Different types of filters are designed to remove a variety of unwanted substances including dust, dirt, and moisture. Main air line filters remove contaminants in compressed air dryers and ensure clean air feeds into and out of dryers. Particulate pre-filters remove contaminants before compressed air enters the dryer. Filters installed at the exit of the dryer clean the dry, compressed air. Also available in membrane air dryers, coalescing filters further eliminate contaminants before drying occurs. Modular style air filters are specifically designed to work in unison with a regulator, or with a lubricator and regulator to form what is referred to as a FRL combo unit. Mist separators remove and separate water and oil from compressed air systems. They ensure only clean, dry air is present and remove contaminants that are too small for ordinary filters. Odor removal filters remove oily odors from compressed air in pneumatic systems. They are typically designed as high-efficiency filters that cover large areas