Boka Reflux Still - How To Build
- 1 History
- 2 Column and Slanted Plates
- 3 Condenser
- 4 Boiler
- 5 Heat source
- 6 Column packing
- 7 Thermometer
- 8 Output Tube & Needle Valve
- 9 Safety considerations
- 10 Cleaning and prep for use
- 11 Operation instructions
- 12 Optional extras
Bokakob created a series of reflux still design drawings that were widely circulated after they were created in 2001. This particular still featured a short (2ft/.67m) column made of 1.5" copper pipe attached to a stove-top pot boiler, and only required three joints to be soldered (the upper and lower plate, and take-off tube). Various discussions on Yahoo and HomeDistiller.org resulted in many design variations and improved efficiencies.
The still was named after its designer Alex, who had a Yahoo username of Bokakob, but web discussions with poor spelling or simple abbreviation led many to refer to the still as a Bokabob, Boka and Bok.
Column and Slanted Plates
The single in-line reflux column of the Boka is meant to carry vapors from the boiler up through packing material for reflux and up to the condenser. Some of the distillate will fall to the dual slanted plates which can then overflow (allowing the distillate to fall back into the packing, which is the basis of the reflux design) or be drawn-off for collection.
The column of your still should be made from copper or stainless steel. Stainless is inert, and copper actually assists in creating a clean flavor in the final product by removing sulfides as vapors come in contact with the copper as they travel through the column.  Availability and local price will probably be the greatest determiner of which material to use. Also note that stainless steel may require more industrial type of equipment and skill to weld than copper which requires simple soldering. Stainless does not suffer as greatly from heat loss, so insulation is less critical. Also note that well polished copper is very pretty. All other things being equal, choose the copper. Synthetic materials other than PTFE are unsuitable.  See rule #8 
Most pipe also comes in varying thicknesses: K, L, M and DWV.   K is the thickest but is commonly not found at retail outlets. L is the most common and is a standard thickness, meaning that it fits the couplers, caps, etc that are sold retail. M is the thinnest and is a better value of pipe for our purposes, but may make attachments more difficult to fit. DWV (which stands for Drain Waste Valve) is the thinnest by far, and subsequently cheapest and best value. Due to its thin walls, it has temperature and pressure ratings and warnings which do not apply to our neutral pressure systems  Your column should ALWAYS be properly vented and no pressure buildup should be possible, even in full reflux. If you have pressure buildup in your column it could explode, regardless of the thickness of the pipe. If you are going to use DWV, made sure that you check the accompanying fittings before you buy to make sure it will all assemble easily and be solderable.
The original Boka was built with a 1.5" (38 mm) column. The width of the column directly impacts the speed at which distillate can be collected, all other things being equal. In actuality, with a narrow column, the heat put into the still has to be lower to keep the vapor speed in the column at an appropriate speed so the reflux can occur properly. With a wider column, more heat can be applied while maintaining proper vapor speed, and the overall yield of properly refluxed distillate will increase. Many Boka builders quickly realized that a 2" (50mm) column yielded almost twice the distillate as the 1.5" (38mm), and was still reasonably cost effective. A 3" (76 mm) column allows for even more speed, but the cost of materials increases exponentially and availability diminishes.
The first decision you'll make when making a Boka is usually the width of your column. This will impact the condenser and boiler sizes you need, and is usually impacted by costs and local availability. Don't forget to look at connectors which can be harder to procure than pipes. (ie, 3" copper male and female screw adapters are pretty rare.) The most common and recommended size for simple hobby distillation is a 2" width. A 2" column will allow for around 1 to 2 quarts or litres output an hour, (depending on column height and desired purity) while a 3" column can almost double that.
One you've figured out which width of pipe you are getting, the next step is to figure out how much pipe you'll need. The packed height (the amount of column height taken up by packing) of the column directly impacts the purity of the distillate you are able to produce. A taller column allows for more packing material, which allows more distance for the vapour to travel while mingling with downcoming reflux and form a temperature gradient, which means higher alcohol content in the distillate and better separation of undesirable fractions (heads; tails).
The recommended packed height of a reflux column is 48" to 60" (120cm to 150cm). Beyond that there is very little benefit in additional packed height. It is possible to make the column shorter than that, however. You can build it as short as you like, but as the packed height is reduced, the capacity of the column to produce high alcohol distillate is reduced, and reflux would need to be increased to maintain pure output, which will slow down operation. Columns with as little packed height as 12" (30cm) can still achieve near azeotrope, but are very slow to run at those purities.
Some people like to talk about column height in a ratio with column diameter. These are accurate in a 2" column, but they break down quickly when diameter changes to other values, and also mislead new distillers into thinking that wider columns require more height - they are independent variables . The recommended column height to width ratios for a 2" column are between 12:1 to 30:1.  For some detailed expaination on the theory behind how packing height and reflux influences column efficiency, and also varies with different packing materials read more at the reflux still design page on the Homedistiller parent site . The interactive design calculators are especially useful for determining how your still will behave and where it's theoretical performance limits lie. 
