Significance of Water in Cooling Tower

Quality water plays a significant role in cooling water system. It is considered that cooling towers and its mechanical components represent the cooling system hardware and the water flowing through the cooling system is the system software as shown in figure 1. The cooling-water system normally has highly efficient pumps supplying both the process and the cooling system. Amount of flow to the process always related to the demand by pump sequencing with multiple pumps or using variable-speed drive controllers with pressure and flow rate as a control signal.

Figure 1 Flow of water in a cooling tower;Tangram Technology

As many researchers have agreed that cooling towers control temperature by rejecting heat from hot equipment or from air-conditioning systems. This is possible by the use of significant amounts of water.

Proper operation, thermal efficiency and longevity of the water cooling system rely solely on the quality of water and its reuse potential.In a cooling tower, water is lost throughbleed-off, drift and evaporation. In order to replace such amount of water lost and retain its cooling function, there is a need for more make-up water which must be added to the tower system. Occasionally water used for other equipment within a system can be reused and recycled for cooling tower make-up with little or no pre-treatment (Homel, 2007).

One of the most potential variables of cooling system is quality of water. The condition of water is often not considered as a potential variable over time. However, if cooling water is left unattended, itsupports biological growth, corrodes or scales equipment.  In the case of efficient process and operation, there must be proper treatment of water for cooling tower usage. An approach commonly employed to remove waste heat from the cooling tower is known as cooling loop. It allows water to evaporate to the environment or atmosphere. The evaporated water should be replaced always by fresh make-up water (Lee, 2005).

Heikkila and Milosavljevic (2001) stated that water play an important role in the operation and performance of cooling tower. In their report, it was described that in heat rejection in cooling tower is convectional transfer between water droplets and the air surrounding the environment. Also evaporation allows small portion of water to evaporate into the atmosphere.

Tyagi et al (2007) also revealed that in hostile weather conditions, the exhaust of the cooling tower remixes with the cooler ambient air and when it cools down the leftover moisture condenses in small fog droplets which creates visible plume.

In another development, (ASHRAE, 1996:Tyagi et al 2007) the plume can travel at few hundreds of meters which can led to visibility and darkness issues.

Furthermore, the operation of cooling tower system depends on water quality of the make-up source. Different sources have different challenges. Typical examples include of surface water sources include rivers, lakes and streams. Also groundwater sources are wells or aquifers. In fact, the location of surface water sources are a major factor because of it high level of suspended debris or silts that can cause fouling if not removed by pre-filtration systems.

In figure 2 represents a schematic water-side free cooling system which consist of two flows; the primary and condenser water flows. The cooling tower chills a flow of condenser water that is passed through a plate-and-frame heat exchanger. The free cooling heat exchanger, the condenser water from the cooling tower absorbs heat from the primary water flow.  Also, the chilled primary flow is used in the same way as the primary flow normally supplied by the chillers. Effectively, the water-side free cooling system permits the cooling towers to work as the building’s chillers. However, the cooling towers engaged atmospheric conditions, rather than a mechanically driven refrigeration cycle, as a heat sink to cool the primary water flow.

Figure 2 Water side flow systems

However, the groundwater sources do not have the seasonal variations which surface water sources have, but depending on the geology of the region. The groundwater can have high levels of dissolved minerals which pose as a problem to many cooling tower systems. The water reuse is one of the wise resources option, where consideration should be given to the quality of water and also how that will affect the efficient operation of the cooling tower system and the system ability to meet the required cooling demand.