super heated (or not)

Just thought I would ask what is the difference in performance between superheated and non superheated steam(in a three and a half inch gauge Britanniia, reason for asking as reference my previous query/post on here. No matter what I have tried (such as removing all the backhead fittings from the boiler I cannot get the loco to run properly with air line connected to the boiler, but have had it going through the super heater tubes,(seperated from the boiler) so I decided to make a direct piping from the boiler t o the cylinders. Just for clarification I may wish to sell the loco and include the super heater tubes as an "extra"

Surely there must be a problem in the regulator not delivering air to the super heater or the connection between the air supply and boiler Once you have sufficient pressure in the boiler, the wheels will turn once that pressure reaches them. The only thing in the way is the regulator or tubing between regulator exit and the superheater.

As stated before, air does not expand as much as steam and consequently you will use more than you expect.

Bob

I agree with Bob, something is wrong. Have you tried pressurising the the boiler with the valve gear and pistons disconnected to see how much air comes out of the pipe? I suspect a blockage.

If the pipework is partly blocked, pressure will be kept high on the boiler side, whilst only enough air gets past to create low pressure on the other side. A blockage will throttle the engine as if the driver were refusing to open the regulator.

I can only report what I've gathered from reading about model superheaters. Their value is uncertain. Full size, they definitely improve efficiency and performance significantly. In small sizes, results seem very mixed. Some engines improve with superheat, others disappoint. I guess it's because the steam still has to be superheated when it gets inside the cylinder, and it cools very quickly on the way. Heat escapes at a furious rate from small models because their ratio of surface area to cylinder volume is much larger than that of a full-size engine. Even if wonderfully super-hot at the firebox end, I suspect most model scale steam cools below superheat before it reaches the piston.

Dave

Since you have disconnected the pipes to the cylinders forget them and just put a plastic pipe on the output and either listen for the air or have it blow bubbles in a galss of water.
Which type of regulator is it? Not unusual to have a mechanism inside the boiler under the steam dome where a screw has dropped out or broken, or perhaps the rod from the handle ends in a square into a square hole which has worn round.

Thanks for the excellent replies, I have checked the boiler outlet, but not through the "regulator" which I also suspect. This is a plumbers ball type valve with a lever to activate, wlll see in the morning

The backhead fittings' presence should not affect an air-test of the engine itself.

Your description and Bob's reply suggests to me that either the regulator is not opening for some reason, or its outlet pipe is choked somehow.

So what might cause these?

A small pipe and its connections might be bunged up with silver-solder or flux, or passages drilled to meet might not quite do so,. but there are two other possibilities.

One is drawing error. The other, erecting-shop error.

I am not trying to be facetious or denigrate your craftsmanship as even the most experienced builder can do this... Make an assembly mistake so simple he cannot think it possible, reinforced by friends all sagely saying, "No, he'd never do owt so daft!" Consequently the problem seems to magnify itself and become even more difficult to solve. I have been caught out many times by such things, though I don't claim Gold Medal quality! For example.....

My club built a loco that refused to run on air for months until by sheer fluke someone not directly involved in building the motion-work originally set out to repair a tiny exhaust-branch leak caused by slight over-drilling, hence discover a gasket had been made and fitted with no hole through it. Yet the builders were all consummate craftsmen too skilled to do something like that! After plugging the drilling the LBSC way, it took a bit of oiled paper and ten minutes' work to make engine run, on air, just as it should.

'

So use the symptoms....

Air through the superheater works. Air through the boiler hence regulator does not.

Is this model's regulator in the dome as prototype, or a smokebox type?

With the superheater still disconnected but the rest of the boiler all fitted up, put some air at low pressure in the boiler, then open the regulator slowly to verify air emerges at the header.

It does? Perhaps the regulator is not opening sufficiently for the engine to work, or something is partially choked.

Remove the dome or regulator cover (according to type); lubricate it with a spot of steam-oil or water to avoid scratching the sealing-faces, and operate it from the handle in the cab, to see if it actuates from fully-closed to fully open, as intended. Small dimension errors might have a significant effect here.

It does that?

Suspect a blocked pipe, or something like a passage or gasket not quite as it should be. The pipe from a dome-mounted regulator to the header is usually straight once past the elbow formed by the regulator body. Remove or lift the valve itself. A bright light shone through from header to regulator may be discernible by looking into the valve body. Or gently push a piece of stiff insulated wire (e.g. a bit of low-power mains-cable) through to see if it meets the regulator. Or try blowing through a length of polythene tube on the header end.

'

These tests should show what is wrong, and you can then work out how to put it right.

.

The loco I mentioned above had the opposite fault!. The regulator worked twice - on its first public operation too - then refused to close! It is the rotary disc type in the dome. The regulator body proved very slightly out of true, preventing the disc from seating correctly.