Most 2" (50mm) columns would have between 36" (90cm) and 50" (125cm) of packed height, plus another 8-10" (20-25cm) for the plates and condenser. All else being equal, and if you have the space and money for the materials, it is better to have a taller column, until you run into the 50"-60" (125cm- 150cm) range, beyond which there is little benefit building taller, and ceiling height also becomes an issue.
The slanted plates (not to be confused with reflux plates or trays such as bubble caps or perforated plates discussed elsewhere) in the Bokakob design are set at an angle below the condenser coil so that the upper plate collects distillate from just over half of the column and directs it onto the other lower plate. The lower plate is angled towards the wall of the column and collects a "pool" of distillate. At the bottom of the "pool" there is a take-off tube which can be opened to collect the distillate. If the take-off tube is only partially opened or is left closed, the "pool" overflows and a notched recess in the lower plate allows this overflow to fall back into the middle of the packing material of the column below. This allows for further reflux and purification of the distillate.
The ideal placement for the plates should be such that the upper plate is just below the condenser, about 0.5" to 1" (15-25mm). In earlier designs the upper plate was connected to the bottom of the condensing coil, but distillate was able to drip down the sides of the column and then would not properly mingle with upcoming vapour. The original design also required the condenser/plate assembly be perfectly rotated to align with the lower plate or collection would suffer. Later designs have the upper plate soldered to the column independent of the coil. So, for proper placement of the slanted plates, you need to know the size of your condenser coil. Building your coil and deciding its placement and measuring where it will end in the column need to be completed before deciding on placement of your slanted plates.
The angle of the slanted plates has traditionally been at 30 degrees of angle from horizontal. The angle is only important to the extent that the take-off tube is submerged in the "pool" of distillate.  More extreme angles (>30º) will require more material to construct and will not provide any benefit, though those using a compound mitre box may only have a 45º slot. You also need to ensure that both plates pass the center of the column, or overlap in a vertical plane. If your plates do not overlap then distillate will fall of the upper plate back into the column, missing the lower plate and collection "pool".
The spacing of the plates, (ie, the vertical space that is left between them) needs to be sufficient to not constrict flow of vapor up the column. Too small of a space may increase vapor speed in the column after the constriction and minimize the time the vapor spends condensing at the condenser coils. The general consensus is that 0.75 to 1" (17-25mm) is best in a 2" (50mm) column. This approximates the cross-sectional area of the column with the packing material in place.  There is a nice diagram of the Boka head here. 
The plates are typically made from a scrap section of copper pipe, which is cut along its edge )parallel to direction of the pipe) and beaten flat with a hammer. Once the column has grooves cut to accommodate the plates, the rectangular plates can be inserted into the grooves and marked with the correct shape, then ground or cut to be smooth. There is no need to pre-cut the plates into elliptical shape before the grooves have been cut. Once to proper shape they can be soldered or brazed into place. 
For those who don't want to go through the trouble of measuring and marking their column with the proper placing of the plates, several generous souls have created easy to use templates that are pre-marked with the proper plate positions for a 2" column. These can be printed and taped to your column in the proper position and the cuts can be made through the template. They can be scaled up 50% for a 3" column, or shrunk down for smaller columns. For a 2" column, ensure that your printer isn't scaling the image at all. This link is to a revised 2010 template 
Cutting, mitres and saw box
Cutting the slanted plate grooves in the column can be difficult. If you are using a template or your column has been pre-marked with the desired cut locations, you can use a Dremel or hand-drill with a black Dremel cutting wheel to cut the column grooves. Others have found success using a hacksaw or two hacksaw blades side by side to accommodate the width of the plates. If you are lucky enough to have a chop saw with a compound mitre box, you may be able to cut the grooves with that. A standard mitre box will not work since you need the angle to be cut around a different axis than a standard mitre box can provide. (A normal one only allows 'yaw', while you need to adjust the 'roll') 
Find a suitable pipe for your column of proper width and length for your needs. Skip to next section and build a condenser. Measure the height of the condenser installed on the column and mark your column. Measure and cut the grooves for the slanted plates just below the condenser, then install the plates and solder into place. (If you are using later plate designs, you may need to install the lower plate first, then bend down its reflux drip tab, and only then install the upper plate.) Drill holes for temperature port and output tube and solder them into place. You probably want to solder each plate and its corresponding tube at the same time so you don't melt nearby solder. Check to ensure the slanted plates overlap inside your column. Check your soldering to ensure the column has no leaks and the output tube functions properly.
A condenser is a section of coiled tubing (usually copper, but occasionally stainless) which has coolant running through it. This coolant reduces the surface temperature of the coil which is placed inside the top of the column. As the boiler is heated, vapors begin to rise through the column. When these vapors come in contact with the condenser, they condense to a liquid and fall back down the column where they are collected by the slanted plates for takeoff or return to the column. To function correctly the condenser must be of such a size or temperature that none of the vapors can exit the column as a vapor, which would be both wasteful and potentially dangerous (i.e., flammable vapors near a heating source).
