What characterizes a natural draft cooling tower?

A natural draft cooling tower is characterized by its reliance on natural convection to circulate air. This process occurs when the warmer air inside the tower rises, creating an upward flow that draws in cooler, denser air from the surrounding environment at the base of the tower. This natural air movement is facilitated by the design and height of the cooling tower, which allows for efficient heat exchange without the need for mechanical fans.

Natural draft cooling towers are typically taller structures that take advantage of the temperature differences between the air and water. As the water is pumped to the top of the tower and allowed to flow downwards over fill materials, it evaporates partially, which cools the remaining water and causes the air at the top to rise.

Other options describe systems that operate on different principles. For instance, using mechanical fans characterizes forced draft cooling towers, which actively move air using electric fans. The operation under vacuum pressure generally relates to systems that require sealed environments, such as certain refrigeration systems, and cooling via chemical reactions pertains to methods that are fundamentally different from the heat exchange processes in a cooling tower. Thus, these distinctions clarify why the reliance on natural convection is a defining feature of a natural draft cooling tower.