Basic Overview of Common Reflux Designs

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kiwistiller
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Basic Overview of Common Reflux Designs

Post by kiwistiller » Mon Jan 11, 2010 3:50 am

A Basic Overview of Common Reflux Designs

This document is intended to be an aid to the novice in identifying and understanding the various different types of reflux design and their pros and cons. The specific parameters of each design are considered outside the scope of this guide. Before we get into the different designs I've whipped up a very brief primer on the basics of reflux theory, which is really needed to understand the still designs. A generalised diagram is provide for each design, and I'll link to a couple of build threads for each type. You'll note that most of the time an actual build of the still will look slightly different to diagrams. There are many ways to skin a cat, and I'd encourage readers to look past the surface detail and try to understand what the builder is trying to achieve with each part of their still.
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What does a reflux still do?
A reflux still (on a hobby scale at least) is generally designed to produce as pure a product as possible. We call this 'neutral' spirits. This contrasts with a pot still, where we desire certain flavours to carry over. Reflux columns are generally quite high (~1m/39in to 1.5m/60in of packed column, plus the head on top), for reasons that will soon be explained.

What is reflux and where does it come from?
Reflux is vapour that has risen up the column, been condensed and returned back down the column as liquid. Some old designs of reflux still only generate reflux passively, that is to say there is no 'active' or 'forced' cooling anywhere in the design except for the output (product) condenser. Reflux in these versions is generated through passive heat loss to the environment. These are notoriously hard to run, and the design is very seldom seen any more so will not be discussed further.

Most (all, really) reflux columns that are seen on this site are of a 'forced' reflux design. This is to say that there is active, purposeful cooling somewhere in the design with the intention of creating reflux to return down the column, through a packing of some sort like copper mesh or stainless steel scrubbers.

How does reflux work?
The thing that makes the column work is the returning distillate, which mingles in the packing or plates and then condenses the stuff coming up, which is then redistilled by the vapour below. My 1.5m of 2" packed with mesh has ~13 distillations (theoretically) over the course of the vapour going up the column (see the parent site for calculators that work out those sort of numbers). As liquid returning hits vapour rising, the water in the vapour tends to condense (takes more energy to boil water than ethanol, right?) which gives energy to alcohols in the liquid that makes them turn into vapour. The end result of a whole lot of this sort of thing is the separation of water and alcohol, with the water eventually falling back down into the boiler as liquid, and the alcohol rising up the column as increasingly concentrated vapour (with the lighter vapours grouping toward the top of the column which helps separate the heads/hearts/tails before takeoff. This is what we call equalibrium).

"Equilibrium:

It is important that a fractionating column, whatever its means of management may be, can reach equilibrium. This is a state where all the rising vapours are cooled back to liquids and all those liquids are being sent down the column for further distillation. This equalibrization process allows for the lightest alcohols to travel up the column and stack there over time. This will allow for a slow draw-off of fores and heads, leaving you with cleaner hearts."


Column height and diameter
When designing a reflux column, height and diameter are two critical decisions that will, along with the method of reflux management you use, determine the performance of the column. Height of the 'Packed Column' (this is the amount of column height filled with packing) determines the purity you can achieve, as more packed height means more of the mingling of reflux and vapour that we discussed above. The diameter of the column determines the amount of vapour and reflux that can be traveling up and down the column at any time, and therefore determines the maximum speed that you can produce at. There are some great calculators in the reflux section of the parent site that can aid you in determining the column parameters you choose (link: http://homedistiller.org/reflux_calc.htm" onclick="window.open(this.href);return false;" rel="nofollow). As a general guideline, for diameter, 2" is really the minimum practical size in terms of production rate, and after 4", the hobby scale designs start to break down a little, they don't scale up well. Column height is best determined as a ratio with column diameter. 12:1 would be the ABSOLUTE minimum. around 20-25:1 would be more normal. More is better, but diminishing returns kicks in pretty quickly, and 30:1 is pretty much as high as you should consider. Anything more than that will probably just hurt the fuel economy of your still.

