Perhaps I 'm being dense this morning but what is the purpose of that 4" "air tube" in the setup above?

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My draft setup is in my basement wet bar and the keg fridge is remotely located in an adjacent room. The air tube allows cooling of the beer lines all the way to the remote tap, and is an extension of the chilled boundary. It is a pipe within a pipe with the chilled air and beer lines in the inner pipe. At the end of the run, the outer pipe is connected to the tap while the inner pipe stops short allowing the air to change directions 180 degrees and return thru the annulus back to the chill box. Other remote storage techniques involve gylcol chillers and pumps, but I was concerned over leaks. I did not have the room to mount the keg fridge in my cabinet under the tap.
Here is the link I used to size the beer lines (length and diameter). You will need to decide beer type, serving temperature, etc.
http://www.franklinbrew.org/brewinfo/balance.html
Here is a schematic of the air shaft tube within a tube:
http://www.micromatic.com/draft-keg-beer-edu/faqs-air-cooled-cid-530.html
Overall, I am quite pleased with the system. It has been in use now for about 5 years. Kegged beer will last for months in the home (assuming you let it!). Just keep the CO2 on.
I used to home brew quite heavily, but life has gotten in the way on that hobby. Thus, I just pump commercial beers for now. The chest freezer is ideal for home brewers that use the 5 gallon soda pop syrup canisters for kegging of home brew. Buried in my basement is my converted 3 keg 2 tier brewery, 150k btu burners, pumped recirculation mash, .....