Cooling water discussion

Perforated & bubble cap plated columns

Cooling water discussion

Postby Doubleuj » Fri Feb 02, 2018 6:14 pm

Ideally water should run in the opposite direction to your vapour BUT, in the case of your rc if you have the inlet up the top and only a trickle of water going it may drain faster than it fills and be ineffective. This is why we say inlet at the bottom on both. However, I still have my rc water coming in the top but looped the outlet water up above the inlet, creating a sort of S trap so it doesn’t drain, seems to work for me :handgestures-thumbupleft:
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Re: virgin run with a bubbler

Postby scythe » Sat Feb 03, 2018 6:42 am

I was just thinking the same thing DUJ.
That would be the only way to get the water to flow from top down, but then it wont self bleed the air bubbles as the cooling water degasses.

So long as your condenser is full of cooling water at all times you will be fine.
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Re: virgin run with a bubbler

Postby db1979 » Sat Feb 03, 2018 7:56 am

Doubleuj wrote:Ideally water should run in the opposite direction to your vapour BUT, in the case of your rc if you have the inlet up the top and only a trickle of water going it may drain faster than it fills and be ineffective. This is why we say inlet at the bottom on both. However, I still have my rc water coming in the top but looped the outlet water up above the inlet, creating a sort of S trap so it doesn’t drain, seems to work for me :handgestures-thumbupleft:


The RC fills up with air because the cooling water has dissolved air in it. Water dissolves more air at low temperature and when the cooling water is heated up, it degasses. If you have clear cooling lines you'll see this happening. It's the same reason why water in a saucepan will have little bubbles stuck on the bottom and bursting and reforming well before the water hits it's boiling point.

Anyway, if the water flow is downward in the RC (top to bottom) then the air bubbles that come out of the water rise up against the flow of water (which is a slow flow once we start collecting) and get stuck in the RC. What was once a RC full of water is now one full of air and water trickles down from the inlet and straight out the outlet.

Cooling water in at the bottom allows the flow of water to be in the same direction that the bubbles of air want to go (up). This takes most of the air with it (some will still get trapped). If the water didn't take the air with it then there would be a point where no flow would occur at all, but the pressure from our pump (which is small now that it's passed the valve) sees to that.

It's the difference between pushing water uphill or allowing it to fall downhill. The pump is doing no work in the second case, other than providing the next bit of water to fall downhill.

I doubt that a loop in a cooling line would stop the air staying in the RC. I think the pressure from the air in the RC will hold back the small amount of water in a loop. Have you tested it out to see how much water is left in the RC after a lengthy spirit run? I would suggest doing a run in each of three different configurations, one with the loop, one without and another without the loop but cooling water in at the bottom. You'll want to have some way of finding the volume of water left in the RC. Or measuring the air that comes out of the RC after putting the still back into full reflux after say an hour of collecting. This could be done by putting a clear container upside down over the top of the water outlet where it exits your cooling lines, simply compare the amount of air trapped in the container in each test.
Last edited by db1979 on Sat Feb 03, 2018 7:57 am, edited 1 time in total.
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Re: Cooling water discussion

Postby Doubleuj » Sat Feb 03, 2018 2:21 pm

i split this topic out of brisvalleys thread.
DB, I really dont think that the RC will fill up with air bubbles because of dissolved air in the water, were not taking it near boiling point, or even the point in the saucepan where those little bubbles form.
consider that the vapour temp is only 78 deg or so and the water is constantly flowing, your outlet temp of your rc water is definitely not 78 deg.
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Re: Cooling water discussion

Postby db1979 » Sat Feb 03, 2018 3:39 pm

I'm not talking about water boiling into vapour and filling up the RC, I'm talking about air, which is dissolved in water whenever water is exposed to air. It's solubility is temperature dependent (as well as pressure but it's effect is less than temperature).

images.png


The above graph shows the solubility of just oxygen in water at different temps. Air is mostly nitrogen (78%) and some oxygen (21%), the rest being other gases. Nitrogen is less soluble in water than oxygen. Based on the above data for oxygen at 20 degrees and 40 degrees, it's possible to calculate the volume of oxygen gas released from the water in the RC as it heats from 20 to 40 as approximately 2 ml per litre of water. This is just oxygen though and the air dissolved in water is about 36% oxygen (the rest is nitrogen). We can then estimate that total air per litre of water left behind in the RC would be about 5 to 6 ml.

A 4" RC that is 100 ml tall will have a volume of 785 ml disregarding the interval tubes. We could assume that an RC might then have half that volume, giving about 400 ml. Based on the above an RC could fill up with air after 80 L of water passes through it.

