The HF drain ... and they are not the only ones to market this particular
type ... hooks to the line that goes to the unloader line on the compressor.
On many compressors there is a mechanism that allows some air to escape ...
a leak by design ... from the compressor cylinders until everything can get
up to speed. Otherwise the motor might stall if the very first thing it had
to do was a 100% compression stroke before it had a chance to get up some
momentum. Hence the term "unloader".
The unloader line hooks the output of the compressor to the unloader
mechanism and as the pressure builds up ... the unloader only lets a small
portion of the output of the cylinder escape so pressure will eventually
build up in the output after a few seconds ... the unloader determines that
things are on a roll and shuts off, allowing 100% of the cylinders output to
go to the tank.
Note that the unloader line is not hooked up to the tank as then it would
always have tank pressure on it. It is hooked up to the output of the
compressor and before the one way valve between the compressor and the tank.
Therefore when the compressor is not running the unloader line does not have
any pressure in it. You can usually hear the compressor\unloader line going
back to its normal non-pressurized state when the motor turns off. At least
on mine it hisses for about 10 seconds after the motor stops running.
And not all compressor\tank setups have unloaders or exposed unloader lines.
Assuming you have an unloader, then you see that you have a line that will
not have any pressure in it when the compressor is stopped and will have
pressure when the compressor is running. And the auto drain takes advantage
of this by sensing the pressure build up in the unloader line when the
compressor first starts running. For a brief period during the transition it
opens a valve at the bottom of the tank and lets the water out ... or at
least that is the way it is suppose to work. On mine there is a 5 to 10
second hiss when the compressor first starts up as it gets rid of the
moisture in the bottom of the tank.
Sort of neat when you think about it. A totally physical non-electronic only
works when the compressor first starts up auto drain. If the compressor
never runs then there is not need to drain the tank. If it starts and stops
a lot then it drains often. The only case it seems to miss is the "you are
using so much air that the compressor comes on and never stops" ... which
means you could put several hours worth of moisture laden air into the tank
and it would not be drained until the next time it starts. But if that is
the normal run pattern for your compressor you may have other issues to deal
with other than the auto drain.
The auto drain that John had is on an electric timer. You set it for how
often is should drain and how long the valve will be open when it does
drain. I had thought about one of those but my compressor is in my basement
and I did not want the Gear Heads version of a Coo Coo Clock clock hissing
at me every hour on the hour. Had the compressor been somewhere out of
earshot I probably would have gone with the electric version.
Arvid
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