Compact VM head, coil in a box

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Compact VM head, coil in a box

Postby manu de hanoi » Thu Nov 06, 2008 10:00 am

Hi all,

I have to follow the VM hype, making 1/2 a millimeter adjusments to the liquid output valve (2 meters high) every 10 minutes is annoying indeed. So, the column is 3 inches, the output is 2 inches, using a valve with screw fittings.

Here are the design constraints :
- the 2 inches stainless screw fittings are expensive, at least 2 males are required for the pipes on each end of the valve (already equipped with female screw fittings). the fittings will be welded on 2 pieces of a 2 inch pipe one connecting to the column the other to the cooling section
- Cooling: I want to use a copper coil because it's much smaller than a stainless jacketed cooler, the problem is that for maintenance, one must be able to disassemble the coil, and that make things much more complex
- distillate liquid output : 10 mm pipe
- My current design so far is in the picture, I am not really satisfied because the coil will touch the bottom of the pipe and retain/slow distillate evacuation. If the coil was vertical then it would require one more screw fitting in order to let the coil be disassembled

I need some suggestions on how to improve the design
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Re: Compact VM head, coil in a box

Postby Nykter » Thu Nov 06, 2008 10:18 am

A traditional VM needs to have the product condenser significantly lower than the exit from the column in order to get the siphoning effect, but since you use a really large one you might not need it that badly.
Its this effect that shuts down the still when there's no more ethanol in the mash.
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Re: Compact VM head, coil in a box

Postby minime » Thu Nov 06, 2008 10:22 am

Hey Manu, you might want the coil to lay on the bottom and cool your distillate before it heads to the collection jar. if you wrapped it in copper gauze it wouldn't slow it enough to back up down the column. Your idea might work like a champ.
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Re: Compact VM head, coil in a box

Postby manu de hanoi » Thu Nov 06, 2008 11:56 am

This head is used on top of a continuous still.
Nykter wrote:A traditional VM needs to have the product condenser significantly lower than the exit from the column in order to get the siphoning effect, but since you use a really large one you might not need it that badly.
Its this effect that shuts down the still when there's no more ethanol in the mash.

Can you explain how the siphoning you mention works on a batch still ? i'd like to understand. Now that you mention it, since there will be a long, 8 mm pipe connected to the distillate output, i'm afraid the vapors will be pulled in the condenser too hard ..... Perhaps drilling a vent would help...Could close the vent if output is too weak.

minime wrote:Hey Manu, you might want the coil to lay on the bottom and cool your distillate before it heads to the collection jar. if you wrapped it in copper gauze it wouldn't slow it enough to back up down the column. Your idea might work like a champ.


Actually the condensate is heading to the rectifying column (i use 2 continuous columns) and I need it liquid but not chilled for energy saving purposes.
If i let the coil touch the bottom, i'm afraid the liquid flow would be blocked and go back to the main column. But thanks for the idea, some scrubbers will do the trick of lifting the coil.....if i manage to coil the pipe to something smaller than 2 inches....
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Re: Compact VM head, coil in a box

Postby decoy » Thu Nov 06, 2008 2:29 pm

you could just wrap 2 lengths of 1.6+ mm of copper electrical wire around the coil so its like a sled...
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Thu Nov 06, 2008 11:32 pm

Nykter wrote-
A traditional VM needs to have the product condenser significantly lower than the exit from the column in order to get the siphoning effect, but since you use a really large one you might not need it that badly.
Its this effect that shuts down the still when there's no more ethanol in the mash.


what siphoning effect? are you implying that the condensing of the vapor in the product takeoff arm is causing a vacuum? if thats the case, how does the pressure balance counter effect (for every action there is an equal, but opposite reaction) "know" not to come in from the product take-off output opening? is there supposed to be a one way valve installed on the product take-off that i haven't heard about?

my understanding of the mechanics (fluid dynamics, heat transfer, gas flow, etc.) involved in a compound reflux column does not include a "siphon effect". the effect may well be present in a potstill and a reflux column (no induced reflux and sealed at the top), where the take-off could be completely filled with liquid and stop air from going in through the take-off tube. then the condensing of the vapor in the take-off tube could cause a vacuum.....if the production of vapor in the boiler was not keeping up with the condensing of the vapor. so if there was a vacuum created....the boiling point of the liquid in the boiler would drop and there would be a surge of vapor created which would fill the vacuum. would this then create a slight over pressurization, forcing uncondensed vapor out the product take-off?

