COOLING TOWERS: FUNCTION OF COOLING TOWER:
Cooling tower cools the warm water discharged from the condenser and feed the cooled water back to the condenser. Cooling towers are used for large size power stations specially in water shortage areas. A cooling tower is a semi-enclosed device for evaporative cooling of water by contact with air. It is a wooden, steel or concrete structure. Inside of cooling tower contains corrugated surfaces, troughs or baffles, tile tubing, perforated trags etc., Warm water from the exit of condenser is collected at the top of tower and allowed to trickle down through these troughs or baffles by gravity. Meanwhile, cold air is drawn from the bottom of tower and forced up in opposite direction to trickling droplets of water.
Thus heat transfer takes place between warm water and cool atmospheric air. As a result water gets cooled which is extracted out and pumped back to condenser for recirculation. To prevent the escape of water particles with air, drift eliminators are provided at the top of the tower.
It may be noted that in cooling tower, the water cools by
(i) evaporation and
(ii) heat transfer to air
In many towers 75% of cooling takes place by evaporation and the remainder by heat transfer.
TYPES OF COOLING TOWERS :
Cooling towers are mainly of two types, namely
1. Atmospheric or Natural Draft Cooling Towers
2. Mechanical Draft Cooling Towers
Ex : Forced Draft Cooling Towers
Induced Draft Cooling Towers
1. Atmospheric or Natural Draft Cooling Towers
Principle of operation and installation: This type of cooling tower cools water by moving air at low velocities through the tower. It is for this reason that the roof of a building is an excellent spot for location. Fig. shows a simple natural draft tower built of cypress or redwood or steel frame work.
Depending upon the capacity of the plant the height of tower varies from 4 m to 30 m or more. It has a number of decks equally spaced as shown so that water may drip or run freely from deck to deck. The warm water from condenser outlet is first distributed over the top deck and from there it drips down. The decks provide a large wetted area to air which moves cross wise through the tower and retard the fall of water. This way water gets cooled by the time it reaches the bottom of tower from where it is pumped back to condenser inlet.
The sides of the tower are open to permit breeze from all directions to flow across all of the wetted decks. The air blowing through the open spaces carries away a fine spray of water which it picks up from the falling water. Louvers are provided around the sides of the tower to prevent the wind from blowing the moisture spray and depositing upon adjacent surfaces where it may be objectionable.
The louvers usually consist of sloping baffle boards which run around the edge of every deck. Since the baffle boards are slanted and overlap, the air which leaves each surface of deck is deflected upward. Thus, most of the entrained moisture is deposited on the baffle boards from where it runs off. In this process, the loss of water due to evaporation is minimised to less than 5% of total water circulated.
This type is widely used because of low operational and maintenance.
2. Mechanical Draft Cooling Towers :
There are two main types of mechanical draft cooling towers.
(a) Forced draft type and
(b) Induced draft type
Principle of operation of either type of these towers is same. Warm water from the condenser unit is admitted into the tower at the top section as shown. It is sprinkled down in the form of broken spray.
Atmospheric air is forced up the tower from the bottom in case of forced draft tower or is sucked from the top in case of induced draft tower. The air velocity may range from 70 to 100 m/min.
Fine particles of falling water come in close contact with the unsaturated air drawn from atmosphere into the tower. Air being at low DBT than that of water, it absorbs sensible heat. It also absorbs certain quantity of water vapour from falling water and in this process some water particles evaporate. Evaporation causes cooling of the non-evaporated bulk of water which drops to the bottom of the tower. The atmospheric air warm or humid leaves the tower from the top of it.
The cooled water collected at the bottom of the tower is pumped into the inlet of condenser.
Cooling towers can be made of wood, metal or masonry. The baffles or filling materials may consist of fine pieces of wood. Reduction-of temperature water in cooling tower (temp. of water at entry-temp. at exit) is called 'Range'(= 7°C). Difference between temperature of leaving water and WBT of entering air is called 'Approach' (5°C). Performance of cooling tower is often expressed in terms of 'range' and 'approach'.
A cooling tower configuration some times, used for large capacity applications has hyperbolic shape, and resembles a chimney 50 to 100 m high.
Thus in a cooling tower heat transfer from water to unsaturated air by difference in dry bulb temperatures, and by the difference in vapour pressure between water surface and air.