Water is used in cooling systems as a heat transfer medium and frequently also as the final point to reject heat into the atmosphere by evaporating inside cooling towers. Depending on the quality of available fresh water supply, waterside problems develop in cooling water systems from:
• Scaling
• Corrosion
• Dirt and dust accumulation
• Biological growth
Any of these problems – or more usually a combination of them – result in costly unscheduled downtime, reduced capacity, increased water usage, high operation and maintenance costs, expensive parts replacements, and acid cleaning operations which reduce the life of the cooling system.
There is no single method of treating cooling water. Selection of water treatment program for a specific system depends on:
1. System design, including system capacity, cooling tower type, basin depth, materials of construction, flow rates, heat transfer rates, temperature drop and associated accessories
2. Water, including make up water composition / quality, availability of pre-treatment and assumed cycle of concentration
3. Contaminants, including process leaks and airborne debris
4. Wastewater discharge restrictions
5. Surrounding environment and air quality
In this course, we will discuss the reasons and means for controlling scale, corrosion and biological fouling.
Critical Parameters
The critical parameters for cooling water are: conductivity, total dissolved solids (TDS), hardness, pH, alkalinity and saturation index.
Conductivity and Total Dissolved Solids (TDS)
Conductivity is a measure of the ability of water to conduct electrical current and it indicates the amount of the dissolved solids (TDS) in water. Pure distilled water will have a very low conductivity (low minerals) and sea water will have a high conductivity (high minerals). Dissolved solids present no problem with respect to the cooling capacity of water, since the evaporation rate of seawater, which has 30,000ppm total dissolved solids, is only 1% less than that of distilled water. The problem with dissolved solids is that many of the chemical compounds and elements in the water will combine to form highly insoluble mineral deposits on the heat transfer surfaces generally referred to as “scale”. The scale stubbornly sticks to the surfaces, gradually builds up and begins to interfere with pipe drainage, heat transfer and water pressure.
The primary maintenance objective in most circulating water systems is to minimize the formation of scale deposits and conductivity can be used as the controlling value after the TDS/conductivity relationship is determined.
pH
pH is a measure of how acidic/basic water is. The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is reported in "logarithmic units," like the Richter scale, which measures earthquakes. Each number represents a 10-fold change in the acidity/baseness of the water. Water with a pH of 5 is ten times more acidic than water having a pH of six.
Control of pH is critical for the majority of cooling water treatment programs. In general, when pH points to acidic environment, the chances for corrosion increase and when pH points to alkaline environment, the chances for scale formation increase.
Alkalinity
The pH values above 7 signify alkalinity. At pH values less than 8.3, most of the alkalinity in the water is in the bicarbonate form, and scale formation is normally not a problem. However, when
the pH rises above 8.3, the alkalinity converts from the bicarbonate to the carbonate and the scale will start to form.
Hardness
The amount of dissolved calcium and magnesium in water determines its "hardness." The total hardness is then broken down into two categories
a. The carbonate or temporary hardness
b. The non-carbonate or permanent hardness
Hardness particularly the temporary hardness is the most common and is responsible for the deposition of calcium carbonate scale in pipes and equipment. Technically any bivalent metal ion such as iron, manganese or tin would constitute hardness, but calcium and magnesium are the two most prevalent forms.
Saturation Index
The saturation index of a water or Langlier Saturation Index (LSI) is a measure of the stability of the water with respect to scale formation. When LSI readings are positive they tend to be scale forming, and when they are negative they tend to be corrosive. Normally readings within 1.0 units from zero are considered stable.
