Apologies in advance for the necro...
My keg washer is a spin on what the OP designed; however, I use a Challenger VI (flat bottom corny) as my primary fermenter, and I cannot seem to CIP the krausen ring without a good toilet brush scrubbing (don't worry, it's dedicated to the brewery & never seen a toilet).
I typically use home-made PBW (although I have used real PBW with similar results) at just over the prescribed minimum CIP concentration (1/3C:2.5gal or 1.06oz/gal) at 145-155degF for 20-30 minutes, but even an hour on the washer leaves some krausen behind. Incidentally, an overnight soak at the same concentration is typically sufficient to kill the ring completely following a hot rinse, so it's not as if the cleaner lacks the necessary cleaning power, but I don't want to wait overnight to clean a keg - especially when I have more than one to clean - but I want to be able to do something else while the keg is washing.
The material differences in my design are 1) my washer is not standalone, it is integrated with my hard-piped, all copper & aluminum brew rig - so no acid cleaners - and 2) I don't have a nozzle on the washer output... instead I drilled a bunch of 1/8" & 3/16" holes in a 1/2" copper cap at the top and seven 3/16" holes winding down the sides of the copper tube. The flow rate seems adaquate (4-5 gpm) and I'm all but certain that I'm getting 100% coverage inside the keg. As the cleaning solution cools, the liquid level inside the keg rises above the krausen as a result of the negative pressure, so the entire ring is even getting a good soak for at least 15-20 minutes of the 30 minute cleaning cycle. My guess is that I'm not getting the penetration necessary to break up the krausen since the cleaning solution simply flows over the krausen ring at the mere speed/force of gravity for those first 10-15 minutes; however, if I simply drilled some holes near the bottom of the tube (so as to spray the top of the inverted keg), I'm afraid I'm still going to still have a ring but with small circles of clean stainless where the streams actually hit the ring.
Has anyone had any success CIPing a krausen ring in under 30 minutes without mechanical intervention?
Is a lower/higher flow rate necessary? Should I up the concentration of PBW? Is a nozzle the answer? Or is it just not possible without a long soak and/or scrubbie?