Standard calculation for BTU?

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Is there a standard calculation for BTUs needed per gallon of still capacity? I have a guy who can design and build a still for me, but he doesn't know how many BTU it needs to be.

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Yes there is, take the specific heat of your wash and temperature and the heat you wish to raise it too and then subtract them, this will give you your BTU's needed to raise the wash to the temp needed. You can read this thread to figure out your specific heat:

The 10% ABV wash example on that thread is:

1030 x .9 x 75 (wash temp) = 69,525 Btu

The temp wanted is 212 F

1030 x .9 x 212 (heated wash temp) = 196,524 Btu

BTU's needed to heat wash to 212 F:

69,525 - 196,524 = 126,999 BTU's

rw

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I'm not completely sure that I understand you question, but I hope that this helps:

For rough estimation purposes, remember that "a pint's a pound the world 'round" and that one BTU is defined as the amount of heat that it takes to raise one pound of water one degree F. Therefore, it takes 8 BTUs to raise one gallon of water one degree F. So take your desired temperature change, multiply it by eight, and you've got your "BTU requirement per gallon of still capacity". Obviously solids content, ethanol content, and other variables are going to change this number slightly, as mentioned in the above post.

But there are other considerations as well. How much time do you want to take? Say you're wanting to take 100 gals of 75 deg F water to 212 deg F in 30 min. The total BTUs required are (212-75)*8*100=109,600 BTUs. In order to accomplish this, you will need to have a boiler capable of putting out 220,000 BTU/hr (twice the "BTU requirement per gallon of still capacity"). Also, you need to think about the capacity of the heat exchanger that is heating your still. Those calculations get very tricky and are typically done by the still manufacturer which is one reason that I find your post a bit confusing...

Also, there is the latent heat of evaporation. This is the heat required to change the phase of your 212 deg. F "water" into 212 deg. F "vapor". In the case of water, this heat is 7,763 BTU/gal and in the case of pure ethanol this heat is 2,397 BTU/gal (big difference, right?) and the latent heat of evaporation of the various mixtures of the two fall somewhere in between. If you count on distilling pure water, you'll have more than enough heat to distill any mixture of ethanol and water.

So if you want to distill 20 gallons of "water" per hour, you're going to need 7,763*20=155,260 BTU/hr. Note that this amount of heat is greater than the total amount of heat required to heat up your 100 gal still. So if you were to spec out the boiler based on a one hour heat up time and were also hoping to distill at a rate of 20 gallons per hour, you'd find that you did not have sufficient BTU output on your boiler to meet your desired distillation rate (at least when the ethanol content is extremely low).

I'd say that if your still designer doesn't already have a grip on some of this basic stuff, you might want to shop around a bit and look at some alternate manufacturers. As I hope the above basic examples have shown, building an actually well-designed still is no simple job and requires either lots of experience, or lots of theoretical knowledge (preferably both).

Either way, each still has its own individual temperment, and I've seen great spirits produced on homemade stills and on shiny new stills built by reputable firms. When in doubt, over-engineer things a bit, and have fun!

Nick

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