So I've been too busy and tired from working on my new house to really do much work on my steam projects besides a few things here and there. Unfortunately, that's given me a lot of time to think about things. See, I'm kinda lazy, and I'm always trying to come up with ideas to repurpose off the shelf industrial and consumer parts for steam. Effort and cost are higher priorities than efficiency at this point. Existing industrial practices are usually well rounded and good enough, and an ounce of electronic control can often replace a pound of mechanical control. Anyways, I had some crazy ideas to make an easier to fabricate boiler. I'm currently aiming for around 15kW thermal with my pellet burner.

First, since I already have a forced circulation setup that seems to pump fairly well, I'm looking at off the shelf sources of heating coils. The last generation coil I made took a lot of work, for a mediocre result. So, I've started looking at HVAC condensers. The good ones are made of brazed copper tubing, with aluminum fins. This one is rated for 175psi at 350f. Obviously these would really only work with a forced circulation setup. I got a free broken air conditioner on craigslist, that I plan to take apart. This seems way better than just bending a bunch of copper tubing into a loose coil. Here is a pre coiled stainless steel coil, perfect for higher pressures.

Anyways, besides that, I don't remember how I came upon this, but appearently a newish thing in industry is to heat thermal oil with fire and then use the thermal oil to make steam. This has a few advantages:

First, you don't need a high pressure heat exchanger exposed to fire. This makes the fired construction a lot more lenient. You no longer need heavy walled pipes, you can look at more conventional radiator style designs. If the coils overheat, they wont burst. The circulation pump is no longer a critical component. If it stops pumping for whatever reason, all the hard work you put into making the fired heat exchanger doesn't go up in a pop of steam. If your circulation pump fails, the oil heats up, starts to boil, which pushes overheated oil out of the heat exchanger and past the temperature sensor, which will sense the temperature and shutdown the burner.

The steam generator is much easier to build. You don't need nearly the surface area that you would for air-water heat transfer. I was thinking of just a big threaded pipe capped at both ends, with some copper coils inside that the oil circulates through. There's no risk from running the boiler dry, as the oil can't get hotter than its boiling point. You can control it just like a firetube boiler.

Scale is much easier to handle. Since the boiling is occuring on the outside of an easily removable coil, it's super easy to clean off the scale.

There are some downsides:

System complexity. You now have two fluid loops. You need a forced circulation loop of hot oil, with temperature monitoring of the burner heat exchanger outlet. If the temperature gets too high, the oil will crack and leave carbon deposits in the heat exchanger. However, the oil loop is pretty simple, since it's not under pressure. Also, since you don't need nearly the surface area in the oil to water heat exchanger, it's easier to make. Electronic controls are no problem for me. Screwing together more pipe is infinitely preferable to having to fabricate something in metal. I would love to have the money, skill, and equipment to make a small firetube boiler.

System size increases. Obviously, this is gonna take up a bit more space. In my vision of a forced circulation oil loop, feeding a boiler made of one piece of threaded pipe with capped ends, and a copper coil connected to the bottom cap for the oil to water heat exchanger, there is a small increase in size compared to a direct forced circulation boiler.

Efficiency losses. Not the kind of efficiency loss that would reduce boiler efficiency, but more like enthalpy/entropy losses that would be present in a regular boiler anyways. With properly sized air to oil and oil to water heat exchangers, efficiency losses won't be any higher. In fact, you can gain some enthalpy efficiency by having the air to oil heat exchanger be counterflow, similar to a monotube but without having to worry about water levels and such. All of this efficiency discussion is really only valid when you're aiming for high pressures and temperatures. Since I'm only looking for 100psi saturated, it's pretty moot. You can still add an economizer and superheater on either side of the air to oil heat exchanger.


Anyways, those are my crazy ideas of late.

One potential problem I would see is decomposition of the oil. Most of the thermal oils with which I am familiar are rated for temperatures in the lower steam pressure range. It would be something to research for any particular setup.

Not too crazy, but I think the high temperature fluid for this has been like the high velocity pump for forced circulation, many looking, but nobody coming up with anything. I read something a long time ago and they were trying/ suggesting to try some very dangerous solutions like mercury? etc.

When the steam generator is moved from the combustion chamber, one thing that is given up is the amount of superheating that takes place in the steam generator itself. Even a firetube benefits from some superheating in the upper regions of the exposed firetubes.

Quote: "ounce of electronic control can often replace a pound of mechanical control."

But generally introduces a ton of problems, I don't know of one mobile steam plant with electronics that doesn't do so without a lot of maintenance. Maybe it's the hobbyish electronics and connectors we have available, who knows, but it seems like they are plagued with connection issues and thermo etc devices that are problematic at those temperatures. There is a guy right now on the British steam car forum with a Likamobile and he's looking for someone to rid it of all the troublesome electronics and make it heat and pressure operated like the Locomobile. I run my Locomobile with no electronics and it is very reliable. Never a bad electrical connection, flat battery or failed electronic device, there aren't any.

-Ron



Edited 1 time(s). Last edit at 06/26/2018 01:56PM by IronChief.

IronChief Wrote:
-------------------------------------------------------
> Not too crazy, but I think the high temperature
> fluid for this has been like the high velocity
> pump for forced circulation, many looking, but
> nobody coming up with anything. I read something a
> long time ago and they were trying/ suggesting to
> try some very dangerous solutions like mercury?
> etc.

This exact method is used in industry for generating (lower pressure) process steam while bypassing the legal requirements for a fired pressure vessel. Your average thermal oil is good to 570f, which still gives a ~200f temperature differential to decrease the required surface area for the oil to steam heat exchanger.

> When the steam generator is moved from the
> combustion chamber, one thing that is given up is
> the amount of superheating that takes place in the
> steam generator itself. Even a firetube benefits
> from some superheating in the upper regions of the
> exposed firetubes.

Yes, this is also one of the things that makes a good counterflow monotube potentially more efficient than any other boiler.

> Quote: "ounce of electronic control can often
> replace a pound of mechanical control."
>
> But generally introduces a ton of problems, I
> don't know of one mobile steam plant with
> electronics that doesn't do so without a lot of
> maintenance. Maybe it's the hobbyish electronics
> and connectors we have available, who knows, but
> it seems like they are plagued with connection
> issues and thermo etc devices that are problematic
> at those temperatures. There is a guy right now on
> the British steam car forum with a Likamobile and
> he's looking for someone to rid it of all the
> troublesome electronics and make it heat and
> pressure operated like the Locomobile. I run my
> Locomobile with no electronics and it is very
> reliable. Never a bad electrical connection, flat
> battery or failed electronic device, there aren't
> any.
>
> -Ron

This is really more of my personal preference. I have a lot of experience with industrial controls, and not a lot of tooling to make custom things like pressure actuated valves and whatnot. The hobby level electronics are most certainly the problem. Industrial grade sensors and controls are as robust as they are expensive. However, the main advantage electronic controls have is that a well designed system will be able to alert you when a sensor fails before things start acting funny.

Hi Z,
Hope all is well with you.

There is a fellow in the club, Craig, who suggests using a liquid salt solution. Same idea as yours. The main concept is to use an ubiquitous heat around the steam generating tubes. I think it's a noble idea. His article is in the latest SACA Bulletin.

An excellent reference point is the nuclear powered submarine. It uses a PWR that circulates to the generating tubes. Note that this system does not super heat. Who cares, it is a low cost heat source. I believe that the system is kept simple to make it work reliably. In other words it generates wet steam, runs through a turbine-generator and condenses to repeat the process.

Another thought is that the system you describe would be suitable to a stationary power plant with constant power output. I would not recommend this system for a Steam Automobile or the likes of it..

I encourage you to keep on with the ideas...I like the thought process!

