pot diameter

[tracker0945]

Just to add my 2 cents worth, seeing this thread has been ressurected.

I believe that the 'practical' result of this (for the benefit of a hobby distiller) is the ability to effectively harness the heat supply.

If using an internal electric element, I think their would be little difference in the evaporation/collection rate observed whatever the boiler design.

If using an external flame I think there would be a noticeable increase in output using a short/wide pot than a tall/narrow one as it it would be more simple to get a better conversion of heat into the boiler (if you get what I mean).


Cheers.

[rezaxis]

Evaporation and boiling are two different things. We can evaporate without boiling, and we can certainly evaporate while boiling. But we're only talking about evaporation.

Evaporation is a surface effect.
If enough energy is applied to the molecules at the surface of a liquid, molecules will begin to leave the surface. If those energized molecules maintain enough of that energy they will stay separated from the liquid.
The total number of possible molecular sites available for evaporation increases with an increase in the surface area.
Of the total number of possible molecular sites available for evaporation in any given surface area, the actual number of sites in which evaporation can occur increases as the amount of energy applied increases.
Therfore it takes more energy to initiate evaporation at all the possible sites of a larger surface area.
Energy is lost in the liquid as evaporation occurs. This is the evaportive cooling effect.
The greater the evaporation rate, the greater the evaporative cooling effect, and the greater the amount of energy input required to maintain the rate of evaporation.

Irving Langmuir explained all this stuff. His works with surface chemistry won him a Nobel prize. Good luck sorting his works out though. It's heady stuff and over my pay grade. Irving is the guy who showed Thomas Edison that by placing an inert gas in his light bulbs would make the filaments last longer, thus putting the final piece in place that brought us all the incandecent lamp.

So, increasing the surface area available to evaporation gives you the ability to liberate more molecules, IF you supply the extra heat required to energize these extra molecules AND supply enough heat to maintain that evaporation rate by offsetting the evaporative cooling effect.

[rezaxis]

Oh, and a lot of interesting chemistry happens within just a couple of molecules thickness of the surface of the liquid.
Never are all the possible molecular sites possible at the surface of a liquid sites with enough energy to liberate that molecule. As these very energized but still unable to leave molecular sites mix around, I think some rectification happens right there at the surface. In other words, the energy input and the resulting evaporation rate affects the composition of the evaporated molecules. This may explain why too energetic evaporating, or more accurately, rectifying at the surface of the liquid, results in indistinct cuts.

[rangaz]

i was interested to know if you would get the same amount of vapor off a smaller surface area as you would for a larger liquid surface area, with the same heat input applied?

Yes,

As liquid vaporises, it changes to a higher energy state and as such, removes a little energy from its surroundings. The amount of energy it absorbs is called the latent heat of vaporisation and is measured in J/kg (energy required to vaporise 1kg of mass). You observe this when wind blows on you when you're wet. The wind evaporates the water, which removes heat from your body.

We make some assumptions like both boilers lose the same amount of heat to the environment, and that the liquids in each boiler have equal mixing.
The energy going in has to be going out somewhere since the boiler doesnt heat up once it gets to boiling point. we lose some to the environment, so the rest has to be going into vaporisation of the liquid. So basically in both boilers, the same amount of energy is going into vaporising, therefore the same amount of vapour will be produced.

In reality liquid surface area makes practically no difference to vapour production rate, but surface area exposed the heat source does. Increasing the surface area exposed to heat will maximise the heat transfer to the liquid and if you double the energy that makes its way to the wash, you will aproximately double the amount of vapour you produce.

The bigger constraint is your column. Excessive amounts of vapor for its size will produce more pressure in your boiler (can be observed if you blow air through a hose and then try blowing the same amount of air through a straw). This generally lowers the concentration of alcohol and raises the concentration of water in the vapour for a given concentration of alcohol in the boiling wash.

Basically, if you're trying to design your still, the most important thing is making sure you're column and condenser are big enough to handle the production rates you intend to achieve. For me, heat transfer efficiency isnt a biggie as gas isnt really that expensive, so I just went for the available and easily modified SS keg

[ZAXBYC]

Just going to throw in my version of things

Guys who say larger surface area increases the amount of vapour produced are right, simple test for this, is using an identicle heat source, boil off the same amount water in a wok and try it in a pan, you'll find the pan takes longer.

But when applied to the boiler, as others have said, and I shall paraphrase them all... the vapor has to go somewhere and not to atmosphere. In our case the surface of your wash isn't open to the atmosphere, the vapour is going through a 1"/2"/3" column up to a condenser before then.

If you have lots of vapour good for you, but can your column and condenser handle it all, and what is your maximum take of rate with the column and condenser? Pointless producing shed loads of vapour only to have the same output rate at the end surely?

Question:
Assuming your condenser was condensing all the vapour, surely producing loads of vapour for the same amount of energy input, must be good for purity, ie the loads of reflux you would get.

[rad14701]

Part of this debate boils down to (pun intended) efficiency of the use of the heat being applied... True, a wok may boil a volume of water quicker than a pan, but it is partly because more of the heat is being used... If you were to place a heat shield around the pot you would get equal, if not better, performance... That's why back in the olden days of moon-shining you used to see pots built into fire-boxes that let the fire heat the sides of the pot as well as the bottom... The thermal dynamics of a heated fluid don't change, it's how you make use of the heat being produced and applied...

So, if you want to get the most bang for your buck using a rim burner you'd be better off making an inverted cone that would hold the heat against the sides of the boiler and either having a gap around the top of the inverted cone or installing a chimney to direct the heat away from the top end of the still...

What good is a 60,000 BTU burner if all you're doing is contributing to global warning and not heating up the contents of the boiler...

[rangaz]

surely producing loads of vapour for the same amount of energy input, must be good for purity, ie the loads of reflux you would get.

