I'm an ex mechanical contractor and have purchased and installed dozens of boilers.
I'm also planning a distillery.
Determine what size boiler you need.
In round numbers:
1 BTU will heat 1 pound of water 1 degree F.
1 gallon of water or wash is about 8.35 pounds so you need 8.35 BTU for each gallon per degreeF.
If you heat your wash from 50F to 172F you need (122 x 8.35) 1018 Btu per gallon.
If you have a 150 gallon still, you need (150 x 1018) 152,805 BTU's output from your boiler, to heat the wash from 50F to 172F in one hour. This does not include heat losses to atmosphere from the still equipment.
In addition to sensible heat you need latent heat to vaporize the ethanol. Latent heat of vaporization for ethanol is 364 BTU per pound.
If your wash is 10% ethanol it will take (150 x .10 = 15 gallons) 15 gallons ethanol weighs (6.58 x 15 = 98.7) (98.7x 364) 35,926 BTU to vaporize the ethanol. If your cycle time is 3 hours you need (35,926/3) 11,975 BTUH for vaporization. Someone check my math.
These are rough numbers that will put you in the ballpark. Add losses from the equipment and a safety factor.
Surely still manufactures can give us BTU heat input to operate their equipment? If they can't i can measure and calculate the losses if I had access to an operating still or two.
Write a boiler specification and put it out for bids. I would be happy to assist anyone or everyone by writing a standard specification for distillers to use when buying boilers. We would all benefit from a set of standard specifications to use when budgeting and purchasing all the equipment for a distillery. Anyone interested?
I also have some friends that wholesale boilers that could offer input.