Why Treat?

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 growths and sludge. These factors are important for another reason: they have a direct effect on equipment life.

SCALE

Deposition of scale is a chemical process due to the concentrations of dissolved salts in the cooling water exceeding their solubility limits and precipitate on to surfaces in contact with the water. The most common scale formers, calcium salt, exhibit reverse solubility in that they become less soluble as the temperature of the water increases. This property causes scale formation in the most sensitive area, the heat transfer surfaces of production equipment.
  Since the thermal conductivity of scale is substantially less than metal, heat removal is reduced and production speeds must be lowered to compensate. In extreme cases, enough material precipitates to physically block the cooling passages, resulting in the affected equipment being removed from production for either chemical (acid) or mechanical cleaning. Scale formation on the condensers of Freon cycle chillers reduces the efficiency of these units from chilled water systems, therefore increasing the power needed to obtain a given volume of chilled water.  Various studies have shown a non-linear electrical power cost increase with increased scale thickness, for instance 0.5 mils of calcium scale results in a power cost increase of 3.5%, while 1.5 mils increases power cost to approximately 12.5%.

CORROSION

Water, while an excellent transporter of heat and quite inexpensive, is also an excellent polar solvent which will dissolve in more or less time just about all known materials. Given this fact, the chemistry of all cooling water treatment programs must begin by addressing corrosion, which is basically an electrochemical oxidation process which results in destruction of the basic metals from which most cooling systems are constructed.

Uncontrolled corrosion is often responsible for physical equipment failure and the plugging of cooling water passages from deposition of corrosion products. More subtle effects, often not linked to corrosion, are loss of production speed and/or control and decreased energy efficiency from deposition of corrosion products on heat transfer surfaces where the deposit acts as an insulator to decrease thermal conductivity.

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 the slime and spore formers 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, resulting in structural weakness and eventual collapse of wooden tanks or towers.

SLUDGE

Under this heading is included dirt, mud, sand, silt, clay, scale salts, and other particulates of airborne origin or entering the system with the 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, of course, a comprehensive water treatment program comprising scale and corrosion inhibitors, microbiocides and dispersants coupled with adequate bleedoff and appropriate equipment.
Water is lost from a cooling tower system through evaporation, leaks in the system , overflow of the cooling tower sump, and "bleed", which is the intentional draining of a certain amount of water in order to decrease the buildup of solids in the system. In a 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 makeup water formula is:
Make-up water = water lost (through evaporation + bleed + leaks + windage)
When water is evaporated or lost from a cooling tower, the solids and chemicals used to treat the tower remain in the system. When water is "bled" from the system, the chemicals lost through bleed must be replaced for the system to remain protected.