Mono-tube steam generators - special adaptations

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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PeteThePen1
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Mono-tube steam generators - special adaptations

Post by PeteThePen1 » Mon Jul 18, 2022 3:05 pm

Hi Steamboating Friends

We in the UK are melting in rather higher summer temperatures that we have ever had. In this house the mechanical ventilation is off to avoid dragging in the warm air and not much activity is to be observed. However, a bit of intellectual activity never did any harm.

I am researching mono-tube boilers and have found an interesting article in the SBA's Funnel (Issue 109 Summer 2001) by Jim White in which he explains the details of his boiler. The thing that is unusual about it is that he uses not one but two coils of tube, so I suppose one should really call it a duo-tube boiler. He points out that the problem with two tubes is that it is nearly impossible to keep the pressure in the two of them the same. To do that he has developed a shuttle valve for which he provides a sketch. I have tried to copy that in CAD but I am puzzled about the dimensions. Below is the sketch from the article and a photo of his boiler that shows the shuttle valve with its single inlet and double outlet.

The boiler coils are 3/16" tubing so I am guessing that the pump inlet might be 1/2" BSP The outlets will be whatever thread standard is used for 3/16" motor vehicle brake pipe.

What dimensions do you think I should use for the shuttle and the bore in which it runs? How big should it be and how long, vis a vis the outlet ports?

Regards

Pete

PS If anybody would like to read the original article, send me a PM or email and I will send you a copy.
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Re: Mono-tube steam generators - special adaptations

Post by PeteThePen1 » Tue Jul 19, 2022 11:48 am

Here is my effort to create this shuttle.

I have assumed that one would use a solid block into which one could tap the appropriate holes for the brake pipe connectors. I have never seen barrel nipples with brake pipe threading. Do they exist? Hopefully the rest should be self explanatory, but I do not feel that the design is very elegant so suggestions welcome.

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Re: Mono-tube steam generators - special adaptations

Post by barts » Tue Jul 19, 2022 3:27 pm

Perhaps this is obvious to everyone, this valve isn't designed to balance the pressure, it's designed to ensure that the flow in both coils is the same - a classic flow divider valve. Note that that if coil 1 has more back pressure and thus receives less flow, the shuttle will be forced to the right, restricting the flow to coil 2 until the net back pressure (valve + coil) is equal on both sides, providing approximately equal flows to both tubes.
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I think your spool may have an issue because if it is forced to one side (as shown in your drawing), pressure is masked from the surface against the plug. Since you're using the inside of the valve spool to regulate flow, making the ends of the spool slightly conical should avoid this. The original design has protrusions on the plug for the same purpose; that also works. Alternatively, make the spool longer and bore the plugs for the spool w/ a loose fit. That would be a better approach with inside admission as used in your design, since it would resist forces that would tend to cock the spool in the bore. As shown there's support only at one end when the spool is shifted completely to one side which would probably jam the spool.

Note that another way of achieving approximate flow balancing is to place a restriction (orifice) in the feed to both coils such that the pressure drop across the orifice is larger than that in the tubes. This is less effective, but certainly simpler; it of course assumes a relatively constant flow. The spool design works well for varying flows.


- Bart
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Re: Mono-tube steam generators - special adaptations

Post by DetroiTug » Tue Jul 19, 2022 6:09 pm

How about a T with two metering jets and check valves on each coil? Spool valves with such small piston face area are very prone to sticking. -Ron
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Re: Mono-tube steam generators - special adaptations

Post by PeteThePen1 » Tue Jul 19, 2022 7:00 pm

Hi Folks

Thanks for your observations and yes, my mistake to say pressure when I really meant flow.

I will re-work the drawing with your suggestions and may even get as far as making a test piece. Apparently the late Mr White found his home made versions worked well but never left us any dimensions.

Regards

Pete
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Re: Mono-tube steam generators - special adaptations

Post by DetroiTug » Wed Jul 20, 2022 1:43 am

DetroiTug wrote:
Tue Jul 19, 2022 6:09 pm
How about a T with two metering jets (/orifices as Bart mentioned above) and check valves on each coil? Spool valves with such small piston face area are very prone to sticking. Constant feed pressure can be achieved with an air over liquid accumulator. Messing with steam cars, I'm getting well versed on weird piping issues and resolutions. -Ron
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Re: Mono-tube steam generators - special adaptations

Post by fredrosse » Thu Jul 21, 2022 2:20 pm

I would second the method of just using two metering orifices to force the flow to be evenly distributed between the two coils. This would be more reliable, provided there is no contamination material clogging the orifices, and this can be assured with a fine screen strainer in the feed pump line.