Like I said, the more vapur you produce, the higher the pressure in your boiler. A still with a partial vaccum will boil at a lower temp, require less heat input for the same amount of vapour and will pontentially produce above 95%ABV. If you have higher than atmospheric pressure, the opposite occurs, giving you lower proof alcohol and a larger heating cost. Its one of the reasons we run our reflux stills slowly during a sprirt run.

[tracker0945]

Part of this debate boils down to (pun intended) efficiency of the use of the heat being applied... True, a wok may boil a volume of water quicker than a pan, but it is partly because more of the heat is being used... If you were to place a heat shield around the pot you would get equal, if not better, performance... That's why back in the olden days of moon-shining you used to see pots built into fire-boxes that let the fire heat the sides of the pot as well as the bottom... The thermal dynamics of a heated fluid don't change, it's how you make use of the heat being produced and applied...

So, if you want to get the most bang for your buck using a rim burner you'd be better off making an inverted cone that would hold the heat against the sides of the boiler and either having a gap around the top of the inverted cone or installing a chimney to direct the heat away from the top end of the still...

What good is a 60,000 BTU burner if all you're doing is contributing to global warning and not heating up the contents of the boiler...

and all this time, I thought that was what I was trying to say


Cheers

[snuffy]

In reality liquid surface area makes practically no difference to vapour production rate, but surface area exposed the heat source does. Increasing the surface area exposed to heat will maximise the heat transfer to the liquid and if you double the energy that makes its way to the wash, you will aproximately double the amount of vapour you produce.

Only as long as you are using an external heater.

Dimensions of the boiler can have a large effect on the heat transfer from an external heat source.

Heat transfer is directly proportional to the temperature differential, thermal conductivity of the wall material and transfer surface area and inversely proportional to the thickness of the wall material.

Length and area have no influence on the latent heat of vaporization. They don't show up in the units used to express this physical constant.

External heat source: large heat transfer surface with high temperature differential. Heat shroud around open flame heat sources. Hot plates: high conductivity material, good contact with heating element, thicker material is better to spread out heat and minimize scorching.
Internal heat source: minimal surface area (optimally a sphere) to reduce cost of insulation.

In either case, the shallower the depth of liquid, the better behaved the boil in terms of surging, frothing and foaming.

Different principles apply to pirate vessels as opposed to boiling vessels. Pirate boiling vessels are another matter entirely.

[rad14701]

I agree that the overall performance is relative to the individual boiler and heat source... I think the average home distiller that only makes occasional runs is just gonna do whatever works for their individual circumstance, regardless of efficiency... The more runs you make, the more important efficiency becomes...

[Socrates]

Full circle time. It is well to remember the original, simple question:

I was interested to know if you would get the same amount of vapor off a smaller surface area as you would for a larger liquid surface area, with the same heat input...

As tends to happen on the net, answers went far afield, with various posters speculating about different heating methods (eg internal vs external), about liquids evaporating from a sidewalk, watching spaghetti boil, ad nauseum. Whew!

One poster actually addressed the question accurately by quoting Avogadro's law:

Avogadro's Law:

(p1 * V1)/(T1 * n1) = (p2 * V2)/(T2 * n2) = constant

p = pressure
V = Volume
T = temp
n = no. of molecules

Easy to see that if you increase the volume, the number of molecules must also increase.

This made clear that boiler VOLUME - not surface area alone - was the key factor, all else being equal. More volume, more vapor (number of molecules). There's just no way around that. While the many speculations were interesting they failed to directly address the question.

The answer: Does decreased surface area decrease the vapor? Maybe. It would, but only if it led to decreased volume. Volume, not area, is the key factor. The real limiting factor is take off.

[snuffy]

Would that be liquid volume or vapor volume, oh Socrates?

[tracker0945]

The real limiting factor is take off.

Please explain.
Are we talking 1/2" v/s 2" column/lyne arm diameter here or condenser capacity or pot v/s reflux still configuration as being the limiting factor?

Cheers.

[RWB]

Would putting some copper shreds or marbles which would rattle more, or something, by making more surface area gas pockets form. That would increase area for the conversion factor from liquid to gas. RIGHT!

[andre.silent]

Hi,

I is hard to go through all ideas and explanations (some of them very scientific).

I would like to add one more simple equation showing the amount of vapors generated during the boiling (you can find it somwhere in the text of the original homedistiller page):
V=P/r
Where
V is the amount of evaporated liquid
P is the heating power
r is the heat of vaporisation (aka latent heat).
This equation reflects the energy conservation law in regards to evaporation process.
Some people may say (and they would be right) that there are some more factors which should be considered (as vapor velocity, pressure, overheating degree etc). These factors are practically negligible in moonshining job.

Coming to the point - the amount of vapors is NOT related to the ANY surface (when the heating power value P is constant)

But (as many pointed out already) the heating power transferred to the liquid may be related to the heating surface (eg. in the case of external heating or in evaporative cooling).

There is however one practical consequence of the pot shape. Once you boil the mash it creates quite amount of foam. In the slim pot there is a danger of "boiling out" the foam into the column and then sometimes even to the outlet. This is more likely at the start up, where the mash is prone to overheating and "bursting out" when boiling starts.
In this respect the wider pot is a little bit more safe.
I would not recomend putting any packing into the pot (Raschig rings or so) - at least I have some bad experience with this idea. They can create flooding (choking) of the boiler and eventually transfer of the liquid from the boiler to the column.

Anyhow the use of "boiling stones" is always very recomendable. (For those who don't know what it is - just several small pieces of porous material eg. clay pottery, which make a boiling more smooth and uniform. They also protect against the initial "bursting out" of heated up mash.

Sorry for answering not only the topic question.


andre.silent