pot diameter

[carlos castenada]

hi all,

i've read a few posts lately about different boiler sizes e,g smaller diamater larger height and smaller height larger diameter.
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 just interested thats all.

[Dnderhead]

I vote, larger surface more vapor.

[Hawke]

Yep, more surface area is definately better. Most of us need to balance this with practicality issues though.

[theholymackerel]

Humm...

Yall are makin' me overwork my greymatter.

If what yall are suggestin' is true then two "perfectly insulated" (zero heat loss) boilers of equal volume, one short and squat and the other tall and slender, both have equal heat added and the short squat one heats up faster and gives off more steam?

I doubt that.

Thou I'm fairly handy with physics, I can't think of any Law that would back that up.

Will anyone enlighten me?

[Hawke]

It won't heat up any faster, with the same heat source, but once it reaches temp, the more surface area lends itself to quicker conversion to vapor.

[theholymackerel]

How so?

[I-GOR]

I vote, larger surface more vapor.

Let's say you got 5 gallons water in a pipe, tipped on end, open on top. Let's say, 6" diameter = 28.26 square inches. It's gonna take a while to evaporate.

Now you got 5 gallons in a larger one; Let's say 12" diameter = 113 square inches , it will evaporate much faster because it has 4 times more surface area.

[theholymackerel]

I'm not doubtin' yall. I just know that any repeatable experiment in physics has a named law to state what happens.

Please state the Law so I can Google it and learn.

As far as I know now, energy can neither be created nor destroyed, so my thinkin' is equal energy into equal sized perfectly insulated boilers will equal equal steam out.

If this is actually wrong please state the Law so I may learn.

[Dnderhead]

Evaporation depends on
1) the temperature at the surface
2)area of the surface
3) air pressure ( hopefully no back pressure in your still)

so this tels me that large surface area and large take off (a place for vapors to go) whould be the best
It whould not make a difference if the take off (lyne arm) is restricted. ( no place for vapors to go) ( as some use 3/8-1/2" )

[theholymackerel]

So far four voices have spoken yea or ney.

Three yeas, and I'm the only ney.

If so many of ya are sure that makes me think again, but I still gotta be like that man from Missouri. Show Me. What postulate, theorum, or Law is it that leads yall to believe this?

Enlighten me. There is alot of sneaky stuff in Physics that ya don't get exposed to unless ya go down a specific path. I want to learn.

Show Me.

[Socrates]

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 (assuming that pressure and temp remain the same). Practical experience seems to bear this out as larger diameter columns (say 2 inches) can be made to process more alcohol than smaller columns (say 1-1/2 inches).

It should be obvious that the column is really simply an extension of the boiler with a larger column adding more volume, more molecules, more alcohol and or desired components to be taken off.

Socrates' Law:

In practical reality the real stricture is not the boiler, but more the column. In the opposite manner, more laws require more lawyers leads to less money retained. Paying attention to laws will divert your time, money and production of good alcohol.

Corollary:

The more laws are discussed, the more money - and alcohol - is diverted to the attorneys, who accordingly, spend the money and drink expensive alcohol, thus minimizing legal efficiency and thereby prolonging litigation and minimizing the distiller's distilling time.

QED

[rad14701]

My vote goes to more surface area releasing more ethanol faster...

<ramble>

Look at it this way... The more area you have from which ethanol can escape the grasp of the more dense water molecules, the better... Those ethanol molecules that can't get out fast enough will be cooled, no matter how minimally, and will remain in the denser water-laden wash... It will then need to be re-heated before it can make another ascent towards freedom - kinda like in-the-wash refluxing...

Along with in-the-wash refluxing we could also take in-the-boiler refluxing into consideration... If ethanol vapor manages to make its way out of the wash and into the boilers head-space, yet cannot make its way up the column, for one reason or another, then it will reach a vapor density which will cause it to drop back into the wash... This would be in-the-boiler reflux... More surface area would also result in more in-the-boiler reflux...

</ramble>

[theholymackerel]

Ok.

Legal and philosophical rants aside, this is the "Research and Theory" section and were talkin' Physics here.

I've heard everyone tell me this and that, and try and convince me that two equally insulated boilers of equal volume that have the exact same heat added will yeild the most vapour from the boiler with the most top surface area.

I think this is a false assumption.

If it is true, it is an easily repeatable experiment and will have a named physical law.

I ask yall proponents of this theory to show me this Law. Don't tell me what ya assume or believe. Tell me what is physically known to be true.

