There are three basic system types: The most common is the parabolic system. In the parabolic system, pressure increases with the square of increasing volume. This means the pressure increases by a factor of four when the volume doubles. Hybrid systems are the second most often seen. In hybrid systems, there is a fixed pressure drop plus a parabolic pressure curve. This is seen in systems with pressure-controlled scrubbers or baghouses that automatically clean to a specified pressure drop. The last type of system is the constant volume system. The volume remains the same in these systems, but the pressure increases with time as the system becomes plugged. This type of system is found in cement kiln ID fans with preheaters and papermill recovery boiler ID fans.