Direct Expansion (refrigerated non-cycling) - Cool compressed air by mechanical refrigeration. Most common way to dry air. A moisture separator removes water vapor. Most use outgoing cold air to pre-cool incoming air before it reaches the refrigeration cycle.

Thermal Mass (refrigerated cycling) - Uses a refrigeration circuit to cool a thermal mass, which cools the compressed air. Cycles on only when the mass requires cooling. More economical to operate than direct expansion dryers.

Desiccant Dryers - Most effective in removing moisture from compressed air. Absorb water molecules from the compressed air stream onto the surface of the desiccant. Designed for continuous operation by automatically cycling air between two desiccant beds. Typically used when freeze-ups must be prevented and in critical processing applications.

Heatless or Pressure Swing - Use a portion of the dry air to purge moisture from the regenerating bed. Cost is higher due to purge air being vented to the atmosphere. Few moving parts keep maintenance costs low. An electric timer commonly controls the cycling airflow. Short-interval cycling results in stable dewpoints.