This is the most popular type of condenser used in a Boka. It is commonly referred to as a 'double helix' coil, though it is not.  It is better described as a single-path double-concentric counter-wind condenser, or its proper name, the Dimroth Condenser.  It is constructed from one continuous piece of copper tubing, which is wound in spiral downwards, and then continued into a second larger spiral wound outside the original spiral back upwards. Both the input and output are at the top of the coil. It is more difficult to make than a single coil, but isn't too difficult with some practice and simple household tools.
There is a great tutorial by Hookline on how to wind this condenser by hand here:  An alternate method is featured here which uses a jig and more tools:  Keep in mind that filling your coil with salt will greatly minimize the risk of kinking the coil, but many users have found removing the salt after coiling difficult or impossible without high pressure. Also, if you don't want to buy a spring-style coil bender, many spring door stops fit 1/4" tubing and work just as well.
For columns narrower than 2" (50mm), a double coil may not be able to be wound narrow enough to fit in the column. For smaller columns a single coil may fit better. This is a Friedrichs condenser  which is constructed from a single piece of copper tubing which runs straight down to the bottom of the coil with no winds and is then coiled in a spiral around the central tube back upwards. This type of coil can be constructed using the same methods as the dual coil, just skipping the first half: 
For either of the above designs, a long coil can create substantial back pressure which may inhibit the ability to use coolant circulated by a pump. The solution is to have a cold-finger run down the center of the coil. A cold-finger is a larger diameter tube down the center that is soldered to the outer coil or coils so that it also carries the flowing coolant. In addition to reducing back-pressure, the cold finger also increases surface area in the coil and increases efficiency.  It requires more tools, cutting, skill and soldering but yields a superior condenser, especially with columns larger than 2" (50mm) Keep in mind that it may be difficult to fit a cold-finger inside the traditional dual-coil used on a 2" (50mm) column. A true double-helix with a cold finger is among the most efficient condensers, creating a uniform spiral path for upward airflow in-line with the coils of the condenser, and is pictured here: 
The condenser needs to sit inside the top of the column in order to condense the rising vapors into distillate that can be collected. Many builders use a cap to hold the condenser perfectly in the center of the column, or to better distribute the weight of coolant lines coming to the condenser. If you use a cap it is critical to ensure that it has enough vent holes that so the column cannot build pressure. If the condenser is of adequate size and temperature it will condense all of the vapors and no cap or seal is needed at the top of the column.  If you do not vent the top of the column, there may not be adequate outlet for the pressure inside the column which would eventually lead to catastrophic failure of the column and/or rapid release of vaporized ethanol near a heat source, both of which can cause serious injury or death. The Boka can run perfectly fine without any cap or lid above a suitable condenser, and accordingly many disillers leave the still open at the top.
A 6"(150mm) condenser in a 2"(50mm)column is strong enough to condense all of the water vapor put off at full boil of plain water from 2400 watts of heat, which requires more condensing power than ethanol at regular distilling temperatures. For most applications, a 6" (150mm) condenser is ample: 
When calculating the length of tubing needed to create a concentric dual-coil condenser for a standard 2" (50mm) column, you should plan on about 8-12 (200-300mm) inches of 1/4" (6.4mm) copper tubing for every linear inch of dual-coil condenser desired, but varies based on the space between the each turn of the coil. (Ie, 9 feet (3m) of tubing will yield about 7" (175mm) of dual-coil condenser, plus the input and output tubes.)  The condenser should be wide enough to fill the majority of the column, but not so wide that it contacts the walls of the column which could warm it and drop its efficiency. Wider tubing can be used though it is more difficult to bend and is less efficient at heat transfer than smaller 1/4" (6.4mm)tubing. Wider tubing can however be of benefit to those planning on using a pump to recirculate coolant, as there is significantly lower backpressure.
You can test your coil is working correctly by holding a glass mirror above the coil during operation and looking for condensation on the mirror. You can also check the temperature of the coolant exiting the condenser to gauge its efficiency. Do NOT use a flame to test for full vapor condensation. If your coil is in place and running, you can increase the condensation rate in four ways: 1. You can lower the temperature of the water or coolant entering the system. For those with re-circulating systems this might mean changing water more often or adding ice to your holding tank. 2. You can increase the flow rate of the coolant. Faster flow rates introduce more coolant and minimize coolant temperature increase, which keeps the condenser more efficient along its entire length. 3. Be sure you have your condenser water lines hooked up the 'right' way. The right way, is the coldest part of the condenser should be furthest away from the boiler, and the warmer part of the condenser, or that part which carries out the water, should closest to boiler. This flow is the 'opposite' direction of the vapor flow. For a single coil or a coil with a cold finger, the coldest part of the coolant should enter the top of the coil and exit through the center line or cold finger. In a dual-coil, the coldest water should enter into the outside coil, and exit the inner coil, though the increase in efficiency for a dual-coil is minimal. 4. Add surface area to your condenser. It is easy and cheap to put a pure copper scrubber into the center of the coil where the largest void is which maximizes condenser contact to vapors and minimizes unobstructed airflow through the middle of a condenser with no cold finger.
