Types of Stills - LM, VM, CM...What do they mean?
Brendan Feb 2013 (AD)
The general pot still consists of a column sitting on top of the boiler. The vapour rises from the boiler and up through the column, reaching the neck at the top of the column, and follows this path down into the product condenser. The only way to control a pot still is by the amount of heat you put into the boiler, whether it be through power or gas. All vapour that reaches the top of the column is condensed and collected (assuming the condenser is adequate).
For illustrative purpose, the following legend applies to diagrams in this thread:
A reflux still, has an additional condenser sitting usually on top of the column (although there are various offset designs, we will ignore these here for simplicity just to demonstrate the basic principles). When the rising vapour reaches this condenser, most if not all of the vapour will condense and fall back down the column (depending on the coolant water flow in the condenser). This process is known as reflux.
Reflux stills are generally referred to in 3 different categories, being LM, VM, & CM.
These anagrams stand for Liquid Management, Vapour Management, and Cooling Management, and refer to the way in which the output collection is managed. In other words, the process which is used to control what, when, and how fast you collect from the output.
Although there are several variations in each category, we will only look at one example for each to avoid over complicating the topic with every still ever designed, and to assist in educating on the meanings of these terms. This way, when you see a still which you haven’t come across before, you can use what you have learnt to determine for yourself which category it belongs in.
As the name suggests, the output is determined by controlling the amount of the actual liquid distillate which is allowed to exit. This is usually achieved by use of a needle valve. A very common example of this type of still is the Boka (or Bokakob, named after it’s inventor).
The column contains two slant plates (pieces of copper), which are soldered in on an angle. As the vapour rises up through the column, it reaches the reflux condenser, condenses into liquid and falls down the column. Some of this falling distillate catches on the top plate which is slanted at a downward angle, and falls down onto the first plate due to the overlap of plates. It is then caught by the second slanted plate on the opposing side which is angled upwards, and fills that space until it overflows and falls back down the column.
The collection is controlled by a needle valve at the bottom plate, which when opened slightly, lets a small amount of the liquid distillate drip from the plate which is collected. By keeping this valve closed for the first half hour or so, the column gets itself into a sort of equilibrium, where the continual rising of vapour and falling condensate, refreshes the liquid held by the slant plates and allows higher/lighter alcohols to be held on the plates.
As the needle valve controls the release of the liquid as described, it is termed a Liquid Management style still.
The Vapour Managed still can come in many various designs, just like the other categories. A common VM design is shown in the picture below. The column contains a reflux condenser sitting at the top of the column (commonly a coil), as well as a tee piece below that branching out into a second condenser called the product condenser. Off of the tee piece is a valve used to control the flow of vapour. A gate valve is usually used here, although ball valves have been used, the gate valve gives much better control which is important in a VM design.
The vapour rises up the column as normal, where the vapour path splits into two at the tee. The vapour that continues towards the top is condensed by the reflux condenser and falls back down the column as explained previously in the description of reflux. With the gate valve closed, the column continues in this cycle reaching an equilibrium where the alcohol components are separated in the column. Lighter alcohols find their way towards the top of the column, and heavier alcohols settle around the bottom of the column.
Safety: Above the reflux condenser must ALWAYS be open to the atmosphere, so that there is no pressure in the still. Alcohol will not escape out of the top if there is enough coolant flow through the condenser. If the top if sealed...KABOOM! (This applies for both VM & LM, or any still were it is not vented to the atmosphere and can be closed off by a valve).
Usually the user would keep the valve closed for the first say half hour of the run, to allow the column to separate the fractions (Lighter and heavier alcohols). At which point, they will ever so slightly open the gate valve to allow a small amount of the vapour through it which will flow through to the product condenser for collection.
As the gate valve controls the release of the vapour as described, it is termed a Vapour Management style still.
The cooling management still will get some extra attention here describing two different examples, but more on that shortly. As the name suggests, the output is controlled by manipulating the coolant water through the reflux condenser. In the other types of reflux stills, a coil is commonly used as the reflux condenser because the aim is to knock the vapour down. However, the reflux condensers in a cooling management still are usually of a different design because at some point, the aim is to let some vapour rise through the reflux condenser...cool hey? Common designs here are a water jacket (larger pipe full of coolant water, flowing around one slightly smaller pipe that carries vapour), and the shotgun condenser (larger diameter pipe carrying coolant water, and several smaller diameter pipes running vertically through it carrying the vapour...appearance like a multi-barrelled shotgun).
