COOLING WATER SYSTEMS
Cooling water systems absorb heat from the processes they are designed to cool. In HVAC systems, the condenser cooling water absorbs heat from the refrigerant through a heat exchanger. In process cooling water systems the cooling water absorbs heat from the process stream that needs cooling. The re-circulating water in cooling water systems is usually cooled by the evaporation of a small amount of the re-circulating flow (typically 1-2%). This evaporation takes place as water flows over and through cooling towers, fluid coolers or evaporative condensers.
Calcium carbonate becomes less soluble with increases in temperature, which happens when cooling water systems absorb the heat from the processes they are cooling. As the water is cooled through evaporation, the concentration of all the dissolved solids in the remaining re-circulating water, including calcium carbonate increases. Left unchecked, calcium carbonate will precipitate as sludge in the cooling tower and as a deposit in the heat exchanger, robbing energy efficiency and possibly causing system shutdown.
Corrosion in cooling water systems can occur for several reasons: 1) The water is constantly being aerated, resulting in a fresh supply of oxygen, 2) chemicals fed to prevent scaling by controlling pH (typically sulfuric acid-containing formulas) may be overfed or 3) anaerobic bacteria can flourish under deposits and in stagnant areas of the system causing severe localized corrosion and pitting. No matter the cause, corrosion must be controlled to prevent the premature failure of valuable plant assets.
Cooling water systems operate at temperatures ideal for biological activity. In addition, as they draw in air to aid in the evaporative process they also draw in all the contaminants contained in that air. As a result, biological problems such as algae, slime, fungus and legionnella must be addressed to avoid loss of heat transfer, plugging of nozzles and strainers and potential health issues.
As cooling towers, evaporative condensers and fluid coolers evaporate water, the dissolved solids in the remaining re-circulating water concentrate. At some point the concentration of dissolved solids will reach a limit and precipitation (scale and sludge) will occur. Therefore, water must be bled from the system so that fresh make-up water with lower dissolved solids can replace it. However, excessive bleed-off wastes water and chemical. As water supplies are scarce and expensive, and where the water quality is poor, the issue of water conservation becomes more critical.
The selection of the scale control program depends on the quality of the make-up water fed to the cooling tower, evaporative condenser or fluid cooler and also on the temperature of the process. Of primary concern are the levels of calcium and bicarbonate alkalinity but other constituents such as silica, sulfate and iron need to be considered as well.
Often scale control can be effectively achieved at an acceptable level of bleed-off with organic polymers alone. However, if the make-up water hardness is very high and/or the temperature of the process is high additional steps may need to be taken. Often this includes the feed of an acid-based material to control alkalinity and pH.
Effective scale control can be achieved without the use of chemical by the proper application of electrolysis. H-O-H Water Technologys Green Machine- accomplishes this objective, virtually eliminating the need for chemical and saving water by reducing the need for bleed-off.
To insure long asset life it is necessary to monitor and control corrosion in the cooling water system. Corrosion inhibitors are often blended with the scale inhibitors in one formulation. They can also be fed separately. Corrosion rates are measured and expressed in mils per year (mpy) measuring metal loss over time. These rates are measured with corrosion coupons or with on-line devices known as corraters.
The control of biological fouling generally requires a different treatment than the one(s) used for scale and corrosion control. Biological treatments are generally divided into two types: oxidizing and non-oxidizing. Oxidizing materials are fed in proportion to make-up, continuously or on a periodic slug-feed basis. Non-oxidizing biocides are slug-fed. Consult your H-O-H Water Quality Engineer or Manager to help you determine the frequency of addition (taking into account such factors as load, system volume) and the right combination of oxidizing and non-oxidizing biocides.
Chemical Feed and Blowdown Equipment
Automatic control of the water treatment program designed to prevent scale, corrosion and biological fouling is essential to the success of the program. Chemical pumps must have the proper materials of construction to handle the specified chemicals and they must be able to handle the output and pressure requirements of the application.
Chemical feed is typically based on make-up water usage or in response to pH changes. Bleed-off is accomplished in response to increases in conductivity (mhos or TDS) or in proportion to make-up water usage.
Biocides are fed on a slug basis based on a time of day and day of the week, or they are fed in proportion to make-up.
One controller typically does all of these functions. In addition control equipment exists that can provide important data (water usage, chemical residuals, alarms on control set-points, etc.) to building automation systems or to sales/service personnel through the Internet or data communication lines.
Most water treatment formulations are liquids that can be pumped from on-site storage tanks with containment to minimize chemical handling and maximize safety. Qualified H-O-H delivery personnel add water treatment chemicals to these tanks.