Connection to water source
The connection between your condenser and your water source will depend largely on your water source. If you are using municipal water from a hose, then you can install a 'garden hose to 1/4" compression fitting' to the input of the condenser. If you are using a recirculating pump, then the pump tubing will have to be adapted to your condenser tubing. Because the coolant does not come in contact with the distillate, any type of tubing can be used for the condenser input and output, including various plastics. Some forethought might be needed to keep heavier tubing from bending condenser lines that exit the column parallel to the ground. Simply bending the lines towards the ground after exiting the column should be enough in most applications. Special care should also be taken to ensure the coolant out line cannot kink and shutoff water flow when it warms up and softens.
If you are using municipal water for your coolant water supply, there should be no issues with pressure regardless of your type of condenser. Many users find that running water through a condenser for several hours can use more water than desired due to cost of water, cost of water disposal, or simply water availability. Those users may choose to use a recirculating system in which coolant is stored in a tank (Ie: barrel, bucket, bathtub, pool) and pumped through the condenser using a pump. The size of the pump needed largely depends on the type of condenser (back-pressure created by dual-coil condensers without cold-fingers can be double that of single coil or cold-fingered coils. Water temperature in the storage tank also impacts the amount of water the pump must re-circulate (Ice can be added to a tank to keep water cold as it re-circulates through the system.) A marine bilge pump or sump pump can be purchased at many local hardware stores or marine supply stores.  Some distillers also opt to route the hot coolant through a heat exchanger such as an automotive radiator, reducing the need for a large cooling reservoir.
Summary of building process
Determine the style of condenser that will fit your column, parts ability, and available tools. Use the guides above to wind your condenser to the proper shape and size, checking for fit in your column. Check the condenser to ensure it has proper flow and isn't kinked closed. Attach any hardware needed to hold the condenser in place on the column and add hardware to connect the condenser to a coolant source and coolant output tube. Soak in dilute vinegar to clean it and make it pretty.
Main Article: Boiler
Choose a boiler that meets your budget, needs and local availability that will be easy for you to fill, empty and clean, or modify it so it will be easy to perform these tasks. Make sure it is stainless steel (or copper). Make any modifications necessary and connect the column.
Main Article: Heat source
The entire still operation depends on the contents of the boiler being boiled so that vapors can rise up the column. It is desirable to be able to be control heat somewhat finely. This heat can come from a kitchen stove for a small Boka (however there will be problems with the elements cycling on and off too coarsly); while a larger Boka is usually heated by a standard propane burner or an electric heater element installed inside the boiler. The heat input required for efficent operation depends on the diameter of the column. 2000W is sufficent for 2" (50mm) columns, but more is needed for larger columns in proportion with the increase in the area of the column, so a 3" will need roughly double that. It is important not to overpower the column, as at best it will wreck the packing's effectiveness, and at worst the still could choke and puke, pushing large amounts of hot liquid out the top of the column.
Vapors moving from the boiler through the column to the condenser would exit the still in the same composition as they boiled off in the boiler at if the column was completely empty (i.e. a pot still)- you can see the initial vapour composition by drawing a line across the water-ethanol phase diagram. To achieve higher purity ethanol than this initial vapour has, reflux needs to occur via a repeated condensation and reboiling inside the column. Packing material is inserted inside the column to provide surface area upon which downcoming cool reflux and upcoming hot vapour can mingle, condense, and reboil, acting as a whole series of pot still distillations. This cycle is continued up the column, with the lower boiling point fractions (like heads components or ethanol) reboiling more readily and rising to the top of the column near the takeoff point, and the higher boiling point fractions like fusel oils or water end up in the lower part of the column, or back in the boiler. This is how we concentrate heads for extraction, and also how we keep taking pure hearts while suppressing tails components like fusel oils. There is good detail on the theory and implications of this on the parent site reflux design section.
The amount of purity that a column can produce is directly linked to the type of packing used (among other factors). While the calculations for calculating theoretical plates are complicated (for details, see the HETPequations on the parent site], the key factor influencing "height equivilent to theoretical plate", and therefore how many theoretical plates there are in your still is packing surface area. In simple terms, the greater the surface area of the packed material inside the column, the more iterations of condensing and reboiling can occur. In many commercial stills, these stages are separated by actual physical plates or trays. In small-scale distillation, other materials are more practical, but the number of stages of reflux, or theoretical number of plates, can be calculated. These stages of reflux are referred to as theoretical plates as they occur inside a packed column. The "height equivilent to a theoretical plate" or HETP which you may come across refers to how much column height is needed to make up one theoretical plate, and is more or less a measure of how well your packing is working. You can find this out for your still design with the parent site's interactive design calculators.
Packing material must be something that won't compress too much, burn, melt, dissolve, or release impurities or undesirable chemicals into the column. Materials must also have as large a surface area as possible, and at the same time offer as little resistance as possible to the gas and liquid flows inside the column. It should be easy to clean, and above all, it should not settle or pack down in the column. An amazingly detailed discussion of packing materials can be found on the parent site, along the reflux calculators that can help you compare packing materials (ie, calculating your height equivilant to a theoretical plate).
