Superheaters

[Garry ColesParticipant @garrycoles69390]

Hi, I have a question about superheaters. I’m building Doug Hewsons BR 2-6-4 Tank loco in 5 in gauge. I’m working around the front end of the boiler and was wondering do I actually need superheating. I will only ever be running this in my workshop, so it’s not going to have a hard life. The cost of the elements is so very expensive. I know I could make them, but getting the s/s materials over to Jersey can be so expensive by post. Any thouhts please.
Garry

[noel shelleyParticipant @noelshelley55608]

I don’t know the design but stainless 316 tube should be available at a reasonable price and then a few beers to someone to TIG weld the firebox end. The headers, wet and dry could be silver soldered or compression fitting. Or you could as you say not bother with super heat, But be warned, once it’s built and running the urge to put it on a track and see what it can do may prove irresistible ! Doug would not have included super heat if it wasn’t worth the candle. Noel.

[duncan webster 1Participant @duncanwebster1]

It will work without superheaters but nothing like as well.

 

[Speedy Builder5Participant @speedybuilder5]

Why introduce stainless as its expansion rate will be different to the copper boiler giving rise to possible joint leaks over its working life.

Use copper with a brazed joint seems to work on my 5″ Speedy.  Think about the eventual life of the model when it passes into other hands and go for copper superheater tubes ??

[SillyOldDufferModerator @sillyoldduffer]

Living in Jersey where post and packing is extra expensive might well change the answer!  As does Garry’s comment that the engine will only be run in his workshop.  Unless there’s a track in Jersey the engine will have to be moved to the mainland!

There’s no advantage in superheat unless the engine is expected to do work.  Saturated steam will look, smell, and spin the wheels on a rolling road or short garden track just as well.   The engine won’t burn fuel as efficiently, accelerate, or pull a heavy load as well, but if that doesn’t matter, don’t superheat.

What I’ve read about superheated models in old magazines suggests they often fail to deliver much, or any, benefit.   Reason, I think, is fairly obvious.  In a small model the surface area of the superheater’s pipework is large compared with the volume of heat contained inside.  Therefore heat leaks rapidly, and the temperature drops!  Even inside the boiler the temperature is well below superheat, so it cools superheated steam.  Assuming the steam inside is still superheated,  the pipes have to get from the smoke-box to the valves and from there into the cylinders.  None of it is well-insulated and performance will be good only if convincingly hot superheated steam fills the cylinder.

At 150psi, saturated steam will be at 185°C, and red-heat in the firebox, say 600°C.   So 415° of superheat in the firebox isn’t difficult.  The question though, is how much of that sugar, if any, reaches the piston!   Much less value in steam that’s merely dry, or a bit less wet than normal.  In a small engine, I fear superheated steam is likely to re-saturate before it can do any useful work.  Anyone measured the temperature inside a model’s cylinder?  How hot is it?

Based on the ratio between surface area and volume of small pipes, my gut predicts most model scale superheaters aren’t ‘worth the candle’.  Guts being highly unreliable though, anyone able to prove me wrong?   Good start would be to measure two identical engine with a dynamometer over many comparative runs; engines identical that is apart from one having a superheater.

The history of superheat in full-size locomotives is interesting.   The scientists identified the advantages of superheat about 40 years before practical men could make it work.   A major problem was that wet steam is usefully self-lubricating and not hot enough to damage animal based lubricants.  Superheated steam is dry and hot enough to destroy natural lubricants, or at least all the affordable ones!  Result, a number of Victorian experimental engines that outperformed saturated steam locos until their pistons, cylinders and valves wore out after an hour or two…

Superheated locos had to wait until the Chemists developed cheap heat resistant lubricants, after which the mechanical part was a doddle.

Dave

[MichaelRParticipant @michaelr]

If you don’t want to be bothered with superheaters just coil the steam feed pipe from the regulator around the smoke box interior and then connect it to the cylinders, the heat in the smoke box will give a degree of steam drying.

I have seen some good performing locomotives without superheaters.

MichaelR

[duncan webster 1Participant @duncanwebster1]

With the trouble SOD has been having I would have thought that he would havd known that guts are very unreliable.

Bill Hall showed conclusively that superheaters work in model locos, but radiant ones for preference. The heat lost from the outside of a short pipe is small compared with the energy flow through if, so the temperature doesn’t drop much. Superheat doesn’t make a lot of difference to pulling power all else being equal, but it reduces condensation on the cylinder walls, so you need to boil less water,  meaning less coal burn. Or conversely for a given amount of coal burn you get a lot more useful steam. Bill Hall did a series of tests reported in SMEE journal which proved this beyond doubt. Not sure whether it ever got published in ME.

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