Monotube Flash Boiler Design

The Ignoble Troll said:

Glad it did not zap you.

Would a fly back work?

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Yes, a flyback transformer would also work,...but I don't have one of those in my spare parts box either. So, I'ld need to order a flyback and incur the same week to 10 day shipping delay. Having a short inside the coil's insulation is pretty rare,...hopefully it's a one-time occurrence for me.

Toymaker said:

Yes, a flyback transformer would also work,...but I don't have one of those in my spare parts box either. So, I'ld need to order a flyback and incur the same week to 10 day shipping delay. Having a short inside the coil's insulation is pretty rare,...hopefully it's a one-time occurrence for me.

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If it happens again, the old school neon sign transformers work well.

Shortly after typing my last post where I mentioned how rare it is to have coil wires inside a transformer develop a short, I remembered that I never had a problem with the coils (aka transformers) on the two previous high voltage boards, which suffered shorts on the high voltage circuit boards, but the coils were undamaged. So I dug through my spare parts box, and lucky me, the coils on the dead circuit boards were the same as the shorted coil. I was able to replace the shorted bad coil with a good one from the bad circuit board, and now my ignitor board is back up and working

So out to the test stand to make a little steam.

Well, a little steam was all I could get. The burner flame keeps going out periodically, causing an auto re-start sequence. So, I'm sitting in front of my computer trying to understand why this is happening.

I've got some ideas,...so back to the test stand for a little investigation.

Problem found and fixed A tiny, nearly invisible tear developed in the silicone tube I use for the fuel line, and because the fuel is sucked into the fuel nozzle via venturi effect, and not forced into the nozzle by a fuel pump, air was being sucked into the fuel line through the tiny tear, creating air bubbles in the line which, upon reaching the fuel nozzle, cause the burner flame to go out. Sometimes, it's the littlest things that cause the biggest headaches.

Toymaker said:

Problem found and fixed A tiny, nearly invisible tear developed in the silicone tube I use for the fuel line, and because the fuel is sucked into the fuel nozzle via venturi effect, and not forced into the nozzle by a fuel pump, air was being sucked into the fuel line through the tiny tear, creating air bubbles in the line which, upon reaching the fuel nozzle, cause the burner flame to go out. Sometimes, it's the littlest things that cause the biggest headaches.

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Nice catch!

@Toymaker

Just a thought...

You have a burn can ECU, you know electronics and your stated goal is to move lots of air...

What about ditching the boiler and getting two of those cheap Amazon/Ebay small turbo chargers.

Rig up one turbo charger as a turbine engine with your flame can burner and then use the second turbo charger as your air blower.

You could use water injection to get more power and help keep the turbos cool.

You could also use the tesla turbine on the final turbo, to power an oil coolant/lubrication pump.

You could ditch the leaf blower this way.


This would take a lot of phase changes out of the equation which would increase efficiency.

If you reworked the final turbo to have a longer shaft, you could have it directly turning a larger blower.

The Ignoble Troll said:

@Toymaker

Just a thought...

You have a burn can ECU, you know electronics and your stated goal is to move lots of air...

What about ditching the boiler and getting two of those cheap Amazon/Ebay small turbo chargers.

Rig up one turbo charger as a turbine engine with your flame can burner and then use the second turbo charger as your air blower.

You could use water injection to get more power and help keep the turbos cool.

You could also use the tesla turbine on the final turbo, to power an oil coolant/lubrication pump.

You could ditch the leaf blower this way.


This would take a lot of phase changes out of the equation which would increase efficiency.

If you reworked the final turbo to have a longer shaft, you could have it directly turning a larger blower.

Click to expand...

One turbo might be enough, just take bleed air from the compressor before it goes to the combustion chamber.

The Ignoble Troll said:

@Toymaker

Just a thought...

You have a burn can ECU, you know electronics and your stated goal is to move lots of air...

What about ditching the boiler and getting two of those cheap Amazon/Ebay small turbo chargers.

Rig up one turbo charger as a turbine engine with your flame can burner and then use the second turbo charger as your air blower.

You could use water injection to get more power and help keep the turbos cool.

You could also use the tesla turbine on the final turbo, to power an oil coolant/lubrication pump.

You could ditch the leaf blower this way.


This would take a lot of phase changes out of the equation which would increase efficiency.

If you reworked the final turbo to have a longer shaft, you could have it directly turning a larger blower.

Click to expand...

I like you're outside-the-box thinking,....but,.... turbo chargers use radial inflow hot-section turbine wheels which are very sensitive to back pressure. If you use the exhaust from a radial inflow turbine to drive a second radial inflow turbine, the first turbine will simply stop working; jet engine designers discovered this problem in the early days of jet engine design.

