Concentric superheaters

[pault]

Hi,
I thought I would chuck in a few views
I have made a number of coax superheaters and tested them a bit.

The ones I have made have had a stainless steel outer tube and a copper inner tube. I have never supported the inner tube, and there is a tenuous argument that the sagging tube might give a little turbulence. In all cases the wet steam came down the centre and back out down the outer tube. From memory they were mostly 20mm outer 7/16” inner pipe, the idea was to give equal area out and back

The non-radiant elements did give a little superheat, the radiant eliments did give significant superheat. This was measured using thermocouples placed into the steam flow in the headers so it is the true temperatures.

Have a look at
modeleng.proboards.com/thread/7812/7-1-4-dyno-car?page=2

[andyhigham]

Paul
My outer tubes are 1/2" 12.7mm dia x 1.2mm wall which gives an inner diameter of 10.3mm area = 83.3 sq mm
My inner tubes are 8mm OD x 0.6mm wall which gives an inner diameter of 6.8mm area = 36.3 sq mm area of outer tube = 50.26 sq mm.
Therefore the area of the anulus is 83.8 - 50.26 = 33.04 which is pretty close to equal.
On the sweet pea the steam pipe to the cylinder is 5/16" OD

[delaplume]

Quote}---"I think you're thinking of Swirlyflo tubes."

Hi Reg.....I'm trying not to go Off Piste so I'll just post the photos that your reply reminded me off when I was on BR as a Diesel Fitter at the old Reading ( Cow Lane 81D.. ) depot....

[delaplume]

Just for consideration}---------- The MAX temp in our taper boiler / Belpaire inner firebox is at the top of its' throatplate .........where by design the max vol of water is also to be found........

Steam, as first generated, will have a certain amount of water droplets within it..So to improve the overall efficiency we pass this "wet" steam back through the original heat source ( the flue-tubes and maybe the firebox if of the radiant type ).... The water droplets will now change into steam and for this to happen we have added extra ENERGY into the final output.....The pressure in the system up to now is governed by the safety valve so the only thing that can change AFTER Superheating is the steam's final TEMPERATURE......Hence the need to use the higher grades of steam oil as mentioned earlier.....

Just for the record my initial training was on Yarrow Y-100 2-Drum and Admiralty 3-Drum Boilers back in the late 1960's....then later on British Railways with the Spanner Swirly-Flo and Stone Vapour steam generators for 1st generation main line Diesel locos to provide steam heating ( and some had water scoops to top-up on the still extant Water troughs !!)...

[RGR 60130]

I’m seeing a lot of confusion here about what we do when we apply heat to water. Maybe I can help clarify a few things in the simplest of terms.

Water as we know can exist in three forms, Solid, liquid and gas i.e. Ice, Water and Steam. If I first talk about the Ice / Water transition then that might help understand the Water / Steam transition.

Consider a block of ice at atmospheric pressure. As heat is added, the ice will reach a temperature of zero degrees Centigrade and start to melt. As it melts the temperature will remain at zero degrees Centigrade. Initially there will be 100% ice. This will progress through a stage of part ice, part water until finally we have 100% water, still at zero degrees Centigrade. It is only after this point that we can increase the temperature of the water until it reaches 100 degrees Centigrade and starts to form steam (in this case still at atmospheric pressure).

At this point it is probably going to be best if we move from this atmospheric pressure scenario to a boiler scenario. For sake of this example the boiler pressure will be 7bar.

Heating the water in the boiler, its temperature will rise and rise until the temperature of the water reaches 165 degrees Centigrade. This is known as the saturation temperature at 7bar. The water vapour above the water will be 100% saturated, i.e. as wet as it can be. Putting more heat in won’t raise the temperature, it will just generate more steam which will vent via the safety valves which are set at 7bar in this instance. This is the Water / Steam equivalent of getting our Ice temperature raised to zero degrees Centigrade in the first example.

Before we go any further I should perhaps say Don’t confuse wet steam with carry-over. Wet steam is a saturated vapour whereas carry-over is drops of water leaving the water surface and getting carried along with the saturated vapour. Mechanical devices may be fitted in the boiler to help stop these droplets leaving the boiler with the saturated vapour.

Now we open the regulator valve and admit the saturated steam into the superheater. What will happen now is the equivalent of us getting a mixture of ice and water until there’s 100% water.

At this point I need to talk about Dryness Fraction. This is the wetness of the water vapour / steam. This will start at 0 and go right through to 1 when the vapour is completely dry and effectively a gas. The whole process will still occur at 165 degrees Centigrade and 7 bar.

Only after this point will we start to superheat the steam (the equivalent of raising the temperature of the water in the Ice / Water example). The temperature of the steam will begin to rise as more heat is added. The pressure will still essentially be 7bar (less any piping losses).

You will note that a lot of the energy transfer to the steam is done at constant temperature. We are not making the steam hotter until we have dried it all and made it into what is essentially a gas.

From Steam Tables, the water at 7bar 165 degrees Centigrade will possess 697 KJ/Kg (Kilo Joules per Kilogram) of energy.

Drying the steam until the Dryness Fraction is 1 will take another 2067 KJ/Kg of energy so by the time we have dried the steam the total energy will be 2764 KJ/Kg.

Then, if we continue heating to a temperature of say 250 degrees Centigrade we will have 2955 KJ/Kg of energy.

Even if we only dry the steam to say, a dryness fraction of 0.7, we will still have a lot more energy than with saturated steam:

697 + (0.7 x 2067) = 2143.9 KJ/kg

Overall it’s still a worthwhile exercise.

You have to remember that we can’t use all this energy to do useful work as a lot of heat energy will go up the chimney.

This is all off the top of my head in a bit of a rush as I need to cook tea and get up to Saudi Shields to see a band. No doubt any errors will be pointed out to me!

Reg

[delaplume]

Hi Reg,

Yes--100% agree---I made a slight error and should have said Water Vapour NOT water droplets....(Most Railway folk will know this as "Priming")....

Just to expand a little if I may ??......When ice changes to water and also when water changes to steam and then when wet steam becomes superheated the temp stays constant untill that process is finished.....The reason is that the energy going into the system is being used to achieve that change-of-state from one to the other...

I'd also forgotten to mention about the Dryness Fraction.....Incidentally if we take 1 Bar rounded up to 15 psi then 7 Bar = 105 psi....A good choice for the average 5" loco....

As both Reg and I point out this is all about ENERGY...

In the "Black Book" it says}----Quote.."The three main advantages of Superheating steam are}--

1) Any entrained water in the saturated steam is converted into additional steam..

2) Cylinder condensation is prevented,

3) The steam volume is increased....

At a working pressure of 225 psi this increase in volume is approximately 30% ...

This results in a considerable reduction in the demand on the boiler to produce steam to the locomotive cylinders resulting in a saving of water and fuel".........End quote.

Now combine that with intelligent use of the cut-off and suddenly a non-stop run from London to Glasgow or Edinburgh or Plymouth or Southampton is achievable..

[delaplume]

This makes interesting reading }------- www.advanced-steam.org/5at/technical-terms/steam-loco-definitions/superheating/