"Nowadays more modern packings, with a lower height equivalent of a theoretical plate (like SPP) allow for a lower height to diameter equivalent."

Reflux ratio, and why does it matter?
Reflux ratio is the ratio of liquid flowing down the column to the amount of distillate drawn off. Alternatively, you can think of it as the ratio of the amount vapour condensed and returned down the column as reflux to the amount of vapour drawn off and condensed as product.

Now, all reflux designs allow you to manipulate the Reflux Ratio - that is the ratio of distillate returned down the column to the amount taken off as product. Depending on your specific design, it's sometimes hard to figure out the exact figure, but this is not necessarily a huge problem, the good thing is you can manipulate it without knowing the exact numbers.

So why does the reflux ratio matter? Because [Simplification warning!] it more or less translates to purity. By returning more or less reflux down the column, we can vary the amount of re-distillations that happen in the column, effecting the purity of the output. By returning more reflux down the column (slowing takeoff), we increase the amount of re-distillations and therefore purity. For the full dose of theory behind this, read the reflux section of the parent site, and even better the book 'The Compleat Distiller'.
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Reflux management
So, the reflux ratio is important. All forced reflux designs are specifically made to manipulate the reflux ratio in a controlled manner. This lets the distiller 'drive' the still the way they want to. For example, most distillers go to complete reflux at the start of the run, to achieve an equilibrium in the column, then slowly begin to bleed off the foreshots and heads, and then decrease the reflux further to get a good speed for hearts. The range of reflux designs, although complicated and confusing for a novice to look at, are all just manipulating the reflux ratio in a different way.

The following section should help you recognise and understand the workings of the various reflux stills discussed on home distiller. There are three main categories, liquid management, vapour management, and cooling management (LM, VM, CM). Power Management is outside the scope of this overview.

Liquid Management
Liquid management is a well established design, first popularised with an 'offset head' design (sometimes called a 'Nixon-Stone'), as seen below:
Inline LM
Inline LM
More recently, an inline version has become more popular, this is often called the Bokakob Inline or Dual Slant Plate design. It looks a lot different but the operation is the same:
Offset Head LM
Offset Head LM
As vapour rises up the column, it passes the takeoff port (in whatever form that may be, slant plates for the boka), and 100% of the vapour is condensed, and the falling distillate collects in a pool at takeoff. A needle valve lets you take off product at a set rate, while the pool overflows back into the column as reflux. This means that you are always taking off quantity X. The amount of reflux is determined by the total amount of distillate produced, less quantity X.

These stills can provide good performance, and have quite a small parts list so are probably one of the cheapest designs to build, don't require too much challenging construction, and are accordingly probably the most built reflux design. The LM has very good heads compression (probably the best of the three main design types). They also allow a reflux ratio of less than 1:1 more easily than a VM. This allows them to operate like a pot still at lower reflux ratios, but (and I'm paraphrasing Husker here, not personal experience) it isn't quite as good as a propper pot still for stripping etc because of the amount of condensation that occurs on the bottom of the slant plates. The LM (assuming a boka type design) is generally regarded as the easiest and cheapest to build.

A little bit of my personal bias is going to creep in here, but I see one serious drawback of an LM. Lets say (just to make it easy on the math) that the column produces 100ml per minute, and X, your take off, is 50ml/min. this means that at the start of the run, your reflux ratio is about 1:1, which is ok (well, just) for a big tall column taking off hearts. Probably not azeotrope, but close. Now as you're distilling away happily, the amount of ethanol left in the boiler is decreasing. This slows the production down from 100ml per minute to 75ml per minute. However, your valve is still set at 50 mls/min, so your reflux ratio is now .5:1, which is way too low. So, you can think of the valve on a LM as controlling the takeoff rate in isolation of the reflux rate. This results in a slowly decreasing reflux ratio, requiring more operator attention.

