Distilling Designs for Thumpers, Doublers & Slobber Boxes

Some moonshine stills have a "thumper" or "doubler" between them and the condensor. This is a jar of sorts, half-filled with liquid (water, mash or tails). See the Moonshine Photos for examples of them.


This acts as a simple second distillation stage. Once its fully saturated with alcohol (hence better to start with something already containing alcohol), and up to temperature, the vapour leaving it will be doing the equivalent of a second distillation (usng the incoming vapour as the heat source) (see the wee applet in Reflux Design ... set it to 2 stages .... so the vapour leaving a 15% wash will be coming off at 65%, after going through a second stage (the thumper) it will be at 85%).

Homedistiller Forum user Usge's pot still shows an excellent example of a thumper in action ...

Please note that we at homedistiller.org do not recommend the use of glass for distillation at any time. While it's tempting to want to have a clear thumper or slobber box to watch the process, the bottom line is that glass is breakable and therefore dangerous. Possibly even catastrophically dangerous.

We recommend copper and/or stainless steel only!

How does a thumper work ? I'll try and describe it ...the main pot heats up, and begins giving off ethanol vapours (say at 40% purity at 96 ° C off a 5% wash)... these get passed into the thumper and try to bubble up through the liquid there. But because the liquid is cold too, the vapour will all condense if given the chance (e.g. small bubbles & water deep enough). A bit later on, the thumper liquid is starting to heat up and increasing in ethanol content .... soon the thumper is at say 84 ° C, and the ethanol content is around 40% - gees - that sounds like a second pot still doesn't it ? So the ethanol starts coming coming out of solution, though this time it will be at around 75-80% - hence the second distillation occurring - all for free, no additional cost or heating involved. Of course all this depends on how effective the thumper is at knocking down the incoming vapour - e.g. bubble size, depth of immersion, % alcohol in the vapour & thumper liquid etc, but you get the basic idea.

Look at the energy involved ..

Heat of Vapourisation/Condensation
Water = 2260 J/g
Ethanol = 855 J/g

So an incoming stream of 40% ethanol will give up about 1698 J/g, whereas to vapourise a 75% rich vapour needs about 1206 J/g. Then given that the flowrate of the 75% product is going to be only just over half of that entering, you have HEAPs of energy left over, to first heat the liquid there during the inital phase, then to superheat the vapour once the thumper is up to temperature. So the stuff coming in can more than happily look after itself in terms of redistilling itself.

I have a spreadsheet Thumper.xls which does these calculations for different scenarios. Play with the spreadsheet, and test the various scenarios ....
  • Thumper vol 1000mL and 0%
  • Thumper vol 1000mL and 30%
  • Thumper vol 3000mL and 30%
With a small thumper volume, the contents are fairly quickly heated up towards the liquid boiling point, but its % alcohol is quite low. For this situation, the vapour then tends to bubble up through the thumper, with no further enriching. What goes in is what comes out - no real increase in purity. The extreme case is one where the thumper acts to strip alcohol from the vapour, and produce a lesser strength vapour.

This situation changes a little if the small thumper is initially charged with a liquid of high % alcohol - eg the tails from the last run, say at 30-40%. In this case, the excess energy available is sufficient to cause the thumper to act like a second distillation stage - once its up to the boiling point of the % alcohol in there, the energy goes into vapourising the vapour there, producing an output quite a bit higher in % than the vapour entering from the pot still. Unfortunately, because of its small volume, this isnt sustained for too long, all the alcohol is driven off, and past a particular point in time, the thumper then simply lets the vapour bubble through & do nothing further, if not actually stripping the vapour & producing less than that off the potstill.

The best scenario for the thumper is where it is of quite a large size, and charged initially with a reasonable % alcohol. Even starting it with the same % wash as that in the pot still is an advantage. In this case, the excess energy from condensing the pot still vapour goes into releasing a vapour of quite a high % alchol, always higher than that exiting the pot still. In this scenario, the thumper is a benefit, as it allows quite an enriching of the vapour to occur, with no additional energy required. The only condition is that the thumper is charged with a liquid of a reasonable % to begin with - if simply filled with water it wont work.

To make the thumper effective you want
  • lots of vapor liquid contact, ie heaps of small bubbles - eg use a screen or simply lots of pinholes in the bottom of the inlet tube.
  • the liquid in the thumper to begin high in alcohol, so a small volume of wash or, maybe better, tails from the last run. Deb recommends .. You put tails or mash into the thumper - not water - and the loss is not a factor compared to the resulting alcohol content :) I highly recommend a thumper!
  • larger is beeter than smaller. For sizing a thumper, Ted suggests ..
    A good rule of thumb is make the thumper twice the size of the amont of distillate in a single run. ie: one run = 1 liter of spirits then make the thumper 2 liters .... as a minimum, have it at least 1/3 the volume, ie ((#gallons of wash * %alc of wash) / 3), or if using tails in the doubler, ((gallons of wash * %alc of wash + gallons of tails * % alc of tails) / 3). I myself prefer to use a larger size to allow for condensation that always seems to overflow the thumper. My grandfathers still was 250 gallons and had two 50 gallon thumpers as well as a 50 gallon slobber pot.
Jim writes ...
    Run your doubling keg in a water bath makes a good drink and it's pure. Do not add no liquid it makes it's own liquid

Note that thumpers aren't the same as slobber boxes (although they look fairly similar. The inlet in a slobber box doesn't extend down into the liquid, so no bubbling & hence further distilling takes place. Rather, it just provides a place for any rubbish (ie foam, mash etc) pushed up the neck of a pot still to settle out before entering the condensor. They sometimes have a small drain cock on the bottom of them to help empty them while the still's running. The original purpose for a slobber box was for stills heated by a fire beneath them - if the heating got too vigorous, and the contents foamed or bubbled up into the lyne arm, that it would settle out in the slobber box rather than go and contaminate the main spirits being collected. They're not needed on well controlled stills.

Slobber Box

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