known as blow-through. Fan assisted natural draft — A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan. Hyperboloid cooling tower On 16 August 1916, Frederik van Iterson took out the UK patent (108,863) for Improved Construction of Cooling Towers of Reinforced Concrete. The patent was filed on 9 August 1917, and published on 11 April 1918. In 1918, DSM built the first hyperboloid natural-draft cooling tower at the Staatsmijn Emma, designed by Frederik van Iterson. Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency. These designs are popularly associated with nuclear power plants. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants and some geothermal plants as well. Conversely, not all nuclear power plants have cooling towers, and some instead cool their heat exchangers with lake, river or ocean water. Thermal efficiencies up to 92% have been observed in hybrid cooling towers. Categorization by air-to-water flow Crossflow Typically lower initial and long-term cost, mostly due to pump requirements. Crossflow is a design in which the airflow is directed perpendicular to the water flow (see diagram at left). Airflow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum volume. Lastly, a fan forces the air out into the atmosphere. A distribution or hot water basin consisting of a deep pan with holes or nozzles in its bottom is located near the top of a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material. Cross Flow V/s Counter Flow Advantages of the crossflow design: Gravity water distribution allows smaller pumps and maintenance while in use. Non-pressurized spray simplifies variable flow. Disadvantages of the crossflow design: More prone to freezing than counterflow designs. Variable flow is useless in some conditions. More prone to dirt buildup in the fill than counterflow designs, especially in dusty or sandy areas. Counterflow In a counterflow design, the air flow is directly opposite to the water flow