Design variants and other names:
Offset Head, Nixon-Stone, Valved Reflux, Bokakob, Boka Inline, Inline LM, Dual Slant Plate.

Commercially available LM stills*:
Amphora Society PDA 1 & 2, stainlessstuff.net, 'Ultra-Pure Still (sold in Aussie).

LM Build/Photo Threads
Pikluk's 2" boka column
Shifty's inline LM (note the difference from a slant plate design - many ways to skin a cat, remember!
Rad's small scale combo still
Linearone's offset head
Uncle Remus's offset head

Vapour Management
The vapour management column is a relatively new design. While LM condenses everything and manages the reflux ratio by splitting the liquid, the VM splits the vapour into two streams, and then condenses each separately:
VM
VM
Because a VM splits the stream of vapour before reflux is generated, with the valve set the valve wide open (assuming a full size takeoff port), you're getting roughly 1:1 reflux ratio regardless of the amount of vapour. While with an LM, adjustment would be needed as the production rate decreases, in a VM the reflux ratio will stay constant and output will fall; lets say my valve is still wide open, at the beginning 100mls/min = 50:50 = 1:1, and later 75mls/min = 37.5:37.5 = 1:1. The valve on a VM sets the reflux ratio, not the takeoff amount, and as the reflux ratio is what you want to set and keep constant over hearts... In my books that's a big tick for VM operation over LM :lol: (but I would say that, I have one)

Also, because of this, the valve settings for VM will remain constant over different charges or the still (which will probably produce at different rates) changes in heat input, and so on and so forth. Anything that varies the rate of production of vapour requires a change in valve settings on an LM. A VM you just leave. Well, for hearts anyway. Any column you choose will require some amount of dicking around for foreshots, heads and tails, but again, the nice thing about the VM is PREDICTABILITY. you can remember what ratio you like to take off heads at, set it, do other things around the shed and change it when you've taken off enough. Instead of having to keep adjusting a valve when tails is looming, you just wait until the thermo budges, knock back the valve a bit til it goes down, when it budges again open her up and take off the measly litre of tails. Or don't even bother. Tails from a reflux are so gross you won't want them anywhere near your product. The VM is generally acknowledged as the easiest design to run.

A VM has drawbacks too. The first is that it's quite hard to discover your reflux ratio (here is my experiment to calculate mine). This isn't a major problem, as you don't really need to know. The other drawback is you are limited in how little reflux you can produce (ie the minimum reflux ratio, because the VM cannot (without simple modification) get less than about 1:1 reflux ratio (unless you have a small takeoff port, in which case the minimum ratio will be more like 4:1), it is (imho) unsuited for flavoured spirits. I personally consider the VM a one trick pony, neutral and only neutral. But then again, if you want flavour, get a pot still.

The other main drawback of a VM is it is less effective for heads extration when compared to a good LM design. This results in a slightly higher proportion of your total run being heads. To get the advantage of both LM and VM in a single build, many combine them, and have a VM takeoff (for hearts) below a set of slant plates for LM takeoff (for heads extraction). Another option for adding LM functionality to a VM stills is to use a horizontal condenser, like the Crossflow Condenser or Thors Hammer (search for them for more info).

Commercially available VM stills*:
stainlessstuff.net

VM Build/Photo Threads
New Moonshiner's VM (not simple but one of the nicest around)
Samohon working out his design
Some modelling and discussion
Manu's 3" Stainless VM
Vman's Stainless VM
Bonnraggarn VM-E-ARC (extra for experts - Riku's design, see 'Designing and Building Automatic Stills)

Note - We're a bit short on complete build photos for the bog standard, 2", copper VM that is most common. If you're making one, please document it!

