Cooling Towers, evaporative condensers and fluid coolers are a specialized heat exchanger in which air and water are brought into direct contact with each other in order to reduce the water's temperature. The warm process water enters the cooling tower from the top through spray nozzles, which is referred to as the distribution system. The water then enters the fill medium where air is also introduced by the cooling tower fans. The fill provides a greater surface area for the flowing tower water. This allows for maximum contact between the air and the water, which allows for greater evaporation rates. This evaporation removes heat from the remaining water, which is collected in the cold water basin and returned to the system to absorb more heat. It is through this process that heat is removed from the building through the evaporation of the water.
Water Issues with Cooling Towers?
Corrosion of cooling towers and condenser system components such as circulating pumps, condenser tubes and tube sheets can become very costly in terms of service disruption, loss of production, increased maintenance and capital equipment replacement. The most common forms of corrosion found in cooling water equipment are;
General Corrosion – overall deterioration of the metal surface, with an accumulation of rust and corrosion products in the piping and sump. On copper condenser tubes it is observed as a surface gouging or a uniform thinning. Generally this is a uniform loss of metal, not localized.
Oxygen Pitting – the second type of corrosion most frequently encountered in heat transfer equipment is pitting. Pitting is characterized by deep penetration of the metal at a small area on the surface with no apparent attack over the entire surface as in general corrosion. Oxygen pitting is caused by the presence of oxygen in the water. Corrosion takes place at a particular location on the metal surface, and corrosion products frequently accumulate over the pit, forming a blister or tubercule. Cooling towers are continually made up with make-up water to compensate for the system water losses due to evaporation. The make-up water is saturated with corrosive oxygen at all times during tower evaporation and therefore creates conditions that are ideal for corrosion.
Galvanic Corrosion – occurs when different metals come in contact with one another in water. When this happens an electric current is generated similar to that of a storage battery. The more active metal (ie. Iron) will tend to dissolve in the water, thereby generating an electric current (an electron flow) from the less active metal (ie. Copper). This can be a common occurrence in the tube sheet in cooling tower condensers due to the copper condenser tubes in contact with the ferrous iron end plate.
Concentration Cell Corrosion – is also a form of pitting corrosion that is localized, but it is frequently called “under deposit corrosion” or “crevice corrosion” since it occurs under deposits or crevices of a metal joint. Deposits of foreign matter, dirt, organic matter, corrosion products, scale or any substance on a metal surface can initiate a corrosion reaction as a result of differences in the environment over the metal surface. Maintaining clean surfaces can best prevent this form of corrosion so that the treatment inhibitors can provide the required protection.
Microbiologically Influenced Corrosion (MIC) – is a form of corrosion caused or influenced by microbiological organisms or organic growths on metals. There are many forms and mechanisms of MIC involving many types of microbiological organisms. The basic causes of MIC found in recirculating cooling water systems are;
- Iron Related Bacteria (IRB) – this class of organisms are responsible for causing corrosion of iron and steel by direct metabolism of iron, by consuming iron in their metabolic process and then depositing it in the form of hydrated ferric hydroxide. (Gallionella ferrugine, Ferrobacilus sp.). Commonly found in cooling water system, particularly in low flow areas.
- Sulfate Reducing Bacteria (SRB) – the best know form of MIC bacteria. These are anaerobic (live without directly metabolizing with oxygen) and they metabolize sulfur in one form or another. (Desulfovibrio, Desulfotomaculum, Desulfomonas).
- Acid Producing Bacteria (APB) – slime forming bacteria which exude various organic acids in their metabolic process. These organic acids (formic, acetic & oxalic acid) cause low pH conditions at local sites, resulting in corrosion at these sites. (Pseudomonos, Aerobacter, Bacillus). Another APB responsible for MIC is Thiobacillus which oxidize sulfur compounds forming sulfuric acid, which is extremely corrosive and leads to localized under-deposit and pitting corrosion, often resulting in pinholes in pipework.
- Biological Deposits (BD) – MIC can also be caused by other forms of organic growths such as algae, yeast, molds and fungus along with bacterial slimes. BD act as traps and food for other organisms, resulting in rapid growth. This complex sets up a corrosion potential between adjacent areas of a metal surface.
Through the process of evaporation of the cooling water, minerals (ie. calcium, magnesium, iron, silica) in the make-up water are continually cycled higher. When mineral concentration exceeds the solubility point, these minerals can precipitate out of solution and promote scale deposits, such as calcium carbonate. The formation of scale deposits can result in a loss of cooling transfer efficiency, translating in increased cooling costs. The heat energy transferred from one surface to another surface is termed as thermal conductivity. Heat transfer occurs at a lower rate across materials of low thermal conductivity than across materials of high thermal conductivity. As seen in the graph below, the thermal conductivity of calcium carbonate scale is very low compared to the metallurgy in cooling tower systems. The energy required to transfer the same heat across any mineral scale formation such as calcium carbonate will greatly increase. The result is greatly increased energy costs as noted in the following table. Scale in cooling water systems can include foreign substances such as corrosion products, organic matter, and mud or dirt. Treatment programs are required to inhibit and control scale crystal formation. Cooling water systems also require sufficient dilution of the recirculating water (bleedoff) to prevent the concentration of minerals from approaching the Saturation Point.
