What is a thermic siphon?

What is a thermic siphon, and what does it do?

On the B&M3713 page, they show new ones being made.
http://www.project3713.com/news2012.html

Video of them being hand flanged:
http://www.youtube.com/watch?v=RFu0AqcmzeI

A syphon is another method of getting more heating surface into the firebox, where it presumably can absorb the most heat.

Imagine two triangles of metal, parallel to each other 6? inches apart, with their long sides welded? riveted? to the top of the firebox and a pipe from the bottom of the triangles to the front of the firebox. The flat box thus formed is full of water that's hopefully getting well heated.

But don't ask us what the advantages and disadvantages were-- we don't know. Plenty of modern locomotives eschewed them.

The energy-efficiency advantage is what Timz described. The firebox is a hotter place than the boiler tubes, so a square foot of firebox wall transfers more heat to the boiler water than a square foot of tube surface. (This is why most modern steam locomotives had combustion chambers: forward extension of the firebox into the boiler barrel, at th expense of moving the rear tube-sheet forward and so decreasing tube length.)
Installing a thermic siphon in effect gives the firebox two extra walls (smaller in area than the "real" walls at the side, but still worth while. Steam locomotives were almost always under 10% efficient (= less than 10% of the energy released by burning fuel wet into turning the wells and pulling the train),so anything to increase the proportion of the heat that went toward converting water into high-pressure steam (and decreasing the proportion that went up the smokestack) was good.

A secondary advantage was in safety. Boiler explosions happen when the top of the firebox overheats and softens (= starts to melt); the way to prevent this is to make sure there is water over the top of the firebox AT ALL TIMES: water absorbs the heat from the metal of the firebox roof and so keeps it cool much better than steam does. The lower pipe of the thermic siphon draws water from low in the boiler barrel, it gets heated in the siphon and flows out the top ... onto the roof of the firebox.

I think the biggest disadvantage was maintenance expense: a siphon is a complex metal shape, attached in complex ways to the firebox front and roof, and so exposed to the hottest conditions in the boiler. I think -- I don't have chapter and verse from an engineering textbook, but this is the interpretation I put on the things I have read -- that the railroads that didn't use thermic siphons avoided them because the siphon and its joints with the firebox front and roof had to be repaired too often.

(Steam locomotive boilers are punishing environments for built-up metal components! It was well-known that you got better efficiency if yu divided the blast pipe into multiple nozzles, so the exhaust steam from the cylinders would be more efficient at creating draft to keep gasses moving from the firebox through the tubes and up the stack:many European railways used such devices ("Kylala" or "KylChap" exhaust systems). American railroads in general didn't: what with large amounts of steam going through them, and immense amounts of cinder-laden gas flowing past them, the nozzles wore out too fast.)

One more claim made was that they enhanced circulation by "pumping" water out of the relatively sluggish area at the bottom of the boiler barrel. This would be desirable because greater velocity increases heat transfer and lessens the likelyhood of solids depositing in a hard to clean area.

also, looking at the pictures of 3713, Good Lord, look at all that drilling ! :o

the thermic siphon theoretically drew cooler water from the bottom of the boiler so it could be heated faster. It also served as a support for the brick arch at the front of the fire box. the 1941 loco encyclopedia shows several types of siphon each touted by its supplier as better than its competitor. look up boilder and fire box design for more insight.

Another round of updates on B&M 3713.

http://www.project3713.com/news2012.html


And more details on Thermic Siphons, I now see the complete picture, and they make perfect sense.

Another idea behind the thermic siphons was that there were no moving parts. Simple convection of heated water rising, drawing in more from below. The bottom of the boiler around the firebox was a location of low circulation (it was called the mud ring for obvious reasons) so this helped keep water moving. Normal circulation in a boiler is rising water over the firebox moving forward to the front tube sheet, where the water is cooling and dropping to the bottom of the barrel, and then moving rearward to the firebox again. Again, simple convection.

They could be trouble to maintain, due to their odd shape and location, but the brick arch did need to be supported, so they provided double duty.

Many ways were tried to get more steam out of a given amount of fuel, some good, some not.