Blowing out sprinkler lines with a compressor is one of the most effective means of winterizing a sprinkler system. Sprinkler lines should be drained of as much water as possible each season, as frozen water will expand and possibly break the underground plastic pipes. But there are limitations on which types of compressors will do the job correctly. Understanding more about the key parts of a compressor will help you make an informed decision about what size to use for blowing out your sprinkler lines.
To properly blow out a sprinkler system, your air compressor needs to provide at least 20 cubic feet per minute. Most irrigation professionals recommend using at least 50 cubic feet per minute. Most air compressors will list their cubic feet per minute rating on the side of the tank. Larger lines, such as pipes that are 3/4- to 1-inch in diameter, will generally require more air volume. Lines that are smaller, such as 1/2-inch lines, may need slightly less airflow. But a good rule of thumb is to provide no less than 20 cubic feet per minute to ensure as much water is blown out as possible.
Once you've established that your air compressor has enough volume to get the job done, you need to make sure it can handle that load over an extended amount of time. Winterizing a sprinkler line requires each zone to be blown out for at least 2 minutes. That means your compressor has to deliver a minimum of 20 cubic feet of air per minute for a continuous 2 minutes. Run your compressor at full capacity for 2 minutes. If the pressure meter drops significantly during that time, it probably will not handle the job.
The size of the tank on the air compressor directly impacts whether it can handle blowing out a sprinkler line. Because a compressor has to recharge a tank once it's nearly empty, it can take a long time for smaller tanks that are 1 to 5 gallons to recharge. Most professionals recommend using a compressor with a tank size of at least 10 gallons. That will ensure that the compressor motor doesn't have to work overtime to keep up.
There are two types of motors that run air compressors -- electric and gas. Electric motors have a limited range of operation because you must have a source of 120-volt electricity near the operating area. When blowing out a sprinkler line, there's usually a lot of water around. That makes using an electric compressor potentially dangerous due to the risk of electric shock. Gas-powered compressors, however, do not rely on extension cords to power the motor, making them great choices for use on wet sprinkler systems. Gas motors have the added ability to be moved anywhere along an irrigation system without bringing a separate power source along.