[heynonny]

When watching a pot of spagetti water boiling (yes it takes longer to boil when watching haha), you might notice the bubbles actually rise from the bottom. now, I've also seen that mash when boiling has bubbles rising from within the liquid to the surface, not from the surface. I would imagine you'd get more vapor from a larger surface area that is <being heated> Boiling is more a function of applied heat than exposed surface area. If you could apply the same <amount of> heat to a smaller area, you should get the same amount of vaporization of alc. Except that prolly you'd get more boiling water (steam) due to concentration of heat in smaller area (Water boils before heat is evenly dispersed through liquid. If you were to put a 'skirt' around your boiler, causing heat contact <transfer> to a larger surface area (!) of boiler, (I have done this on occasion, experimenting) (it works, ie corny kegs make good boilers) then 'heat <calories> applied times time = pints, liters /hr, min, whatever. Some Posts have addressed this issue already.
I'm a grade checker, earth moving, rearranging the landscape. What do I know? Only what I see. Lets see what I catch now, heynonny

[HookLine]

I am with THM on this one. The energy has to go somewhere. At least within the practical limits of the boiler shapes we use, I cannot see the surface area/volume ratio making any difference to vapour output rate. The only difference would be in the degree of surface activity; the larger the surface area, the gentler the surface activity, simply due to the amount of heat energy at the surface (as phase state conversion from liquid to gas) being spread over a larger area.

[Dnderhead]

I gis Grandad was wrong using a sap pan or grand ma using a large pot with a large surface area cooking sauces. maybe pile up my grain the next time I want to dry them all has to do with evaporation. evaporation occurs at the top that is exposed, by heating we exsellerate this . just google evaporation
I believe that by using larger surface it whould be more efficient and less heat whould be needed for the same amount of vapors

[theholymackerel]

I am with THM on this one.

WHAT!?!?!?

I mean, Yes.

YES!!!

Let us unite our ancesters ex-colonial and ex-criminal powers and All Be Damned!

Topsills Only! Hard to Port!

You in the Assumption Barge... prepare to be boarded!

"Hard and True" explinations we'll have, or to Davy Jones' Locker we'll send ye.

Yarr!

[HookLine]

Yes, but who is going to be captain?

[theholymackerel]

You can be Captain as long as I don't have to batten the hatches.

The hatches be damned.

[Dnderhead]

hoist the jolly rogers, I gis you'll be going down then Matey! cause if you don't baton the hatches you'll be taking on water.
bring her around broad side see what we can do.

[Hack]

I think the whole problem with this one is the equal heat part. If you have a larger surface area I think you could get more evaporation, but you'd need more heat to do it, because the larger surface area also means more heat loss. Take a bucket of water left in the sun. How long does it take to evaporate? Now dump the bucket's contents out on a slab of concrete in the sun. Now how long does it take to evaporate?

[Dnderhead]

nop , you got the right idea leave a pot with a gallon of water out , and spread out another gallon , witch one evaporates first?
why? it evaporates from the surface more surface more vaporisation.same as heating but heating accelerates evaporating

[HookLine]

Righteo then, I'm Captain Don'tFuckWithMe Hook, and don't you scurvy stillin sea dogs forget it.

Rule One on the good ship Alembic is that Master H. Mackerel is permanently exempt from hatch battening duty, and is also in charge of the rum supplies. May they never run low, or taste poorly, Master M., upon pain of being flogged, keelhauled, swung from the yard arm, made to walk the plank, and given an unpleasant experience with a stiff east wind.

Rule Two is remember to always talk like a pirate on this day.

Rule Three is that there are no more rules. Except when Admiral Sir Lord Protector of Us All Uncle Jesse Esq. says otherwise. For He is omnipotent.

Weigh anchor, me hearties, we're off to Spirit Island for some lovely treasure!

[MudDuck]

Keep in mind that evaporation and boiling, though similar, are not the same thing. Evaporation only happens at the surface of the liquid while boiling turns liquid into gas throughout the entire volume being heated.

[Socrates]

Mack, we alreay have a law - I wasn't kidding and it was Avogadro's Law (see earlier post). In a (relatively) closed space (and assuming perfect insulation) it's the VOLUME that counts, NOT the area. So I'm with Mackeral on this one. It will not release more ethanol faster. Let me state this again: area alone has NOTHING to do with it.