Cooling Management
This is (unfortunately) a very common design. The cooling management still features cooling of the column before the product takeoff. This condenses a portion of the vapour and sends it back down the column as reflux. It is controlled via the manipulation of cooling water to the reflux condenser. If water flow is reduced, the reflux ratio drops and output increases. If cooling water is increased, the reflux ratio is increased. It is possible to completely stop output by increasing water flow (total reflux). The CM designs has a lot of variations. This is one of the oldest, yet most seen:
Old style CM
Old style CM
The main feature of this design are the cooling lines that travel through the column on their way to the product condenser. The reasons this design is pretty much the worst of the lot have been summed up nicely by rad:
rad14701 wrote: First, the reflux will drip off the entire length [of the tube through the column] and unless perfectly vertical the reflux will end up running down one side of the column rather than dropping into the center... Through tubes would probably perform better if the had a sag in the center so all reflux drips off into the center of the column...

Second, by having the tubes separated, with one at the top and one at the bottom, you are trying to force reflux separation within a small area... If that is the goal then you would need to be able to control the amount of water flow to each individual tube because if the bottom tube gets too cold no vapor will make it to the top, and if it does the top tube might reflux the balance of the vapor... With your design the bottom tube has more surface area than the top which further adds to the complexity...
Because of these problems, a newer design is now being built that is preferred by those people that have them as it gives them more flexibility in their product selection than both the LM and VM. People that want one still that will make both a vodka and a whiskey or rum have turned to this design. It allows you to do both. And with a CM still since it’s nothing more than a pot still with reflux added. They make a great whiskey and rum similar to a pot still. When you use the reflux option it makes an ok neutral.
A debate will always arise between the people that believe you make your best whiskey with a pot still and those that are now making whiskey from their CM stills.

CM3.jpg

This design avoids some of the problems of the older one above by generating all the reflux at the top of the column. The condenser here can be tubes (such as a dephlagmeter), a coldfinger, a jacket, or even a coil mounted inside the column. all will have similar results, as long as the reflux is centered somehow.
The CM still has gotten a bad reputation from the design mentioned above. Because the two through tubes disrupts equilibrium of the column and isn't enough cooling for good reflux. The fact that the reflux condenser can't be controlled separate from the product condenser makes that design an inferior one as well.

Consistent results can be had once good control is added to the cooling system and with a proper designed reflux condenser it is possible to attain 100% reflux that negates the negative rap the original designs had. Also having two valves one to control flow to reflux condenser and the other to the product allows you to make many different products.


One good thing about CM is they achieve great tails compression. To control the reflux ratio, you have to tweak the level of coolant flow to the reflux condenser. It takes some practice to run but like all stills one needs to learn to operate it. Coolant settings are not predictable, either - a hotter day could mean your water supply is warmer, or the still charge could be different. If you are recirculating your water, then it will get warmer, requiring constant attention.

Commercially available cooling managment stills*:
Essencia Express, Still Spirits Reflux / Super Reflux, Spirits Unlimited Eurostill, All of Mile-Hi's designs (at time of writing), Brewhaus Essential Extractor series.

CM Build/Photo Threads
Bushman's 4" Build (double valve, one for reflux condenser and one for product condenser for better control)
Absinthe's CM (cooling lines located at top of column)
Bayou-Ruler's CM (cooling lines also at the top, some rotated)
Snuffy's Coldfinger CM (one of the nicer CM setups)
Chunkystyle's Stainless CM (more advanced design)

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Hopefully this overview can help some novices in understanding and identifying the various stills discussed on this board. Feel free to pm me if you have a suggestion for improving this guide.

Cheers,
Kiwi

* I wanted to provide an indication of what commercial designs were, but I don't really know all of them so I'm a bit stuck. If you can, please add to the commercially available designs lists if you know any by sending me a pm (or a mod I guess) with the name of the still and type. The reason I wanted that bit in is so that people stop saying "hi I have a mile hi still" or 'Hi I have a moonshine still" and expect everyone to know what that is.
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Note: Mod edit by Bushman to update information to reflect more current designs and information
Last edited by Bushman on Tue Feb 25, 2014 7:55 am, edited 8 times in total.
Reason: Added counter-flow clarification to the CM design portion.
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