Cooling Towers - Classifications and Working

Cooling towers - Classifications.
Natural and Forced Cooling towers.
Parts and performance parameters

1. COOLING TOWERS

2. A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Applications include HVAC systems Thermal power plants Oil refineries Chemical plants Electric power generation INTRODUCTION

3. CLASSIFICATIONS Based on Heat Transfer method Base on Air flow generation Based on Air to Water flow

4. HEAT TRANSFER METHOD Based on heat transfer mechanism cooling towers are classified as Dry cooling towers: Uses convection to transfer heat . Heat is transferred through a surface that separates the water from ambient air. Used when cooling water is less

5. Wet cooling tower : Uses evaporation to transfer heat. Water can be cooled to a temperature lower than the ambient air “dry-bulb” temperature. Most commonly used type.

6. AIR FLOW GENERATION NATURAL DRAFT Natural draft utilizes buoyancy via a tall chimney. Warm air inside the tower becomes less dense compared to air outside This induces buoyancy forces. Due to this the denser air outside replaces the less denser air inside. Natural draft towers are typically about 120 m high, depending on the differential pressure between the cold outside air and the hot humid air on the inside of the tower as the driving force.

7. Mechanical draft tower Uses power driven fans to draw air through the tower. They are of 2 types Induced Draft :  A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower.  Forced Draft : A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower

8. Induced draft A fan mounted on the top of the cooling towers sucks the air in by creating a negative pressure gradient Smaller compared to natural draught towers Both counter and cross flow configurations are used.

9. Forced draft Uses a blower fan to force air into the tower Requires lesser power compared to induced , bcoz cool air is being pumped

10. Air to Water Flow Cross flow is a design in which the air flow is directed perpendicular to the water flow. Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity.

11. In a counter flow design the air flow is directly opposite of the water flow. Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.

12. Performance parameters Approach : Difference between the exit temperature and WBT of water. This represents a loss in cooling effect. Usual range is 6 – 8 o C Range : Difference between Inlet and exit temperatures of water. Usual range is 6 – 10 o C. Cooling efficiency : Ratio of actual cooling to the maximum possible cooling possible.

13. Water Losses Evaporation : water that evaporates and leaves along with air. Usually around 1 -1.5% of water. Drift : Fine water droplets entrained and carried away by air. Drift eliminators are installed to eliminate that. Blow down : To maintain a certain solid concentration, some amount of water is removed from cold water basin. Around 1- 1.5 % of the amount. Makeup water is supplied to makeup for these losses.

14. Parts of Cooling tower Frame : Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. Fill : Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. Drift eliminators: They capture water droplets entrapped in the air stream by causing an abrupt change in flow direction of outlet air.