Acceptable? – Most likely. I don’t know your location but I’m unaware of any regulation that would prohibit it.
Ideal? – Not really. Corrosion is an issue. As zdas04 said, it all depends on the corrosion risk you can tolerate.
Two more things to consider
[ul 1][li]If you don’t sufficiently dry your air, water will collect in the underground line. The corrosion issue can be both external and internal.[/li]
[li]Underground leaks are tough to identify and costly to fix (most plants have a hard enough time addressing their above ground, blow-your-hair-as-you-walk-by leaks).[/li][/ul]