The concept behind this design is the column can be held in full reflux by allowing enough coolant water to flow through the reflux condenser, so that all of the vapour is knocked down (condensed) and falls back down the column as liquid. After the column has again reached an equilibrium, the flow of coolant water to the reflux condenser can be restricted, to a point where a certain percentage of the rising vapour is not condensed by the reflux condenser and continues to rise through it and on to the product condenser where it is collected.
As shown in the diagram above, the same coolant water is commonly used for both condensers.
Because the coolant water is manipulated to control the output from the still, this becomes a Cooling Management type still.
Although the aim here is to understand the difference between various types of stills, and how their outputs are controlled, we will briefly discuss column packing here. This is in order to lead into the next section, and for a brief introduction on the interaction between rising vapour and falling liquid distillate. In the previously mentioned reflux stills, the columns are generally packed with a material containing a high surface area which also allows the vapour to pass through it. Most commonly used materials for this purpose are copper mesh rolled up, or stainless steel scrubbers. More on this can be researched further on the Aussie Distiller forum, but the main purpose of this packing is to give a surface that falling liquid distillate can have contact with, which can then be further boiled by the rising distillate. Leaving a column in full reflux will repeat this process all over the packing material, forcing the lighter alcohols to sit towards the top of the column and the heavier alcohols further down the column. This is the process of separating the wash into its alcohol fractions, and collecting these off slowly is the process of fractional distillation. This process is known to strip all flavour from distillate, so the method is used for making neutrals (vodka).
In order to get a flavoured product (whisky, rum, brandy), minimal or no packing is used (as in a pot still), however a proven setup to achieving great flavour retention is a column which utilises plates at various levels throughout as its ‘packing’. ie. The area where the liquid distillate has contact with rising ethanol vapour. Each plate used provides a cleaner product, and a reasonable column height packed with copper mesh can have an equivalent result of 20 or more plates producing a 95% alcohol by volume output. When distilling a whisky or rum, this is obviously not a desirable result, and a pot still or plated column is used.
All the hype at the moment is around bubblers, and for good reason. The bubblers are just scaled down versions of commercial plated columns. These types of distillation columns are renowned for both their flavour retention in the final product, as well as their collection speeds. Without going into anymore about them and what to do with them, which can be researched easily on the Aussie Distiller forum, this section will attempt to educate on the basics of how they work.
Two types of plates are used in hobby distiller’s plated columns, and the choice of which to use comes down to personal preference with almost identical results reported. The two types are perforated plates and bubble caps. On a perforated plate, the plate consists of many small holes (perforated) where vapour can rises up through. On a bubble cap plate, the plate contains either one large, or several small caps with slots cut into them to allow vapour to escape which has travelled up a small tube (riser) through the plate and into the inside of the cap. This explanation will focus on the perforated plate now, however the theory of how they work is identical aside from how the vapour travels through liquid on the plate.
Following the illustration below, here is a brief explanation of what is going on inside...On initial boil up, the vapours begin rising up the column, and as they reach each plate are able to flow through the perforated holes on each plate. Once these vapours reach the top of the column where the reflux condenser is flowing with adequate coolant water, the vapours condense and begin to fall back down the column. Picturing the top plate firstly...as the liquid is falling it will land on the plate, however instead of falling through the holes it is suspended by the vapour rising through the holes which in turn boils the liquid on the plate. The lighter alcohols will boil off of this plate and ascend up the column again as vapour towards the reflux condenser.
As this cycle continues, the depth of liquid on the plate (bath depth) increases to a point where it overflows into a tube going down to the next plate below. This tube is known as a downcomer. The downcomer places this liquid onto the plate, which is boiled by the rising vapour through the holes, and this process continues until all plates are loaded with a bath of boiling liquid. As the system is left in full reflux (all vapour falling back down), the lighter alcohols work their way up through the plates towards the top, while the heavier alcohols and water work their way down lower in the column. At this point in theory, ethanol is boiling off each plate as vapour, while water remains on the plates and flows down the downcomers. The downcomer on the very last plate, ends in some form of a vapour lock, being a J shaped tube, or just a pipe and cup. The idea of this is to allow the liquid overflowing from the bottom plate to return to the boiler, without allowing rising vapour to flow up through the downcomer.
At the point in time where the operator is happy that equilibrium has been reached, the coolant water flow to the reflux condenser is restricted, where a slight amount of the rising vapour will be light enough to make its way through the condenser with turning into liquid. This vapour will then reach the product condenser where it is collected.
As the coolant water is manipulated to control the output from the still, this becomes a Cooling Management still also.
Now that you have learnt these, you should be able to apply your knowledge to any further designs that you come across, and determine for yourself the type of management that the still utilises.
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