COOLING TOWER AND COOLING WATER SYSTEM: A Brief Presentation: Part-1 of 3 - What is Piping

COOLING TOWER AND COOLING WATER SYSTEM: A Brief Presentation: Part-1 of 3

This presentation is published in three parts. This is first part.

HIGHLIGHTS:

  • Basics of Cooling Tower
  • Types of Evaporative Cooling Tower
  • Standard PID for Cooling Tower and data sheets
  • Hybrid Cooling Tower
  • Cooling Water Treatment

NEED FOR COOLING TOWERS:

  • Removal of heat of reaction
  • Condensation
  • Heat of compression

SOME COMMONLY USED TERMS:

  • Dry-Bulb temperature refers basically to the ambient air temperature. It is called “Dry Bulb” because the sensing tips of the thermometer not affected by the moisture of the air.
  • Wet-Bulb temperature is the temperature of adiabatic saturation indicated by a moistened thermometer bulb exposed to the air flow.
  • Relative Humidity is the amount of moisture in the air, as a percentage of the total moisture the air can contain at the current temperature
  • Approach is the difference in temperature between the cooled-water temperature and the entering air wet bulb temperature.
  • Range is the temperature difference between the water inlet and water exit.
  • Cycles of concentration represents the accumulation of dissolved minerals in the re-circulating cooling water.
  • Blow-down – The portion of the circulating water flow that is removed in order to maintain the amount of dissolved solids and other impurities at an acceptable level.
  • Fill – Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer.
  • Drift – Water droplets that are carried out of the cooling tower with the exhaust air.
  • Plume – The stream of saturated exhaust air leaving the cooling tower.
  • Blow-out – Water droplets blown out of the cooling tower by wind, generally at the air inlet openings.

BASICS OF COOLING TOWER (EVAPORATIVE TYPE):

  • Water cooled by evaporation
  • Air flow required to aid evaporation
  • Latent heat of evaporation of water transferred to air
  • Humidity and enthalpy of exit air increases

FACTORS AFFECTING DESIGN AND PERFORMANCE (Fig. 1):

  • Ambient wet bulb temperature
  • Atmospheric conditions (DBT, RH)
  • Range & Approach
  • Capacity
  • L/G Ratio of Tower
  • Filling media
Curves showing different factors with respect to temperature
Fig. 1: Curves showing different factors with respect to temperature

From the above curves this are evident:

  • Tower size varies directly and linearly with heat load
  • Tower size varies inversely with range
  • Tower size varies inversely with approach
  • Tower size varies inversely with wet bulb temperature

IMPACT OF COOLING WATER RANGE:

HIGHER RANGE:

  • Reduced LMTD
  • Bigger Exchangers
  • Lower Velocities on CW side and likely fouling
  • Lower pumping cost

LOWER RANGE:

  • Higher LMTD
  • Smaller Exchangers
  • Higher velocities on CW side and less fouling
  • Higher pumping cost

TYPES OF COOLING TOWERS (Fig. 2):

Figure showing the types of cooling towers
Fig. 2: Figure showing the types of cooling towers

TYPES OF EVAPORATIVE COOLING TOWERS:

Three types based on air flow generation methods:

  • Natural draft, which utilizes buoyancy via a tall chimney
  • Mechanical draft, which uses power driven fan motors to force or draw air through the tower
  • Fan assisted natural draft that appears like a natural draft though airflow is assisted by a fan

NATURAL DRAFT COOLING TOWERS (Fig. 3):

  • Also called as Hyperbolic Natural draft towers, typically about 400 ft (120 m) high
  • Differential pressure between the cold outside air and the hot humid air on the inside of the tower acts as the driving force.
  • No fans are used.
  • Warm, moist air naturally rises due to the density differential to the dry, cooler outside air.
  • Use of natural or mechanical draft towers depends on climatic conditions and operating requirements.
Natural draft Cooling Towers
Fig. 3: Natural draft Cooling Towers

ADVANTAGES:

  • No power cost
  • No rotating parts

DISADVANTAGES:

  • Expensive
  • High construction time
  • Used for flows higher than 250,000 gpm

WHEN TO CHOOSE:

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1 thought on “COOLING TOWER AND COOLING WATER SYSTEM: A Brief Presentation: Part-1 of 3”

  1. Taylor Anderson

    It’s cool that natural draft cooling towers are typically 120 meters tall. Honestly, I’d just heard of cooling towers, so it’s really cool to learn about them. Where can I learn more about the other types of cooling towers mentioned?

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