Kind regards,
Rick H.

This all brings to mind the article in The Steam Automobile in the early 1980s, about a considerably ingenious fellow who built a road-worthy steam car with a "thermal battery" filled with a commercially-available heat-storage/transfer liquid called "Thermal Liquid". A large tank of that stuff was installed in a Ford Pinto, [and what a fine car that was, quoth Mr. Pete with more than a touch of snark] with a large coil of steam-generating tubing submerged in the liquid, and a 20hp Stanley engine added to the Pinto axle. When the fluid was hot, water was pumped into the tubing, and enough steam was generated to propel the car, albeit with a limited range similar to electric cars of that era; something in the 30-50 mile range if I recall correctly.

In the early 2000s, I posted some information about that system on this Forum, with the result that the developer of that experimental car [a Mr. McNeir if I recall correctly] somehow found my post, posted his own comments on the Forum, and wrote to me with some very interesting technical information. For "proof of concept" purposes, he heated the Thermal Liquid with a hydrocarbon fire to "charge the thermal battery", but he noted that the liquid could also be heated directly by solar energy, like with a lenticular or parabolic-reflector solar energy collector/concentrator heating tubes through which the Thermal Liquid was circulated. The result? A genuinely zero-emissions solar-powered steam automobile -- not just "carbon neutral" but "zero carbon". A steam car which runs directly on the heat of the sun. Technically speaking of course, that is nuclear fusion energy, atomic power derived from a rather powerful fusion reactor [the Sun] which is already in operation a few million miles from us. That was his ultimate goal. As I recall, at the time Ken mentioned, in the true spirit of constructive criticism, that heat-storage batteries taking advantage of the latent heat of phase change would be better, IE the considerable "latent heat of liquefaction" of some substances, akin to the immense "latent heat of vaporization" of water.

It is interesting to consider whether the weight and volume of such a heat-storage battery could be improved to the same degree as electricity-storage batteries have been since those days -- ideally without cost increases which would render such a system more expensive overall than other alternatives in spite of the "free energy" source. Perhaps vastly-improved "heat battery" thermal storage media are already commercially available, but nobody in the miniscule light/vehicular steam community has discovered them yet?

I enthusiastically second Rick H's "liking" of the thought process here! This might very well lead in some world-changing directions!

Peter



Edited 2 time(s). Last edit at 06/29/2018 04:35AM by Peter Brow.

Thinking about this for the last few days. I'm not seeing how doubling the heat transfer "problem" could be deemed equally efficient or anywhere near as efficient as a steam generator in the fire. That is the main reason a steam car is inefficient it's the transfer of heat to the water which is slow and with great loss, These systems would make it a two step process with double the loss. Immersing the steam generator coils in heated solution would be more efficient, but the fact remains, it is still done so with additional loss.

I'm not sure what "thermal liquid" is, but that has been the issue with contemporary experimenters, most liquids boil before they reach the proper temperature. Even oil is going to boil (vaporize) (bunker C). The notion that 400 degree oil is going to transfer and make 400 degree boiler water is wishful at best, there would be substantial loss. Remember, it's not a one way street, if the oil is heating the water and the water is cooling the oil. The rest of the heat from both transfer cycles is going out the exhaust (loss). This would be very inefficient. Someone claimed they did this with a road-worthy heavy car and had a range of 40 miles, I'd have to see it to believe it.

To operate a 1-2 ton steam vehicle on the road, requires a large amount of heat with a starting point of 400k to 1M BTU (Doble).

-Ron

Quote

Thinking about this for the last few days. I'm not seeing how doubling the heat transfer "problem" could be deemed equally efficient or anywhere near as efficient as a steam generator in the fire. That is the main reason a steam car is inefficient it's the transfer of heat to the water which is slow and with great loss, These systems would make it a two step process with double the loss. Immersing the steam generator coils in heated solution would be more efficient, but the fact remains, it is still done so with additional loss.

