This page is about compressor auto drains, and how they help keep your industrial air system water free. The words auto drains apply, in this case, to their use in draining compressor tanks, dryers and filters.

Auto drains are used to replace the manual drains that are normally standard equipment on compressor tanks.

Manual Tank Drains vs Automatic Tank Drain

The receiver, or air tank, of your air compressor normally comes equipped with a manual compressor tank drain. This manual tank or reservoir drain is typically located at the lowest spot of the air tank.

Have you ever busted a knuckle trying to drain the tank? Yeah, you know the location makes it almost inaccessible, but that’s not why the drain is there. Gravity works, and water generated by the pump and driven into the tank by the process of compressing air, will flow down to the lowest area in the tank. That’s where the tank drain should be!

In order to keep free water from accumulating inside the receiver, and this is particularly important for industrial compressed air applications in which large compressors can dump gallons of water into a tank daily, this manual tank drain should be opened at least once a day to allow the tank to drain all water completely.

Depending on the size of your compressor and the length of time that your compressor runs each day, it might be necessary to drain water more frequently than that. Better to drain it too often than not enough.

Compressed Air Auto Drains
Compressed Air Auto Drains

This strongly recommended daily ritual of manually draining a compressor tank is often overlooked by routine maintenance, not the least reason of which is the location of the tank drain. This will result in the scenario discussed on the pages of this site that refer to water and water problems here.

Electronic Auto Drains For Air Compressors

This is why an electric compressor tank auto drain is an almost must-have accessory for the compressor receiver in industrial applications, and if you frequently use your air compressor in a home or garage workshop, there too.

Compressor tank auto drain
One example of an electric auto drain. Photo:

Electric auto drains are typically configured as a 2 ported, 2 position valve that incorporates a timer.

The auto drain timer can be set both for the frequency of operation, and for the length of time (the duration) that the auto drain valve is opened, to effectively and automatically rid the compressor tank of water that is generated by the compressing of air.

Many electric auto-drains come equipped with a wire and plug to allow it to be powered from a standard 120 VAC outlet. The auto drain can also be hardwired into a control system that could monitor the functioning of the auto drain automatically.

Compressor Tank Auto Drains Installation & Setup

Installation of the auto drain is fairly simple.

The existing manual drain is screwed out, and the auto drain is screwed into the same port.

Or, if the outlet on the manual drain is threaded, add a nipple and screw the auto drain onto the nipple. If you opt for this route, ensure that you leave the manual tank drain fully open.

Setting the time is straightforward as well. Typically there will be a timer knob, usually calibrated for hours and/or days on which you set the frequency of operation of the auto drain. Once a day? Perhaps twice a shift if the air compressor runs daily and frequently.

There will be another dial or knob to set the duration of the drain cycle. This dial is normally calibrated in seconds and/or minutes. This adjusts the length of time that the drain is open to void water and from the tank.

Receiver auto-drains will mean that the compressor tank gets drained of free water often enough to solve some water related problems in the downstream, air-using applications, and without a maintenance person having to remember to do so.

How often to cycle compressor auto drains?

When you’re setting up your auto drain, start by timing how long it takes to drain the reservoir using the existing manual tank drain valve after the compressor has run for half a shift.

When draining the tank is complete, there should be no free water coming from the drain, the drain bleeding air and water (yes, you want to have air pressure in the tank) until only a slight mist still exits the drain along with compressed air. Set the auto drain to run that same length of time, and for one cycle each 1/2 shift.

Compressor Automatic Drain Valves & Energy Conservation!

Monitor the operation of the auto drain to ensure that is does not run for more than a few seconds after all free water is blown out. If the auto drains continue to run after that time, all that is being accomplished is wasting expensively generated compressed air.

Auto-Drains for in-line compressed air filters

The standard drain for a compressed air filter is a manual one.

This means that a maintenance person has to visit the location on a regular basis to ensure that free water and the “soup” of contamination that the filter strips from the air is drained from the filter bowl before it can be entrained back into the air stream.

An air filter auto-drain will ensure that the filter bowl is drained as frequently as necessary, without operator intervention, resulting in long-term positive effects on the downstream compressed air components that the filter precedes.

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 compressed air, under pressure in the bowl, will then vent to atmosphere through the drain 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.

Auto Drains
Auto Drains

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 up-time 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.

There will likely be environmental or safety issues relating to the outflow from the receiver auto drain or even the filter drain. Please dispose of drained fluids properly.

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 of crud and compressor oil, in your air system, that – along with water – gets to the compressor receiver and then into the air lines to the filter bowl. This accumulation, when exiting the tank or filter bowl when the drain cycles, may plug the outlets in the common drain. A more frequent cycle of draining will help prevent this.

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 shut, and your auto drain won’t work.

If a plugged drain becomes a problem for one of your drains, consider using an electrically actuated ball valve. The ball valve will have a much larger opening meaning it will be difficult plug up.

Did you know that….

“… that at saturated conditions, for every 20?F decrease in air temperature, there is a 50% reduction in the water vapor content?”


Comments? Questions? Photos? Post them here: