Heat Flux and Boiling Regimes: Part 2

What’s really interesting is that if you were to crank up the burner to 11 at Step 5 and allow the surface temperature of the bottom of the pot to continue increasing, the heat flux through the bottom of the pan absolutely falls off the cliff.  That’s peculiar… despite the fact we’ve added more heat, the heat transfer has dropped – how could this be?

In this scenario, the water at the bottom of the pot enters the dreaded Region III (Transition Boiling in ORANGE) or even worse the insidious Region IV (Film Boiling in RED).  In these boiling regimes, large vapor bubbles form sporadically and cling to the bottom of the pot, acting as an insulating barrier.  This “film” of vapor makes it MUCH more difficult for the pot to transfer heat from the burner to the water.

You desperately want to avoid Region III or Region IV boiling inside your steam boiler at all costs.  The unpredictable formation of this vapor “film” causes localized hot spots on the metal surfaces which will cause surface cracks and lead to premature failure.  Region III/IV boiling will occur when years of corrosion/erosion of the tubes thins out the material to a point where the local surface temperature overheats.  It will also occur if a burner is not tuned properly and is overfiring for the rated capacity of the appliance (e.g. a burner is firing at 100HP for a 90HP boiler).  Finally, fouled fireside or waterside conditions can also contribute to this scenario.  If you notice unusually high flue gas temperatures, there’s a good chance parts of your boiler are operating in Region III and/or IV.

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