In Reply to: Re: PV = T posted by Chuck Tribolet on February 23, 2007 at 15:32:28:

If you fill a tank which is at thermal equilibrium with the room from banks that are at thermal equilibrium with the room the tank that is being filled will drop in temperature.

The gas in the banks starts out at room temperature. As you let it into the tank being filled it expands and cools, a lot at first, almost not at all when the tank and the bank equalize. However the net integrated effect is that of the filled tank being cooled.

When you fill a tank with a compressor (and assume adiabatic compression and expansion) the air starts at room temperature and it's temperature is increased by the compressor. When the air expands into the tank it cools, but not back to room temperature since it never sees atmospheric pressure. As you fill, the air going in expands less as the tank fills and stays hot. The net integrated effect is a hot tank.

Yet another way to understand the phenomena is when the banks were filled by the compressor the bank tanks got hot because the work compressing the air heated it. With time that energy was lost and the banks reached thermal equilibrium with the room. That energy was lost and is no longer available to heat a tank that is filled from the compressor.