this "siphon effect" theory is intriguing....could someone please explain how it works (while not breaking any laws of physics). or is it like a jack-a-lope, actually being just an urban legend for those that don't really want to think?
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Thu Nov 06, 2008 11:38 pm

manu wrote-

Actually the condensate is heading to the rectifying column


why?
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Re: Compact VM head, coil in a box

Postby Nykter » Fri Nov 07, 2008 1:09 am

manu de hanoi wrote:Can you explain how the siphoning you mention works on a batch still ? i'd like to understand. Now that you mention it, since there will be a long, 8 mm pipe connected to the distillate output, i'm afraid the vapors will be pulled in the condenser too hard ..... Perhaps drilling a vent would help...Could close the vent if output is too weak.

Hmm well on a classic VM batch still the flow of product vapour is is pulled down through the product condenser by gravity, since ethanol vapour is heavier than air. At the end of the run, when vapour finally contains enough water, it will be lighter than air, reducing this effect, minimising product takeoff, until it finally stops if the still is properly tuned.

I think you won't need an extra vent to minimise output, use the valve instead.

DestructoMutt, this effect is only present in a classic VM which is enough open in the top and in the end of a (tall) Leibig condenser not to allow for any pressure differences. The law of physics i call is a simple one, gravity.

Imagine a vertical tube, filled with a gas heavier than air, open in both ends. What will happen? The gas will slowly flow down through the tube. Now imagine the same tube fillled with a gas lighter than air. The gas will rise.

Now, add a condenser in each one of the ends, still very open, and a connection to a boiler/column somewhere in the middle of this tube adding more gas. The amount of gas has to be less than or equal to the gravity forced flow in the tube. Make sure the condensate from the upper condenser flows back into the column, also by gravity.
If the gas is lighter than air (steam) it will condense in the upper condenser and reflux into the column. If its heavier (ethanol) it will flow down and condense as product.

And please note I'm _not_ suggesting that a mixture of water and ethanol vapour will separate, they wont.

Now, here's the tricky part:

Add a valve in the tube right below the inlet. Close this valve just enough to let the flow of vapour pulled down by gravity be approximately 1/10 of the total vapour entering the tube. Now 9/10 of the vapour will be forced to rise to the reflux condenser, condense and return to the column as reflux.

Now imagine, with the same power and valve settings, the vapour containing more and more water, and less and less ethanol, it will finally be lighter than air. What will happen?

Since the path up to the reflux condenser is in practice unrestricted, all vapour will head to it, and no vapour will be pulled down, by gravity, to the restricted (by the valve) path to the product condenser. Full reflux.

Of course if the amount of vapour leaving the column is large enough, the tube and reflux condenser will act as a restriction, killing this effect by introducing an overpressure just above the valve.

So, a poorly contructed or run VM vill not have this effect.





I think.
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Fri Nov 07, 2008 2:41 pm

Nykter wrote-

Hmm well on a classic VM batch still the flow of product vapour is is pulled down through the product condenser by gravity, since ethanol vapour is heavier than air. At the end of the run, when vapour finally contains enough water, it will be lighter than air, reducing this effect, minimising product takeoff, until it finally stops if the still is properly tuned.


gravity is good. but that's not why vapor goes into the product take-off arm.

you are neglecting:
-the boiler is constantly producing vapor - a gas - gasses expand to fill their container. if you look at a container full of a single gas, at a consistent temperature - the density of the gas will be the same at all points in the container and so will the pressure of the gas (not vapor pressure, but force exertion).
-convection - warm gases are less dense than cooler gases - thus rise. if you heat the bottom of the container, the warmed gas will rise to the top, the cooler gas will sink. [if the container is closed, the pressure will increase because the gas is more energized. if the container is open to the atmosphere, gas will escape. just trying to show i'm not ignoring other factors and reactions.]
-the flow of a gas filling a container is dependent on the physical characteristics of the container, the existing contents of the container and the introduced gas, not just gravity.

ethanol vapor that is at the same temperature as the surrounding air is denser and thus heavier than air. however, if you heat the ethanol vapor sufficiently above the temperature of the surrounding air, you can and will make the ethanol vapor lighter and less dense than air. it will rise, it will also expand to fill its container, if enclosed, such as in a column. (as it cools it will sink/settle, but it cools before sinking/settling, hence it's density is increasing as it sinks/settles.)

