What is the main difference between open and closed circuit cooling towers?

The difference between the two systems lies in the different type of circuit exploited to cool the process fluid.

• Open-circuit cooling tower: water flows downward through a fill material, distributing itself in a thin film. Fans inside the tower draw in cold air from outside, which flows through the fill material, in contact with the “hot” water. A very small part of the water evaporates, absorbing heat from the air, while the remaining part cools and is collected in a basin for reuse. Video of open-circuit evaporative tower operation.
• Closed-circuit cooling tower: the “hot” water from the industrial process does not come into direct contact with the air, but instead circulates in a closed circuit within the tower. A secondary fluid flows in a separate circuit, adjacent to the primary circuit: this is “sprayed” onto the smooth-tube primary circuit and removes the heat of the liquid contained therein. Contact between the air, drawn by the fans, and the sprayed water causes the latter to cool in turn. Video of closed-loop evaporative tower operation.