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:
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 proper 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.
The LM design has one serious setback. 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.
Original Post by Kiwistiller, edited by Bushman, Can be found here.