Basic concept of cooling tower and factors affecting in cooling Tower performance.

Cooling Tower Types

Cooling towers fall into two main categories: Natural draft and Mechanical draft.

Natural draft towers use very large concrete chimneys to introduce air through the media. 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 only by utility power stations.

Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air this helps maximize heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation
.

Mechanical draft towers are available in the following airflow arrangements:

1. Counter flows induced draft.

2. Counter flow forced draft.

3. Cross flow induced draft.

Heat Exchangers

Heat exchangers are devices designed to transfer heat from one fluid to another without the fluids coming into contact. There are a wide variety of applications for heat exchangers, for example: radiators, air conditioning and power plants.

Mechanism of Heat Transfer by Conduction

The transfer of heat energy by conduction takes place within the boundaries of a system.

The rate equation which describes this mechanism is given by Fourier Law

Where    = rate of heat flow in X-direction by conduction in J/S or W,

k = thermal conductivity of the material. It quantitatively measures the heat conducting ability and is a physical property of t he material that depends upon the composition of the material, W/mK,

A = cross-sectional area normal to the direction of heat flow, m2,

dT/dx = temperature gradient at the section,

Thermal Conductivity of Materials

Thermal conductivity is a physical property of a substance and In general, It depends upon the temperature, pressure and nature of the substance. Thermal conductivity of materials are usually determined experimentally and a number of methods for this purpose are well known.

Natural Draft Cooling Towers

The natural draft or hyperbolic cooling tower is designed to take advantage of the temperature differences between the ambient air and the hotter air inside the tower. The design creates a chimney effect that causes the cold air at the bottom of the tower to push the warmer air out the top.

The natural draft-cooling tower further divided in two types.

Cross flow:-
In a cross flow tower, air is drawn across the falling water. In this design the fill is located outside the tower.

Counter flow:-
The fill is contained within a counter flow tower since the air is drawn up and through the failing water. Design selection depends upon conditions at the particular site.