LM, Bokakob, surging / siphoning problems

[der wo]

Many LMs have a pulsing output, when running at 100% output. Independently if this is a problem or not, I thought a lot about the fundamentals the last weeks. Those are my results:

Why does it happen?
It happens when in a tube is not only distillate but also air or vapor and there is not enough space for both to flow independentially. The surface tension of the distillate is the physical cause for the problem.

Generally a LM system has three parts: All three can be partially filled by air/vapor during a run.
1. The tube to the valve: As long the pool is filled, no air/vapor will have space there. Only in potstill mode when the pool is empty, the empty space will be filled by vapor.
2. The valve: At the needle of the valve is NEVER enough space for air/vapor and distillate. When the pool above the valve is filled, no air/vapor will flow here. But when run in potstill mode and the pool has run empty, then there will be not enough distillate to fill the valve completely, so air/vapor will get into the bore of the valve and hinder the distillate to flow down steadily.
3. The tube after the valve: Because the diameter is always larger than the bore of the valve, there will always be empty space, which is filled by air.

So when we run a LM in reflux mode, the tube from the pool and the valve are filled completely with distillate. So here no siphoning will be created. And because normally only a little amount of distillate goes through the valve, the air has enough space in the liebig not to interrupt the distillate flow. That's why a LM used only for neutral alcohol doesn't have a siphoning problem normally.

But when we full open the valve (and the bore of the valve is large enough for true potstill mode), we get air/vapor in all three places.

When does the air/vapor produce siphoning?
It depends mainly on the distillate amount and the diameter of the tube.

large and small diameter

And perhaps the angle of the tubes has a minor effect. Perhaps in vertical tubes air and distillate disturb each other more than in angeled tubes, because in angeled tubes generally the both elements are separated better (the distillate under the air/steam).

angeled and vertical tube

And probably a siphoning causing small diameter tube causes more siphoning the longer it is.

So to avoid creating siphoning at the tubes before and after the valve, we should oversize the diameters of tubes. At least if we want to use the LM in potstill mode too.
But the diameter of the valve can't be larged. A very large valve (for example a full bore ball or gate valve) would not allow to control the distillate amount like we want. We would lose, what a LM is able to do better than any other system (an easy controllable steady output from 0-100%). So if we want to use a small needle valve, which offers a good control, we will always create siphoning there in potstill mode. Except we have a construction, where the valve is always filled with distillate, even in potstill mode. Is this possible? Yes. I have two ideas:

after the valve is a bend upwards

the distillate flow direction is upwards the valve

Both solutions will cause a larger or a second pool, more smearing. So I don't consider them as perfect solutions. But perhaps other members like it or perhaps get inspired to find a better solution.


Here threads with posts about siphoning we had in the past:
http://homedistiller.org/forum/viewtopi ... =17&t=6865
http://homedistiller.org/forum/viewtopi ... 17&t=47184
http://homedistiller.org/forum/viewtopi ... 17&t=64632
http://homedistiller.org/forum/viewtopi ... 17&t=65942


If someone disagrees with me or wants to correct or add something or wants to report about the surging or non-surging of his LM, this is the right thread for.

[Odin]

What is the issue? Just trying to understand what you are facing. Inconsequent collection rates when the valve is fully open, right? After first closing it, that makes sense. You create a liquid bath on the lower plate and that acts like a water column, pushing liquids back faster ... until it is drained.

Der Wo, do you feel the siphoning means that gases are pulled through or some kind of vacuum is build up in the secundairy cooling system?

Again, just trying to understand what you (and others) encounter exactly and what the "siphoning" means.

If you could explain it to me and/or answer my questions, I may dive in with a better understanding.

I run LM's each and every day and the faster/slower take-off rates don't make sense.

Regards, Odin.

[der wo]

Hi Odin,
several quotes from the links I posted:

pintoshine:
After I got it hot, got it equalized and was pulling off the heads, I started encountering problems with flow control out of the needle valve. After some investigation, I discovered that the 1/4 inch tubing, which I ran through a 3/4" x 24" jacket to create a heat exchanger to cool the take off liquid, was pulling a siphon on the needle valve after the air was removed from the tubing.
This caused the flow to suddenly surge, at which point I lost control of the drip count.
I know this sounds trivial but I was running stripped molasses at 48% and it requires tons of reflux to get it near neutral. If the drip count is too high it brings lot of flavor with it. I was trying for a constant 12.5 ml/minute rate and it kept getting screwed up because of the siphon.

bearriver:
It ran fine, but I knew that I wasn't actually getting the maximum takeoff rate figured out. Small amounts, guessing 25-35% of the total distillate would splash over the top plate no matter what I did.

sergiolis:
I have the same issue with my needle-valve. It suddenly changes the stream... Is there a way to know if it is a siphon problem?

