The is auto drains ...2, the second page of information about compressed air system auto drains and getting the water out of your tank, and out of your compressed air lines. Page one is here.
Some compressed air auto drains work on the float principal.
As water and contamination accumulates in the bottom of the bowl, it will ultimately lift a float. The air pressure in the bowl will then vent to atmosphere through the opening, blowing the water and debris from the bowl as it does. When the accumulated water is gone, there's nothing left to "float" the valve operator, and it drops back into the orifice, sealing off the exit from the bowl.
The air pressure in the bowl will keep that drain hole sealed until such time as the water overcomes the air pressure by lifting the float, and the cycle repeats.
Some auto drains in air filters will only work to void the water in the bowl when the air system is depressurized, this allowing the float in this type of air filter to open the drain. Check with your vendor to see what type they offer, as, if you have a system that stays pressurized for long periods of time, this float type of auto drain may not function frequently enough to rid the system of water.
In compressed air systems with a high free water content and long uptimes between system depressurization, you might want to consider using an electric, timed auto drain, the same type that is recommended for use in draining the compressor tank.
The water that exits the devices will contain a range of contaminants, including oil, so it is wise to vent them into a pail. Then the drained liquid can be disposed of properly and safely.
If you have a high incidence of particulates (chunks of crud) in your air system, and in the compressor receiver or filter bowl the water slurry exiting the tank or filter bowl when the drain cycles, may plug most of the outlets in the common drain.
If the frequency of cycle allows the material in the auto drain or float drain to dry out, this may cause the mechanism to stick, and your auto drain won't!
If this becomes a problem for one of your drains, consider using an electrically actuated ball valve which will have larger flow openings less likely to plug up and a more powerful actuation motor to overcome sticking.
Did you know that....