Wood continues to be widely used for the construction of cooling towers. Wood deterioration can shorten the life of a cooling tower from an anticipated 20-25 years to 10 years or less. Cooling tower operation becomes inefficient and repair and replacement costs are excessive.
In the past, redwood was selected for use in cooling towers because of its high strength-to-weight ratio, availability, ease of use, low cost, and natural resistance to decay. Pressure-treated Douglas fir and similar types of wood are replacing redwood due to cost and availability factors.
Wood is composed of three main components: cellulose, lignin, and natural extractives. Long fibers of cellulose give wood its strength. Lignin acts as the cementing agent for the cellulose. The extractives contain most of the natural compounds that enable wood to resist decay. Normally, highly colored woods are most durable.
The extractives in all woods are largely water-soluble, so circulating water leaches them from the wood. Although this leaching process does not appear to affect the strength of the wood, the loss of extractives makes the wood more susceptible to decay.
Cooling tower wood experiences three main types of deterioration: chemical, biological, and physical. Rarely is one present without another; usually, all three occur simultaneously. Sometimes it is difficult to determine which type of attack is the most responsible for the deterioration. Physical and chemical deterioration, which are more visible, render the wood more susceptible to biological attack.
Chemical Attack
Chemical deterioration of cooling tower wood commonly manifests itself in the form of delignification. Delignification is usually caused by oxidizing agents and alkaline materials. Because chemical attack removes the lignin component of wood, the residue is rich in cellulose. The deterioration is particularly severe when high chlorine residuals (more than 1 ppm free chlorine) and high alkalinity concentrations (pH more than 8) occur simultaneously.
Wood that has suffered chemical attack takes on a white or bleached appearance and its surface is fibrillated. Damage is restricted to the surface of the wood and does not impair the strength of unaffected areas. When cascading water has a chance to wash away surface fibers, the wood becomes thinned. In serious cases, the loosened fibers plug screens and tubes and serve as focal points for corrosion when fibers accumulate in heat exchange equipment.
Chemical attack occurs most frequently in the fill section and wetted portions of the tower where water contact is continuous. It also occurs where alternately wet and dry conditions develop, such as on air intake louvers and other exterior surfaces, and in the warm, moist areas of the plenum chamber of the tower. Deterioration occurs as a result of chlorine vapors and the entrainment of droplets of tower water.
Biological Attack
The organisms that attack cooling tower wood are those that can use cellulose as their source of carbon for growth and development. These organisms degrade cellulose by secreting enzymes that convert the cellulose into compounds that they absorb. This attack depletes the cellulose content of the wood and leaves a residue rich in lignin. Characteristically, the wood becomes dark in color, loses much of its strength, and may also become soft, punky, cross-checked, or fibrillated.
Principal cellulolytic organisms that have been isolated from cooling tower wood are fungi that include the classic wood destroyers (Basidiomycetes) and members of Fungi imperfecti. Bacterial organisms that exhibit cellulolytic properties have also been isolated, but their exact role in cooling tower wood deterioration has not been determined. Wood-destroying organisms are common air- and water-borne contaminants.
Moisture and temperature, as well as oxygen, have a marked influence on organism development. When the moisture content of the wood is between 20 and 27% and the temperature is between 88 and 105°F, organisms usually achieve optimum growth and development.Biological attack of cooling tower wood is of two basic types: soft or surface rot and internal decay.
Soft or Surface Rot. Soft or surface rot occurs predominantly in the flooded sections and plenum areas of the tower. Enough oxygen reaches the wood surfaces in flooded portions to support growth. Surface rot is more readily detected and less serious than internal decay.
Internal Decay. Classic internal decay, the worse of the two types of biological attack, is generally restricted to the plenum area, cell partitions, access doors, drift eliminators, decks, fan housing, and supports. Because the decay is internal, it is difficult to detect in its early stages. Even areas affected by severe decay can have a sound external appearance.
Internal decay is rarely found in flooded portions of the tower, such as the fill section, where the wood is completely saturated with water. The water excludes oxygen, which the wood-attacking organisms need for their growth and development.
Physical and Other Factors
In addition to supporting biological growth, high temperature can greatly affect the wood integrity. Continuous exposure to high water temperatures (140°F or higher) causes significant changes in structure and accelerates loss in wood substance. This weakens the wood and predisposes it to biological attack, particularly in the plenum areas of a cooling tower.Other factors also influence the deterioration of tower wood. Areas adjacent to iron nails and other iron hardware are susceptible to deterioration. Slime and algae growths and dust and oil deposition support the growth of soft rot organisms. Affected areas can lose much of their strength and crumble easily.
Preferential erosion of spring wood is relatively common in tower fill. In severe cases, significant losses occur in a very short time. Extremely high concentrations of cooling water dissolved solids should be prevented where tower wood areas are exposed to alternate wetting and drying. Although salts show little tendency to attack wood, their crystallization in dry areas can rupture wood cells.
Preventive Maintenance
Preventive maintenance is the only effective method of protecting cooling towers from deterioration. Prevention is relatively easy in flooded sections of the tower, where chemical and biological attack is limited to wood surfaces. Preventive measures for the nonflooded portions of the tower, where internal decay is the primary concern, are more difficult. To ensure the success of a program, it is important to adopt appro-priate measures before infection reaches serious proportions.
Flooded Sections. Control of chemical and bio-logical surface attack in flooded portions of a cooling tower may be accomplished through a water treatment program, which should include the use of nonoxidizing antimicrobials to control slime and prevent biological surface attack.
When chlorine is used to minimize chemical attack, it should be closely controlled. Free residuals should be restricted to less than 1 ppm, preferably to a range of 0.3-0.7 ppm.
