Cooling Tower

Cooling Tower (CT) Principle ?

• To decrease the temperature of hot water entering the CT.
• Cooling by evaporation is the principle used.
• Heat transfer from Water to Air.

• High difference between Wet Bulb Temperature ( WBT ) and Dry Bulb Temperature ( DBT ) encourages more Heat Transfer ( HT ) between air and water inside the cooling tower. This is due to the fact that when WBT = DBT , air is fully saturated i.e 100% relative humidity (R.H) and no longer accepts water, thus no more HT.

• Thus more difference between WBT and DBT => low R.H. => greater capacity to hold water => effective lowering of water temperature.

• Final outlet water temperature will always be < WBT of entering air. ( suppose DBT = 30°C, WBT = 25°C find R.H from humidity chart, this implies the capacity of air to hold water i.e. hot water can maximum transfer heat till its drops 

A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature.

Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.

Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.

With respect to drawing air through the tower, there are three types of cooling towers:

·         Natural draft  — Utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. Warm moist air is less dense than drier air at the same pressure. This moist air buoyancy produces an upwards current of air through the tower.

·         Mechanical draft — Uses power-driven fan motors to force or draw air through the tower.

·         Induced draft — A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The faninduces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fin arrangement is also known as draw-through.

·         Forced draft — A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high static pressure. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through.

·         Fan assisted natural draft — A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan.

Working of CT ?

• Hot water from HE enters cooling tower
• It is sprinkled from top with the help of nozzles
• fills are used for better Heat Transfer between air and water.
• Normally water is brought down to room temperature and is collected in the concrete basin at the bottom.
• This water is re-used again and the same process follows.
• small amount of water gets evaporated during the entire process hence make up water is used .

Types of CT

1. Natural Draft ( Hyperbolic CT ) :

• ADVANTAGES : No fans, motors or gearboxes required, usually used for large quantity of water flow.
• DISADVANTAGE : Large space required
• uses the difference between ambient air temperature and the air inside tower.
• Hot air rises upwards and cooler air is drawn inside through bottom.
• Two types :

A) Cross flow Natural Draft CT: water and air flow are perpendicular. Fill is located outside CT.