A SCOTCH MARINE BOILER FOR MY TRIPLE EXPANSION ENGINE

[crueby]

Ooooohhhhh, okay!  I had not looked close enough at the plan cross section in the very first post, sorry!  Never seen that arrangement before, so I assumed it was the usual  burner-at-one-end-chimney-at-the-other style. So, as you say, not part of the pressure vessel, though it will take a direct hit from the burner flame coming through the lower pipe.   

 

[Jasonb]

regarding expansion, if we said the metals could get upto 500deg C which they are unlikely to then the difference in expansion across the diameter will be 0.21mm or 8thou. ( 0.83mm - 0.62mm ) so provided the "snug fit was eased a bit to sat 5thou all round then the aluminium won't apply any force to the boiler shell

copper boilers used to be caulked withsoft solder and that never melted out at 200deg C or less and is why 648 is OK and aluminium unlikely to melt.

[Vixen]

Yes I took it a sAluminium at both ends, and agree that too snug a fit may cause problems with expansion and depending on the amount of use  galvanic attack could be an issue.

But regarding heat the plates are a bit like the 648 will they see direct heat ?and if so what flame temperature and more importantly material temp, a typical air/propane torch may only give 750degC temp in the components and that is with the flame directly applied

You need to be careful how you interpret that graph. That graph shows the flame temperature of propane/ air to be 1950*C and that has only recently been demonstrated to be hot enough to melt pure copper down tubes. It appears that the flame tube (if that is what it's called) is 2.25" diameter, so will do little to cool the flame. Therefore, I would be very concerned at the flame playing directly on the return end plate. The return path through the smaller boiler tubes will remove most of the heat and so reduce the flue gas temperature, so I am less concerned about the smoke box end.

Why make-do with what's in the scrap box? Why not work to MIL TBD41 standards?
For those who are not familiar with that standard, it stands for "Make It Like The Bl**dy Drawing For Once"

[Jasonb]

I can melt steel with my propane torch, just depends on how thin it is and the output of the burner and where you hold it in the flame.

Do we actually know what John is going to fire it with, Harris suggests the Stuart paraffin blowlamp, Flame temp of paraffin 1100degC

There are other variations from the drawing so would really need to start from scratch if sticking to them.

[simplyloco]

Yes I took it a sAluminium at both ends, and agree that too snug a fit may cause problems with expansion and depending on the amount of use  galvanic attack could be an issue.

But regarding heat the plates are a bit like the 648 will they see direct heat ?and if so what flame temperature and more importantly material temp, a typical air/propane torch may only give 750degC temp in the components and that is with the flame directly applied

You need to be careful how you interpret that graph. That graph shows the flame temperature of propane/ air to be 1950*C and that has only recently been demonstrated to be hot enough to melt pure copper down tubes. It appears that the flame tube (if that is what it's called) is 2.25" diameter, so will do little to cool the flame. Therefore, I would be very concerned at the flame playing directly on the return end plate. The return path through the smaller boiler tubes will remove most of the heat and so reduce the flue gas temperature, so I am less concerned about the smoke box end.

Why make-do with what's in the scrap box? Why not work to MIL TBD41 standards?
For those who are not familiar with that standard, it stands for "Make It Like The Bl**dy Drawing For Once"

Hi Mike.
If there was a drawing available I would have probably attempted to make it to that.
However, in it's absence I did my own thing, as most competent model engineers would do. I will leave it as it is and see what happens in practice.
John

[Laurentic]

On a full size Scotch boiler the 'flame tube' is usually called the furnace, or furnace tube.  Originally Scotch boiler were coal fired so would have had a grate longitudinally ecxtending about 1/2 to 2/3rd down the furnace tube with a brick wall at the end, the hot gasses would then go over the wall to the end, which is called the reversal chamber, as the flue gasses then reverse direction and come back down to the front of the boiler to the smoke box and then up the chimney. 