addressing just one physical characteristic of the system is inappropriate. gravity is just one part of the total equation, you must consider the effect of the constantly produced vapor from the boiler - this produces an over pressuritzation in the column - gases expand, especially warmed gases. the production of a dense gas from the boiler also forces out/expels lower density gases (atmoshperic air). we then end up dealing with system containing ethanol, water and the other byproducts of fermentation, but lacking atmospheric air. the ethanol vapor splits into two streams in the vapor management head (top part of the column) not because of gravity, but because the vapor is doing what a gas does - it's expanding to fill its container. gravity is pulling the condensed liquid down the condensors, but has fairly little effect on the movement of the gas/vapor.

additionally you must consider the flow of heat in the system, and how the heat affects the flow of gas/vapor and liquids. heat flows from hot to cold. warmed air rises, but can transfer its heat to something else and then sink again. water vapor gives up its heat faster than ethanol (a characteristic that is exploited in distillation), thus condenses out of gas form faster than ethanol.

if you want to think of an effect that might have some play in the column, consider the venturi effect - a gas or liquid flowing in a pipe, past a small opening in a pipe, gat generate a vacuum in the small pipe such that the contents of the small pipe are sucked into the larger pipe. but this doesn't happen in the column of a still, just ask anybody who has leak in their column. for the effect to come into to play, the large pipe needs to shrink in diameter at the point of the small pipe, such that the gas/liquid in the large pipe has to speed up as it passes the small pipe.
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Re: Compact VM head, coil in a box

Postby manu de hanoi » Fri Nov 07, 2008 6:12 pm

meanwhile a 2 inches 2nd hand valve was aquired for 9 usd.
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Re: Compact VM head, coil in a box

Postby snuffy » Fri Nov 07, 2008 7:58 pm

I think you need to move the two condenser coils much farther away from the valve.

The valve has to be at vapor temperature or the vapor will start condensing in the valve itself. Conduction through the walls of the takeoff chamber and valve will cause the vapor to collapse too soon and severely limit the amount of output.

The reflux condenser needs to be at least two feet above the vapor takeoff so there is sufficient head to the height of the vapor column. Too short and there will be little or no pressure to drive the vapor into the valve and product condenser.

Also, you can probably use a valve much smaller - 3/4" or smaller. The valve opening is very small compared to the cross-sectional area of the reflux column.

Maybe minime can tell us the cross sectional area of his valve openings at various stages of the run and the height of his vapor column above the takeoff? I think his reflux column is 3" diameter, yes? These ratios are very important to getting VM working properly.
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Re: Compact VM head, coil in a box

Postby decoy » Fri Nov 07, 2008 8:51 pm

id like to hear more on your theory about alcohol vapor going down the tube to condensor because its heavyer..
it dosent sound right... im inclined to think its more so presure related...

is there a source i could perhaps have a read of..?
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Re: Compact VM head, coil in a box

Postby HookLine » Fri Nov 07, 2008 10:39 pm

snuffy wrote:The reflux condenser needs to be at least two feet above the vapor takeoff so there is sufficient head to the height of the vapor column.


Two feet?

The bottom of the reflux condenser is only an inch or two above the take-off port in my VM column, and most VM columns, and they still work fine.
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Fri Nov 07, 2008 11:13 pm

snuffy wrote-

I think you need to move the two condenser coils much farther away from the valve.

The valve has to be at vapor temperature or the vapor will start condensing in the valve itself. Conduction through the walls of the takeoff chamber and valve will cause the vapor to collapse too soon and severely limit the amount of output.

The reflux condenser needs to be at least two feet above the vapor takeoff so there is sufficient head to the height of the vapor column. Too short and there will be little or no pressure to drive the vapor into the valve and product condenser.


nixon and mccaw discuss this misconception in their book (the compleat distiller).

so to pass on the knowledge of giants; snuffy, you're wrong.

nixon and mccaw's book is "only" $25.00. it's well worth the price. so is ian smiley's.
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Re: Compact VM head, coil in a box

Postby Nykter » Sat Nov 08, 2008 2:43 am

DestructoMutt wrote:you are neglecting:
-the boiler is constantly producing vapor - a gas - gasses expand to fill their container. if you look at a container full of a single gas, at a consistent temperature - the density of the gas will be the same at all points in the container and so will the pressure of the gas (not vapor pressure, but force exertion).

