloud for several meters, and the steam temperature was reading 140 C, so I decided to increase the feed pump; I got to 15 before I started to see noticeable traces of water in the steam, so I decreased the feed pump to 13, which produced a steady flow of dry steam at 130 C. I turned off the burner, leaving the feed pump un-touched, and waited for the water to flow smoothly from the boiler. The measured flow rate was 2.44 liters/min which equates to 322.8 lbs per Hr.
Given: 1 BHP = 34.5 lbs steam per Hr at atmospheric pressure and 100 C.
Therefore, at 72% power level, my boiler's output of 322.8 lbs/Hr divided by 34.5 = 9.36 BHP.
Since 1 BHP = 13.15 HP than 9.36 BHP x 13.15 HP/BHP = 123 HP at 72% output power.

 

[Steamchick]

Hmmm. Soon we will hear about "max. power" no doubt.
I just wonder, because I am an amateur and "simply don't know"....
The feed pump is a centrifugal pump, so I think perhaps the flow rate changes subject to the back-pressure on the pump...(?). So if the flow rate with burner ON" and just steam exiting the pipe at 130deg.C. has pressure "P" at the immediate boiler exit point, does the water without heat have the same pressure "P" - and therefore demonstrate the same mass flow?
If the pressures are different, I think there should be some extra conversion factor between water and steam conditions, due to back pressure affecting pump efficiency (leakage?)... but if the same, then Toymaker is right with his analysis. - I think?
I guess at full power we'll find out what's what because (if I remember correctly) Toymaker has already determined the power from fuel developed for his burner's max. fuel consumption.
Would the 72% power be appropriate to compare such figures?
Or maybe I am completely confused and we should measure the pressure at the outlet of the pump to see if the steam back-pressure has any influence on the pump's flow rate?
But "Well done Toymaker!"

K2

 

[The Ignoble Troll]

Hmmm. Soon we will hear about "max. power" no doubt.
I just wonder, because I am an amateur and "simply don't know"....
The feed pump is a centrifugal pump, so I think perhaps the flow rate changes subject to the back-pressure on the pump...(?).

Using a regulator with a tank return let's you access a constant output pressure off of a variable displacement pump, which with a fixed orrifice should equal linear throttling, not sure if one is in play here. Otherwise yes, the output of a variable displacement pump is on a curve with back pressure changing the flow non linearly.


Going to a positive displacement pump and a return to tank regulator will give one very precise linear flow control.


But to be honest it doesn't sound like it would be needed here because the electronic controls are theoretically going to be making continual adjustments.


Awesome project.

 

[Toymaker]

Hmmm. Soon we will hear about "max. power" no doubt.
I just wonder, because I am an amateur and "simply don't know"....
The feed pump is a centrifugal pump,

I do have hopes of one day using a centrifugal pump as the feedpump, but for now, the feedpump is a standard, 3 piston, positive displacement, pressure washer pump (see post #332).

so I think perhaps the flow rate changes subject to the back-pressure on the pump...(?). So if the flow rate with burner ON" and just steam exiting the pipe at 130deg.C. has pressure "P" at the immediate boiler exit point, does the water without heat have the same pressure "P" - and therefore demonstrate the same mass flow?
If the pressures are different, I think there should be some extra conversion factor between water and steam conditions, due to back pressure affecting pump efficiency (leakage?)... but if the same, then Toymaker is right with his analysis. - I think?
I guess at full power we'll find out what's what because (if I remember correctly) Toymaker has already determined the power from fuel developed for his burner's max. fuel consumption.

Would the 72% power be appropriate to compare such figures?

The difference in flame output between 72% and 100% is fairly large, so I suspect the difference in steam output will also be quite large,....time and testing will tell

Or maybe I am completely confused and we should measure the pressure at the outlet of the pump to see if the steam back-pressure has any influence on the pump's flow rate?
But "Well done Toymaker!"