When they were coal fired there was a sacificial fusible plug on the top of the furnace tube over coals.  If the water level was lost then the plug overheated, melted and the boiler water then poured through, put the fire out and prevented the furnace overheating from the coals, sagging down, fracturing and causing a boiler explosion, which would (and did) have killed anyone nearby and the boiler might well have ended up about 100 yards down the road, or a couple of fields away.  If you ever get to see a photo of a boiler explosion, it is quite horrifying, the energy released so suddeny is horrific, people were killed and the damage done to buildings exceedingly extensive, and, as I said, the boilers used to end up down the end of the street.  It was partly due to those occurances that 'proper' boilers in the UK are subject to statutory cold thorough inspections internally and externally every 14 months, or within that period, followed by a working inspection upon return to service when the safety valves were checked for blow-off at the correct pressure under steam and at full firing.  Why 14 months?  The statutory inspections all started when there were plenty of factories and mills in the UK, and they all used to shut down at Easter, and Easter being a movable feast 14 months covered it!

Later Scotch boilers were oil fired, a pressure jet flame being projected down the furnace tube.  Sometimes the brick wall in the furnace was retained, to slow the gasses down, sometimes not.  If the oil burners were changed to gas fired at a later date - and at one time when gas was a lot cheaper than oil a lot of various boilers were changed to gas firing, it was found that the gas flame burned with a far longer envelope than the oil flame, giving rise to the tube ends on the rear tubeplate burning away.  A solution, to avoid endless requirements for tube end repairs by welding each year and possible tube leaks before the year was up, was to fit ceramic protective rings in the tube ends.

In their day Scotch were a renowned, reliable and very respected boiler used extensively both on land and at sea

None of the above applies to the Scotch boiler in this thread thankfully, but just saying!  Provides a bit of background info to the model.   

Chris

[Laurentic]

Standard MIL TBD41 - Make It Like The Bloody Drawing For Once, - bit revolutionary that Mike, for us model engineers! 

Chris

[Jasonb]

I suppose if there was a bit of a worry about the flame heating the plate at the far end it would not be difficult to add a circular disc mounted on a central stand off to act as a baffle, something you see on model and full size firebox doors

I see that PM Research vertical boilers have a cast aluminum cone on the top to collect the flue gasses and direct them to the central chimney and that mounts very close to the top tube plate. Anyone read of reported problems with this arrangement?

[Jasonb]

Touching on the flame temperature again, I quite often see those new to silver soldering wanting to know what to use or why they end up with  a blob of solder rather than a nicely flowed joint. Many think that a MAP gas torch should do the job as they have read that MAP gas burns hotter than propane at 2700degC

Well it does but they miss the point that it is not the temp of the flame but the actual output of the burner that matters be that in watts or BTU. So their Map torch with maybe a 12 to 15mm burner dia will actually only be putting out 2 or 3 KW where as the usually suggested Sievert 28mm nozzle puts out about 7KW and therefore gets the job hot enough even though the flame temp is less.

[simplyloco]

Taking Jason's point about a baffle, it will be quite easy to mount a thin steel plate on the 3 mm stay that holds the two ends on. I'll just use a nut as a spacer between the plates. I'm confident that the alloy smoke box won't pose a problem, after all, it is 300 mm away from the burner and also 5 mm thick.
Potential problem solved!
John

[Jasonb]

I was thinking central to the furnace tube but a bigger disc central to the barrel would also work. Less than 3mm would do, I think my Fowler has 1mm thick or whatever that is in SWG. but depends what you have to hand, could always flatten that yarrow boiler top tube and cut one from that

EDIT that flattening of a tube made me think you could make the baffle slightly curved and then that would help return the gasses

[simplyloco]

SNIP

EDIT that flattening of a tube made me think you could make the baffle slightly curved and then that would help return the gasses

Agreed on the curve. I have plenty of alternative material.

[simplyloco]

Following some good advice I've fitted a stainless steel baffle plate to the big end cap, sandwiching some ceramic fibre insulation for good measure! It ain't gonna melt..
I've also done one side of the smoke box cladding. I'm torn between cap screws and slotted csk ones. I prefer the former...
The box is now so snug I'll have to relieve the outer tube to relax the chimney to get it vertical!
 
Fiddly stuff!

[simplyloco]

Cladding attached and up to steam in less than ten minutes!
It held 85psi without trouble. I'll take it further when I fit a proper water gauge.

[Vixen]

 

Congratulations!!!   Well done!!!  It's a steamer!!!

That's an impressive blow lamp firing it. Just as well you protected the far end plate.

Cheers   

Mike