you are neglecting that this is an open system, at least above the column, all gasses are exactly the same pressure, that is atmospheric, they have already done their expansion.
DestructoMutt wrote: -convection - warm gases are less dense than cooler gases - thus rise. if you heat the bottom of the container, the warmed gas will rise to the top, the cooler gas will sink. [if the container is closed, the pressure will increase because the gas is more energized. if the container is open to the atmosphere, gas will escape. just trying to show i'm not ignoring other factors and reactions.]
All densities are counted for at their actual temeratures in a still, ethanol vapour at 78C, steam at 100C, Air at room temperature.
DestructoMutt wrote: -the flow of a gas filling a container is dependent on the physical characteristics of the container, the existing contents of the container and the introduced gas, not just gravity.
True
DestructoMutt wrote:ethanol vapor that is at the same temperature as the surrounding air is denser and thus heavier than air. however, if you heat the ethanol vapor sufficiently above the temperature of the surrounding air, you can and will make the ethanol vapor lighter and less dense than air. it will rise, it will also expand to fill its container, if enclosed, such as in a column. (as it cools it will sink/settle, but it cools before sinking/settling, hence it's density is increasing as it sinks/settles.)
It will never get hotter than 78C at the top of a column.
DestructoMutt wrote:addressing just one physical characteristic of the system is inappropriate. gravity is just one part of the total equation, you must consider the effect of the constantly produced vapor from the boiler - this produces an over pressuritzation in the column - gases expand, especially warmed gases.
Expansion has already taken place when the vapour reaches the top of the column, see above.
DestructoMutt wrote:the production of a dense gas from the boiler also forces out/expels lower density gases (atmoshperic air). we then end up dealing with system containing ethanol, water and the other byproducts of fermentation, but lacking atmospheric air. the ethanol vapor splits into two streams in the vapor management head (top part of the column) not because of gravity, but because the vapor is doing what a gas does - it's expanding to fill its container. gravity is pulling the condensed liquid down the condensors, but has fairly little effect on the movement of the gas/vapor.
additionally you must consider the flow of heat in the system, and how the heat affects the flow of gas/vapor and liquids. heat flows from hot to cold. warmed air rises, but can transfer its heat to something else and then sink again. water vapor gives up its heat faster than ethanol (a characteristic that is exploited in distillation), thus condenses out of gas form faster than ethanol.
if you want to think of an effect that might have some play in the column, consider the venturi effect - a gas or liquid flowing in a pipe, past a small opening in a pipe, gat generate a vacuum in the small pipe such that the contents of the small pipe are sucked into the larger pipe. but this doesn't happen in the column of a still, just ask anybody who has leak in their column. for the effect to come into to play, the large pipe needs to shrink in diameter at the point of the small pipe, such that the gas/liquid in the large pipe has to speed up as it passes the small pipe.

I wont argue to that, I'll have to admit that this is far beyound my knowledge.

But, you might be right. It might be an urban legend. its just that I have found it in a couple of distilling books, for example "Designing and building automatic stills" by Riku. Maybe in the compleat distiller too, I dont have it handy now. And it made sense to me.

But, how would you explain the autoshutoff property of a VM?
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busy day

Postby manu de hanoi » Sat Nov 08, 2008 3:03 am

Your message contains too few characters.
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Re: Compact VM head, coil in a box

Postby manu de hanoi » Sat Nov 08, 2008 3:04 am

Thanks all for your comments.
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Re: Compact VM head, coil in a box

Postby snuffy » Sat Nov 08, 2008 8:36 am

Hook: I should have said works better - see the Thor's hammer VM and VM ARC designs. The additional column height provides a buffer that improves stability and increases heads compression if used with a reservoir at the top.

Destrutomutt: In The Compleat Distiller, 2nd Edition, page 200, bold NOTE:

the vapor control head should operate at the temperature of the vapor emerging from the top of the column. It must not be cooled [emphasis in original] by its proximity to the top reflux condenser , or by ambient conditions. If any of vapor [sic] condenses inside the head, the separation principle will be compromised. ... A gloved cold finger or an an external cooling coil would be bad choices for this application because they do cool the outer surface, and that would therefore cool the still head.


