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.

Tuesday, 10 November 2015

Managing moisture

It has been a busy few days with the Powerspout.  With autumn well on the way, the weather has been warm and wet.  The flow available to the turbine has been gradually increasing so I have been able to change from the reduced core stator to the 42 pole, full core one. This will now see the turbine set up for its winter period of peak output.

Whilst doing the change of stator, I also changed the bearing block.  I had to deliver on a promise to Michael Lawley that once winter flows arrived, I would run another trial of ceramic bearings.  The first trial was done in June and the bearings lasted just 4 weeks.

This second trial started yesterday, - and finished yesterday ! Just 8 hours !  Inspection of the failed bearings showed much the same as the first trial: pitted balls causing locking up of the race, but this time there was also abrasion of the PTFE spacer ring, leaving particles of PTFE everywhere.  I think this has to be the end of the road for the ceramic bearing dream !

In the course of this repeated delving into the turbine, I have progressed the idea of trying to run the Smart Drive compartment as dry as possible.  This I've done by stopping the ingress of moist outside air by blocking off the ventilation ports and de-humidifying the captive air inside with bags of silica gel.

The idea is to create a warm, dry environment for the electrical side which will inhibit corrosion and promote insulation.

It seems easily possible to obtain very low levels of relative humidity.  Ten bags of silica gel, each of 100g, brings the humidity down to just 10% within an hour of closing the housing, and this is with an outside ambient humidity of 98%.

The rise in temperature is to about 6 ℃ above ambient when the power output to grid is 300 W. At the reading showing this morning, 22.4 ℃, I am happy with this, although as power output increases in coming weeks, and with it greater heat output from the alternator, the rise in temperature will have to be watched.

To touch briefly on the theory of humidity and its relationship to dew point, if the relative humidity inside the housing can be kept at this 10% level and the temperature in the housing does not rise above 36 ℃, it will completely prevent any condensation (which is dew) forming on the bulkhead, even when the temperature of the bulkhead gets down to 0 ℃.  Since it is condensation forming on the bulkhead and then dribbling down over electrical components which probably causes most of the issues with insulation breakdown, this would be a significant advance.

The unknown will be how often the silica gel bags will need 're-charging', and the answer to that will be how often the compartment is opened allowing an air change with moist outside air.  I'll be keeping my inspections there to a minimum.

For those interested, 25 silica gel bags of 100 g cost £18.59 and a useful dew point calculator can be found here.

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