Chlorine should be supplemented by nonoxidizing antimicrobials to control biological surface attack. When a combination program is possible, chemical attack can be held to a minimum and biological degradation controlled effectively.
Nonflooded Sections. Although soft rot or surface attack may occasionally occur in the non-flooded portions of a tower, loss of wood structure is not as severe as it is in the flooded areas because wood is not eroded by cascading water.
Internal decay is the principal and most serious problem in nonflooded areas. Cooling towers should be inspected thoroughly at least once a year. When internal decay occurs only as white pocket rot, the affected areas can be very small and easily missed. Because internal decay usually remains undetected until extensive damage has occurred, it is important to look for signs of internal decay in structural members. Sometimes, such decay is evidenced by abnormal sagging or settling of the tower wood. At other times, it is necessary to test for soundness with a blunt probe. Unexpected softness in an apparently healthy wood beam is a sign of decay. When decay is not evident, samples of wood should be examined microscopically to detect the presence of internal fungi.
Infected wood must be replaced to retard the spread of infection to adjacent, structurally sound members. A weakened section shifts additional weight to sound sections, causing them to crack and become more susceptible to the spread of internal decay. Infected wood should be replaced with pretreated wood.
Several different wood preservatives are available, including:
- creosote
- ammoniacal copper arsenite
- acid copper chromate and copper naphthenate
- chromated copper arsenate
- pentachlorophenol
- fluoride chromate arsenate phenol
- chlorinated paraffin
Periodic spraying with an antifungal is an effective preventive maintenance step if performed on a regular basis. However, diffusion of antifungal typically penetrates the wood to a depth of in. or less (even when wood is incised and pressure-treated). The protection provided by spray application of antimicrobials is temporary, especially where wood surfaces are contacted by flowing water, and the protective barrier formed by the antimicrobials is easily breached by cracking of the wood.
Examination
Cooling tower inspections, conducted on a regular basis, should always include collection of tower wood samples for examination in the laboratory. Because several types of wood deterioration are likely to occur, various laboratory examinations should be employed.
Macroscopic examination of wood reveals the degree of erosion, surface structure, and depth of surface attack. Macroscopic study can also be used to assess physical aspects of the wood and to determine the presence and extent of chemical and biological decay. Breaking of specimens reveals the degree to which the wood is brash and is useful in assessing loss of structural strength.
Microscopic examination is useful for determining the extent of microbiological deterioration. A Microtome can be used to prepare thin sections of wood. These sections, usually 25 µm thick, show the internal structure of the wood. They indicate the existence and extent of infection and show whether it was caused by bacteria or fungi. From this, it can be determined whether any fungi present are cellulolytic or noncellulolytic organisms.The zone of inhibition test can be used to determine the susceptibility of the wood to fungal growth and decay when inoculated with wood-destroying organisms.
Briefly described, a zone of inhibition test involves placing two in. square wood samples on nutrient agar that has been seeded with a wood-rotting organism, such as Aspergillus niger or Chaetomium globosum. Plates are then incubated for 7 days at 82°F, after which the wood samples are evaluated for their degree of resistance or susceptibility.A complete zone of inhibition exists when there is a clear area around the test block that is free of fungal growth. This preventive barrier is created by antifungal application or natural properties present in the wood that inhibit the growth of wood-rotting organisms.
A partial zone of inhibition exists when some growth (white zone) of the test organism occurs around the block but fungal spores (black zone) of the test organism show retarded growth. This partial zone reflects continuing presence of some residual antifungal or natural inhibitive properties in the wood.
No zone of inhibition exists when growth of the test organism or other organisms inherent in the wood occurs. This growth, on or around the block, indicates that the wood is susceptible to fungal attack, and corrective measures must be taken to prevent fungi from spreading to sound members of the tower.
A zone of inhibition test evaluates both the residual effect of antifungal treatment and the degree of resistance restored by an application of antifungal. Sometimes, pressure-treated wood can yield zone of inhibition results that suggest the need for antifungal application. This occurs when the treatment has not leached from the wood in sufficient quantities to yield a zone of inhibition in the test. If the wood possesses inhibitory properties, growth will not occur on the block.
Spraying
Direct Spraying. Manual spraying of cooling tower wood with pneumatic assist (similar to spray painting) can be an effective method of treating cooling tower wood. A concentrated antifungal with the appropriate EPA end-use registration is applied directly to the wood with spraying equipment handled by an experienced operator or team of operators. This method is most effective because small areas can be covered thoroughly with the antifungal, and close attention can be given to spraying joints, holes, and other critical areas, such as cracks or splits in wood beams.
Direct antifungal spraying is hazardous to the operator unless proper precautions are taken. This procedure should be entrusted to a competent commercial spraying company with proper equipment and experience.
Steam Spraying. Steam spraying through a permanent piping arrangement has also been used to treat cooling tower wood. The antifungal is forced into the steam and transported into the tower cell by the steam. Distribution piping must be designed properly to ensure complete coverage. Because steam dilutes the antifungal, a much more dilute solution is applied than with direct spraying. As a result, a smaller quantity of toxicant penetrates the wood and more frequent spraying is needed to keep the wood fungistatic.
Because diffusion and spray methods penetrate only the outer surfaces of wood, preventive maintenance should be started before infection begins and the interior parts of the wood lose much of their natural resistance.
Sterilization
When a tower has suffered a serious infection that the normal corrective programs are not likely to control, consideration can be given to sterilization. In this process, wood temperature is elevated to 150°F for a period of 2 hr. Longer periods must be avoided to limit the loss of wood strength that occurs.