Scrubbers and structured mesh
Common household pot-scrubbers are one of the most efficient materials that can be used for packing, and they are easily found at many home retailers. Is is essential that they be made from either pure stainless steel or pure copper. Many scrubbers can be coated with the right material, but have steel inside and will quickly rust or disintegrate upon repeated use. You can hold a magnet against a scrubber to quickly and easily test if it has steel inside. If a magnet sticks to a scrubber, don't use it. There are many discussions linked here on different brands that have proven to be pure:    
Some brewing supply companies like The Amphora Society (who also ship worldwide quite cheaply) or Brewhaus also carry copper scrubber-like material, known as structured copper packing, which is usually far more cost effective than having to buy lots of scrubbers, and without the uncertainty about composition the materials that you have with scrubbers.
Scrubbers or mesh should be copper or stainless steel. Stainless has the advantage of not degrading, while copper has the benefit of aiding the removal of sulfides from the product, but will need cleaning occasionally, and eventual replacement.
Scrubbers or mesh are the most desirable commonly used packing material, as they achieve the best (i.e. lowest) HETP.
Raschig rings are hollow cylinders made of unglazed ceramic material. They are made in many sizes but the 1/4" diameter is perfect for this kind of column. They may be more difficult to locate than other packing materials, and be more expensive. They do have the benefit of being more durable and last longer than many of the other materials. They are a less effective packing than mesh or scrubbers, achieving a lower HETP and also requiring something inside the column to contain them.
Glass marbles can be used as packing. They are pretty common and are very easy to clean. Their low surface area and need for a container, however, makes them the least desirable of common packing materials.
Packing needs to stay in your column while the still is running. If you are using something other than mesh or scrubbers, you will probably need to plan on including some type of screen or soldered supports to keep the packing material from falling out of the column and into the boiler. The type of support you will need will vary depending on the type of connection between your boiler and column, and the type of packing used. Again, if you are using mesh or scrubbers, this is likely to be unnessesary.
Removing packing for pot still mode
If there is no packing in the column, there is nearly no surface area in the column upon which vapors can reflux. Some condensation can form on the walls of the column, however, so insulation, or a detachable head that can be used without the whole column is desirable.
This allows a Boka to function as a pot still. This is commonly referred to as a de-tuned or un-packed Boka.
A thermometer allows you to monitor the vapor temperature as you distill. At the beginning of the run, the temperature will allow you to know when your still has begun to reflux. Later, the temperature lets you judge the purity of the distillate output without having to measure it with an alcometer. While it is often shunned by potstillers, in reflux distillation the thermometer becomes a invaluable tool to indicate when to begin collecting the distillate, and when to cut to tails, or to assist in adjusting the correct amount of reflux. Temperature is the primary feedback in optimizing your distillation, as it relates directly to the purity of ethanol vapour at that point (see the ethanol / water phase diagram).
The original Boka design called for the thermometer to be placed in the packing, near the middle of the column. The theory was that the temperature should be uniform in the column and temperature changes in the middle of the column could be noticed before the distillate being collected higher up changed, thus allowing for better cuts. Later designs call for the thermometer to be placed above the top of the packing, just below the upper slanted plate, shielding the thermometer from falling distillate. Distillate that drops down into the packing during the reflux process can have a different temperature than the vapor and make it more difficult to properly identify the minute changes in vapor pressure that denote change in purity or composition of the vapor. 
Both locations have their merits, but the consensus would be that placement above the packing is most useful. Placements further down the column can provide advance notice of changes occuring before they influence output, but thermometers there are best seen as suplementary to the essential top thermometer.
Many methods can be used to get a thermometer reading from the column. A plain hole can be drilled into the side, and plugged with a cork stopper, which has a hole into which the thermometer is inserted. For heavier thermometers, a short section of 1/4" or 3/8" tube can be soldered into a hole in the column, and then the thermometer can be sealed into this tube using cork, flour paste, or pure PTFE plumbers/thread tape. Alternatively, if you are using a digital thermometer with a metal probe, you can use a compression fitting to keep the probe in place and seal the column. 
The temperature readings only give an idea of when vapor purity or content are changing and some distillers, especially of pot stils, feel that over-dependence on a thermometer can be a crutch that will prevent good cuts. A thermometer isn't 100% necessary to run a reflux still, but will make it much easier to run in an optimal fashion, as undesirable fractions like tails can be very hard to detect using taste and smell at high alcohol levels, and by the time you taste them, it's too late!
Many reflux distillers will change valve settings or cut to tails based on a 0.1°C change, so it can be difficult to use a glass thermometer. Digital thermometers, while sometimes inaccurate to several degrees, are often more sensitive to minor changes in temperature. Sensing fine changes in temperature is much more important than a highly accurate temperature reading, and thermometers should be chosen according to these needs.  
