Requirements for commercial vat pasteurizers (in short):
-Be able to heat and hold milk at 145+ for 30 minutes (161 for 15 seconds is not an option for vat pasteurizing)
-Stir constantly and effectively so every particle of milk is up to temp, the whole time (which also means the vat must be round, not square)
-Any outlet valve (if there is one) must be a close-coupled leak-detection valve ($2000+ new), or have no valve at all (pump milk out instead)
-Airspace must be heated to 150+ the whole time
-Have indicating and recording thermometers for milk and airspace, with a 24 hour temp-recording chart (Anderson's are about $2000)
-Have a way to chill the milk to 80-100 degrees, unless you pump it out first and chill it some other way
-And of course, all 3A stainless steel with extremely smooth welds
For consideration, a C Van't Riet 35 gallon vat pasteurizer takes at least an hour to bring 40 degree milk to 145. I'm not sure about the larger models. Cooling takes 15 minutes of cold water running through the jacket, then another 15 to coast down to 90.
The cheapest option I have seen is a steam tilt kettle (no valve needed) that has a lid custom welded with fittings for the thermometers, and a motor and paddle added. You can't chill in it if it's a self-enclosed unit. They come in every size from 2-200 gallons. A pasteurizer under 4-5 gallons could be heated on the stove, but beyond that you need side and floor heating for even, fast heating. Look here:
http://www.ruggleshill.com/_pdf/MA_MicrovatPasteurizer.pdfMike Viera in Sunnyside, WA makes custom pasteurizers out of steam kettles.
BTW, any milk for making pasteurized cheese must be pasteurized at the final processing facility. So a cheesemaker can buy pasteurized milk, but must re-pasteurize in their facility.