Reflux Coil and Coldfinger

[Rebel_Plague]

I am building a reflux coil with a cold finger and am wondering what kind of clearance does the condenser need on the inside of the column. I picked up some more copper for my still some 3/8 id copper tubing and a 1/2 x 12" air trap. And Coiled a small piece of tubing to see how well it fit inside a scrap piece of 2" before I Unit it. It seems a little snug ....??? Here is some pics.

[pounsfos]

for a 2' I would personally use 1/4' (because that's what I did use)

that's very snug, but I doubt you will have problems, gas is gas and will fit through any gap.

just make sure it is vented to the atmosphere

with this size, the only problem I see is that it might flood, or somehow keep some liqwuid up in the coil due to the size

but err 1/4' fits very nicely

[meatheadinc]

Yep little snug.
You will lose efficiency when touching the column wall, but given enough water flow it will still knock down vapour.
I agree with pounsfos. 1/4 inch would be much better.
Given that you have a neat 3/8 coil you will do 1/4 with ease.

[YHB]

If the coil touches the wall then the wall gets cold and the wall will become another condensing surface.

I, like you, used 3/8" and ended up with a "snug" fit. Having to push and pull the coil very forcibly every time I wanted to fit or remove it.

I put the coil back on the mandrel and went over the full length of the coil with a very small hammer giving light taps. This flattened the coil ever so slightly and made it a very nice sliding fit.

Having gone to all lengths possible to avoid flattening the tube when making the coil, it seemed a shame to deliberately reshape it with a hammer, but the deformation is so small you cannot see it.

The 3/8" coil makes a big improvement to the water flow and reduces the pressure loss allowing smaller circulation pumps to be used than those needed for 1/4" coils.

[meatheadinc]

YHB has a point 1/4 can be very restrictive.
If using recirculated water, a double 1/4 with 3/8+ cold finger, with coils running parallel will have the same flow, with more surface area

[DAD300]

What you've got will work. But when everything is tight you are counting on the very tip of the coldfinger and the first coil to do 90% of the work.

Lowering the coldfinger below the coil more will expose more surface to the vapor, but reduce water flow to the tip...and spreading the distance between your coils.

Or, using a single wound coil will actually expose more surface area to vapor, because the vapor could reach the the upper coils easier.

[Stonecutter]

What you've got will work. But when everything is tight you are counting on the very tip of the coldfinger and the first coil to do 90% of the work.

Lowering the coldfinger below the coil more will expose more surface to the vapor, but reduce water flow to the tip...and spreading the distance between your coils.

Or, using a single wound coil will actually expose more surface area to vapor, because the vapor could reach the the upper coils easier.

I’ll have a 12” long x 3/8” coil. For a 2” pipe. Was DAD300 suggestion about a single wound coil including the cold finger? or will a single wind without the finger be enough?

[DAD300]

You should need the cold finger with your described coil.

if it isn't enough, put some SS Scrubbie in the center of the coil.

[Stonecutter]

You should need the cold finger with your described coil.

if it isn't enough, put some SS Scrubbie in the center of the coil.

Holy Cow DAD300 I don’t know how I missed your reply. I’ve been procrastinating big time on this coil. I’ve got next week off in which I plan to finally do the damn thing and get this coil twisted up . I will update with pictures.

Thanks for continuing to help us newbies out.

[shadylane]

You should need the cold finger with your described coil.

if it isn't enough, put some SS Scrubbie in the center of the coil.

I'll have to disagree with you about a scrubbie making an RC more efficient.

What the scrubbie will do is occupy space and make vapor have to travel faster.
The higher vapor speed decreases dwell time and the vapor has less of a chance to condense.

[Stonecutter]

What the scrubbie will do is occupy space and make vapor have to travel faster.
The higher vapor speed decreases dwell time and the vapor has less of a chance to condense.

That is definitely backwards to my thinking. I’m not saying it’s wrong Shady. I don’t have any experience with CCVM rigs or much experience with scrubbie/vapor behavior.
Wouldn’t the vapor slow down after coming into contact with the scrubbies obstruction? Is it that vapor pressure would increase thereby increasing vapor speed?

[cob]

#1 Wouldn’t the vapor slow down after coming into contact with the scrubbies obstruction? #2 Is it that vapor pressure would increase thereby increasing vapor speed?

#1 no

#2 Venturi effect.

[subbrew]

Vapor output is controlled by power so scrubbie is not changing output. For a given output volume the larger the path the slower the vapor goes. So by restricting the path with scrubbies the pressure increases to speed up the vapor so the same amount can flow, just now via a smaller path.

If you actually did slow down the vapor you would have less volume going past the scrubbie. You still have the same volume coming up the riser. That vapor coming up the riser has to go someplace.

[Stonecutter]

#1 Wouldn’t the vapor slow down after coming into contact with the scrubbies obstruction? #2 Is it that vapor pressure would increase thereby increasing vapor speed?

#1 no

#2 Venturi effect.

I’m going to humbly take your word on it Cob. Thank you for taking the time.
I wiki’d Venturi effect and it’s obvious I’m going to need an empty house, a couple drams and a smoke to venture far enough down that rabbits hole to have even a mild understanding of such effects.

