Sometimes even the simplest of things can go misunderstood for far longer than one is happy to admit…especially once one realizes just how simple the problem is once boiled down, or in this case–condensed.
Those who have large air compressors know that it is important to “blow down” the sump in the bottom of the tank to remove water. When air is compressed, so is the moisture in it…..eventually some of this moisture condenses on the walls of the air tank and runs to the bottom……where it sits………and promotes corrosion…..which will lead to a rather unpleasant catastrophic failure of the pressure vessel–likely ruining everything and everyone around it. To mitigate this, you briefly open a drain cock on the bottom and let it blow out the water until only air is expelled. Simple enough. …But……..not always.
The compressor in question is a nice big new rotary screw type that can, and often does, run all day. The outlet from the tank flows through a refrigeration type dryer to remove moisture before the air flows into the line and to the various pieces of equipment connected to it. It is a shame that this dryer is not inline between the compressor and tank instead……the result would be dry air in the tank to begin with. But, the dryer would have to be rather huge to effectively deal with the volume and temperature of the air flowing from the compressor and so very rarely would anyone build a unit this way…..so, like all others, one must periodically open the drain cock and let the water out. This compressor, however, has a semi translucent section of line between the tank and that drain valve–this is to locate it conveniently.(Yay!) ….it also means that with the right lighting, you can tell if there is water in that tube or not…..this is, to me, sort of a neat feature…….but being able to see what was in there also lead to my frustration–afterall, if it was not visible to me I never would have paid it this level of attention.
So, I go open the valve–just a little…..letting the water flow out until there’s just a gurgle of air. Then close it. And before my eyes the tube fills back up with water. OK….do it again. Same result. ….and again. ….and of course it does it again. After two or three good shots and then letting it blast out some significant air (valve full open for a second or two and then shut) all appears well and dry. I walk off to other tasks.
A bit later I walk back by. The compressor is still OFF….it has NOT cycled or been run since draining. ….I take a glance at that tube and what do I see?…..more water!!! What the heck? Ok…blast the water out again, etc. Similar story as before but less this time………and……….as if just for spite…..within a half hour there’s more water collected in there. Perplexing.
And then the truth of the matter arrives in the form of remembering stuff I learned in third grade. Maybe it wasn’t really third grade, but my high school physics teacher always explained things with, “Just like you learned in third grade…….(phenomenon explanation follows)” In this case, the realization was with regard to Charles Law which states that if a given quantity of gas is held at a constant pressure, its volume is directly proportional to the absolute temperature.
Now by some simple mathematical substitution, it becomes obvious that if you kept the volume constant (aka–the tank) but altered the pressure, its temperature would be changed. This is why compressors have after-coolers and why your air conditioner works and to a lessor extent, why it rains. Which brings me to my realization:
When I vented the tank….the pressure dropped……since the tank is a constant volume, the temperature fell by some amount. At this point, the moisture in the air which was at equilibrium as a gas was able to condense from a vapor to a liquid and then fall to the sump in the tank in order to establish a new equilibrium at the new pressure and temperature combination. In effect…..the more I blew down the water drain, the more water I “made” in the bottom of the tank.
It was simply raining inside the tank immediately after my draining it. Every. Single. Time. Simple.
….and I feel a little bad that it took me 5 times of blasting it to reach this utterly simple conclusion.