Output Tube & Needle Valve
The lower slanted plate inside the column of the Boka collects distillate that has been condensed. This distillate forms a pool which will eventually overflow and fall back down into the column as reflux. If all of the distillate falls back into the column, the still is said to be in '100% reflux' or 'total reflux', or 'equilibriating'. At the lowest point of this pool, a small tube serves as an outlet where the distillate can be drawn off. If this outlet is opened completely and no pool is able to form or fall back into the column, the still has a very small amount of reflux, mainly generated by condensation on the underside of the slant plates due to cold reflux falling on them and passive heat loss throughout the column (these two things are what prevent a Boka from being completely the same as a pot still when detuned). The rate of distillate removed from the column directly controls the amount of distillate that can pool and fall back into the column, and thus the reflux ratio. This demands a very precise method for controlling the amount of distillate that is removed from the output tube. A needle valve is the best choice, and gives the best amount of control. A true needle valve has a needle inside that seats into a hole such that flow is minimized or stopped. Many similar valves are less efficient and lack the amount of control needed to properly adjust a column for the best amount of reflux, usually measured in a specific number of drops per minute of drawn-off or collected distillate.
The output tube should be made of the same material as your column so that it can be soldered or sealed accordingly. It is best to drill and solder this tube after soldering in the plates, ensuring that it is placed in the lowest possible position above the plate for maximum drainage. The templates in the 'slanted plate section' show the proper drill location if you can't properly locate it. If you have a copper column, use a copper outlet tube. (Some users find that it may be easier to solder the lower slanted plate and outlet tube at the same time since it requires the area to be heated only once; While others find that doing them simultaneously only encourages solder to leak out of the lower plate when the outlet tube is being heated for soldering.)
Needle valves are not available in copper, and are usually available in either brass or stainless steel. Brass valves are far more common locally and much less expensive, but some brass contains lead, and should be pickled on a regular basis for safety. Many of the brass fittings are of sub-standard manufacturing quality, and may be very inefficient at getting proper control (ie, it may be hard to get exactly 3 drops per second if that were your desired output.) Stainless steel needle valves tend to be much higher in quality, and can be one of the most expensive parts of the still. A good stainless steel needle valve may cost as much as 10-15 times as much as a brass equivalent and be harder to procure locally, but will be lead-free and highly adjustable and accurate  If you use brass, don't forget to pickle it to get rid of any lead that may be in or on it. Read about pickling brass here: 
Compression fitting needle valves are very popular, as they will not require soldering.
Note that nearly every commercially available needle valve has some sort of seal or seat. This seal may be made of plastic, rubber or PTFE (teflon). Try to ensure you get a needle valve with a PTFE seal, as this is a safe material. Take caution if you solder your valve into place not to overheat it which may ruin the seals inside. Some of the DIY options below are available without any seals and may be a better choice for distillers concerned about synthetics, or if PTFE is unavailable.
The most common size of output tube is 1/4" (6mm), though you may choose a 3/8" (9mm) output tube. The larger size isn't necessary for flow, but the larger needle valve will give you better flow when fully open (for stripping or when using your Boka without reflux as a pot still) . Others feel it is overkill and sticking with 1/4" (6mm) is cheaper and perfectly acceptable .
This is a very cheap and elegant DIY needle valve with no synthetic seals. Here is a rough plan for another DIY needle valve that could be 100% copper and pretty cheap to make. This forum thread includes some info on adjusting a common brass needle valve to be more finely adjustable.
Because the lower slanted plate which holds the 'pool' of distillate sits directly in the vapor path, the pooled distillate is constantly being heated back up towards vaporization threshold again. This makes for a more efficient reflux, but results in drawn-off and collected distillate being very hot when collected. Some Boka users choose to run a secondary external condenser on their output tube just after the needle valve . Secondary condensers range from Liebig arms to a graham condenser that runs off the same coolant used for the main condensing coil inside the column .
This being said, many users have no secondary condenser, or just use a long output tube that gives the distillate more time to air cool before being collected, or use a cooled parrot. The primary reason for having cool distillate is so that accurate proof/ABV readings can be made as the distillate exits the still. This is typical of real-time measurements such as those provided by a parrot or parrots beak.  (A parrot is not an animal. It is a device used to hold distillate and a proof hydrometer which allows distillate to flow out while giving a proof reading, so the hydrometer is always visible and measuring. They are typically made of copper.)
A Boka is a very simple still to use, and when used correctly, is very safe. Proper precaution, common sense, and general disilling safety should be used at all times. All stills used to distill spirits by their very nature will contain or be on a heat source, and be full of flammable vapors. Never leave your still unattended and ensure that you have proper fire retardants nearby (ie, a fire extinguisher or ready water supply.) Improper use of a still can result in serious injury or death.
If your condenser is working correctly, there should not be any vapor leaving the still. Check your still for leaks at the beginning and during each run using a mirror or flashlight. There is absolutely no need to seal the top of the still and doing so can cause incredibly dangerous build-up of pressure. This high pressure can cause your still to explode which could kill you, and then the released flammable vapor would ignite and explode and kill you again. Make sure your Boka is open above the condenser or has a proper pressure release valve that cannot fail.
