Thumper(s) to increase boil capacity

[tommysb]

If I can easily get hold of 30l or 50l (7-15Usgal?) beer kegs, at what point do we get diminishing returns by arranging them in a thumper style setup in order to increase overall system capacity?

Rather than using an 100l boiler, I was thinking to set up 2*50l kegs (or even 3*50l) in a thumper configuration, and load each keg with half the wash.

First keg is heated, vapour passes into keg 2, condenses, until second keg becomes hot enough to evaporate (that's how a thumper works right?

Points where we need to be careful:

We need to be able to hold enough 0f the liquid that's evaporated from keg1 in keg 2 without overflowing. Once keg 2 starts to evaporate, of course it can take more product from keg 1. But I presume we could not load the 2*50l kegs to the same capacity as a single 100l keg. SO lets imagine something like we can fill a single keg to 80%, 2 kegs to 70% overall system capacity, 3 kegs to 65%,....

Heat losses in the system:
Increasing number of thumpers increases losses, decreasing heating efficiency. Again, stacking more thumpers increases the effects of this.

So what's the practical limit? I would imagine that some sort of system with 2 to 3 kegs would allow more overall boiler charge, but not 2 to 3 times a single keg boiler.

Anybody have thoughts or insights about this? The kegs are readily available. The other big advantage is that I guess it would allow some stripping of solids in the thumper kegs. Yes you might also get some higher ABV off a single run than a single pot design, but I guess you pay for it in time and energy anyway.

What's the theory behind thumpers typically being smaller than the boiler?