Cooling Towers

Cooling towers are used to reject heat from buildings or processes. Large residential buildings require cooling towers to reject heat. The heat is generated by electrical equipment inside the buildings, the generation of heat by humans, and heat generated through solar gain, heating the building by the sun. but the residential buildings are not only the places that must be heat rejected.

Large power stations use cooling towers to reject heat and almost all modern industrial processes require some means of rejecting heat.

In the past cooling could be provided by heat sinks, such as rivers, lakes, or cooling ponds. But these options are now less viable due to the tightening of environmental pollution legislation and rising cooling demands.

Modern cooling towers require as little as one square foot of area to obtain the same cooling effect as 1000 square feet of a natural heat sink. Additionally, water in cooling towers is recirculated which drastically reduces operating costs which reduce operating costs compared once-through cooling system.

This typical cooling tower arrangement shows water being taken from the cooling tower to a condenser, then back to the cooling tower. The cooling water temperature increases as it passes through the condenser and reduces as it passes through a cooling tower. The cycle repeats continuously.

Cooling towers
Cooling towers

Components of the Cooling Towers:

  1. Distribution system: The system where the hot water distributes over the heat exchanger.
  2. Spray nozzle: Spray nozzle from a part of the distribution system.
  3. Drain: The drain is used for draining the water from the system. The drain is the lowest point of discharge within the cooling tower basin and usually the point of blowdown.
  4. Drift eliminator: The drift eliminator consists of the parallel blades arranged on the air discharge side of the tower to remove and drain water droplets from exiting the air stream. The shape of the drift eliminator requires the entrained water droplets to change direction several times prior to being discharged out of the tower. Air passes through the drift eliminator unhindered but water droplets impinge upon the drift eliminator condense then drip back down onto the fill and into the cooling tower basin. Drift eliminators reduce water losses considerably, as much of the evaporated water can be reclaimed.
  5. Fans: Fans are used to force or induce airflow through the cooling tower and are available in different designs. The three most common fan designs are the axial, centrifugal, and cross-flow types. Fans are chosen for a particular application based on certain characteristics such as desired volumetric flow rate, the pressure differential across the fan, and the space available.
  • Large or industrial cooling towers use axial fans or no fans.
  • Small or medium size cooling towers use axial fans or centrifugal fans.
  • Small air conditioning units use axial, centrifugal, or cross-flow fans.
  1. Fill: The fill is essentially a heat exchanger. Its purpose is to increase the contact surface area between the water and air. A large contact surface area yields a much greater heat transfer capacity. The spray nozzle distributes water evenly across the entire fill. This ensures the fill is always operating at maximum efficiency.
  2. Louvres: The louvres from an arrangement of horizontal blades at the air inlet that prevent the escape of failing water while allowing free entry of ambient air. Louvres also reduce the likelihood of wildlife accessing the internals of the tower.
  3. Makeup valve: It is necessary to add water to the system to replace water losses. Water added to the cooling water system is known as makeup water. A makeup valve is a simple float lever mechanism used to control the amount of water that enters the tower.

When the water level in the basin is high, the valve is closed.

When the water level in the basin is low, the valve is open.

The makeup valve is a proportional response device and will open and close proportionally depending upon the water basin level.

  1. Overflow: The overflow is a drain that prevents the collecting basin from overflowing. If the water level in the basin is too high the water will enter the pipe and drain away. This prevents an overflow situation.

How Cooling Towers works:

The incoming warm water enters the distribution’s manifold and discharges through spray nozzles inside the tower. The spray nozzles spray the warm water evenly over the entire fill. The water passes downward through the fill while air passes upwards. As the water travel through the fill, some of it evaporates which causes the remaining water to be cooled. The cooling water that did not evaporate falls due to the collection basin due to the gravity while the air continues its path upwards through the drift eliminator and axial fan. Then exit through the top of the tower.

 

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