Industrial Water Engineering, Inc.

CHEMICAL WATER TREATMENT

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

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Large cooling tower at Electric Utility.

Cooling towers are used in the arid Southwest to conserve water.  Water is a limiting factor in economic growth.  Cooling towers are major water users and proper use is critical to conserve valuable water.

A cooling tower allows water to be reused.  For example, in a cooling tower used for comfort air-conditioning (HVAC), water is recirculated at the rate of 3 gallons per minute (gpm) per ton of refrigeration.  A 1000 ton unit will circulate water through a heat exchanger at the rate of 3000 gpm.  As the water passes through the condenser heat exchanger, the water removes heat from the  refrigeration process.  The cooling water temperature will increase about 10 degrees F.  The water is cooled by evaporation in the cooling tower back to the original temperature.  To cool this water, approximately 1% of the recirculating water must be evaporated.  For this 1000 ton unit, that means that 30 gallons per minute are lost through evaporation.

As the water evaporates, the dissolved minerals concentrate in the water.  Using tower makeup water as the benchmark, the makeup water is said to be at one cycle of   concentration.  If the tower water contains twice the dissolved minerals as the makeup, then it is at two (2) cycles of concentration.  If the tower water contains three times the dissolved minerals it is at three (3) cycles of concentration.  As the water's dissolved mineral concentration increases, the water will become supersaturated with minerals such as silica, magnesium silicate, calcium sulfate, calcium carbonate, and others.  Chemicals are added to allow minerals to remain soluble without depositing as hard mineral scale in the system. At some point, even the chemicals are overcome and if nothing was done scale would form.  Some of the salty tower water is removed by a process called blowdown or bleed.

Modern cooling tower controllers measure the electrical conductivity of water. Water conductivity is measured in micromhos.  As water becomes saltier, the conductivity will increase.  If a tower makeup water has an electrical conductivity of 400 micromhos, then at 2 cycles it would be 800 micromhos and at three cycles it would be 1200 micromhos.  Cycles are easily calculated by dividing the tower conductivity by the raw water conductivity.  In the above example, if the tower water was 1200 micromhos and the makeup water was 400 micromhos, then the tower would be operating at 3 cycles of concentration (1200/400=3).  The tower controller is set to open an electrical bleed valve when the water reaches a preset conductivity reading.

Water is added to the tower to makeup for water lost to evaporation and bleed.  It is here that conservation is noticed.  Evaporation loss is a function of heat loads and cannot be altered.  Bleed loss is determined by the following formula:

   bleed gpm=evaporation loss-gpm/(cycles of concentration-1)

The following illustrates water use for the 1000 Ton cooling tower:

If the tower is operated at 1.5 cycles of concentration it will use 47.3 million gallons of water per year.  If it operates at 4.0 cycles, it will use 21.02 million gallons of water per year.  The water saving in this example is 26.28 million gallons.  In Albuquerque, this water has a value of approximately $29,070.

Water treatment is used to obtain the highest possible cycles of concentration while protecting the equipment from scale and corrosion.  Water treatment is a misnomer.   Chemicals are added to the water to protect metal surfaces from scale, corrosion, and microbiological fouling.  Water carries chemicals to those metal surfaces.