FOULING, SCALING, AND CORROSION
Fouling reduces water flow and heat transfer. It can be caused by the collection of loose debris over pump- suction screens in sumps, growth of algae in sunlit areas, and slime in shade or dark sections of water systems. Material can clog pipes or other parts of a system after it has broken loose and been carried into the system by the water stream. Scaling also reduces water flow and heat transfer. The depositing of dissolved minerals on equipment surfaces causes scaling. This is particularly so in hot areas, where heat transfer is most important. Corrosion is caused by impurities in the water. In addition to reducing water flow and heat transfer it also damages equipment. Eventually, corrosion will reduce operational efficiency. It may lead to expensive repairs or even equipment replacement.
Impurities have at least five confirmed sources. One is the earth’s atmosphere. Water falling through the air, whether it be natural precipitation or water showering through a cooling tower, picks up dust as well as oxygen and carbon dioxide. Similarly, synthetic atmospheric gases and dust affect the purity of water. Heavily industrialized areas are susceptible to such impurities being introduced into their water systems.
Decaying plant life is a source of water impurity. Decaying plants produce carbon dioxide. Other products of vegetable decay cause bad odor and taste. The byproducts of plant decay provide a nutrient for slime growth.
These three sources of impurities contaminate water with material that makes it possible for it to pick up more impurities from a fourth source, minerals. Minerals found in the soil beneath the earth’s surface are probably the major source of impurities in water. Many minerals are present in subsurface soil. They are more soluble in the presence of the impurities from the first three sources, mentioned above.
Industrial and municipal wastes are a fifth major source of water impurities. Municipal waste affects bacterial count. Therefore, it is of interest to health officials but is not of primary concern from a scale or corrosion standpoint. Industrial waste, however, can add greatly to the corrosive nature of water. It can indirectly cause a higher than normal mineral content.
The correction or generation of finely divided material that has the appearance of mud or silt causes foul- ing. This sludge is normally composed of dirt and trash from the air. Silt is introduced with makeup water. Leaves and dust are blown in by wind and washed from the air by rain. This debris settles in sumps or other parts of cooling systems. Plant growth also causes fouling. Bacteria or algae in water will result in the formation of large masses of algae and slime. These may clog system water pipes and filters. Paper, bottles, and other trash also cause fouling.
Scaling
Prevention
There are three ways to prevent scaling. The first is to eliminate or reduce hardness minerals from the feed water. Control of factors that cause hardness salts to become less soluble is important. Hardness min- erals are defined as water-soluble compounds of calcium and magnesium. Most calcium and magnesium compounds are much less soluble than are corresponding sodium compounds. By replacing the calcium and magnesium portion of these minerals with sodium, solubility of the sulfates and carbonates is improved to such a degree that scaling no longer is a problem. This is the function of a water softener.
The second method of preventing scale is by controlling water conditions that affect the solubility of scale- forming minerals. The five factors that affect the rate of scale formation are:
• Temperature
• TDS (total dissolved solids)
• Hardness
• Alkalinity
• pH
These factors can to some extent be regulated by proper design and operation of water-cooled equipment. Proper temperature levels are maintained by ensuring a good water flow rate and adequate cooling in the tower. Water flow in recirculating systems should be approximately 3 gallons per minute per ton. Lower flow levels allow the water to remain in contact with hot surfaces of the condenser for a longer time and pick up more heat. Temperature drop across the tower should be 8 to 10 degrees F (4.5 to 5.5 degrees C) for a compression refrigeration system and 18 to 20 degrees F (10 to 11 degrees C) in most absorption systems. This cooling effect, due to evaporation, is dependent on tower characteristics and uncontrollable atmospheric conditions.
Airflow through the tower and the degree of water breakup are two factors that determine the amount of evaporation that will occur. Since heat energy is required for evaporation, the amount of water that is changed into vapor and lost from the system determines the amount of heat. That is, the number of Btu to be dissipated is the heat factor. One pound of water, at cooling tower temperatures, requires 1050 Btu to be converted from liquid to vapor. Therefore, the greater the weight of water evaporated from the system, the greater the cooling effect or temperature drop across the tower.
Total dissolved solids, hardness, and alkalinity are affected by three interrelated factors: evaporation, makeup, and bleed or blow-down rates. Water, when it evaporates, leaves the system in a pure state, leaving behind all dissolved matter. Water volume of evaporative cooling systems is held at a relatively constant figure through the use of float valves.
Fresh makeup water brings with it dissolved material. This is added to that already left behind by the evaporated water. Theoretically, assuming that all the water leaves the system by evaporation and the sys- tem volume stays constant, the concentration of dissolved material will continue to increase indefinitely. For this reason, a bleed or blow-down is used.
There is a limit to the amount of any material that can be dissolved in water. When this limit is reached, the introduction of additional material will cause either sludge or scale to form. Controlling the rate at which dis- solved material is removed controls the degree to which this material is concentrated in circulating water.
Identification
Scale removal depends on the chemical reaction between scale and the cleaning chemical. Scale identification is important. Of the four scales most commonly found, only carbonate is highly reactive with cleaning chemicals generally regarded as safe for use in cooling equipment. The other scales require a pretreat- ment that renders them more reactive. This pretreatment depends on the type of scale to be removed. Attempting to remove a problem scale without proper pretreatment can waste time and money.
Scale identification can be accomplished in one of three ways:
• Experience
• Field tests
• Laboratory analysis
With the exception of iron scale, which is orange, it is very difficult, if not impossible, to identify scale by appearance. Experience is gained by cleaning systems in a given area over an extended period of time. In this way, the pretreatment procedure and the amount of scale remover required to remove the type of scale most often found in this area become common knowledge. Unless radical changes in feed-water quality occur, the type of scale encountered remains fairly constant. Experience is further developed through the use of the two other methods. Figure 8.1 shows a water-analysis kit.
Field tests, which are quite simple to perform, determine the reactivity of scale with the cleaning solution. Adding 1 tablespoon of liquid scale remover or 1 teaspoon of solid scale remover to 1/2 pint of water pre- pares a small sample of cleaning solution. A small piece of scale is then dropped into the cleaning solution.
The reaction rate usually will determine the type of scale. The reaction between scale remover and carbonate scale results in vigorous bubbling. The scale eventually dissolves or disintegrates. However, if the scale sample is of hard or flinty composition and little or no bubbling in the acid solution is observed, heat
should be applied. Sulfate scale will dissolve at 140 degrees F (60 degrees C). The small-scale sample should be consumed in about an hour. If the scale sample contains a high percentage of silica, little or no reaction will be observed. Iron scale is easily identified by appearance. Testing with a clean solution usually is not required.
Since this identification procedure is quite elementary and combinations of all types of scale are often en- countered, it is obvious that more precise methods may be required. Such methods are most easily carried out in a laboratory. Many chemical manufacturers provide this service. Scale samples that cannot be identified in the field may be mailed to these laboratories. Here a complete breakdown and analysis of the problem scale will be performed. Detailed cleaning recommendations will be given to the sender.
Most scales are predominantly carbonate, but they may also contain varying amounts of sulfate, iron, or silica. Thus, the quantities of scale remover required for cleaning should be calculated specifically for the type of scale present. The presence of sulfate, iron, or silica also affects other cleaning procedures.