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.

Monday, 1 February 2016

More on moisture

Eleven weeks ago, I wrote about Managing moisture. It described my attempt to create a warm and dry atmosphere in the electrical side of the turbine by packing in a kilo of silica gel beads and blocking the ventilation ports. The aim was to improve insulation resistance and reduce corrosion. What's happened since?

At the time I sealed the compartment, generation was only 300W, so temperature inside did not immediately rise very much.  But as winter generation picked up (it has been > 780W since 1st December), the temperature rose to 18-20℃ above outside temperature, reaching nearly 40℃ on warm days. There was nothing worrying about this. But at about this time I began to recognise that the warmer the SmartDrive operated, the lower was its output, about 6 watts less per 5℃ temperature rise (see last diary entry and also here).  After thinking about it and deciding that maximal efficiency was my main aim, I re-established ventilation by putting back the louvred ports in two of their three locations. Ever since, the temperature inside has been lower at between 19 and 24℃.

Restoring ventilation predictably made relative humidity inside rise. When sealed, it was as low as 10%, but unsealed it rose to 20 - 38% because more air exchange occurred with humid, outside air.  The inside humidity now fluctuates in the 20-38% range, tracking the humidity of outside air, and the temperature in the compartment. I have been assuming the silica beads are still taking up moisture as the inside RH is well below the outside value. Later on I'll take the bags out and weigh them to see just how much water they've taken up.

Today, I've been measuring the temperature of the water powering the turbine.  It was 8℃.
It's the temperature of the bulkhead that I really want to know and I'm assuming it will be the same as the water. The temperature of the bulkhead determines when moisture will condense on its surface and this measurement was the only factor I'd never got round to measuring in my quest to predict the conditions which will create condensation.

In the diary entry "Managing moisture" I gave a link to a dew point calculator.  This is a handy tool for working out at what point condensation will occur, handy because the computation is complicated: there are three variables to consider: the relative humidity of ambient air, the temperature of the air, and the temperature of the surface on which condensation is to occur. 

Convenient as the calculator is, a graph gives an alternative way, a more visually predictive way, of appreciating how relative humidity and ambient temperature can be manipulated to avoid condensation. These two variables, temperature and humidity, are the only ones which can be manipulated, since the temperature of the bulkhead obviously cannot be changed.





To understand the graph requires an understanding of humidity: humidity is the amount of water carried as invisible water vapour in a mass of air; the amount of water vapour carried will depend on how moist the air happens to be: deserts are dry, rain forests are wet; but the maximum amount of water air can carry in either region depends on the temperature of the air: the warmer the air, the more water; the cooler the air, the less water.  When a body of air at a certain temperature and carrying a certain amount of water (as invisible vapour) comes into contact with a surface which is colder, the layer of air immediately above the surface is cooled; in being cooled it finds it can no longer carry as much water vapour as the warmer air further away from the surface, and if it cools below a certain point, water comes out of being in a vapour phase and condenses on the cold surface as liquid, first as misting, later coalescing to droplets.

What the graph shows is a family of curves. Each curve describes the ambient humidity and ambient temperature at which condensation will occur on a surface having a given temperature. Several curves are needed because each shows the humidity / temperature relationship for a different surface temperature. In my graph, curves are shown for surface temperatures of 0, 4, 8 and 12. 

The measurements I took today recorded the temperature of the bulkhead as 8℃, the temperature in the compartment as 23℃, and the humidity in the compartment as 28%. The red arrow in the graph has its point at the place for the two temperature measurements: where today's compartment temperature 23℃ intersects with the 8℃ curve.

At this point it can be read on the Ambient Relative Humidity (RH) axis that dew (condensation) will form if ambient relative humidity is 38%.  It will of course form too if the RH is at any figure higher than that. Since the ambient RH inside the compartment today was only 28%, it follows that condensation would not have been possible under today's conditions.

We can use the graph in an alternative way to see at what ambient temperature condensation will occur when the RH is 28% and the bulkhead temperature is 8℃: it will be 29℃ (as best as can be discerned from this crude graph).  Now 29℃ is a temperature which might easily be reached if ventilation was blocked off so we might conclude that blocking off ventilation would be a bad idea from the condensation point of view (it will certainly be bad from a power generation viewpoint); but if ventilation was blocked off, ambient RH can be expected to be lower, maybe only 20%, perhaps less, perhaps as low as 10%, and then it can be seen by extrapolating the green 8℃ line to the right, that ambient temperature could be allowed to rise to perhaps 45℃ before condensation would occur, but only so long as the bulkhead temperature is at 8℃.  And if the bulkhead was colder, say 4℃, we would have to jump to the 4℃ curve which gives a whole new set of temperature / humidity relationships.

You can begin to appreciate what a complex business condensation is, and yet it is a very precise and predictable subject ... if only one knows the values of the variables. 

What a fun occupation it is measuring things and using the data to better understand the world about us!  If it prevents a V-Clamp board failing from poor insulation resistance caused by dampness, it will be useful as well as fun.  Here's to hoping!

No comments: