Tipo Heat Exchangers Manufacturers, Cooling Towers Manufacturers, and Industrial Chillers Manufacturers-Heat Exchangers Manufacturers, Cooling Towers Manufacturers, and Industrial Chillers Manufacturers

Blog

  • /
  • Cooling tower in India
NCT 84

Cooling tower in India

COOLING TOWER PERFORMANCE

Core Performance Indicators

Cooling tower performance is measured by its ability to reject a specified heat load while achieving the required cooling range with minimal approach to the wet-bulb temperature, all within constraints of energy use, water consumption, and cost.

1. KEY THERMODYNAMIC PARAMETERS

Range

  • Definition: Temperature drop of the water as it passes through the tower.
    Range (°F or °C) = Hot Water Temperature (HWT) – Cold Water Temperature (CWT)
  • Significance:

Directly proportional to the heat load rejected.
Q=mw×Cp×RangeQ=mw​×Cp​×Range

  • where mwmw​ = mass flow rate of water, CpCp​ = specific heat of water).
    • Higher range = Greater heat rejection.

Approach

  • Definition: How close the cold water temperature gets to the ambient wet-bulb temperature (WBT).
    Approach = Cold Water Temperature (CWT) – Wet Bulb Temperature (WBT)
  • Significance:
    • Primary measure of cooling tower thermal efficiency.
    • Lower approach = Higher tower capability and efficiency (but requires larger tower/fan/more energy).
    • Theoretical limit: Approach can never be zero; practical limits are typically 2.5–4°C (5–7°F).

Wet-Bulb Temperature (WBT)

  • Definition: The lowest temperature to which air can be cooled by evaporating water into it at constant pressure (measured by a psychrometer).
  • Significance:
    • The absolute thermodynamic limit for water cooling in an evaporative tower.
    • Performance varies daily/seasonally with WBT.

Cooling Tower Capability

  • Definition: The percentage of design heat load a tower can reject under actual conditions.
    Capability (%) = (Actual Range / Design Range) × 100
    (at same approach, or adjusted for actual WBT).
  • Significance: Indicates if the tower is underperforming or oversized.

2. PERFORMANCE CURVES & CHARACTERISTICS

Tower Characteristic Curve (KaV/L)

  • A dimensionless parameter representing the tower’s “degree of difficulty” or required size.
  • KaV/L = f(Range, Approach, WBT)
    • KK = Mass transfer coefficient
    • aa = Area of contact (fill)
    • VV = Volume of fill
    • LL = Water flow rate
  • Used by engineers to select/size a tower for given conditions.

Typical Performance Graph

  • Plots Approach vs. Water Flow Rate (or Range) for different Wet-Bulb Temperatures and Fan Speeds.
  • Shows that:
    • For a fixed WBT and flow, increasing fan speed lowers approach.
    • For a fixed WBT and fan speed, increasing water flow increases approach.
    • Lower WBT improves performance (lowers approach for same conditions).

“Approach Degradation”

  • The increase in approach over time due to:
    • Fouled fill (scaling, biological growth, debris).
    • Poor water distribution (clogged nozzles).
    • Damaged drift eliminators (increasing air pressure drop).
    • Fan/motor degradation.

. WATER-SIDE PERFORMANCE METRICS

Cycle of Concentration (COC)

  • Definition: Ratio of dissolved solids in blowdown water to make-up water.
    COC = TDS_blowdown / TDS_makeup
  • Typical Range: 3–7 cycles.
  • Significance: Higher COC reduces water consumption but increases scaling potential. Optimizing COC is key to water and chemical cost savings.

Evaporation Loss

  • Rule of Thumb: ~1% of circulation rate for every 7°C (12.5°F) of Range.
  • Calculation:
    E=Q×Range×0.00085E=Q×Range×0.00085 (in gpm, °F)
    Or E=0.00153×Circulation Rate (m³/h)×Range (°C)E=0.00153×Circulation Rate (m³/h)×Range (°C)

Drift Loss

  • Water droplets lost with exhaust air.
    • Modern towers: 0.0005% – 0.005% of circulation rate.
    • Older towers: Up to 0.2%.
  • Significance: Wastes water and can spread treatment chemicals.

Blowdown (Purge)

  • Intentional discharge to control dissolved solids.
  • Calculation:
    B=E/(COC−1)B=E/(COC−1)

Make-up Water Requirement

  • M=E+D+BM=E+D+B
    Where MM = Make-up, EE = Evaporation, DD = Drift, BB = Blowdown.

4. AIR-SIDE & ENERGY PERFORMANCE

Fan Performance

  • Static Pressure: Resistance faced by the fan (fill + eliminators + louvers). Increases with fouling.
  • Fan Power: Proportional to Air Flow³ and static pressure. Small reductions in airflow save significant power.
  • Variable Frequency Drives (VFDs): Used to modulate fan speed based on load/WBT, saving 30–50% of fan energy.

Air Flow Rate

  • Determined by fan size, speed, and tower static pressure.
  • Higher air flow lowers approach but increases fan power and drift.

Effect of Ambient Conditions

  1. Wet-Bulb Temperature: Single biggest external factor. Performance declines as WBT rises.
  2. Dry-Bulb Temperature: Minor direct effect, but correlates with WBT.
  3. Relative Humidity: High humidity raises WBT, reducing cooling potential.
  4. Altitude: Affairs air density; towers at high altitude require more air volume for same mass flow.