K2

Thanks K2

By definition, 1 BHP is the power (energy) needed to evaporated 34.5 pounds of water, already at a temperature of 212° F, per hour, at atmospheric pressure. So, no pressure in from the feedpump, and no pressure out. My boiler has input and output pressure sensors, and so far, they're both reading "no pressure". Additionally, the pressure dial gage on feedpump is also reading zero.

Now, since I'm not feeding my pump water at 212° F, the calculations I've made for BHP are probably on the low side, as my feedwater is about 93° F, so I'm adding a lot of additional BTUs into the water just to raise the temp up to 212. So for now, I'm making ball-park-estimates.

 

[The Ignoble Troll]

My mistake, I thought the feed pump you were increasing was the fuel pump output not the water flow.

 

[NapierDeltic]

Maybe I'm wrong but I believe even swapping to final fluid with its own viscosity curve might require another final tuning - lots and lots of diagrams to get closer to the truth... Luckily, electronic control comes in support. Care should be taken, though, for stable vs unstable processes - which I, for myself, couldn't predict. Only experiments will show.

 

[Toymaker]

<snip>
I guess at full power we'll find out what's what because (if I remember correctly) Toymaker has already determined the power from fuel developed for his burner's max. fuel consumption.
<snip>

K2

Yes K2, I did post max fuel burn data back in post #405,...along with a short video of the burner output.

At 95% power setting the burner consumes 10 LPH (Liters Per Hour), which equates to 103 KWH or 138 HP-Hours.

It's gonna be interesting to see how many BHP I get at 100% burner output, because the numbers are already starting to look "off". Seems likely I will need to re-run some of my tests

 

[Steamchick]

OK, so 103kWh for 1 hour is running at 103kW... (or 138HP).
and your water flow prediction is 123HP at 72% pump flow. - suggesting 90% efficiency if the burner is at max, not "72%"... (I don't believe you have 125% efficiency, but suspect the pump is not 100% efficient? So the 72% isn't the same mass flow of water against "steam production back-pressure" as the mass flow of cold water issued from the pipe).
I agree that there is some calculation for a) heating water to boiling temperature - at the pressure in the boiler where it boils.
Then b) more heat (latent heat of vaporisation) to convert water to steam,
and more heat c) to finally heat the water if the boiling is not happening at the end of the boiler. (I.E. some superheating occurring if the boiler tubes contain steam for the final length that is heated, after steam is produced).
But these are standard calculations made for all boilers - using steam tables - so determining the steam temperature at ~130C and knowing the pressure - I missed that one?) should help you determine some of the numbers to do the sums?
Can you connect (say) a 5-gallon drum of water to the water pump inlet, then time the rate of emptying of cold water, and do the same when making steam? (using the same 72% settings, etc?). That should give the difference of flow against water and atmospheric pressure versus steam at 130C? (25psi?).
K2

 

[The Ignoble Troll]

OK, so 103kWh for 1 hour is running at 103kW... (or 138HP).
and your water flow prediction is 123HP at 72% pump flow. - suggesting 90% efficiency if the burner is at max, not "72%"... (I don't believe you have 125% efficiency, but suspect the pump is not 100% efficient?

Piston pumps, in the hydraulics world, generally have an efficiency in the low 90's.

Being and axial pump, 89%-91% would probably be an okay assumption.

When mathing out efficiency for a *quality* water axial pump I would typically use 90%.


1714 psi/gpm(constant for 100% efficient pump) *.9 = 1542psi/gpm per hp(745.7W)

So using 1542 as a constant, with a known pressure and wattage, should give the theoretical flow which can be cross checked against the pumps displacement at x RPM.

If the unloader is being used in throttling, then the above goes out the window unless you know if the back pressure is cracking the unload piston open a little or if the unloader is very sensitive and is simply open or closed.

I assume here that the unloader is fully closed as the back pressure would be very low unless there is a nozzle or other restriction inline to provide sensing back pressure.


