Physics Question involving an Air Compressor
 

I have a general understanding of why an air compressor needs to be drained periodically (and lord knows I've taken enough in hock with what sounds like a quart of water sloshing inside). But why does my portable air tank, even after years of service, not suffer the same malady? The manufacturers of the portables must be aware of this, I've not seen one with a drain.
I had a difficult time with college physics, took me two attempts to pull a hard earned D, so please don't use big words in your explanations.
Jim

Once the air is compressed enough to condense the moisture in the air, it falls to the bottom of the compressor tank (where the drain is). The air outlet is at the top of the compressor tank, so water "should" never be blown out of the compressor tank, and into the portable tank.

Specific Humidity is a ratio of how much water is in the air.

Relative Humidty is a ration of how much water is in the air, vs how much it can hold.

Dew Point is the temperature where water will start coming out of the air.

If you compress air, the pressure will rise and the temperature will rise. But you also increase the mositure content. It will stay in gas form, as long as the temp stays above the dewpoint. However, as you compress the gas, the dewpoint will also rise. When the compressed air cools, it will form liquid water.

If you fill your tank with cold compressed air, most of the mositure will have already fallen out.

Typically I'm filling the portable while the compressor is running. I was thinking the air inside a running compressor would be a fairly homogenous mixture but it's believeable that the (relatively) hot, dry air stays on top and the cold, moist air sinks.
Would the problem be worse using a flat (pancake) compressor or a tankless one?
Jim

Much worse with a tankless