Cooling towers have optimum temperature and sunlight exposure to trap a variety of life forms and nutrient sources, causing a perfect breeding ground for biological fouling such as algae, fungi and bacteria. Some of these forms circulate throughout the condenser system, while others attach themselves to convenient surfaces. Corrosion is frequently found beneath these deposits. The organisms that grow in such systems consist primarily of algae, fungi and bacterial slimes.
Algae are the most primitive form of plant life and together with fungus form the family of thallus plants. The form of algae found in open recirculating systems are the blue-green algae, green algae, and brown algae. Large masses of algae can cause serious problems by blocking the air in cooling towers, plugging water distribution piping and screens, and accelerating corrosion by concentration cell corrosion and pitting. Algae must be removed physically before a system can be cleaned since the mass will provide a continuous source of material for reproduction and biocides will be consumed only at the surface of the mass, leaving the interior alive for further growth.
Fungi are also a thallus plant similar to the unicellular and multicellular algae. They require air, water and carbohydrates for growth. Fungi and algae can grow together; the algae living within the fungus mass are furnished with a moist, protected environment, while the fungus obtains carbohydrates from the algae.
Bacteria are microscopic unicellular living organisms that exhibit both plant and animal characteristics. They exist in rod-shaped, spiral and spherical forms. There are many thousands of strains of bacteria, and all recirculating waters contain some bacteria. The troublesome ones, however, are bacterial slimes, iron bacteria, sulfate reducing bacteria, and pathogenic bacteria.
Cooling towers have been found to provide ideal breeding conditions for pathogenic bacteria such as Legionella pneumophila. Legionnaires’ Disease is a potentially fatal form of pneumonia and is thought to be transmitted to humans via airborne water deposits. PACE Chemicals Ltd. has introduced a Legionella Awareness and Risk Management maintenance plan with our Policy Guideline LEG 2015-07. The purpose of this guideline is to provide information and guidance to our customers to minimize the potential for Legionella contamination in their recirculating cooling water systems. Consult with your PACE representative for our Policy Guideline LEG 2015-07 and become familiar with this recommended cooling tower maintenance plan.
Bacterial Fouling Control
Biological fouling in cooling tower systems can be controlled by developing a water treatment program that includes the addition of Biocides. Biocides can include oxidizing and non-oxidizing type treatment products. To provide effective control, regular tests should be performed to maintain the Total Aerobic Bacterial count <10,000 CFU/ml with dipslide field tests. This can be monitored with a combination TTC/Malt Agar dipslide that tests for both aerobic bacteria and yeasts & mold.
PACE Chemicals Ltd. Treatment Programs for Cooling Towers
PACE Chemicals Ltd. has developed a comprehensive treatment program to inhibit:
- Scale Deposition
- Biological Fouling
Corrosion & Scale Inhibitors
PACE Chemicals Ltd. SCALE PRO cooling tower products are complex corrosion/scale inhibition formulations that provide;
- The industry’s most powerful mild steel corrosion inhibitor for all-organic cooling water formulations, providing a significantly better performance compared to commodity phosphonate programs, particularly in corrosive soft water conditions.
- Extensive synergistic phosphonates to provide superior scale inhibiting performance by increasing the solubility of mineral salts.
- High concentration of co-polymer – phosphonate ratio for superior calcium carbonate scale inhibition, even in moderately stressed scaling water conditions.
- Most resistant inhibitor to degradation from oxidizing biocide treatment.
- Soft metal corrosion inhibition.
- Very cost effective due to the highly concentrated formula.
- Higher biodegradability and better environmental profile versus other available programs.
Biological Fouling Inhibitors
Recommend dosing cooling towers with two biocides (oxidizing & non-oxidizing) with an automated dosing system proportional to the system volume to inhibit biological fouling,
BromMax 7.1 is a stabilized oxidizing bromine biocide that provides:
- Powerful biofilm removal properties with single feed, pre-activated solution.
- Effective algestat and algaecide.
- Enhanced stability over sodium hypochlorite.
- Effective at high pH ranges (>9).
- Shown to inactivate Legionella pneumophila bacteria (both planktonic and sessile)
MICROCIDE CA-39 or MICROCIDE WT-640 biocides which will provide;
- Non-oxidizing broad-spectrum cationic based biocide to inhibit Algae, Pseudomonas sp. Bacteria and Sulfate Reducing Bacteria.
- Effective algaecide and bactericide over a wide pH range.
- Better fungal control compared to other quaternary ammonium compounds.
- Less tendency to foam and relatively safe to handle with minimal risk of skin irritation.
Consult with your PACE representative for more information on new construction or improving your current water treatment. Email us at firstname.lastname@example.org if you would like a survey of your cooling water system.
Refer to our Cooling Tower technical bulletins below for more information regarding cooling tower treatment programs.