The majority of ventless dryers sold in the U.S. are condenser dryers, which don't exhaust air. Instead, they use a dual loop airflow system that’s much more efficient. The first airflow loop draws a small amount of ambient air into the dryer—much less than is needed in a conventional (vented) unit. The air passes through the condenser for initial heating. The heated air is then pushed into the drum, where it heats up the wet laundry and causes water to evaporate.
Instead of venting that hot, wet air outside, the air is looped back into the condenser where it's cooled down—that's the second airflow loop. From there, the air that’s already inside the condenser is reheated and sent back into the drum to repeat the process until the clothes are dry. The evaporated water either goes down a drain pipe or collects in a tray that the user must empty after a cycle.