I had the same issue with my plate chiller. Emailed the manufacturer and they sent me a pretty detailed process for cleaning. Here is the jist from Duda for my chiller..
As far as cleaning, backflushing with boiling water, and then using starsans before a brewing day to sanitize are our preferred methods. There are of course many other methods, but the rest seem to have both pros and cons:
PBW - I love PBW. Does a great job cleaning, but doesn't sanitize like starsans will. This is a good option if your filter didn't do a good enough job, or if you don't want to flush with boiling water.
Starsans after a day, rather than before for sanitizing - no point in this. Starsans won't clean as well as PBW will, and the risk you run into is some people like to let the exchanger just soak in starsans. Seeing as starsans is mildly acidic, this isn’t good for the long term life of the exchanger.
Lye solution - this is actually the method I prefer, submerging the exchanger in water and lye, but it is technically not recommended. The lye water will clean and sanitize better than any other chemical out there, but it can do some very minor damage to the chromium coating on the stainless steel, taking away a layer of its protection from corrosion. So you might have to replace the exchanger after 20 years if you go this route. This is also a much more dangerous chemical than the others mentioned, you want proper hazmat gear for this method. But its still the way I go, because the cleaning is second to none.
Baking - throw the unit in the oven, 400 degrees for 3 hours. This is my boss's preferred technique. No chemicals involved, absolutely sanitizes, burns out anything caught inside, and removes any fluids from inside the unit. My objection to this method is that some types of trub, if already inside, can get cooked into the unit and make things worse. “
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