Air flow generation methods

Regarding the intake of air through the tower, cooling towers are divided into three types:

• Natural ventilation take advantage of the buoyancy of tall chimneys.Warm moist air naturally rises due to density differences compared to dry, cool outside air.Warm moist air is less dense than dry air at the same pressure.This moist air buoyancy creates an upward airflow through the tower.

• Mechanical Ventilation Air is forced or drawn through the tower using an electric fan motor.

• Induced Draft A mechanical ventilation tower with a fan at the exhaust (top) that pulls air up through the tower.A fan directs hot, humid air out of the exhaust.This creates a lower intake air velocity and a higher exhaust air velocity, reducing the possibility of exhaust air being recirculated back into the intake.This fan/fin arrangement is also known as a drawout.Forced ventilation mechanical ventilation tower with blower at the inlet.Fans force air into the tower, creating high inlet velocity and low outlet velocity.Low outlet velocities are more susceptible to recirculation. Since the fan is located in the air intake, the fan is more prone to complications from freezing conditions.Another disadvantage is that forced draft designs typically require more motor power than equivalent induced draft designs.The benefit of the forced air design is its ability to operate at high static pressures. Such setups can be installed in tighter spaces and even in some indoor environments.This fan/fin geometry is also known as blow through.Fan Assisted Natural Ventilation A hybrid type that looks like a natural ventilation setup, although the airflow is fan assisted.

Hyperboloid cooling  tower 

On August 16, 1916,Frederik van Iterson was granted a British patent (108,863) for an improved construction of a reinforced concrete cooling tower.The patent was filed on August 9, 1917 and published on April 11, 1918.In 1918, DSM built the first hyperbolic natural draft cooling tower at Staatsmijn Emma, designed by Frederik van Iterson.Hyperbolic (sometimes erroneously called hyperbolic) cooling towers have become the design standard for all natural draft cooling towers because of their structural strength and minimal material usage.The hyperboloid shape also helps to accelerate the upward convective airflow, improving cooling efficiency.These designs are often associated with nuclear power plants.However, this association is misleading because large coal-fired power plants and some geothermal power plants often use the same cooling towers.Conversely, not all nuclear power plants have cooling towers, and some use lake, river or sea water to cool their heat exchangers.Hybrid cooling towers have a thermal efficiency of up to 92%.

Classified by air-water flow

Crossflow

Usually lower initial and long-term costs, mainly due to pumping requirements.Crossflow is a design in which the airflow is perpendicular to the flow of water (see photo at left).One or more vertical faces where the airflow enters the cooling tower meets the fill material. Water flows through the fill under the force of gravity (perpendicular to the air).The air continues through the filling and thus through the water flow into the open plenum.Finally, a fan blows the air into the atmosphere.The distribution or hot water basin consists of a deep pan with holes or nozzles in the bottom, located near the top of the crossflow tower.Gravity distributes the water evenly over the fill material through the nozzles. Cross flow V/s Counter flow

Advantages of Cross-Flow Design:

• Gravity water distribution allows for smaller pumps and maintenance.

• Non-pressurized spray simplifies variable flow.

• Disadvantages of cross-flow design:

• Freezes more easily than counterflow designs.

• Variable flow is useless in some situations.

• More prone to dust accumulation in the fill than counterflow designs, especially in dusty or sandy areas.

• Counterflow.

• In a counter-flow design, the air flow is in direct opposition to the water flow (see illustration at left).The airflow first enters the open area below the fill medium and is drawn vertically upwards.Water is ejected through pressurized nozzles near the top of the tower and flows down through the fill, opposing the air flow.

  Advantages of counterflow design:

• Water spray distribution makes the tower more frost resistant.

• Water splitting in the spray makes heat transfer more efficient.

Disadvantages of counterflow design:

• Usually higher initial and long-term costs, mainly due to pumping requirements.

• Difficult to use variable water flow as spray characteristics may be negatively affected.

• Typically noisier due to greater fall height from bottom of fill to cold water basin.