Archive for February, 2009

Sugaring season soon; high-efficiency evaporator concepts

February 14, 2009

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.


bug fix for Zareba SP10 solar fence charger; electric fences in winter

February 2, 2009

Last spring I mentioned in a post that I had trouble with water leaking into a brand new Zareba SP10 solar fence charger, and that I traced the problem to a woefully underdesigned seal in the housing.  I am pleased to report that the replacement provided by Tractor Supply is still going strong some seven months later in the dead of winter.  So, for anybody who plans to put up one of these triangular Zareba solar fence chargers, here’s the “patch” (literally).  Mount the charger normally (on a post, facing south), then apply a generous helping of 50 year durability outdoor-rated silicone sealant to the seam where the solar panel meets the housing at the top, just under the screw holes.  You apply the silicone after it’s mounted since the screw holes are pretty close to the seam between the housing and the solar panel, and the adhesive bond between the silicone and the black plastic of the unit is not super strong – also for this reason, try to spooge some silicone down in the crack between the module and the housing, and run some down the sides for good measure (but not all the way around; you want any condensation that does find its way in to run out).

As mentioned, the unit I subjected to this treatment was still going strong last weekend, and I’m hopeful that it will continue to work for a long time yet.  The subject of electric fences in winter deserves a bit more comment; I’ve taken the approach suggested by various folks and rewired our fences so that the bottom two wires (8 and 16 inches off of the ground) are connected to earth during the winter.  This is benficial both because wet snow would otherwise short out live wires, draining the battery, and because dry snow is a good electrical insulator, reducing the ability of the fence to shock animals in winter.  The idea is that animals in the course of trying to get through the fence, animals will touch both the live high wires and the grounded low wires, getting the maximum potential shock from the fence charger.  My mom reports that when she’s patrolled the orchard margin with fresh snow on the ground, there are deer tracks nearby but they seem to be giving the fence a respectful margin, suggesting that they have learned their lesson.  When the snow melts in spring we will reconnect the lower wires to the charger, the better to keep porcupines and raccoons out.  In any case, we still have the lightweight plastic deer netting on the inside of the fence posts, to act as a secondary barrier in case there’s a problem with the charger.  This netting is not physically strong enough to keep out a determined deer or moose, but apparently they don’t like it, apparently becuase they fear getting tangled up in it.  Between the electric and the netting, we have so far been successful (knock on wood) in holding the hordes at bay.

Fiddling for a contradance

February 1, 2009

Friday night my classmates and I played for a contradance in South Strafford, VT, and it was an absolute blast!  It was the culmination of a series of music classes given by an accordionist named Jeremiah McLane, and the dance followed a public dinner given by the active locavore food group.  Dinner consisted of all New England ingredients, and consisted of shepard’s pie, roasted vegetables, Waldorf salad, fresh rolls, sweet cider, and bread pudding with custard and berry sauce.  The downstairs room of Barrett Hall was packed, and the food was really good. It felt old fashioned and modern all at once – it seems as if at least that part of Vermont has attained a sufficient critical mass of crunchy people to rediscover the sort of social structure that dominated in a pre-TV era.

Afterward the dance started upstairs.  The band was nine people: three fiddles, two accordions, a flute, a hammered dulcimer, a guitar, and a keyboard.  Having never played for such an event before (besides sitting in as a minor contributor to Flagstaff’s “Just Desserts” dance band and chocolate appreciation society), I was a bit nervous, but had brought some bottles of this fall’s cider and passed it around to help settle the nerves.  There was a lot of fiddling around with the sound system – Jeremiah who plays in several bands once remarked that a musician is fated to spend a third of his time messing around with the sound system, and another third playing with crummy sound.  It was the first time I’d dealt with monitors, which make for a strange experience – you hear yourself playing and it sounds sort of like it’s coming from your instrument, only much louder.  The sound guy tries to put an appropriate combination of all the instruments into the monitor mix; what I heard was mostly the three fiddles and the rhythm (guitar and keyboard) without much flute or accordion, but it was perfectly sufficient – there have been times in practices where I was sandwiched between accordions, and had the strange experience of playing loudly and hearing my fiddle make accordion sounds.

Anyway, the sound guys did a great job, and we started in to play.  We had a few sets of reels, a couple sets of jigs, an a set each of polkas and marches, as well as a couple of waltzes to end the first and second halves of the dance.  Playing contradance music is challenging, at least for an amateur of modest ability such as myself – the nature of the dancing is that the tunes must be played up to a certain tempo to be enjoyable, and especially for the notey-er reels it sometimes feels like hanging on for dear life.  But one we got a few sets into the dance and I realized I wasn’t going to crash completely, I relaxed a bit and found that I was having a lot of fun.  It seemed the perfect number of instruments and ability balance for my level of skill – there were times when the others carying the melody seemed to get in trouble and I could hear myself coming through clearly from out in the hall, giving me the thrilling-but-nerve-wracking feeling that I was carrying the melody torch over Sue and Rob’s reliable rhythm; other times I lost the thread of the tune or wasn’t sure we whether we were going to A or B, and was comforted to hear my companions carrying the melody staunchly forward.  Overall there were a couple thin spots but we never had a train wreck as a band, and my correspondants in the audience (Alexis, our friends Joshua and Kelsey, and well-wishers from our Thursday night music get-togethers) reported that the music was solidly danceable.  My fingers though tired never cramped to the point where I couldn’t carry some reasonable approximation of the tune.  Jeremiah was directing and would call out various melody instruments to be featured with the rest dropped out; I got a turn through one of my favorites, an infectiously manic French-Canadian reel that opens with a two-measure syncopated broadside and contrasts nicely in the beginning of the B part with three nice long descending notes useful for catching ones’ breath.  In the heat of the moment I had the impression that I pulled off a satisfactory if workmanlike rendition and kept more or less to the beat; I’m looking forward to hearing the disk that the sound guys cut to see how a more objective memory fares.

Anyway, it was a blast, and I want to do it some more.  As I understand it there is no shortage of minor dance events that are looking for a band that won’t charge much (or anything), and that the greater challenge is keeping the members of a large band focused and practicing.   For journeymen musicians such as ourselves there is strength in numbers and a 9-member band is none too many; the holy grail would be to attain a sufficient level of skill that 3-5 people could reliably carry a dance; obviously it would also be easier to find times when everyone could get together for practice.