[Stonecutter]

That helps my poor brains to understand a bit better subbrew. I’m better at drinking the shit than understanding how it comes out the spout.


Edit: Either way I do believe I have enough pipe to wind a double coil so I won’t need any scrubbies in it.

[shadylane]

That is definitely backwards to my thinking. I’m not saying it’s wrong Shady. I don’t have any experience with CCVM rigs or much experience with scrubbie/vapor behavior.

It's counterintuitive but that's the way it works.
A friend of mine has been playing with a CCVM and he uses a small "beard" of copper mesh hanging down off the bottom of the RC, he says it makes the coil adjustment less touchy and allows his rig to run a lower reflux ratio if he wishes. It also does a good job of guiding the reflux so it drips onto the center of the packing.

[still_stirrin]

Here’s a better explanation: The scrubby increases the conducting surface area that the vapor transfers its heat into. And as it cools below it saturation temperature, it condenses. And, the (condensed) liquid form of the fluid requires only a small fraction of the space that the vapor does, so the velocity is NOT adversely affected by the scrubby. It actually WILL IMPROVE heat transfer (condenser efficiency).

And keep in mind that copper is much better than stainless steel at conduction of heat. So, if you can use copper scrubbies, you”ll be glad you spent the extra money.

Dwell time for the vapor in contact with the condenser is also important. So, you don’t want fast moving vapors … they’ll just puff right through the condenser. Small diameter vapor tubes are a problem here. Plus, as the vapor passes through a pipe/tube, the friction causes a pressure drop. And you know what happens to a vapor when the pressure is lowered … the vapor EXPANDS. And when it does that in a tube, the velocity INCREASES, sometimes significantly.

If the tube is long enough the flow can actually choke, reaching mach velocity in the pipe, and not allowing any more mass flow even with significant pressure on the source. To demonstrate this, dry breathing through a drinking straw. You won’t get enough air through fast enough to breath … you’ll suffocate.
ss

[Stonecutter]

Shady, I think Deplorable runs with a bearded coil as well. I’m definitely interested in anything that helps with reflux “touchy-ness”.

[shadylane]

Here's my thinking based of fuzzy tube RC experiments

The only way a scrubbie can help is if it's thermally bonded to a cold surface.
As in soldered to a cold tube, even then there's a limit.
Because the mesh holds reflux and gets saturated with liquid and obstructs the flow of vapor.

[Stonecutter]

Ok this is obviously a miniature crash course in vapor behavior and I want to get out it in front here and say that I do understand that one should examine as many factors as possible when determining ones setup.

So within the scrubbie “matrix” vapor velocity could be weighed less and heat transfer considered more and when determining riser pipe size and length, vapor velocity rather than heat transfer is considered. We’re currently only talking about behavior inside of the condenser and an empty riser. It would seem obvious that both friction and heat transfer should be heavily considered when determining an ideal packed column environment.
BTW I do have a big role of copper mesh i use for any scrubbie stuffs and excluding the boiler my rig is all copper.

[DAD300]

QUOTE "Here’s a better explanation: The scrubby increases the conducting surface area that the vapor transfers its heat into. And as it cools below it saturation temperature, it condenses. And, the (condensed) liquid form of the fluid requires only a small fraction of the space that the vapor does, so the velocity is NOT adversely affected by the scrubby. It actually WILL IMPROVE heat transfer (condenser efficiency)."

the scrubby in contact with the condenser, adds cold surface to the condenser, amplifying its affect.

Reflux dropping through the rising vapor also slows the vapor (by reducing it) the same way.

[shadylane]

Here's a thought.
I've never seen a commercially designed reflux condenser that used anything resembling scrubbies.

[kimbodious]

Scrubbies wrapped in around RC coil get cold. When the vapour hits the cold stuff it condenses. DAD300 gave me the tip years ago about a cold finger extending below the RC coil. It helped to detune the RC giving more scope for finer control. Nice one Dad!

[Sporacle]

Great thread, love the input from everyone.
My two cents, I think we might be discussing apples and oranges.
I don't believe that the addition of scrubbies to a undersized or poorly designed reflux condensor will improve the ability of the RC to knock down vapour.
You still need to cool the thermal mass that is knocking down the vapour and the scrubbies are thermal mass.
So if your RC can't condense the vapor due to poor design or flow then scrubbies won't help.

If you have a well designed RC that has good flow then the addition of the scrubbies may allow you to slow the speed and pressure, due to the additional thermal mass being able to be cooled effectively.

There is a whole vapor vs liquid and the reduction of pressure that the condensation causes that I can't and won't figure out.

In conclusion, you can't fix a crap RC with scrubbies but you may make a good RC a bit better with scrubbies in my opinion

I've got a feints run coming up, I might scrubbie vs no scrubbie on my CCVM and see who blows past at 4000w first right at the start of the run, then let it sit for an hour or so reheat and do my run

[kimbodious]

wrong thread, sorry

[Saltbush Bill]

It seems a little snug ....??? Here is some pics.

Might pay to scroll to the top of the post, for those that haven't.