Most plastics and sythetic materials are inappropriate for use in the distillation process (HDPE containers can be used, but only in the fermentation process). Strong ethanol (especially HOT/BOILING and in vapor), is an extremely strong solvent, and there are additional solvents in the heads fractions such as acetone. Most materials are simply not safe to use. Also, storage of high proof ethanol in plastic containers is not a good idea either. Some people do this, and even a few lower commercial products are stored in plastic. However, this community STRONGLY recommends not using plastics. Some places to be cautious (even if your still was commercially manufactured), is using plastic tubing to join the copper tubing, or plastic tubing on the â€œtake offâ€ of the still (i.e. after the vapor has been condensed). This subject seems to always generate lots of heated discussion. But this community is VERY serious about keeping plastics out of the distillation process, and all parts of your still (Borrowed from a FAQ on the main site, by Husker ). The ONLY exception to this rule currently is PFTE/Teflon, which has been scientifically tested in a distilling application 
No lead solder
Lead in solder can and probably will find its way into your still. Only use solder that is clearly labeled as lead-free solder. If you aren't sure or if your solder isn't labeled, don't use it. Solder is cheap, and lead-poisoning is extremely serious.
Proper collection container
Ensure that your collection containers are stable and don't tip over into or are near your heat source. Also ensure that your collection container is large enough that it can collect any distillate that is collected if you aren't paying attention. An overflowing collection container can be really dangerous. You may choose to set your collection container inside a large pan that can collect any overflow in a worst case scenario. Some mashes produce some serious foaming when boiled and can cause the still to puke, which is when foam and mash rise up the column and exit the still. This can cause your collection container to overflow, which could make a mess or start a fire. Do not overfill your still boiler.
Don't spill, and cleanup immediately if you do
High proof distillate can be very flammable, and the flame can be nearly invisible in normal light or daylight. If you spill, clean up the spill immediately and be smart about discarding rags used to clean up spilled distillate. Spilled distillate can catch on fire, as can rags used to clean spills - a bucket of water handy to throw them in is a good idea.
Be aware of your local laws
Building, owning, possessing, selling, buying, watching, using, running, or storing a still may be illegal where you live. Storing, selling or buying the distillate that comes out of your still will likely be even more illegal. Check you local and federal (where applicable) laws to ensure you are in compliance. Don't rely on what you think or have heard about what is legal. Don't rely on common sense or folk tales about what is allowed. Check your local laws and don't break them. If you don't like your laws, call your local legislator and get them changed.
More links to still safety
Cleaning and prep for use
After building a Boka, the still requires cleaning to remove any contaminants that could negatively affect your distillate. The following process should help prepare your still for proper use. This multi-step process borders on over-kill, but only takes a little bit of extra time to ensure your still is perfectly clean, so many endorse it for peace of mind alone.
Clean while building
It is best to progressively clean it during the building process. Take time to scrub your materials with soap and water after purchasing them but before building to remove anything they came in contact with during shipping and storage. Doubly so if you are using recycled pipes or fittings. The inside of pipes can be cleaned with a soapy scrubber pushed through the pipe with a broom repeatedly. Many also choose to polish their materials before building which is far easier than trying to accommodate bits of soldered on metal that make later polishing harder. Continue to clean each piece off after soldering or assembly, taking extra caution to remove any excess flux.
Post-build cleaning runs
This section taken from the Reading Lounge 
Soak and Scrub
A really good way to start is to soak any parts small enough in a weak acid solution - use dilute vinegar, which you can reuse for the next step. Soak it for a good few hours, or overnight. This is really an optional step, but is a nice idea for things like coils. You can augment it by giving the insides a bit of a scrub as well with a kitchen scourer (don't use your packing for this). Keep the vinegar solution.
Vinegar Cleaning Run
Next, pour the vinegar solution into your boiler, attach the still head, and bring it to a boil. It is beneficial to run without cooling for a period, effectively giving the condenser an acidic steam - the problem if you don't run steam through it is that some areas (like the top of a liebig condenser or coil) may be untouched in normal, condenser-on operation. There is no fire danger - but there is still vapour being generated, so this is a good time to double check that your still is always open at some point to the atmosphere. Of course, normal caution is needed with hot steam, don't scald yourself, and it won't be very pleasant to breathe. If your column has packing, it isn't necessary to have it in at this stage.
After steaming it for 20 minutes or so, turn on the condenser(s), and return your still to normal condensing operation. Check that the condenser(s) are knocking down vapour. A reflux condenser may struggle to knock down all this vapour, don't worry about that at this stage, it's a lot easier to condense ethanol / water mix. Run with the condenser(s) on for 20 minutes or so. This is a really good time to get a (glass!) mirror, and check for vapour leaks in any seals and solder joins, brazing, etc. The mirror will fog up if held up to a leak.
Shut down and dump your vinegar.
Give everything a comprehensive rinse out with water. Most of your still should be pretty shiny on the inside by now. If there are visual patches of flux and crap still in there, go back and do some more soaking and scrubbing before you continue.
