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Author Topic: Too much superheat?  (Read 377 times)

mrsgoggins

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Too much superheat?
« on: July 29, 2017, 04:46:16 PM »

Thought I might share the following, for interest really.

I have just stripped down the steam plant on my picket boat to attend to a few niggles including blistered paint and oxidised steam delivery piping.
the boiler is a 31/2" return flue of my own design and build, having a feed water heater coil in the smokebox and a steam drier/superheater in the combustion chamber. A picture of the latter is attached and shows heavy oxidation of the copper superheater coil. The combustion chamber temperature will be high as there are no cross tubes in the main flue.
Normal operating pressure is 60 to 70 psi which is maintained during sailing and the 3/8" x 7/16" twin slide valve engine runs well despite blistered paint. The other picture shows one of the cylinder covers with significant bluing of the internal face. Valve chests and valves show similar colouring.
I suspect that the high temperatures occur only at low steam flows but I am remaking the superheater coil to reduce the length by about 50% and move it away from the blow torch effect of the main flue.
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rhavrane

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Re: Too much superheat?
« Reply #1 on: July 29, 2017, 05:38:43 PM »

Bonjour,
Your topic is very interesting because it raises a question "wet" or "super heet / dry" steam for our little machines.
Personally, as my boats are often too fast, I like using them at 2/3 bars (30/5 psi) with a wet steam to save autonomy and have lubrication even if there is no more oil in the lubricator after a while.
An old "knowing man" had told me that I even could use no oil as soon as steam was wet enough because there is not much effort at 2/3 bars.
My other concern would be that you can have flakes in your copper pipe when heated empty, not good for the plant.
I hope forum members will share their experience on this topic because, as often, we can find several answers to a question.
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mrsgoggins

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Re: Too much superheat?
« Reply #2 on: July 29, 2017, 06:00:44 PM »

Hi rhavrane,
Thanks for your comments. There is no evidence of copper oxide reaching the engine but yes, it is a risk.
I saw superheat as a way of avoiding "lumps" of water being thrown out of the funnel and making a mess of my model.


Keith
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steamboatmodel

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Re: Too much superheat?
« Reply #3 on: July 29, 2017, 11:56:08 PM »


Hi Keith,
Most of our size engine are not designed to use real superheat (exceptions are those into flash steam). You should defiantly shorten and reposition you coil. What most of the modelers referee to as super heaters are really only driers, which is what we need in our models.
Regards,
Gerald.
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hammer

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Re: Too much superheat?
« Reply #4 on: August 02, 2017, 09:59:17 AM »

That looks a great boiler. You obviously know what you are doing. But wouldn't a separator tank stop the globules of water going up the chimney? 
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steamboatmodel

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Re: Too much superheat?
« Reply #5 on: August 02, 2017, 04:19:33 PM »


That looks a great boiler. You obviously know what you are doing. But wouldn't a separator tank stop the globules of water going up the chimney?
That is why you use one, too stop the water and oil globules from going up the stack and hopefully allow only the steam to go up.
Gerald
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mrsgoggins

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Re: Too much superheat?
« Reply #6 on: August 02, 2017, 05:00:44 PM »

Superheat was only part of the plan to avoid lumpy steam, other parts being a cyclone and a separate drains tank, all insulated to minimise condensation. It all worked fairly well so we shall see what happens now that I have reduced the length of the superheater coil by

Thank for the comments.

Keith

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