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.
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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.
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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.