Calculation of Condenser Size

Theres quite a few variables involved, so its hard to cover every situation.

You have to consider how hot the vapour is, what its composition is (eg water takes 3x as much cooling to condense than ethanol), how cold the cooling water is & how hot its going to get, and how well the cooling water is flowing around the tube, then even stuff like if the tubes are vertical or horizontal.

There are very detailed equations about that let you work this out precisely . But you can also use a few simplifying guesses to sorta give an idea of what size it could be, for far less effort. The results may not be quite as accurate, but they'll give you an approximate idea of what size is involved (so to be safe/certain you might even just double them !)

The hardest bit is the"heat transfer coefficient". This descibes whats going on inside & outside the cooling coil or tube, as well as the heat transfer properties of the tube (eg plastic vs copper). Rather than doing all the calculations, we can use a "typical" value. For "organics" being condensed using water in a shell heat exchanger, this value can be typically 700-1000 W/m2C. Lets use 850. This is for industrial condensors, where the cooling water will be flowing past the tube at a fairly good velocity. For the situation where the cooling coil is just sitting in a big tub of water (and nothing is stirring the water), this value will be more like 100-200 W/m2C (lets use 150 W/m2C).

Just remember that the size of the pot is irrelevent. Its the heating element we need to match.

If you have a "reflux" condensor somewhere in your still as well, this will be taking out some of the heat too. Subtract away the amount of heat used there, from the total. So then your distillate condensor might not be as big in that situation.
Input your Condensor Requirements
Cooling water input Temperature ° C
Cooling water outlet Temperature ° C
Tube Diameter mm   (1/2 inch = 13 mm)
Heat Input W
eg "Heating element" - "Cooling done in Reflux condensor" if seperate
Vapour Temperature ° C
Select Heat Transfer Coefficient
Jacketed Condensor (eg tube condensor) approx 850 W/m2C
Natural Convection (eg coil in a tub of water) approx 150 W/m2C
or guess your own value .. W/m2C
Typical length needed : m.   ( inches)
Typical water flowrate needed : L/min

Note that you don't need such a long condenser if you keep the outlet temperature of the water cold. But then the flowrate of water needed increases. Yours to choose.

If you're going for the "coil in a tub" approx, the "cooling water outlet temperature" refers to how how the tub would typically get to at the end of the run. The maths isn't strictly true, but should do as a bit of a guess. You can multiply the flowrate by how long you'd be running the still for in order to work out how much water the tub would need to hold.

As always, they're only as accurate as the guesses used, and the guesses I've shown above in the examples might not match your situation in practice. But doing this should give you a basic starting point from which to experiment. If in doubt, double the size you estimate, and you should be OK (though your pocket a little bit lighter).


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