Cooling Towers four major problems

In today's world of expensive energy, it is more vital than ever for heat exchange equipment to be kept free of insulating deposits that promote high-energy consumption.
The four principal sources of these deposits in the case of water-cooled systems are scale, corrosion, biological growth and sludge.

These factors are important for another reason, they have direct effect on equipment life.

1- Scale

Caused by the precipitation of calcium and other salts of limited solubility, scale, in addition to its high insulation value, progressively narrow pipe internal diameters and roughens tube surfaces, thereby impending proper flow of water.

In compression refrigeration systems, scale translates into higher head pressure, hence an increase in power requirements and cost. For example, 1/8" of scale in a 100-ton refrigeration unit represents an increase of 22% in electrical energy compared to the same size unit free of scale. Similarly, in absorption systems, scale creates a higher back-pressure on the concentrator, increasing energy expenditure. While scale formation proceeds more rapidly in open recirculating systems owing to the concentration effect on evaporation, once-through systems are not exempt from scaling if high temperatures are combined with silt and iron.

2- Corrosion

Dissolved gases, improper pH control or formation of differential aeration cells under deposits causes corrosion to be undoubtedly the single most significant factor leading to the premature deterioration of HVAC equipment and piping. In its most insidious form, piping is only revealed when the equipment fails. Failures of this type can be catastrophic, leading to costly downtime for repairs and equipment replacement, personal discomfort and even total plant shutdown.

3- Biological Growths

Apart from their well known role in promoting corrosion, through the formation of acid metabolic products or through the mechanism of concentration cells associated with deposits of all kinds, the uncontrolled multiplication of bacteria, algae and fungi, results in bio-film formation on heat exchange surfaces and has now been recognized as an important contributor to impaired heat transfer efficiency in cooling water systems.

In fact, it has been unequivocally demonstrated that because of the unique surface characteristics of bio-films, their hydrodynamic and insulating properties far exceed those of an equivalent thickness of scale or corrosion deposits.

Of particular concern are slime and spore former which are difficult to control because of the protection afforded by the polysaccharide sheaths that they secrete and the organisms that metabolize either cellulose or lignin, results in structural weakness and eventual collapse of wooden tanks or towers.

4- Sludge

Under this heading is included dirt, mud, sand, silt, clay, scale salt, and other particulate of airborne origin or entering the system with makeup water. Very often these suspended solids are tightly bound and cemented by corrosion products and organic matter.

Where abrasive, sludge deposits can damage pump seals and in addition to their insulating nature can also promote "under-deposit" corrosion.

The answer to the aforementioned problems created by scale, corrosion, bio-fouling and sludge is, course, a comprehensive water treatment program comprising scale and corrosion inhibitors, microbiocides and dispersants coupled with adequate bleed-off and appropriate equipment.

Water is lost from cooling tower system through evaporation, leaks in the system, overflow of cooling tower sump, and "bleed", which is the draining of a certain amount of water in order to decrease the buildup of solids in the system. In poorly designed system, high winds can blow some of the water out onto surrounding areas. "make-up" water is the water brought into the system to replace lost water, regardless of the cause of water loss.

The make-up water formula is:

Make-up water = water lost (evaporating + bleed + leaks + windage). When water is evaporated or lost from a cooling tower, the solids and the chemicals used to treat the tower remains in the system. When water is "bled" from the system, the chemicals lost through bleed must be replaced for the system to remain protected.