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POWER STATION ZERO DISCHARGE
COOLING TOWER - "Friend or Foe."
December 12, 1990

COOLING TOWER SYSTEMS "Basic Principles"

In Power Stations, water is recirculated between the condenser and cooling towers to dissipate waste heat. What appears to be a simple process, in reality, is dynamic and complex. Substantial transfer of material and energy occurs. The process is constantly changing, consuming, and discharging.

Evaporation in the Cooling Tower transfers the waste heat to the environment at ambient conditions. The quantities of HEAT and EVAPORATION can be estimated by knowing the MASS FLOW and TEMPERATURE DROP of the recirculating water.(The heat capacity of water = 1 BTU per pound per degree F; 1 gallon = 8.33 pounds).

EXAMPLE: With: 30,000 GPM RECIRCULATION FLOW (250,200 LBS/MIN) @ Delta T = 10 F

Then: BTU/MIN = 2,502,000

Since evaporation consumes 1,000 BTU/LB, the MASS RATE (in pounds) equals TOTAL HEAT RATE/1000 (2,502 LBS/MIN above). This equals 300 gpm Evaporation Rate (2,502/8.33). Makeup is added to the Tower to account for evaporation and other losses. WATER LOSSES occur from the system (Drift, Windage, Blowdown, sampling, condenser leakage). In many plants, cooling water is consumed for ash quenching, scrubber makeup, area losses, etc.).

TOWER MAKEUP is supplied mostly from fresh water. However, wastewaters are often directed to the system. These include plant drains, boiler blowdown, demineralization wastes, etc.

MAKEUP = Evaporation + Losses - Recycled Wastes.

Short term variations occur (tower basin level fluctuation, batch recovery of waste streams, load swings, and periodic withdrawals). The recirculating system is a large "fly wheel" as it responds to many changes.

Since EVAPORATION IS PURE WATER VAPOR, dissolved salts from the INPUT concentrate in the Tower. The degree of concentration is limited by losses that occur (salts are lost from the system by several means as referenced above). The ultimate salinity that results is further influenced by chemical additives, air stripping of volatiles, dust, bio-reactions, precipitation, and dilution.

Conventionally, the term CYCLES is used to describe the concentrating effects of salinity in cooling tower systems.

CYCLES = MAKEUP/BLOWDOWN (Expressed as mass flowrate or as Total Dissolved Solids; For expediency, some inexactly use the conductivity ratio).

However, CHEMICAL PROCESSES alter the composition of the SALINITY. Therefore, the term "Cycles" is generally of little meaning in modern systems. These PROCESSES (pH control, chemical additives, stripping, bio-reactions, sidestream treatment, etc.) increase salinity and change the nature. In sophisticated systems, "Cycles" is of little value for controlling operations.

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