RE: Surge Boiling
- Hi, Harry,
I think at least part of your concern is a direct result of using
a potstill, especially a potstill with a column. With a column-
type potstill with the thermometer placed at the head, just
before the vapor enters the condenser, head temperature
is intrinsically and wildly non-linear, for the following reason.
As energy is added to the wash in the boiler, the wash
temperature will increase in a more or less linear fashion,
but as long as the wash temp is less than boiling temp,
relatively small amounts of vapor travel up the column to
heat the thermometer at the head, and any changes in
head temp occur much more slowly than wash temp
In the last very few degrees before wash temp boiling,
however, heated vapor starts moving in the column, and
head temp rockets up, and by the time you actually see
head temp in the range you expect for distillation, your
wash may already be at a full boil. If that wash is a foamy
one, like rum, you may already be puking foam through
I also run a potstill with an unpacked column, and it's a
great still design for flavored boozes, and I also have to
deal with the head-temp non-linearity you must deal with.
I use a digital kitchen thermometer with a remote probe
for my head thermometer, and it includes a programmable
temperature alarm. I start each run with that alarm set to
40C. (Using propane heat) I turn the gas on full until the
alarm goes off. I then turn the gas down to perhaps 1/3
and watch like a hawk as the head temp increases. At
perhaps 60C I turn the gas on lowest and watch the head
temp creep up to whatever the wash's starting temp is.
If I'm stilling a rum wash, I may set the alarm at 30C.
As far as the wild fluctuations *after* you first start
boiling, I haven't a clue.
Oh, And two comments inline.
I hope this helps.
"boe_didley" boe_didley@... boe_didley wrote:
>I've been a member a while and have spent time reading up and
>So eventually, i built a simple still. It is a 4.5ltr boiler with a
>38mm copper column, about 90cm tall. I have a 45o elbow from the top
>nto a jacketed condenser, which is very big. In total length, the
>condenser is 1.5m long. I wasn't sure what sort of temperatures or
>pressures i was dealing with, i had the copper tubing lengths so i
>just made it big.
>The column is fixed to the boiler lid with a bulkhead fitting and
>i've made gaskets to seal the rim while little bulldog clips hold the
>My heating for the boiler is a hot plate, 2K adjustable.
>The first time i ran the thing, it heated up nice and slowly, but
>when it got to 60oC, the temp rocketed and the whole thing boiled up
>nto a mess.
>The second time i tried a double boiler to try and act as a buffer.
>It worked but only slightly. I did a water run to test the system,
>but it's impossible to hold a temperature.
>Do i have this wrong? If i only use plain water will it be impossible
>to keep at a steady temperature below boiling? Because it's just
>water, will it just try and get to boiling point and that's it?
>If i had alcohol mash to distill, would the temperature go up moreNO
>I added a few thermometers inbetween run 1 and 2 for control and it
>helped a lot with the hot plate settings. I'm constantly clicking it
>on and off but there's so much lag between the knob and any real
>world change, it's near impossible to control. The temp difference on
>run two was 90oC at the boiler and 14oC at the take of point on the
>So it seems that the hardest part is temperature/boiler control. How
>do other people do it? Is it easier when there's an alcohol component
>in the water, so there's two boiling points? Would an indirect system
>be better? Should i get a better hotplate or do i need to take a temp
>reading somewhere else?
>Thanks for any input you guys can give.
Zymurgy Bob, a simple potstller