> But how can I keep the temperature below 80 degrees in the top
> without adjust on the water? I don't need to adjust on the power
The temperature in the column is a RESULT of the alcohol vapour purity there.
for a graph of vapour temperature vs purity.
So to get the top of your column down to 80C, you need to have pure alcohol up there.
NOT vice versa. Making the top of your column cold does not make the alcohol pure (if so, we would only need 2 inch tall columns with a very cold section at the top).
So the question is "how do make the alcohol vapour pure" up there ?
The answer is that it needs to have undergone a whole series of redistillations before it gets up there. See http://homedistiller.org/refluxdesign.htm
for more details. To get those redistillations happening, you need to have some liquid coming down the column, to "clean" the uprising vapour. The vapour will take alcohol out of the falling liquid, and the liquid will take some of the water content out of the vapour. To get this working well, you need to spread the liquid out very very thinly. So you pass it down over a packing that has lots and lots of surface area (but doesnt take up much volume). For this to be optimum, it needs to be done at the dew point of the vapour (so insulate the column so not to get excessive heat loss, and dont stick cold through tubes @ 10C where the vapour wants to be at 83C). The more of the liquid you can provide (REFLUX) the more effective the process will be. The taller the height, also the more effective.
So in a nut shell, to obtain good purity (as seen by a low head temperature), you need:
* a tall column (at least 1m, preferably 1.2-1.5m),
* with effective packing (stainless steel scrubbers or raschig rings),
* at a stable temperature,
* with sufficient reflux (ratio of 4:1 to 10:1)
The key is the reflux. By changing the ratio of how much liquid gets returned to the column, you will have total control over the temperature that you see at the head of the column (provided the height & packing are sufficent).
Thats the principle behind the Offset Head design. Tall column with good packing. Condense all the vapour into liquid. Control how much you want to draw off & collect - by default the remainder will be returned back down the column as reflux.
If the valve is fully open you may be collecting say 150 mL/min depending on the amount of heat you supply. Normally you'd close that right down until you were only collecting say 20 mL/min. That means you are refluxing 130 mL/min (87% of the vapour being returned as reflux).
Notice through all of this I've never touched the power input. I leave that on full, to keep every thing at the maximum rate. The only times you touch the power is (a) if you have over powered the still, and the diameter of the column is fundamentally too small, or say the condenser. (b) If you dont have control over the reflux, and have a fixed amount of liquid being returned - in that case you are altering the ratio by using less vapour - but thats less than ideal timewise.