Most of these models, when they do run on steam, run on saturated (wet) steam.
Look at the following chart:
Anything underneath the "bell" is a combination of steam and water, aka saturated steam. To the right of the "C" is super-heated (100% vapor) steam. To the left is of the "C" is entirely water. Solids (ice) do not appear on this particular graph.
Above the "C" is super-critical steam: a condition where liquid and vapor have equivalent densities and therefore are in distinguishable from each other. Many power generation stations run in the super-critical realm.
So you can see that saturated steam from 1% to 100% super-heated (dry) steam at 5 bar (~60 psi) is going to be around 140deg C (284 deg C). The extra energy is in converting the water to vapor. (Latent heat of vaporization).
So it takes a hell of a lot of energy to go from water to dry steam. The small boilers that tend to run these machines just don't have the burner capacity to safely get there.
I run my mono-tube boiler around 5 bar (75 psi). I don't have a water temp gauge attached. If I did, I could use the graph to determine my percentage of saturation.
As for why brass cannot be used with super-heated steam? I don't know. Perhaps it has to do with the often silver soldered connections?
Brass should never be used for steam fitting as the zinc in it reacts with the steam (zincifies) and forms a white powdery crust on the brass and makes the metal porous and brittle, All steam fittings should be made from bronze or gunmetal containing no zinc.
And thats why they suffer from the dreaded dezink as well as running on saturated steam at about 1bar
I played with flash steam at onetime using Pisces 2 style engine and it would burn the inlet valve out on a regular basis
Im always doubtful about claims for super heating on small model boilers and often think they should really be called steam dryers
From a personal point of view I tend to use bronze for my simple low pressure wet steam plants as brass simply doesnt cut the mustard.
Just my two penneth
De-zincification in brass happens not only in the presence of steam but also any moisture.
It does take a considerable length of time and is hardly worth most of us worrying about.
If you are worried that the steam engine you want to leave for the grandkids might fall to bits then use a resistant quality brass, such as water taps are made from , or better still gunmetal.
Lots of very good informaton here! Thanks.
So if the engine is going to be run just occationaly, it is probably nothing to wory about? Guess the arguments saying "Never run a brass engine on saturated steam" is from the days when steam engines did real work.
Saturated steam is probably not all that satturated just by running the steam pipe a turn around the burner?
For small toy boilers, at atmospheric pressure, ie 15psi (1bar), brass boilers and fittings are fine, though dezinc of the boilers is always going to be a possibility, I left water in one of my Mamod boilers for two month, when I returned to it, the brass had sucummed to dezinc. And there were spots all over the boiler. When the white powder was removed, there was pinning of the boiler, do not ever fire a boiler in this state. As I have seen a video of one blow by a member on another forum. He was ok but got quite a fright, (and respect) for the possibilities of a boiler failure. Daft thing was he had thought twice about running it, and decided "just one last time"!
If you intend on running at higher pressures than 15psi then I would take the advice above, and use bronze bushes, and copper boiler tube.
They will always be those on both sides of this fence. I expect there will be a differing argument to what i have posted but this is my experience. And I will be making a small boiler with brass fittings in due course, despite me knowing about the use of Bronze, as simply I like brass finish/colour, and ease to work over bronze which is much harder material.
Although its a matter of choice thats up to the builder. On a copper boiler that should have a long life given care not to mention the cost of materials it seems a pity to use a material to make threaded bushes that could fail over time when for a little extra cost and very little extra work a bush that will last the life of the boiler canbe made
Also most if not all boiler inspectors in the UK may fail or refuse to test the boiler thats assuming its to be run in public and the builder should want to go down that route
Just my two bobs worth as Iv no intention of restarting the brass verses bronze argument. For UK builders have a look at the code of practise and although it is only a recommendation and not law it has some very pertinent points.
Also remember the formula given in KN Harris boiler book is incorrect for boilers using fire tubes as the pressure is external the formula is true for the main drum were the pressure is internal
That's a good point, my post above is only meant to be referring to pot boilers, (ie externally fired by meths, or gel fuel) if using gas, or internally fired boilers, with tubes. For strength and durability, I think personally I'd build from copper and bronze. Most big toy steam manufacturers these days do build with these materials. But they also tend to operate at higher pressures 30-60psi, where regulations do come into play. Toy steam for the most part, working under 15psi hasn't attracted much attention from the health and safety board. Though most exhibitions won't let you run without proper certificates, and insurance.