hi john,

alec farmer of Reeves fame popularised coaxial superheaters and they were adopted by don young starting with his Mountaineer 3.5"g loco.

the late jim ewins was highly critical of them and explained why they could not be as effective as ordinary superheaters with return bends.

so you might like to bear this in mind.

cheers,

julian

They appear to work in practice, but i can understand the criticism – however, you set them up the cold steam coming in cools the hot steam as it comes back out.

There's some good stuff here http://en.wikipedia.org/wiki/Countercurrent_exchange

The optimal solution would be for cold steam to enter at the smokebox end and leave at the firebox end and find its way to the cylinders down an insulated pipe.

in practice both coaxial and standard superheater tubes are more like a countercurrent multiplier **LINK** which act to make the steam hottest at the firebox end and actively keep it's temperature down at the smokebox end.

In practice, you just don't need the levels of superheat that make this matter.

Neil