When air is compressed, some of it turns into water vapour inside the air lines. No matter how long your compressor's air line is, water will gather in the line. The water will flow into your tools when you use them or it will be sprayed out into the room whenever you use your air nozzle. Water should be removed from air lines daily. It's easy to remove the water if you set up your lines with a water trap.
Walk the length of your air line and find the lowest point of the line. If your line is on the ground prop up one end slightly with a wood block so that you create a low point. Make it closer to one end if possible.
Cut through the pipe with a hacksaw. Glue a "T" fitting between the two ends if it is a PVC line. If it is a steel line, use a pipe wrench to remove both pieces where you cut through. Take the pieces to a plumbing shop and have them put threads on both ends and then screw a "T" into the steel pipes to join them.
Glue an 450 mm (18 inch) piece of pipe onto the end of "T" facing straight down. Screw an 450 mm (18 inch) pipe with both ends threaded, onto the end of the "T" if its steel. When you pick up your 450 mm (18 inch) pipe, purchase one that has threads on both ends.
Glue a threaded female reducer onto the end of the PVC pipe facing straight down, and then screw an air outlet jack to the end of the reducer. Screw a threaded female reducer to the end of the steel pipe facing straight down, and screw an air outlet jack to the end of the reducer.
Plug an air hose with air nozzle into the air outlet jack. Water will run down into the 45 cm (18 inch) pipe and become trapped there. Use the air nozzle to blow the collected air out of the line.
Most air lines are 20 mm (3/4 inch) steel, or 30 mm (1 1/4 inch) PVC. Purchase the appropriate size for your line. The reducers will typically be 19 to 12 mm (3/4 to 1/2 inch) for steel, and 30 to 12 mm (1 1/4 to 1/2 inch) for PVC. The air output jacks will always be 12 mm (1/2 inch).
Never point an air hose with water in it at anyone or anything that can be damaged by water under pressure.