Ethanol Cleaning Run
To be completely thorough, you should do an alcohol cleaning run as well. Use any old wash, pretty much whatever you can make the cheapest and easiest. Alternatively, you can use cheap box wine or something (avoid beer - it's hard to get the hop oils out afterwards), pretty much any source of ethanol you like. DO NOT use denatured alcohol for this (never put that through your still).
This second cleaning run can double as a practice of still operation. You can put the packing in for this. Do not repeat the steaming step we did with the vinegar run, as the ethanol vapour is of course flammable, and also heavy - it sinks. Just run the wash in the normal fashion, and have a play around with heat / cooling / takeoff rate to get the hang of it. See how the ABV changes. Play around with the reflux management, see how temperature and output respond to your actions.
Don't treat this as drinkable, but do keep it, clearly labeled as cleaning run alcohol - you can use this for future builds.
Some distillers prefer a used, patina coated copper still, but others like them bright and shiny. Olddog has provided a good walkthrough to polishing a still. 
It is a good idea to clean your still after each use. Simply rinse the still with water thoroughly just after using to keep things clean and fresh. If your still pukes (Mash boils over and rises up into the column), you might need to do more scrubbing with water, or take out your packing and clean out any particulates caught there. Some users go to the extra length of running the still with boiling water every so often to really clean things out, and some users also run a vinegar and water solution or a citric acid solution every 5-20 runs to keep things as clean as possible. Copper packing will benefit from weak acidic cleaning on a reasonably regular basis (twice a year-ish)to remove built up sulfides and oxidation. Do not be alarmed at the patina that develops inside a copper column, this is normal.
It has been mentioned elsewhere, but part of your cleaning should be a regular pickling of any lead-containing brass parts that you have. Usually this is just some joints and probably your take-off valve. The pickling process will prevent any lead that may be in the brass from contaminating the distilling process. A solution of two parts white vinegar to one part hydrogen peroxide (common 3% solution) will remove tarnish and surface lead from brass parts when they are soaked for 5-10 minutes at room temperature. The brass will turn a buttery yellow color as it is cleaned. If the solution starts to turn green and the brass darkens, then the parts have been soaking too long and the copper in the brass is beginning to dissolve, exposing more lead. The solution has become contaminated and the part should be re-cleaned in a fresh solution.
Running your Boka as a reflux still:
Running your Boka as a pot still: If you have built your Boka with a detachable head, remove the packed column part of your still and run the still with the head attached directly to the boiler. If you don't have a detachable head, you'll need to de-tune (remove the packing from) the column. Then you open your output valve completely and adjust the temperature on your boiler to give the proper output. (The proper output is usually determined by your recipe, but generally, the lower the heat, the slower the output and the more distinct the cuts will be. Or, slower is better) There will be only a small amount of reflux like this.
With low reflux, a pot still can be run hard and fast, yielding low wines, which are later run a second time at a lower temperature to give final output in a spirit run.
Some users choose to leave a small amount of packing in the column so they can run a wash in a single run. Based on your mash ABV, just the right amount of packing and a slow boil and low careful output can yield a final product on the first run. Too much packing will purify the distillate too much, and remove too much of the flavor, defeating the purpose of a pot still.
(Note: air temperature outside the still can have a significant impact on the amount of reflux. If it is really cold outside the still (ie, 30-40°F, 0-5°C) there will be a significant amount of reflux. Conversely, hot outside temperatures will drastically reduce the amount of reflux on the walls the Boka column. If you are not in a controlled environment (ie, indoors) you may have to adjust the minimal packing in your Boka during the year to maintain consistency as outdoor temperatures fluctuate.
Husker's instructions for running a detuned LM are here.
Some builders choose to add a threaded connection to the column just below the slanted plates. This allows the head to be quickly removed for cleaning, to better access and remove the packing, or to remove the packed section of the column and attach the head to the boiler for a stripping run or to use the still as a pot still for flavored spirits. Making this union out of threaded copper adapters can be extremely cost prohibitive. Brass couplers are more affordable, but may contain lead and need proper pickling as regular maintenance.   Alternatively, you can use a common non-soldered slip-joint, and seal the seams on the outside of the column using flour-paste upon assembly and before distillation. Finally, triclamp flanges and clamps with a PTFE gasket can be used, as can triclamps in conjunction with an easy flange. A triclamp / easy flange connection here in combination with a beer keg boiler allows for very easy detune into a pseudo-pot still, as triclamps and easy flanges are compatable with the flange on a uncut beer keg.
In taller columns (especially wide and tall columns), it is desirable to center the returning reflux halfway or two thirds of the way down the column. This is because as purity drops towards the bottom of the column, the reflux tends to clump and stream together instead of spreading efficently throughout the packing. A centering ring helps break up those streams and prevent reflux 'channels' from running down the column walls. These could be spaced in the packing, but should not be allowed to have the packing fill the aperture as it could cause the still to choke. The easiest option is to have a joint (such as a triclamp union or a slip joint) in the column at the halfway point or 2/3rd down, and use the joint to place the collar inside the column . This also means that the still column can be broken down into smaller parts for storage, and less physically able distillers may find it easier to assemble the still in small parts.