We deal with this question on a regular basis as we offer jacketed/insulated Batch Pasteurizers.
Electric Heat: As someone mentioned in this thread already a circulation heater with an expansion tank in a closed loop will do the trick. Depending on how fast you want to heat and what temperature you want to obtain. To heat a 100 gallon vat with milk from 38F to 145F will require about a 45kw heater.
On Demand Water Heater: Another great option, it offers some flexibility via natural gas, LP or electric. Set it up in a closed loop with an expansion tank. Draw back is on demand water heaters cap out at 185F, so if your looking to get to that temperature it will cause some issues.
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