Siphon killing my needle valve control

[pintoshine]

Yesterday afternoon, I started up my SS column, that I refer to as Toni.
I have a needle valve very near the output of the reservoir at the bottom of the liquid/gas separator which makes this an inline BOKOKOB style unit.
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.
I corrected the problem by drilling a tiny air hole in an elbow immediately following the valve. This allowed air to break the siphon and allow the valve to continue to drip normally without the additional suction.

siphonfix.jpg

I just wanted to let everyone know about this unforeseen control problem, in case any of you using a heat exchanger after your inline takeoff point experience the same difficulty with flow control.

[DestructoMutt]

Pint -
a mechanical engineer friend explained to me that the cooling of the fluid (liquid or gas) causes the volume of the fluid to decrease (when our vapor changes into a liquid, also the change in volume from hot liquid ethanol to cool liquid ethanol). the decrease in the volume of the fluid (liquid or gas) induces a vacuum effect. this same phenomonom occurs when the vapor (a gas) goes through the vapor takeoff heat exchangers on our potstills (the huffing and puffing that occurs). my friend said that if the volume of the cooler/condensor was decreased (your hint of stuffing a scrubbie into the heat exchanger) the vacuum effect could be reduced or eliminated. alternatively, one could use a stepped volume heat exchanger, e.g.: starting with 3/4" o.d. tube inside 1" o.d. tube at 4 - 6" long; stepped down to 5/8" o.d. tube inside 3/4" o.d. tube at 6 - 12" long; stepped down to 1/2" o.d. tube inside 3/4" o.d. tube at 12 - 24" long; stepped down to 3/8" o.d. tube inside 5/8" o.d. tube at 18 - 36" long. (please note: the given sizes are only an example - they may have to be resized to actually fit the reducer fittings available and to avoid coolant flow restrictions and/or blockages).
he also said something about a possible venturi effect and the ideal gas law, but i'm not a mechanical engineer - maybe somebody else can expound on the theory/practice of fluid mechanics for us.

[pintoshine]

I have explained the huffing to several others who have encountered it. I can get my column to huff if I turn it up too high.

This siphon phenomenon is slightly different and involves no gas other than air and is one I have encountered a few times before. In this case it was a matter of the tubing internal diameter being smaller than a drop. So once the internal surface became wet enough for the drops to cling all the way across the tube, they started accumulating rather than flowing. Once enough accumulated, gravity pulled the whole volume down at once and caused a vacuum on the outside of the needle valve. The vacuum caused much more liquid to exit the reservoir than was expected and cause its behavior to be sporadic.

Now that it is repaired I have been collecting at 190+ for 18 hours now at 12.5ml/minute. Rum is sure hard to get the flavor out of. If this keeps up I won't have to distill anymore until the fall when grapes come in.

[DestructoMutt]

according to the mechanical engineer -

air is a gas, so is steam (water vapor), so is ethanol vapor, ....

heating air causes its volume to increase and cooling it causes its volume to decrease.

an easy experiment, is to take an empty 1 gallon, metal container (one of the rectangular shaped ones with the screw on lid), put one or two cups of water in it and then se the container on a burner to heat up the water. once the water is boiling, remove the container from the heat and quickly screw on the cap. the force exerted on the container by the cooling vapor will become readily apparent.

same mechanics are involved when gas/air/vapor is cooled in an heat exchanger. if the supply of vapor is not sufficient to keep up with the decrease in the volume of the cooled vapor as it enters and traverses the heat exchanger, a vacuum is created. a decreasing volume in the heat exchanger can/will make up for the decreased volume of the vapor.

[nutmg1]

according to the mechanical engineer -

air is a gas, so is steam (water vapor), so is ethanol vapor, ....

heating air causes its volume to increase and cooling it causes its volume to decrease.

an easy experiment, is to take an empty 1 gallon, metal container (one of the rectangular shaped ones with the screw on lid), put one or two cups of water in it and then se the container on a burner to heat up the water. once the water is boiling, remove the container from the heat and quickly screw on the cap. the force exerted on the container by the cooling vapor will become readily apparent.

same mechanics are involved when gas/air/vapor is cooled in an heat exchanger. if the supply of vapor is not sufficient to keep up with the decrease in the volume of the cooled vapor as it enters and traverses the heat exchanger, a vacuum is created. a decreasing volume in the heat exchanger can/will make up for the decreased volume of the vapor.

I think Pintoshine solved this by drilling the hole

[Oaty]

Alteratively, I think the problem could be corrected by an increase in the diameter of the take-off tube. It appears you are using standard 1/4 inch tubing. An increase to 3/8 should alleviate the problem. 3/8 tubing has an o.d. of 3/8 in & an i.d. of 1/4 in. 1/4 tubing is tiny by comparison.

In adddition, I noticed the valve you are using. It is not strictly speaking a needle valve. It is commonly called one. Even by the manufacturer.It is a valve. It has a needle. It is not a needle valve. You buy it off the shelf at any home center. It's really used mostly as a valve for your refrigerator ice-machine.However, it allows very little control of the takeoff volumes. A quarter turn of the handle changes the volume tremendously. It's almost an on-off valve. Which is all that is required for the ice machine. This in and of itself could cause your problem. You have very little control of the drip rate.

