Even with regular tank draining, some moisture always makes it into the air lines. Without a water separator or filter at the point of use (right before the tool or application), this moisture reaches your equipment. A simple $20-$50 filter-regulator with an auto-drain solves most water problems.

What you'll see

Water sputtering from air tools or spray guns even though the tank is drained regularly. The water appears at the tool, not at the tank. The piping between the compressor and the tool is picking up condensation (the air cools as it travels through the pipe, causing more moisture to drop out). This is especially noticeable on long pipe runs, in cold weather (cold pipes condense more water), or with spray painting where any moisture ruins the finish.
Before you assume this is the problem

If water is pouring out of the tank drain in large quantities, deal with that first -- proper draining reduces the load on downstream filters. If you already have point-of-use filters and still get water, the filters may be saturated or the drain on the filter bowl is clogged. See: Infrequent Tank Draining.

How to diagnose

  1. Check if point-of-use filters exist

    Look at the air connection right before each tool or work station. Is there a filter, regulator, or water separator in the line? If it's just a hose connected directly from the wall outlet to the tool, there's no filtration at all. Any moisture in the piping goes straight to the tool.
    Result: Filter present = check if it's working and draining. No filter = install one.

How to fix it

  1. Install a filter-regulator-lubricator (FRL) unit

    Install an FRL combination unit at each point of use. The filter catches water and particles, the regulator sets the correct pressure for the tool, and the lubricator adds oil for air tools (skip the lubricator for painting). Size the filter for the flow rate of the tool. A 1/4 inch unit with a 5-micron element is suitable for most hand tools. For painting, add a coalescing filter for ultra-clean air.

  2. Drain the filter bowls regularly

    Filter bowls collect the separated water. Most have a manual drain at the bottom -- open it when the bowl is half full. Or install an auto-drain bowl that empties itself. A filter bowl that fills completely stops filtering and sends water straight through to the tool. Check daily in heavy-use environments.

  3. Consider a main-line water separator

    For systems with long pipe runs, install a water separator right after the compressor (after the tank, before the main distribution piping). This catches the bulk of the water before it enters the piping system. Combined with point-of-use filters, this two-stage approach handles most moisture problems without needing an expensive air dryer.

Common mistakes

Don't install the filter right next to the compressor and expect it to protect a tool 30 meters away. Moisture condenses all along the piping as the air cools. The filter needs to be at the point of use -- right before the tool. Also: don't forget to drain the filter bowl. A full bowl is worse than no filter because it can actually inject collected water back into the air stream when the air flow creates turbulence in the bowl.

Parts & tools

Filter-regulator unit (match port size and flow rate). Wall-mount bracket. Pipe fittings and adapters. Coalescing filter (for painting applications). Replacement filter elements (keep spares).

Review safety precautions before starting →

Safety

No special hazards. Install filters with the system depressurized.