It's not about thermal efficiency, it's about ease of construction. The transfer of heat is only lossy if you don't maintain a good counterflow thermal gradient, which really only exists in counterfired monotube generators. A thermal oil system can be an advantage in this situation as you can build a good counterflow heat exchanger and just pump oil without having to worry about evaporation zones. You just control pump speed based on the outlet temperature. It's potentially slightly more efficient than your average firetube boiler.

In a firetube boiler, the 2000f flue gas hits the 600f superheater, then it hits the bottom of the boiler at 350f. In a hot oil setup, it hits a 600f superheater, and then hits the bottom of the oil coils which are 500f, and works its way up the coils which steadily get colder to 350f at the top. Ideally, you would have another counterflow heat gradient when transferring the heat from the oil to the water, but I'm just gonna pump it into some coils inside a pipe full of water because it's easy.

In any case, since we only want 350f steam, you're not really going to notice a difference as long as nothing in between the flue gas and the boiler water ever gets colder than 350f. (unless you take the oil exiting the boiler and run it through an economizer)

Quote

I'm not sure what "thermal liquid" is, but that has been the issue with contemporary experimenters, most liquids boil before they reach the proper temperature. Even oil is going to boil (vaporize) (bunker C). The notion that 400 degree oil is going to transfer and make 400 degree boiler water is wishful at best, there would be substantial loss. Remember, it's not a one way street, if the oil is heating the water and the water is cooling the oil. The rest of the heat from both transfer cycles is going out the exhaust (loss). This would be very inefficient. Someone claimed they did this with a road-worthy heavy car and had a range of 40 miles, I'd have to see it to believe it.

Since I'm only looking for 100psi steam, I'll have a good 200f temp differential between the steam and oil to work with. Your average synthetic gear oil has a boiling point of 600f.

In any case, I have no idea if I'm even going to try this idea yet. I might do a small scale prototype just to see what happens though.

So, regardless of oil or not, I noticed a common design with industrial oil heaters. They all have this double helical coil design, and I think I would like to replicate this coil arrangement in my new cart boiler. I attached a picture I made in powerpoint of the design. It looks easy to build and quite efficient. There is a large combustion chamber in the middle, inside the first helical coil. This would provide good radiation heat transfer, especially for a yellow flame pellet burner. Then the flue gas takes a narrow path down in between the outside of the inner coil and the inside of the outer coil. It turns around again and passes by the outside of the outer coil before exiting the top.

I won't put a pancake coil on the top, so I'll have to put an insulated cap there instead. Since there's no gap between the coils, I won't have to snake wires through them. I'm thinking of welding four vertical posts for the inside of the inner coil, another set for between the coils, and a third for the outside of the outer coil. The coils should just slide in from the top. Then just make a sheet metal outer enclosure. I'm thinking 60 feet of 3/8in copper tubing for a ~15kw pellet burner. A 5x circulation rate gives me 35cc/s. Easily done by my 25mm x 50mm cylinder driven by a 45rpm gear motor. That means I'll need ~4 feet of 1/4in copper tubing to connect my cylinder to my valves in order to keep heat away from the cylinder on my circulation pump.



Edited 1 time(s). Last edit at 07/02/2018 10:06AM by zimirken.

Hi Z,
Seams like it would work. You know the acid test...right?

When you build it and it gives you acid indigestion because its not working the way its supposed to.

Good luck,
Rick H.

Hi Zim
Nice picture of you in the Bulletin…you're part of the center-fold, in color!

It was nice seeing you at the Spring Meet. I enjoyed evaluating your Cart! Your forced circulating system put out some major steam. BTW I agree that recirc. pump did a nice job, proven and can be used on your next boiler build.

Perhaps you can make it to the September Meet?

Kind regards,
Rick

Just thought it would be cool to share a report I found.

It's a study of a converted IC engine bieng driven by supercritical steam. They got >20kW shaft power from 45kW electric boiler in. That's pretty good efficiency.

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