I'm a newbee , but I think you could get much better relux control using a valve such as made by 'Swagelok' or any other industrial valve, particularly one with a regulating stem, such as the ss-1rs6. With these you'd get up to 10 turns of control with slight changes in reflux per turn.With this particular valve a change of fluid coefficent of from 0- 0.7 is acheived over 8 complete turns of the valve. In layman's terms ...'that's alot of control'.I can send you a PDF file of their catalogue and even contact info. If you would like.

[manu de hanoi]

had similar problem with controllin the wash flow in the preheater/condenser of my continuous setup, guess what ... I did the same... a small surgical hole in a tactical point. Seems simple but it takes some thoughts ....

[sergiolis]

Hi Pintoshine, 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? thanks in advance

[bearriver]

Hi Pintoshine, 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? thanks in advance

I don't think Pinto posts here much if at all. He has his own forum. You can go there and ask him.

Otherwise, can you post a larger photo of the still head (above the sight glass)?

Under what circumstances does the stream change? How does it change?

I use that same exact Parker brand needle valve on my LM, without any issues.

[sergiolis]

Hi bearriver, I appreciate very much your help. 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... Hound Dog thinks maybe the valve is defective. Today I ordered a new one because I'm getting crazy about that issue and when I'm running I have to be watching it almost all the time.
I have a sight-glass over the packing. And the odd thing is that even at 2400W the boiling liquid is bellow the sight-glass. So I know I'm not reaching flooding state (from the bottom at least) but suddenly at any time the needle-valve increase the stream too much.

[Edwin Croissant]

Does it also does that when you disconnect the product condensor cooler?

[sergiolis]

Hi Edwin, nice to hear from you again!! As you know I changed the stem type but the problem persist.
Sorry Edwin I don't understand your question. My english is terrible!!! What do you mean? when I'm running I never disconnect the product condenser. I try to keep the water draining at about 50ºC.

[Edwin Croissant]

When you disconnect the product condenser cooler from the needle valve and catch the hot distillate directly does the flow changes in the same fashion?

Nothing wrong with your english, its a cooler not a condenser
Trust it make more sense now

[bearriver]

Sorry... I still have more questions than answers, and will probably have more.

Does the surging happen with the valve wide open, when it is partially closed, or both?

You are using 3/8" for the takeoff tubing with a 1/2" Parker brand valve, correct?

Is there any mesh anywhere in the takeoff line?

Have you inspected the ID of the entire takeoff line to ensure there is no foreign material creating a restriction? Note: A good design should accommodate for this.

How tight is that scrubber packed in the top of your reflux condenser coil? The still head should be well vented to the atmosphere so that no pressure of any kind builds up.

[sergiolis]

Edwin: Sorry I never tried. It happens when I'm in the middle of the run so I've never tried to remove the condenser.
bearriver: When I'm collecting product the needle valve is always partially closed so it happens then.
I'm using a 6mm needle valve with a copper tube of 6mm too
As far I know the tube is clean, no mesh
Its a problem of excess liquid
Before I was running with a copper condenser double helix and without the scrubber and the issue was the same
Really it's a weird problem!!!

[sergiolis]

But I will try it my next run Edwin.

[bearriver]

\
Really it's a weird problem!!!

Yeah it is. I don't even know where to speculate. Everything seems to be sound.

If it is somehow a siphon, you are better off re-orientating the secondary condenser into a vertical posistion. That will give a better chance for air to move up the takeoff branch and meet the valve, thus killing any hypothetical siphon.

Here is an outdated pic of my setup, which shows what I'm talking about.

[skow69]

I remember the long discussion of this previously. I sounds to me like it could be exactly the same thing that pint was talking about. Try drilling a pinhole in the tubing just after the valve. If it doesn't work you can just solder it shut.

EDIT: In your case the effect would be magnified by the 6 inches or so of tubing above the valve. That and whatever is held in the column above the plate would be a reservoir of liquid. When the siphon starts, that would all have to drain before it could break the siphon.

Please let us know. I have high hopes that this could solve that problem that you have been fighting for so long.

[shadylane]

pintoshine
Do you have a picture showing the rest of the still?
Also It's nice to have you back

[pfshine]

pintoshine
Do you have a picture showing the rest of the still?
Also It's nice to have you back

Pint hasn't been on in years. Anyhoo. If I wre you I would check your power supply as well. If its not reliable you could be getting surges in power. That would make a slight overpressure pushing your distilate out faster than usual. Just a thought. I'm probably talking out my backside though.

[sergiolis]

Thanks a lot for your help guys!!
I will try both solutions. I will drill a 1mm hole and I will place my liebig vertically. Hope springs eternal !! In 2 weeks I will let you know..
Yes, I see Edwin, I could try it without the liebig condenser(product cooler) too....

[Edwin Croissant]

Maybe you are experiencing a milder form of what is described in this thread.

[sergiolis]

It was a siphon problem. A 1mm hole has solved all my problems. wonderful !!!!!!!
Tiny solution........ Thanks a lot skow, edwin and others