My concern is that the horizontal product condenser may rob too much heat from the valve and junction. If the valve is below condensation temperature, vapor could condense on the surface nearest the column and not make it past the gate to the product condenser.

All these questions can be answered by experimental design. That would be worth a separate thread. I am building a modular set up that should answer these and other questions. I'm still getting bids from welders. I expect early next year I'll have the experimental apparatus working. I will publish results when available.

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Re: Compact VM head, coil in a box

Postby manu de hanoi » Sat Nov 08, 2008 12:17 pm

yup snuff, I agree there's nothing good in condensing before the gate valve, and added extra 4 cm buffer length in the coil housing for that matter, remember I use stainless. It also made me consider insulating the valve. A priori, I dont see a bit of condensation here and there being that detrimental to the matters at hand.

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The reflux condenser needs to be at least two feet above the vapor takeoff so there is sufficient head to the height of the vapor column. Too short and there will be little or no pressure to drive the vapor into the valve and product condenser.

That may explain why some people here see little impact from the "ethanol heavier than air pushing vapor in the output" effect, that's because they didnt add the extra 2 feet....
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Hook: I should have said works better - see the Thor's hammer VM and VM ARC designs. The additional column height provides a buffer that improves stability and increases heads compression if used with a reservoir at the top.

I would gladly comment on the heads compression but since it's ©®™©®™©®™ and little info is leaked, I'll keep my continuous heads separation knowledge and xp for myself.
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Sat Nov 08, 2008 5:11 pm

nykter wrote-

DestructoMutt wrote:
you are neglecting:
-the boiler is constantly producing vapor - a gas - gasses expand to fill their container. if you look at a container full of a single gas, at a consistent temperature - the density of the gas will be the same at all points in the container and so will the pressure of the gas (not vapor pressure, but force exertion).


you are neglecting that this is an open system, at least above the column, all gasses are exactly the same pressure, that is atmospheric, they have already done their expansion.


the vapor leaving the surface of the liquid in the boiler is expanding, and this happens continuously, it is not static. the vapor is not at atmospheric pressure, it is minutely above because it is flowing from a large area (surface of the liquid in the boiler) up into a reduced volume container (the coulumn, which is usually packed). the system, as whole, contains vapor at a consistent pressure.

DestructoMutt wrote:
-convection - warm gases are less dense than cooler gases - thus rise. if you heat the bottom of the container, the warmed gas will rise to the top, the cooler gas will sink. [if the container is closed, the pressure will increase because the gas is more energized. if the container is open to the atmosphere, gas will escape. just trying to show i'm not ignoring other factors and reactions.]


All densities are counted for at their actual temeratures in a still, ethanol vapour at 78C, steam at 100C, Air at room temperature.


not quite - the vapor that is produced by the boiler is a homogenous mixture, ethanol, water and the other by products of fermentation. and the vaporized water is not at 100deg C, otherwise we could just heat the mixture to 78.5C and collect pure ethanol, which we can't do.

DestructoMutt wrote:
ethanol vapor that is at the same temperature as the surrounding air is denser and thus heavier than air. however, if you heat the ethanol vapor sufficiently above the temperature of the surrounding air, you can and will make the ethanol vapor lighter and less dense than air. it will rise, it will also expand to fill its container, if enclosed, such as in a column. (as it cools it will sink/settle, but it cools before sinking/settling, hence it's density is increasing as it sinks/settles.)


It will never get hotter than 78C at the top of a column.


uhhh... wasn't talking about in a column in the first part of that quote, so....ok.


DestructoMutt wrote:
addressing just one physical characteristic of the system is inappropriate. gravity is just one part of the total equation, you must consider the effect of the constantly produced vapor from the boiler - this produces an over pressuritzation in the column - gases expand, especially warmed gases.


