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Cooling Tower Useful Terms and Cooling Tower Performance

Some of the commonly used useful terms, in the cooling tower industry are
  1. BTU( British-Thermal-Units):
    It is the requirement of Heat-Energy to increase the temperature by 1oF of one-pond of water between 32 and 212oF.
  2. Cooling Range:
    It is the temperature difference between cooling tower entering hot water and leaving cold water is cooling tower range.
  3. Approach:
    It is the difference of temperature between the cooling water leaving temperature and the WBT (Wet Bulb Temperature). It is one of the most important parameter in deciding the cost and size of the cooling tower. It also decides the cooling water temperature.
  4. Drift:
    It is those water-droplets thrown out of the cooling tower by the outgoing exhaust air. It is a loss of water from the cooling tower, so it can be reduced with the help of baffle-like device called as drift eliminators.
  5. Heat Load:
    It is the heat that is to be removed from the cooling tower circulating water.
  6. Ton:
    Tons in an evaporative cooling is 15,000 BTU’s/Hr. The refrigerator ton is 12,000 BTU’s/Hr.
  7. Wet Bulb Temperature (WBT):
    It is the theoretical lowest temperature that water can attain by evaporation. WBT is most important parameter in tower design and selection and can be measured through psychometric chart.
  8. Dry Bulb Temperature (DBT):
    Is the entering air temperature measured by dry bulb thermometer.
  9. Pumping Head:
    It is the pressure requirement of the circulating water from the basin of the CW pumps to overcome the pressure drop across discharge valves, filters, condensers and finally to top of the cooling tower distribution header.
  10. Make-Up:
    It is the amount of water required to compensate for the following: -
    • Drift Losses
    • Evaporation losses
    • Slowdown or Bleed off losses
  11. Bleed Off: It is that circulating water which is taken out from the system in the form of Bleed off, to maintain the dissolved solids, hardness and alkalinity with in limiting range. Evaporation of circulating water increases the Cycle of Concentration (COC), unless controlled by bleed off.

Cooling Tower Performance

Cooling tower performance depends upon 4 major factors:
  1. Range
  2. Heat Load
  3. Ambient wet bulb temperature or RH
  4. Approach

Range

Range is difference in temperature between the hot water entering the tower and the cold water leaving the tower. A high cooling tower range is the indication for satisfactory cooling tower performance. It is represented by:
Range is determined by the process and not by the cooling tower. Thus range is a function of process heat load and the circulating water flow.

Approach

It is the temperature difference between the leaving cooling water temperature and the WBT (Wet Bulb Temperature).
Thumb Rule:
Lower the approach, bigger is the size of the cooling tower and vice-versa. Normally approach of 2.8oC is what manufacturer can guarantee.
Approach decides or govern the size of the cooling tower to be selected.

WBT or RH

It is measured by covering the bulb of thermometer with a water-soaked wet cloth. It is one of the important element in performance calculation of evaporative cooling towers. During cooling tower pre-selection based on wet bulb temperature, following needs to be considered:
  • WBT design value, at any point of time should not exceed 5% of the time. Selected WBT shall be near to the avg. max. WBT in summer.
  • Specified Wet bulb temperature can be ambient which is temperature in the cooling tower area or specified as inlet which is temperature of the air entering the tower and is affected by circulation of tower.

Heat Load

Heat load of the cooling tower is indicated by the equation: Where,
Q = Heat load in Btu/hr.
m = Mass of cooling water in Ibs/hr.
Cp = Specific heat of water.
ΔT = Difference between hot and cold water temperature.
Cooling Tower dimension and cost increases with increase in heat load. Designing over sized and undersized cooling tower is not desirable and correct thing to practice.

Relationship between Range, Flow and Heat Load

With the increase in heat load both range and flow increases. With the increase of range there are two possible cases:
  • Acceptable and Economical Case:
    By increasing the inlet cooling water temperature, keeping the exit cooling tower temperature as fixed.
  • Unacceptable and Uneconomical Case:
    By reducing the exit cooling water temperature, keeping the inlet cooling tower water temperature as fixed. Uneconomical because approach is reducing.

Relationship between Approach and WBT

WBT design value is established based on geographical location of the project. Keeping the approach as fixed and by changing the WBT (increasing), resulting in smaller cooling tower.




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