The example of unenclosed sidewalk evaporation is not relevent.

OTOH this is really moot question as all pots have a stricture called the column or takeoff which is really the limiting factor, not to mention the relatively slow rates of takeoff. A two inch column is capable of taking off the same amount of alcohol REGARDLESS of the boiler volume. Heat is more relevent than volume. Accordingly, any reasonable boiler volume, which includes all the boilers I've seen here, are more than capable of producing enough alcohol to take off.

Quit feedin the lawyers and distill.

Let me make a guess: the outcome of this thread will have little if any effect on how any of us distill. May I ask: are any of you planning on changing the exposed area (not volume) of your boiler design based on this thread?

[Dnderhead]

the column is restricting the vapors so if you make a more efficient boil, 1) you could use less heat or2) you could could produce more vapors
and use a larger column. and to answer your question why does it matter? if you are building/designing a still and have a choice then Id pick one with the larger surface.

[Trid]

Righteo then, I'm Captain Don'tFuckWithMe Hook, and don't you scurvy stillin sea dogs forget it.
Rule Two is remember to always talk like a pirate on this day.

Speaking of which...
*points to calendar*

[punkin]

May I ask: are any of you planning on changing the exposed area (not volume) of your boiler design based on this thread?

Not that i have an opinion either way, i've never thought about it or tried to discover which was true, but i'm with Dunder on your question.

If i was designing and building a new tank for a boiler, i certainly would take this into account if it proved to be correct.

Atm, i have no plans to do this though...


Another linked question would be, if you designed a tube shaped, cylindrical boiler, would more heat be absorbed from the same heat source if it was laying on it's side with a skirt around it presenting more surface area for a gas burner to heat it? or would it be the same time to boil standing upright with the skirt around the bottom?

[rezaxis]

AVAST!!!!!

Physics Forums > Physics > General Physics

Rez submits the following to the above forum:

Two vessels of the same volume, in the same location and open to the atmosphere. One is tall and of a small diameter, the other is short and is of a larger diameter.

Each vessel supplied with an identical heat source. Each vessel filled with an equal volume of an identical liquid.

The goal is to reduce the volume of the liquid in both vessels by 1/2.

I know it will take exactly the same amount of energy to bring each vessel to the boil.

The question is will the vessel with the larger surface area evaporate the liquid faster than the smaller one?

Is there a formula or known and named relationship the would allow me to say "If I doubled the area available to evaporation, I would see an X increase in the rate of evaporation."
________________
Re: Important clarification, though: boiling and evaporation are not quite the same thing. Boiling is specifically the internal vaporization of a liquid due to adding heat to bring the vapor pressure above atmospheric pressure. Evaporation, typically, is a surface effect where water is vaporized due to the vapor pressure at the surface's temperature being above the vapor pressure of water in the air.

So since the two concepts were mixed and matched, I'm going to provide two answers:
-Water will boil (roughly) equally fast in two different shaped vessels given the same heat input.
-Water will evaporate faster from a vessel with more surface area exposed to the atmosphere than from one with less surface area exposed to the atmosphere.

This also means that if you heat the water up to slightly less than the boiling point, the one with more surface area will evaporate faster and as a result, keeping the temperature steady will require more heat input.
_________________
Rez then asks:
Now, does this evaporation rate vs surface area relationship have a name or a formula?
_________________
Re: Simply that evaporation is proportional to surface area. The proportionality depends upon things such as the properties of the specific liquid and the temperature.
_________________

The replies came from two people with about 12 and 19 thousand posts and both are mentors in a physics related forum. I'm going to assume they know what they're talking about.

Oh and HAAARRRRRRRRRRRRRRRRRRRRRR!!!!!!!!!!!!!!!!

[snuffy]

...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 ?.

The short answer is yes. For the reason that THM gave. (8=X

The question is malformed and should be made to walk the plank. Or be keelhauled.

The reason wide shallow boilers are preferred is that they minimize the pressure differential between the heated bottom and the liquid surface. Tall skinny boilers (very slightly) superheat the vapor at the bottom and it expands as it rises giving frothy, foamy, surging boils. The practical limits are imposed by the cost and strength of materials.

It's not surface area, it's depth of liquid. They are inversely related.

Reductio ad adsurdum: Thin film evaporator or plate flash boilers at one end of the continuum and building a boiler out of a bazillion feet of vertical 1/4" copper tubing at the other. QEMFD.

PS. We need to add a pirate smiley.