Almost all RC jet engines marketed today use the design you've described (without the second turbine), and they are all fuel hogs as this simple design is not very fuel efficient.

Toymaker said:

I like you're outside-the-box thinking,....but,.... turbo chargers use radial inflow hot-section turbine wheels which are very sensitive to back pressure. If you use the exhaust from a radial inflow turbine to drive a second radial inflow turbine, the first turbine will simply stop working; jet engine designers discovered this problem in the early days of jet engine design.

Almost all RC jet engines marketed today use the design you've described (without the second turbine), and they are all fuel hogs as this simple design is not very fuel efficient.

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There are people who've succeeded in building free-turbine turboshafts from turbochargers, so I think that radial flow turbines can work with another stage downstream (the one I've seen online used an axial flow power turbine- routing the exhaust back out for a second radial stage is rather inconvenient).

RC jets generally use an axial flow turbine, probably for packaging reasons (makes the back of the engine narrower).

The lousy fuel economy is because R/C turbines have poor overall pressure ratios, rarely better than 4:1 even in the largest examples. But I'd be hesitant to say that a small steam turbine could be better, especially if you have no condenser. The Brayton cycle has some significant advantages over the Rankine cycle in terms of efficiency potential (no losses to latent heat of vapourisation and generally higher temperature of the hot side, mostly).

Now if you want to be a smart Alec, you might note that the exhaust from a gas turbine is hot enough to run a steam boiler...

Nerd1000 said:

There are people who've succeeded in building free-turbine turboshafts from turbochargers, so I think that radial flow turbines can work with another stage downstream (the one I've seen online used an axial flow power turbine- routing the exhaust back out for a second radial stage is rather inconvenient).

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Hmmm, so the one you saw online used the exhaust from an axial flow turbine to power a radial in-flow turbine ?
I doubt you will be able to find a single commercial jet engine using a radial in-flow hot section turbine followed by a second hot section turbine either radial or axial.

Nerd1000 said:

The lousy fuel economy is because R/C turbines have poor overall pressure ratios, rarely better than 4:1 even in the largest examples.

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Certainly low pressure from the single stage compressor is a contributing factor to low fuel efficiency, but it's not the only factor. Another problem comes from compressor and turbine blade operational clearances which are much the same for both RC engines and larger aircraft engines. In a large aircraft engine the required clearance between the rotating blades and the stationary wall is tiny when compared to blade length. However, that same clearance becomes a much larger percentage of overall blade length in the much smaller RC engines.

Another inefficiency suffered by small RC engines is a direct result of the limitations of metallurgy. Even in aircraft jet engines, the vast majority of air moving through the engine is used for cooling purposes and is not a part of the combustion process taking place in the burner. Even the best turbine blades used today cannot survive adiabatic flame temperatures from burning kerosene, therefor, hot combustion gases must be cooled by mixing them with air from the compressor before they impact the turbine blades. Blades used in larger aircraft engines are constructed with internal air channels which route air from the compressor through the inside of the blades and out through tiny holes on the leading edge of each blade, thereby further cooling the blades, buffering them from hot combustion gases. Finally, during the fabrication process, commercial turbine blades are subjected to near melting temperatures in a kiln for several hours and are allowed to cool in such a manor as to force the blade's metal to become a single crystal. Crystals are much more heat resistant than other metal forms and can withstand higher temperatures than castings or worked metal.

All of these additional processes allow commercial engines to operate at much higher TITs than RC engines. (TIT = Turbine Inlet Temperature), and in the jet engine world, higher TITs result in greater efficiency. At the time I stopped working in the gas turbine industry (40 years ago), I knew that both GE and Garrett were testing all-ceramic turbine blades in pursuit of higher TITs,...I don't know how successful they were.

RC engines do not use these advanced manufacturing processes or internal blade cooling designs, and therefore must run at lower TITs, resulting in lower efficiency.

Nerd1000 said:

But I'd be hesitant to say that a small steam turbine could be better, especially if you have no condenser. The Brayton cycle has some significant advantages over the Rankine cycle in terms of efficiency potential (no losses to latent heat of vapourisation and generally higher temperature of the hot side, mostly).

Click to expand...

I think you'll find that the advantages of higher temperatures in the Bryton cycle are more than offset by the substantially higher gas pressures found in the Rankine cycle. Compressor outlet pressure in gas turbine engines almost never exceed about 300 psi compared to 3000 psi found in many steam turbines. Higher pressure means higher mass flow through the turbine, resulting in greater power density and greater efficiency.

I have no interest in doing a math comparison between Bryton and Rankine cycles. For me, it's not a question of which one is the most efficient, but rather a question of which engine can I actually build in my DIY shop.