Supporting > Boilers
A SCOTCH MARINE BOILER FOR MY TRIPLE EXPANSION ENGINE
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. :old:
Chris
Laurentic:
Standard MIL TBD41 - Make It Like The Bloody Drawing For Once, - bit revolutionary that Mike, for us model engineers! :lolb:
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
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