The objective of all cooling water treatment is not to treat the cooling water -- it's to protect the equipment. Paul Puckorius begins a series on proper water treatment.

The objective of all cooling water treatments is not to treat the cooling water. You may say, "What?" This often is discussed -- even presented by chemical suppliers and some users -- as the primary objective of the water treatment chemicals program. It often is emphasized by the need to control the cooling water pH, conductivity, treatment residuals, hardness and alkalinities, etc. Yet these water qualities are guidelines to preventing problems on the cooling-water-contacted equipment.

As I have said in cooling water workshops during the past 20 years, the objective of all cooling water treatments is to protect the cooling-water-contacted equipment. The cooling water must carry those protective inhibitors to those surfaces requiring protection. Treating the water is not the main objective.

So you ask, what problems need to be controlled or eliminated? Just two, in most cases:

• That which interferes with heat transfer (or in cooling towers, heat rejection), commonly referred to as deposits.
  
• Deterioration of the water-contacted equipment, referred to as corrosion.

More recently, another possible concern is Legionnaires disease, mainly in cooling tower systems.

Although only two problems need to be controlled or eliminated, we often subdivide these problems into a number of categories:

Cooling Water Deposits

• Scales -- CaCO₃, Ca₃ (PO₄)₂ CaSO₄, SiO₂ and others.
  
• Foulants -- mud, silt, iron, manganese.
  
• Microbiological slimes -- algae, bacteria, fungus.

Cooling Water Corrosion

• Aluminum.
  
• Mild steel.
  
• Copper alloys.
  
• Stainless steels.
  
• Galvanized steels.
  
• Bimetallic couples.
  
• Microbiologically induced corrosion (MIC).

While each of these subcategories is a concern in some cooling systems, specific cooling water treatment needs depend upon the design of the cooling system you are treating.

Types of Cooling Water Treatment Needs

The three different types of cooling water systems -- cooling towers, closed cooling and once-through systems -- require different cooling water treatments (table 1).

Cooling Tower Systems. Often referred to as "open-recirculation" cooling syst ems, cooling tower water systems usually are plagued by the most problems; thus, they require the greatest amount of water treatment -- both quantity and variation. This is due to their being open to the atmosphere and, more specifically, that they are highly aerated water systems.

Figure 1 illustrates an open-recirculating cooling tower water system. It shows that air and water mix in the cooling tower, which means impurities in the air enter the cooling water. These include not only dust, dirt, gase s and microbiological organisms but also vegetation debris and trash. It also means that water is being evaporated, leaving higher levels of minerals in the cooling water, resulting in scale, foulants and corrosion. Thus, scaling, corrosion, fouling and m icrobiological growth --SCFM, for short -- all can occur in open-recirculation cooling tower systems. These systems operate by discharging some of the cooling tower system water -- called bleed-off or blowdown -- to rid the system of some of these contami nants.

In cooling tower s ystems, it is not economical to achieve 100 percent control these problems. Instead, water treatment should be sufficient to provide water-contacted equipment with satisfactory life expectancy -- usually five to 10 years, possibly longer. This goal usually results in about 80 percent to 90 percent protection from corrosion, deposits and biologicals. Water treatments required for this protection will be discussed in future issues.

Closed Cooling Systems. A closed cooling water system is shown in figure 2. This system is typical to an automobile cooling water system: There is no water evaporation and therefore no air water scrubbing, air contamination nor mineral concentration. These closed systems should have little or no water losses, so the cooling water will not cause scale deposits or foulants such as mud or silt. Therefore, the primary concern is corrosion control, and this must be 100 percent. Should corrosion control be less than 100 percent, where will the corrosion products go? Right! They stay within the system. They can cause iron oxide deposits, so 100 percent corrosion control is essential. Usually microbiological control is minimal except if the water treatment provides nutrient (sodium nitrite) that can create a microbiological problem.

Treatments for these types of systems will be discussed in future issues.

Links

• Water Works: Complete Column Archives
• Water Works:
  Cooling Water Treatment vs. Cooling Systems, Part 1
• Water Works:
  Cooling Water Treatment vs. Cooling Systems, Part 3
• Water Works:
  Cooling Water Treatment vs. Cooling Systems, Part 4
• Water Works:
  Cooling Water Treatment vs. Cooling Systems, Part 5
• Water Works:
  Cooling Water Treatment vs. Cooling Systems, Part 2
• Puckorius & Associates Inc.