In steam systems, the amount of work the feedwater pump performs is on the order of 1% of the steam plant output. To that, one could add double the discharge pressure, plus poor pump efficiency, and only use about 3% of the power plant's output, something that we all could afford. For a 100 PSI steam plant, forcing 100 PSID across the orifices will force good balancing of nearly equal flow to both coils. This assured balance will only be assured if the differential pressure across the orifices is a high fraction of outlet steam pressure. With a variable flow feedwater pump, at 1/2 flow rate, the differential across the orifices would only be (1/2)^2 = 1/4 of the original flow differential pressure, so the balancing criteria is not as good, but still may be acceptable.

Let us know details if available (plant feed flow, operating steam pressure, type of feed pump, etc.), I would size the orifices if you wish.
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Re: Mono-tube steam generators - special adaptations

Post by Andre » Sat Jul 23, 2022 8:35 pm

Good day, I am Andre and interested in a steam engine and building a boiler for use on a boat. I use to work as a tool and die maker in South Africa. At present I am sailing on a yacht in Portugal so I am without a workshop, like I have my hands tied. Looking at the drawing of the balancing valve there is 2 x 8 dia holes, from what I can remember the tapping size for 1/8 bsp is 8,2 mm . That would be more or less what you are looking for.
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Re: Mono-tube steam generators - special adaptations

Post by PeteThePen1 » Tue Jul 26, 2022 5:02 pm

Hi Fred and Andre

Thanks for your observations and thanks for the offer Fred. I must apologise for the slow response but the PC's power supply died and I have been without it for a few days. Happily it is back and I can now get some things done.

The shuttle valve is a suggested addition to a mono-tube boiler built by a friend and fellow SBA member John Emmett. I am hoping to copy his boiler for my proposed steam canoe project. As the first stage of that I have started to draw it in 3D using Alibre Atom. Being new to that software progress has not been fast. Our discussion also led to the idea of writing some articles for Funnel magazine on mono-tubes. One has been finished and submitted and the second one on mono-tube boiler controls is nearing completion. It is probably too 'academic' as it is based upon my reading of the literature and some conversations with those who have mono-tubes. However, it may be of interest to other steamboaters when finished.

Anyway, John's boiler uses 8mm OD Kunifer brake pipe. The connections to the pipe are standard motor vehicle connections which in the UK would be 14mm X 1.5 mm thread. (I think the US would call this 5/16" pipe which uses 1/2" x 20 unf fittings if I have read the tables correctly). My assumption is that all the rest of the pipe fittings would be BSP not least because they are readily available in the UK. John uses an electric feed pump which I think is a diaphram pump but I have no details (I must nag him about that!). The target is to have wet steam so we are talking 100C to 120C I would imagine as the boiler has a water separator. John commented at one point that the ideal sensor for an electronic control system would be a 'steam dryness' sensor. No doubt some clever person has worked out how to do it but I have never seen them mentioned before.

Regards

Pete
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Re: Mono-tube steam generators - special adaptations

Post by barts » Tue Jul 26, 2022 6:12 pm

Since our small steam plants suffer from considerable condensation due to volume/surface ratios, keeping the steamfeed just slightly superheated might suffice - and be easier - than sensing dryness. This could be done of course by measuring both temperature and pressure and using a saturation temp/pressure lookup table to compute the amount of superheat.

I'm somewhat allergic to electronic controls in a steam plant (save perhaps a electric fuel pump somewhere), so a more desirable approach might be to extract steam from a spot about 3/4 of the way to the output and separate the water out at that point. Control could be effected by insuring that the trap continues to release water (back to the hotwell if well controlled, otherwise to the (oversized) condenser).

It's worth examining a unusually readable patent on the solid fuel Herreshoff coil boiler:

https://herreshoff.org/wp-content/uploa ... 183054.pdf

There, the steam from the monotube is discharged into a vertical drum, and the independent steam pump is driven by a feed line taken from the center of the drum. As long as the capacity of the feed pump is sufficient to provide all the water needed, the water level in the drum will be maintained at the approximate center; as the water level drops the feed pump will speed up due to getting pure steam; as the water level increases, the feed pump will slow because its feed steam has entrained water. Engine feed steam is taken from the top of the drum and further slightly superheated to insure dryness. Note that such a boiler could be built today for our steamboats. The steam drum & control mechanism provides more stability in steam conditions that typically seen in monotube boilers.

Not bad - a feedback control system from 1876.

- Bart
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