Indirect dry cooling is also known as "Heller System"
Indirect dry cooling means, air is used as a secondary cooling medium.
Water is still needed as primary coolant, however the cooling water flows in a closed cycle and is never in contact with the cooling air. The heat transfer between air and cooling water is achieved by convection and not by evaporation like in a wet cooling tower.
The Heller system does not need any make up cooling water and does not generate plume. It means that, like the ACC it is a "dry" system. Cooling water from the cooling tower flows back to the turbine condenser to condense steam from the turbine exhaust.
If a Direct Contact jet condenser is used, the mixed cooling water and condensate are then extracted from the bottom (hot well) of the condenser by two 50% duty circulated water pumps.
The major part of the water flow is returned to the tower for cooling. The cooling deltas (water to air fin tube heat exchangers) are arranged inside or outside the cooling tower and are divided into parallel sectors These deltas dissipate the heat from the cycle. These type of systems are used for capacities of over 60 MW.