Expansion has already taken place when the vapour reaches the top of the column, see above.


no it hasn't. you can see this by poking a hole in the side of your column, the vapor escapes and expands to mingle with the atmosphere. the vapor has more energy (heat) than the atmosphere, it will stop expanding when it reaches energy equilibrium with the atmosphere.

just because one book, article or story says something doesn't make it true, or accurate or correct.

in regards to the autoshutoff of vapor management stills: i don't know, but researching the phenomonon has led to a better understanding of the things that do happen and why they happen. for example - the vacuum created by the condensig of a vapor - that is something that is discussed as fact, but is it? i don't believe so - if a vacuum were created by the condensing of vapor, the boiling point of liquids in the general area of the vacuum would drop, and any ethanol that was still warm (78.1C) would revaporize in the vacuum and thus fill the vacuum with vapor again (it's what happens in a vacuum) - so the vacuum is self canceling - end result is that the volume of vapor entering a condensor self regulates to match the temperature of the condensor so that the system (the condensor) is in equilibrium at some point along the condensor.

snuffy - my apologies, i'm still using the 2001 edition of the compleat distiller. where in they discuss that a buffer region is created in the column, and that super cooling is not really a viable concern. again, my apologies.
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Re: Compact VM head, coil in a box

Postby Nykter » Sun Nov 09, 2008 12:24 am

Phew.
Im only talking about the stuation above the column.
Uh nevermind.
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Sun Nov 09, 2008 1:30 pm

Nykter wrote-

But, how would you explain the autoshutoff property of a VM?


i want to give Nykter credit for making me contemplate this phenomonon.

pulled out the old thermodynamics, fluid dynamics and physics texts....then "the compleat distiller"....then it dawned on me...

we don't allow the top of the column to reach 100deg C. (in vapor management or cooling management heads), the temperature at which pure water (ignoring elevation and localized high/low weather conditions), boils (before the system is shut down due to a lack of production). the lower boiling point "stuff", the "stuff" that is wanted, has been removed from the system, so we turn the system off.

side note: evaporation (and sublimation) occurs at the surface of a substance. boiling occurs throughout a substance (within and at the surface). the bubbles you see in a boiling liquid are areas where the molecules have acquired enough energy to phase jump. the bubbles grow for two reasons, pressure reduction as they near the surface of the liquid and they act like a bus, picking up additional passengers on their way to the surface (the bubbles trigger the creation of other bubbles).

we now return you to your regularly scheduled programming:
when the wash is boiling - vapor is climbing the column - the vapor is condensing and vaporizing - some molecules are rising due to evaporation, but a much greater volume of molecules are rising and falling because of boiling. the temperature of the column effects how high each chemical can go due to the ation of boiling. if the top of the column is kept below the boiling point of a chemical, that chemical won't get to the top of the column. with induced reflux and a vapor management or cooling management head, the water that does make it to the top of the column is sent back down the column, cooling the top of the column. if you run the boiler long enough, you will eventually start producing "pure" water.

the auto-shutoff isn't so much a shut-off as a "gap" in production. most operators reduce the heat supplied to the boiler once boiling commences. this reduced heat is sufficient to produce vapor when the boiler contains ethanol, water and the other by products of fermentation. so, between 64deg C and 98deg C, there is a semi-constant production of vapors from the boiler, but between 98deg C and 100deg C, there is a noticeable lag in vapor production. if however, one were to increase the heat to the boiler when the temperature of the column were to reach 96deg C, the gap in vapor production would diminish. it's this lag or gap in vapor production that we see as an auto-shutoff effect.
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Re: Compact VM head, coil in a box

Postby punkin » Sun Nov 09, 2008 1:45 pm

Don't think so. I've read some theories on how it works, don't understand em completely (i'm not real smart when it comes to thinkin things out or science), but i don't reckon this theory fits with what happens. The change from 95 to fast falling alchohol content is fast. But it's not abrupt. There's no reason why it should have a lag as it's happeneing on a curve.
Your theory does not explain why different angles of takeoff apparently cause this behaviour to change or cease operation. I don't think most people with vm's adjust temp too much, i know Hook turns his down once he starts to get equilibrium, but i've found it to make no difference.

I run my VM flat out from start to finnish. In fact i've taken to not even plugging the controller in now. The only adjustment i make is to the gate valve from closed to a few drops a sec for foreshots, a little faster for heads, wide open for hearts then slow it down to quarter or so when the temp fluctuates to drag the last 1/2 litre of 94-95+ out.

Spose you could call it tails, but i drink it and it's good, so not what the LM boys call tails either.

