NATURAL DRAFT COOLING TOWERS - Their CONSTRUCTION, WORKING, TYPES AND ADVANTAGES & DISADVANTAGES.

NATURAL DRAFT COOLING TOWERS

A NATURAL DRAFT Cooling Tower is so called because it do not use fans for drawing air into the tower, instead it uses the natural air which is drawn into the tower naturally by the process of Natural Convection for cooling the hot water. It may be Cylindrical or Hyperbolic in shape. Hyperbolic shape offers superior structural strength and resistence to ambient wind loadings. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m³/hr.  These types of towers are used mainly by Utility Power Stations. 

A NATURAL DRAFT  COOLING TOWER consists of following main parts :

HOT WATER INLET

The hot water inlet is generally provided at the top of the tower, below the Drift Eliminators and above the fill. It is connected to a series of nozzles that sprays the water downwards on the fills present. 

NOZZLES

Nozzles are connected to Hot Water Inlet and are present above the fill. Uniform water distribution at the top of the fill is very much essential to achieve proper wetting of the entire fill surface.  Nozzles sprays water from the top of the fill to uniformly wet the entire fill.

COLD AIR INLET

The cold air inlet is generally provided at the bottom of the tower, above the cold basin and below the fill. The cold air enters into the towers through this.

LOUVERS

The Louvers are present at the Cold Air Inlet. The purpose of louvers is to equalise the air flow into the fill. Also they help in retaining the water within the Coling Towers. They are mainly used when the flow of water and air is crossflow. Many counter flow towers do not require Louvers.

FILL

Fills are present inside the tower below the nozzles to provide a large surface area and time for air and water contact . Most towers have fills made of plastic or wood as it facilitate good heat transfer by maximising water and air contact.

Fill can either be splash or filmtype. In splash fill, water falls in successive layers and is continuously broken it into smaller droplets, thereby wetting the entire fill surface.

Plastic splash fill provides better heat transfer than the wood splash fill.

Film fill consists of thin, closely spaced plastic surfaces(PVC) over which the water spreads uniformly forming a thin film which is in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is more efficient than the splash fill and provides same heat transfer in a smaller volume.

DRIFT ELIMINATORS

Sometimes it happens that the droplets of water are carried out of the cooling tower along with the hot exhaust air. The concentration of impurities in these droplets is same as that in the water entering the tower. These may lead to environmental pollution. To  reduce this baffle-like devices, called as Drift Eliminators are installed above the nozzles. Drift Eliminators allow only the air to pass through it thereby removing water droplets from the air.

COLD WATER BASIN

The cold water basin, is located at the bottom of the tower.  The cooled water that flows down through the tower and fill is collected by the Cold Water Basin. These collected cold water is then discharged and used for cooling other hot process fluids .

WORKING OF NDCT’s :-

The hot water that is too be cooled in the cooling tower is pumped to the top of the tower at the hot water inlet. The hot water inlet is connected to a series of nozzles which sprays this water over the fills. Simultaneously air is introduced from the bottom of the tower which flows in upward direction. The air is circulated inside the cooling tower by Natural Convection. The fills provide a large surface area and time for air and water contact . The water then flows down through plastic or wooden fills. This causes drop formation. While flowing down, the water emits heat which mixes with the air flowing upward, causing it to cool down. Part of the water evaporates, causing it to emit more heat. Water vapor can sometimes be observed over the cooling tower. The cooled water falls down into the cold water basin which is discharged and can be then used for cooling various hot process fluids.

  

ADVANTAGES OF NDCT’s :-

1. FAN IS NOT INSTALLED AS NDCT’S RELY ON NATURAL AIR
2. POWER SAVING
3. EASY TO CLEAN
4. NO MECHANICAL NOISE
5. NO CORROSION PROBLEMS
6. MAINTAINANCE IS LOW
7. NO REPAIR PROBLEMS
8. NO RECIRCULATION OF AIR OCCURS AS THE PLUME IS REJECTED AT HIGH LEVEL
9. ENVIRONMENT FRIENDLY

DISADVANTAGES OF NDCT’s :-

1. A VERY LONG CONSTRUCTION TIME IS NEEDED TO CONSTRUCT NDCT.
2. HUGE WATER FLOW REQUIRED
3. NOT FOR ALL CLIMATIC CONDITIONS
4. PERFORMANCE DEPENDS ON PREVAILING WIND VELOCITY AND DIRECTION.
5. AIR MOVEMENT DEPEND ON INDUCTION FORCES
6. UNOBSTRUCTED LOCATION NECESSARY.
7. NATURAL DRAFT COOLING TOWERS REQUIRES VERY LARGE AREA.

       The NATURAL DRAFT COOLING TOWERS are of two types :

1. SPRAY TYPE NDCT’S

2. SPLASH DECK TYPE NDCT’S.