"Tracyman,
It's easy to get wrapped around the axel with the boiling point stuff. But it's one of the fundamentals that is an absolute ""must"" to understand.
Quote:If your mash is boiling, then that means the water AND the alcohol are both boiling
Correct. In an ideal world, when you distill alcohol, you're dealing with a binary liquid. That is, you have a solution that's a mixture of pure water and pure ethanol. When these two components are mixed together they don't act independently -- they act as a whole. So when you want to bring it to the boiling point (the temperature at which the internal vapor pressure equals the pressure exerted on the liquid) you get rising vapor. The boiling point of water is about 100 C and the boiling point of alcohol is about 78.4 C. When the two are mixed together, the alcohol doesn't vaporize when the temperature reaches 78.4 C. Instead, the solution as a whole will reach its boiling point somewhere in between, depending on the relative concentrations of water and ethanol. So the higher your abv, the lower the boiling point (BP) ... and the lower the abv, the higher the boiling point.
Now, the next thing you need to understand is that once a given liquid (any liquid for that matter) reaches its boiling point, the temperature will not increase any further. You can add as much energy as you want to a pot of boiling water, but its temperature will never increase beyond 100 C (the latent heat of vaporization). You can produce a greater volume of vapor, but the temperature of the water (the liquid) will remain constant.
However, there's another fundamental involved with a boiling binary liquid, the concentration of the lower BP component will be higher in the vapor. In our case this means that concentration of EthOH will be higher in the vapor than it is in the wash. For example, a 10% abv wash will have a boiling point of about 93 C. The vapor that is produced from this wash will have an EthOH concentration of about 53% abv ... and that's why we do what we do.
So, since the vapor has a higher concentration of EthOH, that means that the wash is slowly losing more EthOH than water -- the abv of the wash decreases throughout the run. This means that the boiling point of the wash starts to creep up since its abv is decreasing.
This is why many experienced distillers don't bother with a thermometer in the kettle. At best, it can be used to give us an estimate of the wash abv. But that's all. And we know that we can't control the boiling point of the wash -- that's determined by the abv. So that kettle thermometer doesn't really help us much.
Quote:One thing that confuses me is that most thermos are placed in the vapor path---wouldn't you want it in the LIQUID, if you wanted one at all?
Well you might get some hot debate on this. Personally, I don't think the thermometer in the vapor path gives you much either. Sure, it can give you some idea of where you are in your run, but I'll never adjust my rig based on vapor temp. All of my adjustments are based on the collection rate I want. The vapor temp is just something to watch (like for unexpected changes). Do you really watch your speedometer when you drive? When refluxing, your collection rate controls your reflux ratio, hence your abv -- the temperature is irrelevant. It is what it is.
Anyway the key things to keep in mind are:
- EthOH and water acts as a whole, not independently.
- The boiling point of the wash is basically determined by the abv.
- You can't control the temperature of a boiling wash.
- Adding more energy will only increase the volume of vapor (not the wash temp).
- If you can't control the temp of the wash, why bother measuring it?
I didn't intend to be so long-winded. But this is the good stuff. The better you understand it, the better you'll be able to drive your rig ... and experiment.
Regards,
--JB"
