Manual twist drain to to release built up liquid and debris from the system. located on the bottom of the bowl Compressed air filters protect your equipment from dust, dirt, oil and water. Dust will wear down your equipment (like the air motor in grinders, pneumatic actuators and it can make pneumatic control valves get stuck. Especially when the dirt and dust is combined with oil (which is often the case with compressors), a sticky mess can form inside valves, motors and other compressed air equipment.