Austrian Federal Railway (ÖBB) 80 series with heat exchanger

I initially believed that the apparatus installed on top of this loco was an air cooled condenser. However after further research I learned that this is actually a heat exchanger. It preheats intake air before it enters the firebox using waste heat from the exhaust. It's a heat recirculator just like a feed water heater. Judging by the fact that it was only equiped on one example of the 80 series which was otherwise very successful, it's reasonable to guess that it was not as effective as a feed water heater. This follows with the fact that air is a poor conveyer of heat compared to water. The addition of bends in the chimney most definitely increase the back pressure in the cylinders and reduce drafting efficiency. Drafting efficiency is the most important type behind cylinder efficiency in a steam locomotive, so features that reduce drafting efficiency need to have a really good benefit to justify introduction and this preheater certainly doesn't.

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