Cooling Towers

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    In a power plant large amount of heat is rejected  which is a byproduct. This heat can not ignored & it is rejected from the primary system (CW system)  to a secondary system (atmosphere).

    Physically atmosphere is treated as a heat sink in which heat is dumped  with the help of air or water.

Cooling system are three distinct types:-

  • Once-through cooling:- Waste heat is transfer  with the help of cooling water which is receive & discharge to a rivers or lakes sea. 
  • Evaporative cooling:- Waste heat is dissipated to the atmosphere by evaporation of a small portion of the cooling water. it require cooling towers.
  • Dry cooling:-Heat is dissipated directly to the atmosphere. it require air cooled heat exchangers.
    In once through cooling system water is taken from natural resources & after cooling process hot water is dumped in to the again in natural resources so there is a thermal effect on the natural water resources.
    So once through cooling is then replaced by evaporative cooling in which natural draft of mechanical draft cooling towers are used. A cooling towers recirculate water, which can be an advantage that better control of water quality is possible than once through cooling system. But in evaporative cooling temperature of cooling water is not too low as compare to once through cooling. So the quantity of cooling water is more required as compared to once through cooling. Also air is not an efficient cooling medium. In evaporative cooling makeup water is required.

Design inputs for cooling towers:-

  • Water flow
  • Water temperature
  • Local weather conditions, siting
  • Environment closely studied.
  • Energy conservation attempted in tower design & operation.
  • Optimum water usage.
  • Increased heat-transfer capability.
  • Improved with the turbine/condenser circuit.
  • Hyperbolic towers
  • Evaporative mechanical draft towers
  • Large and small towers
  • Dry and wet/dry towers
  • Cooling-tower maintenance
  • Chemical treatment and noise control.

 

Cooling system terms:-

  • Approach:-Difference between cold-water temperature & measured wet-bulb temperature.
  • Blow down: - Water discharged from the system to control concentration of salts or other impurities in the circulating water.
  • Cold-water temperature: - Temperature of the circulating water leaving the tower.
  • Cooling range: - Difference between the hot-water temperature and the cold-water temperature.
  • Cycles of concentration: - Compares dissolved solids in makeup with solids in the circulating water. Since chlorides are soluble, for example, cycles of concentration are equal to ratio of chlorides in circulating water to chlorides in makeup.
  • Drift: - Water lost from the tower as liquid droplets entrained in the exhaust air. It is independent of water lost by evaporation. Eliminators control this loss in cooling towers.
  • Heat load: - Heat removed from the circulating water within the tower. It may be calculated from the range and the circulating water flow and is generally expressed in kj/hr. 
  • Hot-water temperature:-Temperature of circulating water entering the distribution system.
  • Makeup:-Water added to the circulating water system to replace water lost from the system by evaporation, drift, and blow down & leakage.
  • pH:- Number between 0 and 14 indicating degree of acidity (below 7) or alkalinity (above 7): 7 denotes a solution that is neutral.
  • Pumping head: - Minimum pressure required to lift water from basin level to the top of the tower and force it into the water-distribution system. Pumping head is equal to static head plus friction loss and pressure drop through the distribution system.
  • Recirculation (recycle):- That portion of the exhaust air which re-enters the tower. Unit: percent of total entering-air quantity which is exhaust air.
  • TDS: -Total dissolved solids contained in solution in cooling-system water.
  • Water loading:- Circulating water flow expressed in litter/squire meter of effective horizontal wetted area of the tower, a factor in the unit's ability to meet design cold-water temperature.
  • Wet bulb temperature: - Temperature indicated by a psychrometer. Also known as the thermodynamic wet-bulb temperature or the temperature of adiabatic saturation.
 
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