For what it's worth, that axial pump has such a tiny heat sinking area that there should be no valve between it and the steam tubes, pwm (as you are using) is superior for flow control because back pressure opening the unloader is going to be stupidly hard on the suction and discharge valves due to water heating from the recycled unloader and the resulting cavitation of the heated water. (Around 53° most pumps start to die iirc). The water in an axial pump is also removing the heat from the wobble plate and pistons so you really want good flow.

You definitely don't want to be throttling with just a needle valve or similar inline restriction valve.


However:
Throttling with a restriction type valve can be done if you add an inline unloader down-stream from the pump and run the extra unloaders output through a long hose or into a reservoir to allow cooling. Note the down-stream unloader will need to be set much lower then the pump unloader.

While I am rambling:
Other factors that affect axial water pump longevity is that they typically (I've never seen one that does not) need a minimal inlet pressure and are unsuitable for suction from a reservoir because of cavitation, they really suck at pulling in water.

So if you are going to measure output volume, don't attach the pump suction side to a reservoir without an inline booster pump (something variable/dynamic like a centrifugalpump). Flow would be best measured at the output with the pump attached to your existent pressurized water supply ie. Your home.

Sorry if this is all known stuff.


Back to my bridge...

 

[Toymaker]

OK, so 103kWh for 1 hour is running at 103kW... (or 138HP).
and your water flow prediction is 123HP at 72% pump flow. - suggesting 90% efficiency if the burner is at max, not "72%"... (I don't believe you have 125% efficiency, but suspect the pump is not 100% efficient? So the 72% isn't the same mass flow of water against "steam production back-pressure" as the mass flow of cold water issued from the pipe).

Back pressure caused by steam production should show up on the feedwater pressure sensor and displayed on the LCD screen; also, the dial gauge mounted on the feed-pump should have shown something, but both gauges didn't move. I suspect that any steam produced backpressure was so small that it would not have reduced flow rate. Seems likely the problem is somewhere else.

I agree that there is some calculation for a) heating water to boiling temperature - at the pressure in the boiler where it boils.
Then b) more heat (latent heat of vaporization) to convert water to steam,
and more heat c) to finally heat the water if the boiling is not happening at the end of the boiler. (I.E. some superheating occurring if the boiler tubes contain steam for the final length that is heated, after steam is produced).
But these are standard calculations made for all boilers - using steam tables - so determining the steam temperature at ~130C and knowing the pressure - I missed that one?)

Yep,...the missing pressure is the problem. Watching the video, it certainly appears the steam, wet and dry, is exiting under at least a few psi, but all three of my pressure sensors show "0" psi.

should help you determine some of the numbers to do the sums?
Can you connect (say) a 5-gallon drum of water to the water pump inlet, then time the rate of emptying of cold water, and do the same when making steam? (using the same 72% settings, etc?). That should give the difference of flow against water and atmospheric pressure versus steam at 130C? (25psi?).
K2

I've already been using a water bottle as my feed water supply, (it's the 5 gallon, clear plastic, water-cooler bottle in the lower left corner in the photo in post #514). The tests I've run to date have been too short to allow the bottle to be a useful measuring tool.

 

[The Ignoble Troll]

Back pressure caused by steam production should show up on the feedwater pressure sensor and displayed on the LCD screen; also, the dial gauge mounted on the feed-pump should have shown something, but both gauges didn't move. I suspect that any steam produced backpressure was so small that it would not have reduced flow rate. Seems likely the problem is somewhere else.



Yep,...the missing pressure is the problem. Watching the video, it certainly appears the steam, wet and dry, is exiting under at least a few psi, but all three of my pressure sensors show "0" psi.

It's exciting with velocity with no back pressure because the system is totally open.

Even if the exit tempurature was steam at 200°C, it is just showing the thermal pick up by the rapidly expanding, unrestricted, water vapour.

If you restricted the output, say with a critical flow nozzle, you would have back pressure as the water vapour/water would have a velocity reduction and would switch to mass flow at the choke...