2400w incolloy element, twenty eight to thirty litres of 40% odd low wines, 1.4m of coppermesh packed into two inch copper.
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Re: Compact VM head, coil in a box

Postby DestructoMutt » Sun Nov 09, 2008 2:35 pm

punkin wrote -

The change from 95 to fast falling alchohol content is fast. But it's not abrupt. There's no reason why it should have a lag as it's happeneing on a curve.


if a majority of the material that boils below 98degC is eliminated from the system, and the remaining components boil at 100degC or over, there will be a period of decreased vapor production. and you're correct, the production does fall gradually as 98degC is reached.

keep in mind though that water needs more energy to vaporize than ethanol does, and if you supply the energy slowly, the time of the gap from 98degC to 100degC will appear longer than if you supply the energy more quickly.
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Re: Compact VM head, coil in a box

Postby rad14701 » Sun Nov 09, 2008 4:10 pm

If you don't push enough heat for the water vapor to travel up the column without collapsing, then it will appear as though there is a lag or that the flow has "shut off" after the ethanol in the boiler is exhausted... The denser water vapor will just condense and reflux in the column and boiler without rising to the take-off port...
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Re: Compact VM head, coil in a box

Postby punkin » Sun Nov 09, 2008 4:53 pm

Of course though, if that was truely what was happening, it would happen in all the designs, not just VM :roll:
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Re: Compact VM head, coil in a box

Postby snuffy » Sun Nov 09, 2008 9:47 pm

VM is a dynamic process based on water and alcohol vapors having lower and higher densities than air. Density of gas or vapor makes a big difference on how they flow in a open vertical column.

Pure water vapor is lighter than air. Not only does it rise in the column, it "floats" up and reduces the pressure inside the column relative to the atmosphere. This will prevent it from flowing out the side arm and descending the product condenser.

Pure alcohol vapor is heavier than air. It is "pushed" up the column by the pressure of the vapor below it. As it rises up the column, the pressure in the column increases above atmospheric.

A mixture of water and alcohol vapor will vary in density depending on the %abv of alcohol. At 41%abv, the density of the water/alcohol vapor is the same as air. This is the cutoff point where VM heads stop producing. By coincidence, it is also the cutoff point where US distillers have to stop collecting product. I take this as a sign (like the existence of beer) that God loves us.

When the %abv of the vapor is greater than 41%, the vapor will flow down the product condenser. The farther the fall before the vapor hits the condenser, the greater the pressure differential between the top and bottom of the vertical leg of the product discharge tube. Once the vapor is cooled, the volume it occupied is replaced by air that initially filled the discharge tube and was pushed down by the water/alcohol vapor as it initially entered the tube.

This downward flow is a fluid siphon and the hydrodynamics of siphons apply. There is a nice demonstration experiment showing how carbon dioxide (which is heavier than air) can be siphoned just like water.

Because the top of the column and the bottom of the product discharge tube are open to the atmosphere, the column is almost at ambient pressure. The very slight pressure differential is solely due to the difference in density between the vapor and air.

It might be more useful to speak of fluids, rather than vapor or gas. The pressure head on the fluid in the column is caused by and directly proportional to several things: 1) the density of the fluid coming up from the boiler; 2) the size of the opening to the product discharge tube; 3) the vertical distance from the top of the discharge tube and the condenser; 4) the ratio of the cross-section areas of the column and the valve opening; and 5) the height of the column above the discharge tube to the condenser.

As long as the fluid coming out of the boiler is above 41%abv (which is the same as saying that it is denser than air), it will flow like a liquid up the column and out the valve opening.

Note that the %abv is continuously changing and that change is monotonically decreasing. It should be possible to measure the differential pressure in the column just below the product takeoff with a sensitive slant-tube manometer - there are nice designs for this in the Amateur Scientist back when C.L. Strong was running it. If the manometer was sensitive enough (and they can be made very sensitive) it would give an accurate index of the %abv in the column vapor.

That's the best I can do at summarizing the dynamics of VM.
Time's a wasting!!!
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Re: Compact VM head, coil in a box

Postby manu de hanoi » Sun Nov 09, 2008 10:12 pm

May I add that the heavier than air factor may be negligible compared to the "stream splitting" effect once a certain vapour speed/pressure is reached ?
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Re: Compact VM head, coil in a box

Postby HookLine » Sun Nov 09, 2008 10:29 pm

Manu, can you please reduce the data size of those photos. They chew up nearly 2 MB of bandwidth every time we reload the page, and also chew up storage and bandwidth for Uncle Jesse. You should be able to get them down to about 70K each, without losing any clarity or detail.

Thanks
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Re: Compact VM head, coil in a box

Postby manu de hanoi » Sun Nov 09, 2008 11:08 pm

will do tonight
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