Additionally, as the RC loses water volume and is replaced with air the temperature difference will get higher, leading to faster degassing and a greater rate of air accumulation.

Less than 80 L of water could cause the RC to fill up with air.

If you don't trust my numbers I can type the calculations up for you. I used the molar mass of oxygen along with the formula pv=nrt to solve for volume.
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Re: Cooling water discussion

Postby Doubleuj » Sat Feb 03, 2018 3:54 pm

I knew I shouldn’t have got in a argument/ discussion with a teacher :laughing-rolling: :laughing-rolling:
But... that graph shows the solubility of oxygen in water, ie how readily the water disolves and accepts oxygen atoms in a type of mixture.
What is waters ability to release the O2?
I think a simple experiment could help.
If I ran a given amount of water through the rc at a stable temp, then shut off the flow and measured the volume left in the rc we would know if there is any loss due to this theory.
How many L do you reckon I’d run through at room temp of say 25-30 degrees?
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Re: Cooling water discussion

Postby RuddyCrazy » Sat Feb 03, 2018 4:13 pm

Ok as pointed out in my build thread about cooling. Yes I do have the RC with the water input where the vapour first meets and have the same at the PC where the cold water condensers the vapor at contact. With my 3" pc I twisted the pipes to create a flow and the vapour current hitting a cold spot and going thru a vortex as it becomes liquid is the best way to go on my setup.

Cheers Bryan
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Re: Cooling water discussion

Postby RC Al » Sun Feb 04, 2018 9:54 am

Speaking of cooling water temps, in a nixon stone discussion i read it was observed that you get higher production rate with a hotter RC coil (outlet water in the 70's) - the less heat taken out of the of the vapor gives the product a higher chance of reboiling in the column, BrisV's cleaning run mentioned the coil only cleaning part the way up - too much flow rate? the now 30-40c fluid would just flood back down the packing?

I think this concept applies to any VM still? obviously they are all a bit different and I realize that many of the shorter bokas rely on that RC to increase reflux/abv, but for the guys with the "overkill" coils, this may be an untapped area of production rates?

Sorry to thread jack a already split thread, but the topic had an appropriate title :)
Last edited by RC Al on Sun Feb 04, 2018 9:58 am, edited 1 time in total.
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Re: Cooling water discussion

Postby RC Al » Sun Feb 04, 2018 10:06 am

(expletive) I meant LM Doh!

And it was prob a reverse LM i was reading, but i think the concept still applies
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Re: Cooling water discussion

Postby db1979 » Mon Feb 05, 2018 5:13 pm

Doubleuj wrote:that graph shows the solubility of oxygen in water, ie how readily the water disolves and accepts oxygen atoms in a type of mixture.
What is waters ability to release the O2?
I think a simple experiment could help.
If I ran a given amount of water through the rc at a stable temp, then shut off the flow and measured the volume left in the rc we would know if there is any loss due to this theory.
How many L do you reckon I’d run through at room temp of say 25-30 degrees?

The solubility is a measure of how much can dissolve at a specific temperature (and pressure) and if there is more than this amount dissolved it will form bubbles very quickly. How long does it take for a bubbles to form when you open a carbonated drink? They form straight away. The carbonated drink is under high pressure but if you compare a warm soft drink (where far more bubbles form in a shorter period of time) to a cold one we can see the effect temperature has on bubble formation with time.

If you want to experiment, I recommend installing some clear tubing on your cooling outlet. Compare the amount of bubbles you see if your cooling outlet is at the top of the RC compared to the bottom. I would guess that you'll see very little bubbles coming through the clear tubing if your cooling outlet is at the bottom. This effect will probably be less pronounced during full reflux as the high flow will probably be enough to take all bubbles out of the RC.

If you see less bubbles coming out when the outlet is at the bottom then it's because they are getting stuck in the RC.
Last edited by db1979 on Mon Feb 05, 2018 5:15 pm, edited 1 time in total.
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Re: Cooling water discussion

Postby db1979 » Mon Feb 05, 2018 5:19 pm

Bryan1 wrote:Ok as pointed out in my build thread about cooling. Yes I do have the RC with the water input where the vapour first meets and have the same at the PC where the cold water condensers the vapor at contact. With my 3" pc I twisted the pipes to create a flow and the vapour current hitting a cold spot and going thru a vortex as it becomes liquid is the best way to go on my setup.

Cheers Bryan


It's not as critical that the PC plumbing has the cooling enter at the bottom as your bubbler will still run the same, but if the PC fills up with bubbles it'll be less efficient and the product will come out warmer. It'll take a much longer time for PC to fill up with bubbles though.
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