At that point you would see back pressure as you would be using pressure to suppress the waters boiling point.


Otherwise the steam just vents at atmospheric pressure with substantial, relative, velocity.

 

[Steamchick]

Thanks Troll. I think I understand your description of the effect of what is being tested, but how does that relate to the Enthalpy calculation so we do have a calculation with more power out than we know is going in? Toymaker is clever, but creating energy blows away all human knowledge of thermodynamics I think? - Hence I think we have got something wrong in the measurement (well, Toymaker) or (My?) calculations.
I think the difference is that when pumping cold water, the pump achieves greater mass flow than when pumping against the hot boiler converting the water to steam. I think it needs a pressure sensor telling us pressure at the boiler outlet? - Not assume it is atmospheric, because there will be a pressure drop down the pipe as the steam expands and accelerates.
Can anyone who is familiar with the maths help us (me) here?
Ta,
K2

 

[Toymaker]

Thanks Troll. I think I understand your description of the effect of what is being tested, but how does that relate to the Enthalpy calculation so we do have a calculation with more power out than we know is going in? Toymaker is clever, but creating energy blows away all human knowledge of thermodynamics I think? - Hence I think we have got something wrong in the measurement (well, Toymaker)

Back in post #528 I mentioned, something was amiss with my power numbers, and that it seems likely I will need to rerun some tests.

or (My?) calculations.
I think the difference is that when pumping cold water, the pump achieves greater mass flow than when pumping against the hot boiler converting the water to steam. I think it needs a pressure sensor telling us pressure at the boiler outlet?

The boiler has pressure sensors on both input and output (from post #362)

, but since there's no restrictors in the tube, both sensors read "0".

- Not assume it is atmospheric, because there will be a pressure drop down the pipe as the steam expands and accelerates.
Can anyone who is familiar with the maths help us (me) here?
Ta,
K2

 

[The Ignoble Troll]

Thanks Troll. I think I understand your description of the effect of what is being tested, but how does that relate to the Enthalpy calculation so we do have a calculation with more power out than we know is going in? Toymaker is clever, but creating energy blows away all human knowledge of thermodynamics I think? - Hence I think we have got something wrong in the measurement (well, Toymaker) or (My?) calculations.
I think the difference is that when pumping cold water, the pump achieves greater mass flow than when pumping against the hot boiler converting the water to steam. I think it needs a pressure sensor telling us pressure at the boiler outlet? - Not assume it is atmospheric, because there will be a pressure drop down the pipe as the steam expands and accelerates.
Can anyone who is familiar with the maths help us (me) here?
Ta,
K2

You are both clever! No doubt there.

But right now my money is also on the steam being a negitive pressure at the exit of the tube due to speed.


The axial pump is one of the best positive displacement pumps in existence. Basically 3 syringes, so long as it is getting sufficient flow to avoid drawing vacuum bubbles at each intake, then the output should be 100% perfectly in tune with its driven RPM.



I'd suggest measuring the pump outflow for a given number of rotations.


I'd also suggest adding a tyre stem valve to the reservoir and putting the feed water under just a few psi with a bicycle pump. Nothing crazy just like 10-12psi to help force the water into the pump, because, again, those pumps suction is very poor.


Do you have a link to where you bought the pump so I can look at it's stats?


Pumps don't develop pressure, down stream plumbing does and it's industry normal for steam and hot water units to operate with a pressurized boiler, because pumping cold water is so much easier vs pumping from a heated water source which makes the pump flow dynamic as the water vapour pressure is so much higher.

I'd suggest establishing mass flow measurements are critical here.

 

[Toymaker]

You are both clever! No doubt there.

But right now my money is also on the steam being a negitive pressure at the exit of the tube due to speed.

I agree. The outlet pressure does, and should, read 0, or perhaps even a bit negative.
But I also agree with K2 that the input pressure should show at least a small pressure as the feed pump pushes against the steam pressure inside the boiler tubes. I suspect that pressure is so low that my sensors are not seeing it.

