The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Sunday, 19 February 2017

Yet more on moisture

Dealing with the problem of moisture in the electrical side of my Powerspout is proving more difficult than I'd anticipated.  A year ago I started putting bags of silica gel in there in the hope I could create a warm, dry atmosphere. I also blocked off the ventilation louvres to stop moist outside air exchanging with the dry inside air.  But it hasn't worked out as well as I'd hoped.

Initially the strategy seemed promising; relative humidity in the compartment could be got down to 10% (ambient outside being in the 90's) and the lack of ventilation made it warm but not too hot. The warmth did have the disadvantage of reducing electrical output slightly but the reduction was only by 1 watt per degree Centigrade rise and I came to accept this as a price worth paying.

What has become evident in the longer term is that the silica gel doesn't keep the absorbency it had when first purchased; it cannot be re-charged to be as good as when new; I have been re-charging it by putting the bags of beads in a fan oven at 120℃ for 2 hours, but even prolonging the time to 3 or 4 hours doesn't seem to drive the last 7-8 mls of water out of each 100g bag; in consequence, when put back in the SmartDrive housing they only last for 3 weeks before needing to be re-charged again. 

More recently I've tried a new tactic: if a compartment is COMPLETELY sealed it ought to be possible to get the relative humidity down to a low level where it will stay; but the sealing does need to be thorough: ALL possibility of moist outside air exchanging with dry inside air needs to be removed;  to achieve this for the electrical side means blocking two openings in addition to the louvred ports which I have already blocked: the drain hole on the floor of the compartment and the notch in the bearing block. The notch is there to allow any water getting past the seal on the shaft to dribble down, and so not come close to the outer shaft bearing.

But though plausible in theory, this tactic doesn't seem to work in practice; admittedly I haven't blocked the notch in the bearing block because it's tricky to do, but plugging the drain hole seems not to have done anything to extend the period before the silica gel needs replenishing. This is disappointing. The drain hole gives direct communication with very moist air and spray from beneath the pelton; blocking it ought to have produced a benefit.  That it hasn't leaves me scratching my head for the next bright idea...

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