Industrial Water Engineering, Inc.

CHEMICAL WATER TREATMENT

toll free 1-800-543-6519

GSA # GS-10F-7884A (1/31/2007)

DOD Cage # 0MV90

 

 

Earth's fresh water supply is finite. Cooling towers cool industrial processes and air-conditioning. Cooling towers are a major water consumer and a water conservation device. Without cooling towers, water use would be much greater because all systems would operate at 1 cycle of concentration. Let’s explain this.

Water is cooled by evaporation. Approximately 1% of the recirculating water evaporates and removes heat. As the water evaporates, dissolved minerals concentrate and eventually will form deposits throughout the system. City well water is called makeup water. As the makeup up water is evaporated, water is lost and the dissolved minerals stay in the bulk water. The water’s mineral concentration increases as evaporation continues. When the tower water contains twice the mineral concentration of makeup water, the water is said to be at two cycles of concentration. The concentrations will continue to climb unless a portion of the tower water is removed and fresh makeup water is added. Water that is removed is called bleed or blowdown water.

The desired cycle is maintained by an instrument called a conductivity controller. The conductivity controller measures the electrical conductance of water in micromhos. Albuquerque’s makeup water varies from 350 to 600 micromhos. When makeup water is 500 micromhos, the water at two cycles will measure 1000 micromhos. At three cycles, the conductivity will be 1500 micromhos. The amount of bleed water is calculated as follows:

            Bleed gallons = (evaporation gallons)/(cycles-1.0)

Water is added to makeup loss from evaporation and bleed. This is called makeup water. System water use is summarized:

           Makeup water gallons = evaporation water gallons + bleed water gallons

If evaporation is 100 gallons per minute, lets calculate water use at various cycles.

As the water concentrates and is controlled at higher cycles, then water use is minimized. As you can see, a cooling tower saves water. At water costs of over $1.00 per thousand gallons, the owner is advised to operate at highest possible cycles.

Cycles are maintained with a conductivity controller. The controller measures the conductance of water. As the water becomes more salty, the conductance will increase. Conductivity is measured in micromhos. The controller is set to blowdown the system with a electrically actuated valve. If the tower water is set to control at 1500 micromhos, when the water concentrates above this number the bleed valve is opened and water is removed from the tower. When the water conductivity goes below 1500 micromhos, the valve is closed. As makeup water is added, water treatment chemicals are also added. Without bleed and water treatment chemicals, mineral deposits, corrosion, and biological growth will occur.

If mineral deposits form, corrosion and heat transfer loss are inevitable. To prevent mineral deposits, water treatment chemicals are added that increases mineral solubility. As the water becomes saltier with evaporation, the mineral concentration increases. Several potential problems can occur as the cycles increase.

SCALE OR MINERAL DEPOSITS

As the cycles increase, the potential for mineral deposits increase. The two most common minerals that form deposits are calcium carbonate (limestone) and silica. Calcium carbonate deposits are found in the hot side of heat exchangers.

Calcium carbonate deposits restrict heat transfer. Once formed, calcium carbonate may be removed either by brushing or by acid cleaning. Water treatment chemicals such as CW-7200 are useful to preventing calcium carbonate deposits. A maximum control point is a Langlier Saturation Index of 2.5. If the water concentrates more than this, pH of the water must be controlled by acid additions.

Silica can form deposits with magnesium and calcium. These deposits are very hard and are found in the hot sides of heat exchangers. If fouling occurs, removal is very difficult and hazardous material such as ammonium bifluoride or hydrofluoric acid must be used. Silica levels should not exceed 200 ppm with proper water treatment chemical additions.

CORROSION CONTROL

As cycles increase, the water becomes more scale forming. Mineral concentrations increase especially the carbonate ion. 

Wet-dry areas of cooling towers are subject to under-deposit corrosion as minerals accumulate in low water flow areas. Water flow is critical in high-cycle towers.

Galvanized towers will experience corrosion called "white-rust" that is the formation of zinc carbonate. The only solution to "white-rust" is lowering cooling water pH with supplemental sulfuric acid feed. Modern controllers make this a reliable option.

Mild steel and copper corrosion is easily maintained with the corrosion inhibitors found in CW-7200 or CW-7044 cooling water treatments. Corrosion monitoring is recommended.

The user is cautioned against removing minerals from water in an effort to increase cycles. Water that is pretreated by either softening or reverse osmosis is  very corrosive to parent metals. Mineral removal prevents scale deposits and increases corrosion to unacceptable levels.

Modern cooling towers use a PVC honeycomb fill material. The outside edges go to dryness and other "wet/dry" areas also will form deposits. These areas contain a soluble calcium carbonate and sodium carbonate salt. Poor tower design causes this problem. Sandia Laboratories has redesigned towers to improve water flow in these areas and the fouling is greatly reduced.

Airborne dirt and debris are washed from the air in the cooling tower. Tower basins accumulate several inches of mud each year from this process. Debris such as cottonwood fibers can plug heat exchangers, control equipment, and solenoid valves. Once mud forms on the tower basin, microorganism will grow in the mud. These organisms will accelerate corrosion of tower basin metal and rapidly multiply. Biocides added to control biomass do not penetrate mud and control organisms. OSHA recommends cleaning all cooling towers used in comfort air-conditioning twice per year. The cleaning allows biomass to be controlled and minimizes the risk of Legionella Pneumophilla. This bacterium causes Legionnaires Disease. Side-stream filtration also helps to keep towers clean.

As cycles increase, the margin for error decreases. Outdated control systems should be replaced with new computer-interfaced equipment. Old systems will typically control a tower at plus or minus 200 micromhos of the conductivity setpoint. New systems will maintain at plus or minus 10 micromhos.

BIOCONTROL

Water treatment companies provide an arsenal of microbiocides. These products are registered with the US EPA and with the New Mexico Department of Agriculture (NMDA). Your water treatment service person is required to have a NMDA license to apply these microbiocides. This pesticide applicator license requires a thorough knowledge of microbiocides and their applications. If in doubt, ask to see a copy of the license.

Biocides can be categorized into two classes:

• Oxidizing

• Non-oxidizing

Non-oxidizing biocides function as a poison and kill specific types of microorganisms. One non-oxidizer cannot control all types of organisms; so two non-oxidizers must be used.

Some common non-oxidizers are:

• Glutaraldehyde

• Isothiazoline

• Quats

• DPNPA

Oxidizing biocides control all microorganisms. These biocides include:

• Chlorine

• Chlorine Dioxide

• Bromine

• Hydrogen Peroxide

• Ozone

• BCDMH

• Bleach

Oxidizing biocide may be fed continuously to maintain a 0.1 to 0.3 ppm free chlorine residual or may be slug fed to maintain 0.5 to 1.0 ppm free chlorine residual.

The frequency of biocide addition is depends on the system and time of year. Biocide frequency increases with sunlight exposure and water temperature. Summer requires more biocide treatment to control the system. In winter, weekly additions may be enough to control the system. In summer, the frequency may be three times per week. If manpower is only available every two weeks, then you need to use an automatic system that adds biocides on a preprogrammed basis.

With current technology, we operate to 1500 micromhos. If you desire higher cycles, then supplemental control loops are required. Most systems cannot justify operation at higher than 1500 micromhos or 3-cycles.