The stream change at any moment but usually when i'm getting a big flow-rate, let's say 40ml/min, suddenly the stream increase to 60 or more so I lose the equilibrium and the temp increase too because it's too much. (But sometimes the stream increased when I was on heads too at about 15 or 20 ml/min) On my last running it was like that, I was increasing the power and the take-off slowly until 40ml/min. The progression was ok but after some minutes at 40ml/min the needle-valve lost the control. Why it happens is what I don't know.


Here a video with a funnel and a tube showing siphoning:

Here the difference between abgeled and vertical tube:


If this behaviour reduces the maximum output, it is a problem in potstill mode.

[still_stirrin]

Indeed, this condition results from the tubing ID being too small. Surface tension of the liquid causes the miniscus to close within the tubing, creating pockets of vacuum. Gravity continues to pull the liquid down, but due to the pockets of air in the liquid channel, it "gulps" in surges.

A larger tube, as you suggested, would eliminate the closed channel flow and the resultant low pressure pockets of air. The liquid flow would stabilize then.
ss

[der wo]

Here a comparision large diameter tube vs small tube with the same water flow:

[Odin]

Does the video prove that the outlet is not the problem?

Odin.

PS: Not trying to do suggestive questioning, but - maybe with my eyesight failing - I didn't see much difference.

[der wo]

Sorry for the bad video quality. (And for the first two pics. I know it looks like giraffe necks)
I think it is visible, that in the first half of the video with large diameter the flow is much better. Not only visible but also audible.
I am sure with more differing diameters and more water pressure I could produce a more impressive video.
Do you think, the diameter of the outlet is not a factor?

[00Speedy]

Sorry for the bad video quality. (And for the first two pics. I know it looks like giraffe necks)
I think it is visible, that in the first half of the video with large diameter the flow is much better. Not only visible but also audible.
I am sure with more differing diameters and more water pressure I could produce a more impressive video.
Do you think, the diameter of the outlet is not a factor?

I think also, it is well visible and audible.
And i hear the gurgling sound from the funnel.
This is the same sound from the Boka Head by me, but i think it was solved by the Tube in Liebig under the Valve.

Der wo with giraffen in the bathtub plays .

[NZChris]

I use a 1/2" gate valve and tube. I was going to cut a slot in the bottom of the gate to give me finer control if I needed it, but I'm happy with it how it is.

[der wo]

May the wales forgive me (at least still_stirrin knows what I mean).

NZChris,
sorry I have forgotten, what stills you run (although R2D2 looks like a potstiller). Do you use your LM for neutral only? Is it a 3" column?
Generally a gate valve would work for sure great for running potstill mode. Interesting to hear, that it is manageable for slow pull off too.

[NZChris]

A new 2" Boka. I've used it once each for neutral and pot still modes. I doubt I'll use it much in pot still mode as I have a very good swan neck for that boiler.

[Yummyrum]

I think this thread could bring up some interesting points der wo

When I was making my LM/VM head with external reflux line I also tried it with water before fully assembling it .
The flow was erratic and syphoning was bad . I was about to throw it in the bin .... then I retested it with Methylated spirites ( cheap source of high ABV alc ) and it performed perfectly .

My point is that you are absolutely correct that the surface tension has everything to do with it. .and larger diameters are less effected

Second point is that a design should be tested with the ABV that it is likely to experience. ..... and in the case of a stripping head , that ABV will potentially drop close to water .

[der wo]

You are right, yummy. Just looking at wikipedia:

Surface tensions of:
Ethanol 20°C: 22.55 mN/m
Water 20°C: 72.75 mN/m
Water 80°C: 62.6 mN/m

So ethanol vs water is a huge difference.
But also cold and warm distillate perhaps makes a difference. Considering that the longest tube in a LM is the tube after the valve normally, a cooling jacket could make a difference too.

And when I remember the videos from Speedy, the output of the water runs looked worse, although the distillate flow was much lower than with alcohol (same heat input produces less water steam than alcohol vapor).