The axial pump is one of the best positive displacement pumps in existence. Basically 3 syringes, so long as it is getting sufficient flow to avoid drawing vacuum bubbles at each intake, then the output should be 100% perfectly in tune with its driven RPM.

I'd suggest measuring the pump outflow for a given number of rotations.

Much easier said than done The best I could to do would be to measure RPM & volume flow/time.

I'd also suggest adding a tyre stem valve to the reservoir and putting the feed water under just a few psi with a bicycle pump. Nothing crazy just like 10-12psi to help force the water into the pump, because, again, those pumps suction is very poor.

One of the advantages of using a pressure washer pump is that it was made to be connected to standard house and yard plumbing connections, so when I want a little psi on the input, I connect it to one of my outdoor water connectors and open the valve; instant pressure. This particular model does suck water from a bucket or water bottle.

Do you have a link to where you bought the pump so I can look at it's stats?

See post #332 to see the converted Toma pressure washer with it's 36 VDC motor. Here's a photo which includes specs. This pressure washer is widely available online and in some big-box stores, but since it's sold as a home use pressure washer, specs like volume per revolution are not available

Pumps don't develop pressure, down stream plumbing does and it's industry normal for steam and hot water units to operate with a pressurized boiler, because pumping cold water is so much easier vs pumping from a heated water source which makes the pump flow dynamic as the water vapour pressure is so much higher.

I'd suggest establishing mass flow measurements are critical here.

 

[The Ignoble Troll]

Thanks, I did check that post but the numbers make no sense.

It would need to draw 17amps, and put out like 10hp to be 5,800 psi at 2.7 gpm

(5,800 • 2.7/ 1543 = 10.14)

So I assume there is some embellishment.

Can you measure the RPM of the original motor?

Is their ANY numbers stamped into the pump? They frequently have the dispacment encoded and stamped on the pump body.


It might be fine sucking from a bottle, but if you hear any slight roughening of it's noise as you dial up the flow, you will know you are producing vacuum bubbles.

The safest number to rely on for its output is the published gpm because that is used for nozzle orifice selection - so making the assumption that it is correct and having the stock motors rpm gives you displacement.

 

[The Ignoble Troll]

Oh and BTW, nice conversion! Very clever to use a DC motor on a stock positive displacement pump for your tests.

 

[The Ignoble Troll]

Do you have safety valving in place yet?


Seeems to me...

If you do then a restriction at the boilers exit, such as a venturi or delaval, would let you build back pressure and provide a set exit speed for the steam, if you design it right.

A venturi would let you use a manometer to measure vacuum and derive mass flow

A de laval would let you use pages of equations, but would be the loudest therfore the most fun.

From that you could calculate the mass flow.

www.engineeringtoolbox.com/amp/speed-sound-d_519.html

https://www.efunda.com/formulae/fluids/venturi_flowmeter.cfm

 

[Toymaker]

Thanks, I did check that post but the numbers make no sense.

It would need to draw 17amps, and put out like 10hp to be 5,800 psi at 2.7 gpm

(5,800 • 2.7/ 1543 = 10.14)

So I assume there is some embellishment.

Chinese marketing,...all too common to inflate the specs.

Can you measure the RPM of the original motor?

No. The pump housing (actually the swash plate housing) also served as the front plate for the motor, and contained the front bearing. Since the original motor no longer had a front plate or bearing, I tossed it to a motor recycling collector.

Is their ANY numbers stamped into the pump? They frequently have the dispacment encoded and stamped on the pump body.

This is the only marking I could find; FB-2. Does it mean anything to you??

It might be fine sucking from a bottle, but if you hear any slight roughening of it's noise as you dial up the flow, you will know you are producing vacuum bubbles.

Good advice, thanks

The safest number to rely on for its output is the published gpm because that is used for nozzle orifice selection - so making the assumption that it is correct and having the stock motors rpm gives you displacement.