Dan,
I have had that design of air-plumbing in my unheated garage for about 10 years. In November, before the deep-freeze sets in, I drain all drops in the piping and I drain the compressor.
It gets humid here in the summer, but it is relatively dry air from Nov to April.
The only thing I failed to put in my air-plumbing is the pressure release valve in the vertical line near the compressor.
By the way, I used a high-pressure-rated flexible metal line/hose (200psi rated - for water heater) between the compressor and the black-pipe.
