Cooling Tower and It's Energy Efficiency

Basic Principles of Cooling Tower and It's Energy Efficiency

Principles of Cooling Tower: 

Water-cooled chillers are used in conjunction with cooling towers. Cooling towers are not required for air-cooled chillers. A cooling tower rejects the heat collected from the building as well as the chiller's compression work. In the HVAC industry, there are two types: induced draught and forced draught.

To draw air counterflow to the water, induced draught towers include a huge propeller fan at the top of the tower (discharge end). For the same capacity as forced draught towers, they require much smaller fan motors. Induced draught towers are thought to be less prone to recirculation, which can lead to decreased performance.

Fans on the air inlet of forced draught towers push air either counterflow or crossflow to the movement of the water. Forward curved fans are frequently used. They require more fan power than induced draught systems but can supply external static pressure when necessary. This is significant if the cooling tower requires ducting, a discharge cap, or another piece of equipment that causes a pressure drop.

Trays or nozzles distribute condenser water throughout the tower. The water flows over the fill within the tower, increasing the air-to-water surface contact area significantly. The water is collected in a sump, which can be built inside or separate from the tower. The latter is preferred in cold climates where condenser water can be stored indoors. 

Single or many cells can be found in either tower type. Isolation valves can be used to separate the sections of cells that are headered together on both the supply and return sides. This method enables the addition of more cells as more chillers are activated, or the utilization of more tower surface area by a single chiller to reduce fan effort.

Standard Operating Conditions

Cooling towers are rated by the Cooling Tower Institute (CTI) at 78°F ambient wet bulb, 85°F supply water temperature, and a 10°F range. Because it is typical (but not required) to have a temperature range of 10°F, the cooling tower flow rate will be 3.0 GPM/ton, which is higher than the chilled water flow rate of 2.4 GPM/ton. 

The increased condenser water flow rate is required to meet the heat generated by the compression activity. Cooling towers are extremely adaptable, with applications spanning a wide range of techniques, ranges, flows, and wet bulb temperatures. Low wet-bulb temperatures can provide lower condenser water temperatures in many climes, improving chiller performance.

The Cooling Tower Method

Cooling towers expose condenser water directly to the ambient air in a waterfall-like process. The process can cool condenser water below the ambient dry bulb temperature.

A combination of sensible and latent cooling is used to cool the water. A portion of the water evaporates, resulting in latent cooling. The cooling tower process is depicted on a psychrometric chart at ARI conditions in the example on page 18. As the temperature of the wet-bulb falls, cooling towers rely more on sensible cooling and less on latent cooling.

Low-temperature air can hold very little moisture, resulting in large plumes. In some cases, winter tower selection necessitates a larger tower than summer conditions.

When selecting cooling towers for winter use, extra care should be taken.

Water Treatment

Condenser water has all the proper ingredients for biological growth; its heat, exposure to air and provides surfaces to grow on. additionally, the constant water loss makes water treatment even a lot of difficult. each chemical and ozone-based treatment systems square measure used. an intensive discussion on the topic of water treatment is on the far side of the scope of this Guide however it suffices to mention, that it's necessary to provide the correct operation of each tower and therefore the hair-raiser.

 Energy Efficiency and Cooling Towers

Cooling towers use electricity to run their fans. Induced draught towers should be used since they typically require half the fan horsepower force that forced draught towers to do. Piggyback motors, multi-speed motors, or Variable Speed Drives are also recommended for fan speed control (VFDs).

Furthermore, to make use of the varying speeds, the sensible control logic is required.

For heat rejection devices, ASHRAE 90.1-2001 specifies the following:

Fan speed control is required for any fan motor with a power rating of 7 12 hp or higher. The fan must be able to run at two-thirds speed or less and have the required controls to change the speed automatically. (6.3.5.2)

Exclude: 

Multiple refrigeration circuits are served by condenser fans.

Condenser fans that serve flooded condensers

Installations in climates with a CDD50 greater than 7200.

Up to one-third of the fans on a condenser or tower with numerous fans, provided that the lead fans meet the speed control criteria.

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