Sugaring season soon; high-efficiency evaporator concepts

It’s definitely still winter here, but if you’re feeling optimistic you can imagine that it’s starting to slip away.  Days are longer, sometimes high temps over 30, and crusty old snow in the woods pockmarked by deer tracks – puts me in mind of the start of sugaring season.  It is said that during some period of blockade (perhaps the war of 1812) Benjamin Franklin floated a plan to make the US independent of tropical sugar by vastly expanding maple syrup production.  It’s an interesting concept; by my best napkin estimate an acre of sugarbush can produced enough calories to feed a person for a year – not as efficient as most field crops, but a lot less work and a lot less environmental upset.  To make this any kind of reality we would however need a much more efficient way to do the evaporation, as compared to the primitive wood and oil-fired evaporator technology now in use.

We don’t sugar here in NH (small town lot with no maple trees), but it was a fixture of the season growing up in Maine when I was a kid.  We had a small Leader evaporator made from a welded-up oil drum with a galvanized sap pan that we’d operate outdoors, producing 5-15 gals per year from around a hundred red maple taps.  It’s classic New England, but from an engineering perspective it seemed really inefficient.  The single-wall steel firebox shed most of its heat out the sides, the combustion was not well-controlled, and the wood used was typically marginally dry softwood slabs or other low grade stuff that wasn’t worth burning as stovewood.  More fundamentally, a huge amount of recoverable heat goes up in steam – about 2200 joules per gram.  The small home evaporator systems make only the most pathetic attempt at recovering the energy in the steam – a preheater pan sits over around 20% of the evaporator – and ironically the condensate that drips off the preheater falls right back into the pan.  So the first step in improving the efficiency would be to make sure to condense all the steam possible onto a heat exchanger delivering the cold sap to the evaporator – and arrange for the condensate not to fall back into the pan.  Still, even if you delivered the maximum amount of heat possible into the cold sap (around 420 J/g to raise it from0C to 100C) you would only have recovered around 20% of the available energy in the steam.  To really boost efficiency, you need to use the steam to boil more sap.  The problem is that the steam will only condense on surfaces colder than 100C – and you need a temperature higher than 100C to drive heat into boiling sap (because sap contains sugar, it boils at a temperature slightly above 100C at one atmosphere).  This could be accomplished with some kind of refrigeration cycle, but that’s too complex for hard-times engineering.   Also energetically efficient but similarly complex is the recent adoption of reverse osmosis by sugarmakers, with attendant tending of  finicky high pressure pumps and delicate membranes. What’s called for is a relatively simple, elegant, robust solution in the (quite literally) steampunk style (also cf. handy apocalypse guide) .

One solution is to condense the primary steam against a secondary evaporator, the contents of which (sap at lower sugar concentration) are held below atmospheric pressure.  This is a convenient way of lowering the boiling point.  This is not a revolutionary concept; I have demonstrated it (sort of in reverse) by boiling water on the lid of my pressure cooker – the steam within the cooker is condensing on the lid at a temperature well above 100C due to elevated internal pressure of about 2 atm absolute, such that the temperature of the lid itself is sufficiently above 100C to initiate vigorous pool boiling of pure water on its surface.   Naturally, the condensate from the secondary evaporator would have to be pumped out of that chamber; a mechanical pump would certainly do, but in the spirit of low-energy passive design it occurs to me that at least on the coast of Maine (and in many hilly regions where syrup is produced) it is not difficult to find 32 feet of vertical relief; a stable vacuum of the desired level could be established by maintaining a fluid column of condensate in a tube and controlling its escape from the secondary condensate sump with a float valve.  A glance at a steam table indicates that a pressure of around 0.6 atm is sufficient to decrease the boiling point of water by around 15C, which should be sufficient to drive boiling in the secondary evaporator.   The reduced-pressure secondary evaporator also provides a convenient means of drawing fresh sap into the system – again regulated by a float valve.  On the other hand, the partially concentrated syrup must of course be pumped from the secondary evaporator into the (atmospheric) primary evaporator.  This could be accomplished by a hand pump if the system is monitored steadily (the amount of shaft work required being relatively modest) or in concession to modern technology a small diaphragm pump.  Condensate (either from the primary or secondary evaporator) would again be used to preheat the incoming feed of sap.

By means of this dual-stage evaporator, the energy required to make syrup could be decreased from approximately 2600 J/gram of sap plus losses to around 1100 plus losses.  Of course, still greater efficiency could be achieved by condensing the evaporated steam from the secondary evaporator against the underside of a tertiary evaporator (operating at perhaps 0.2psi absolute) and so on ad infinitum.  A primary boiler stage operated at elevated pressure is also conceivable.  But a doubling (or better, given concomitant thermal conservation measures of a more pedestrian nature) of evaporating efficiency as an initial goal seems reasonable.  I have not pieced together all of the mechanical and fluidic aspects in my head, but my dim mental picture is quite satisfying, combining the favorable aesthetics of the African Queen with that of a backwoods moonshine still.  Also, the potential for catastrophic explosion is almost too good to pass up.

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