Desiccant dryers reach very low dew points by adsorbing moisture onto a drying bed, then regenerate that bed. The difference between heatless and heated designs is how regeneration happens, which affects energy use and cost.
Heatless dryers
Heatless dryers regenerate the offline bed using a portion of the dried outlet air, typically 15–18% purge. They are simple, reliable and cheap to buy, with a fixed dew point, but the purge air represents a steady energy cost that grows with dryer size.
Heated dryers
Heated dryers use an electric or blower heater to regenerate the bed, reducing purge air to around 5–8%. They cost more to buy but use less compressed air, so they become economical at larger flows or where electricity is expensive.
Choosing between them
For small flows, the simplicity of a heatless dryer usually wins. For medium and large systems running continuously, a heated or blower-purge dryer lowers operating cost. Consider dew-point stability, serviceability and the cost of purge air in the lifetime calculation.