Classification of Cooling tower
The cooling tower might be classified into several types, but they are broadly categorized by the following considerations:
1. Whether there is direct or indirect contact
2. The mechanism used to provide the required
airflow
3. The relative flow paths of air and water
4. The primary materials of construction
5. the type of heat transfer media applied
6. The tower’s physical shape
The general classification of the cooling tower is briefed below:
Classification
By Build
👉 Package Type
👉 Field Erected Type
Package Type
This type
of cooling tower is pre-assembled and can be simply transported on trucks as
they are compact machines.
The capacity of package-type towers is limited and
for that reason, they are usually preferred by facilities with low heat
rejection requirements such as food processing plants, textile plants,
buildings like hospitals, hotels, malls, chemical processing plants, automotive
factories etc.
Due to the intensive use in domestic areas, sound
level control is a relatively more important issue for package-type cooling
towers.
Field Erected Type
Field-erected
type cooling towers are usually preferred for power plants, steel processing
plants, petroleum refineries, and petrochemical plants.
These towers are larger in size compared to package-type
cooling towers.
Classification
based on the heat transfer method
👉 Wet Cooling Tower
👉 Dry Cooling Tower
👉 Fluid Cooler
Wet
Cooling Tower
This type of cooling tower operates based on the evaporation
principle. The working fluid and the evaporated fluid (usually water) are one
and the same. In a wet cooling tower, the warm water can be cooled to a
temperature lower than the ambient air dry-bulb temperature, if the air is
relatively dry.
Dry
Cooling Tower
This tower operates by heat transfer through a
surface that separates the working fluid from ambient air, such as in a tube to
air heat exchanger, utilizing convective heat transfer. Dry cooling tower does
not use evaporation.
Fluid
Cooler
This tower passes the working fluid through a tube
bundle, upon which clean water is sprayed and a fan-induced draft applied. The
resulting heat transfer performance is much closer to that of a wet cooling
tower, with the advantage provided by a dry cooler of protecting the working
fluid from environmental exposure and contamination.
Classification
based on air draft
👉 Atmospheric Tower
👉 Natural Draft Tower
👉 Mechanical Draft Cooling Tower
Atmospheric
Tower
An atmospheric tower consist of a big rectangular
chamber with two opposite louvered walls. The tower is packed with a suitable
tower fill.
Atmospheric air enters the tower through the
louvers driven by its own velocity. An atmospheric tower is cheap but inefficient.
Its performance largely depends upon the direction
and velocity of wind.
Natural
Draft Tower
The natural draft or hyperbolic cooling tower makes
use of the difference in temperature between the ambient air and the hotter air
inside the tower. As hot air moves upwards through the tower (because hot air
rises), fresh cool air is drawn into the tower through an air inlet at the
bottom.
A natural draft tower is so called because natural
flow of air occurs through the tower.
Two factors are responsible for creating the
natural draft:
- A rise in temperature and humidity of air in the
column reduces its density, and
- The wind velocity at the tower bottom.
Due to the layout of the tower, no fan is required
and there is almost no circulation of hot air that could affect the
performance. But in some cases, a few fans are installed at the bottom to
enhance the air flow rate. This type of tower is called a ‘fan-assisted’
natural draft tower.
The hyperbolic shape is made because of the
following reasons:
- More packing can be fitted in the bigger area at the bottom of the shell;
- The entering air gets smoothly
directed towards the centre
because of the shape of the wall, producing a strong upward draft;
- Greater structural strength and stability of the shell is provided by
this shape.
The pressure drop across the tower is low and the
air velocity above the packing may vary from 1-1.5 m/s. The concrete tower is
supported on a set of reinforced concrete columns. Concrete is used for the
tower shell with a height of up to 200 m.
These cooling towers are mostly only for large heat
duties because large concrete structures are expensive.
Mechanical Draft Cooling Tower
B. Induced draft
A mechanical draft tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge.
This produces
low entering and high exiting air velocities, reducing the possibility of re
circulation in which discharged air flows back into the air intake.
Classification
based on airflow pattern
👉 Crossflow
👉 Counterflow
Crossflow
Crossflow 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. The air continues through the fill and thus past the water
flow into an open plenum area.
A distribution or hot water basin consisting of a
deep pan with holes or nozzles in the bottom is utilized in a crossflow tower.
Gravity distributes the water through the nozzles uniformly across the
fill material.
Counterflow
In a counterflow design the air flow is directly
opposite to the water flow (see diagram below). 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 airflow.
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