But if you are a fuel tank, condensation is bad... no, it is evil. Here's what happens:
- The sun goes down.
- The tank cools off.
- Because it is cooling, the air inside contracts, wanting to pull a vacuum
- But you have a vent on the tank (thank goodness!), so damp night air flows in thru the vent.
- And the moisture in the damp air condenses on the tank walls and the surface of the fuel itself.
- Eventually, the moisture makes it to the bottom of the tank.
- The sun comes up
- The tank heats up.
- The air in the tank heats up, and grows in size.
- The air exhausts thru the vent line, leaving the water in the bottom of the tank - after all, it is covered by a thick layer of diesel - it is effectively out of circulation.
- Lather, rinse, repeat.
So how can you prevent it?
My friend Drew over at Sail Delmarva, who does this professionally for large industrial tank farms recommends this product from H2Out Systems:
This is a flow-thru container made of diesel-compatible materials, and filled with silica gel. You know, the same stuff found in those little "Do not eat" packets in there with your new digital camera or your vitamins (have you ever been tempted to eat one?).
This device gets installed in your fuel tank vent line. When there, the silica gel adsorbs the water from the incoming air; only dry air gets to the tank. The silica gel is treated with cobalt chloride which serves as a moisture indicator. When it turns from blue to pink, it is time to regenerate the silica gel. Regeneration consists only of heating the silica gel in a low temperature oven for a few minutes.
According to Drew, there is some self-regeneration, depending on the size of the filter and the size of the free airspace in the tank. When the dry air in the tank expands on heating, it passes thru the filter in the outbound direction, and strips moisture out of the filter.
Pretty simple, eh? But there is one important caveat: the moisture filter must be protected from contact with diesel - either by design of the installation or by process constraints. If diesel gets into the media, it is ruined.
So I have this one installed - I'll let you know how it works in about 6 months.