Superheat coil

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Ralph B
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Superheat coil

Post by Ralph B » Sat Nov 21, 2009 11:59 am

Is there a ROT to determine how many square feet surface area is needed per (say) 100 degree (F not C) of superheat at 150psi steam pressure? This would be on a Roberts type watertube boiler, initially burning oil but will eventually burn coal. (coal is not readily available in my current location)


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Re: Superheat coil

Post by Edward » Sat Nov 21, 2009 1:58 pm

Dear Ralph ,
I would have thought that it would be difficult to have such a ROT as I think there are too many variables :
1) Area of pipework exposed to heat/flue gasses .
2) Material/conductivity of superheater and wall thickness .
3) Temperature of flue gasses .
4) Time that the steam to be superheated spends in the
superheater ie : flow rate.
Not being an engineer I've probably missed several factors and no doubt some of our experts will comment .

I suspect that your best hope would be an empirical approach : find out what others have done , plot their results and from that work out what best meets your needs .
I hope that the second law of empiricism will not apply : (having done all of the above research and applied and made the superheater it it is found that one's result itself becomes part of the empirical research and the size needs doubling {roughly} )

Best of luck Edward .
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Re: Superheat coil

Post by barts » Sat Nov 21, 2009 5:07 pm

Rules of thumb regarding superheater sizing are indeed difficult. I'd recommend no more
that 15% of the total boiler surface area to be super heater. Some superheater issues:

* radiant vs convective heat transfer (does the superheater "see" the fire?)
* proximity to the fire
* variation in degree of superheat with load

Super heaters that use convection tend to produce additional superheat under heavy
load, those that use radiant heat transfer tend to decrease amount of super heat
as the boiler output goes up, so a blend of the two makes of easier control. Super heaters
that see the fire can easily overheat if fired heavily to raise steam while the engine is
stopped. Keep in mind that we often have transient conditions in our plants... throttling
down suddenly after running hard will produce significant amounts of extra super heat above
that from just adiabatic throttling, since the tubing & fire are hot and the amount of steam
to be heated has suddenly dropped. In full size practice, super heaters were often flooded
during boiler startup and shutdown to prevent overheating.

Your design should consider:

* make sure that the super heater piping is readily replaceable. Any water carry-over
from the boiler (this happens on our small plants sometimes, particularly when blowing
the whistle while carrying a lot of water) will flash into steam when it hits the super heater, leaving
any minerals, boiler treatment, etc. behind on the tubes. This will end up insulating the tubes
over time and make them less effective, and may lead to failure due to overheating over the
long term.

* if your boat is used where the power plant failing would place you and your passengers in
danger, make sure you have a way of revising the plumbing so as to continue on w/o
super heat.

* if the super heater can be isolated from the main steam plumbing w/ valves, make sure
it has its own safety valve so it's not a bomb if all the valves are closed.

* Engines will require lubrication with super heated steam. Do not attempt to use slide
valves w/ super heated steam. I've done it, and the the valve required frequent resurfacing
due to galling. For moderate superheat, piston valves will work fine.

* Consider using stainless steel tubing for the super heater - it's readily replaceable,
and will resist over temperatures conditions during temp. engine shutdowns.

* At 150 PSI, saturated steam temp. is roughly 365 F; 100F super heat brings you to
465. Allowing a bit for temperature excursions, let's call that 500F. The extra temp. results
in a drop in strength of about 25% in bronze, so using valves rated at 200 psi would be
a good idea. Teflon seated valves won't work at these temps., so most ball valves,
etc, cannot be used. Arranging your plumbing so only the main engine steam is superheated
will result in a simpler, cheaper configuration. Your engine throttle valve should be metal on
metal (a stainless plug into a bronze body worked well for me). Cotton rope won't work as
an insulating material when steam is superheated.

Hope this helps -

- Bart
Bart Smaalders Menlo Park, CA
Ralph B
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Re: Superheat coil

Post by Ralph B » Mon Nov 23, 2009 12:43 pm

Bart, Edward

Thanks for some good info. I was thinking that there might be some sort of ROT, granted it is complex but thought there might be a better method than WAG which is pretty much what I am doing. What I was going on was assuming 50% steam flow (125lbs per hr on 12hp) since most of the time I don't plan on running a flank bell. Only the main steam line will have superheat, aux steam will run saturated. I was not planning on too much carry over as the main steam line comes from the top of the steam dome, a good 10% above high water level. I don't know how much shrink and swell I will experience but I hope not to do too many crash bells (ahead flank to back emergency) or all stop/ahead slow to ahead flank.

That idea of bypassing the superheat coil is a good one, although I dont know if practicable for me. I planned on a standpipe to the top of the steam dome and the main steam line exiting the drum inside the fire box. As the coil will be inbetween the water tubes and fire most heat transfer will be convective.

Got to run will add more later...thanks again for your insight it really is valuable.
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