Page 209 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.2 Cooling tower 207

Induced draft: The draft of hot, moist air is created by fans mounted at the air exit/tower top. This
produces low entering and high exit air velocities, reducing the possibility of moist air recirculation
into the air intake. This is the most common type for moderate-sized cooling towers where the liquid
3
2
load is around 12e20 m /m $ hr.
Classification based on airflow pattern

Crossflow Towers: These have airflow perpendicular to water downflow. Air enters through one or
more vertical faces of the tower to meet the fill material and flowing water (perpendicular to the air)
that descends through the fill by gravity. Water distributor in these towers consists of a deep pan with
holes or nozzles in the bottom. The exiting air in larger towers usually flows into an open plenum area.
Crossflow towers are thermodynamically less efficient and are mostly for small capacity. These
generally use induced draft, but crossflow natural draft towers are also in use.
Counterflow Towers: These have airflow opposite to water flow. Air first enters an open area below
the fill media and then moves up vertically through water, which flows downward by gravity through
the fill. Counterflow configuration ensures higher thermodynamic efficiency and is used for large
capacity towers. These can have either forced or induced draft configuration.

Classification based on the heat transfer method
Wet Cooling Towers: These lower the water temperature by evaporative cooling. A small part of the
water evaporates in the tower by absorbing the heat of vaporization (latent heat) from the remaining
water, which gets cooled in the process. Thus cooling in a wet cooling tower is a combined heat and
mass transfer process and the heat transfer is predominantly by latent heat. These are the most common
types of cooling towers and shall, henceforth, be referred to as cooling towers.
Dry Cooling Towers: These are basically air-cooled exchangers used for cooling circulating water by
the transfer of sensible heat. Since air and water are not in direct contact, there is no evaporation loss or
contamination of water, and this is particularly advantageous in areas having water shortage. The cooling
water recirculates in a closed loop between the process equipment/heat exchanger and the cooler.
Fluid Coolers: In these coolers, the circulating cooling water passes through a tube bundle, upon
which water is sprayed and a fan-induced draft is applied to cool the external surface of the tube. This
is a case of circulating water surface cooler and the cooling tower combined as a single unit. The
resulting heat transfer performance is close to a wet cooling tower with the advantage of protecting the
circulating cooling water from air exposure and picking up dirt. Fluid coolers avoid cooling water
contamination due to exposure to atmospheric air and are used when the cooling water quality is
important. 2

7.2.2 Components of a typical cooling tower
The schematic diagram of a typical “induced-draft countercurrent” cooling tower is shown in Fig. 7.2.
The basic components are described below e
Cell: Mechanical cooling towers are constructed with one or more square cells. The cells are
placed side by side, sharing a common partition. Air entry/exit is through the opposite sides. The cell

2
More on the quality specification of cooling water is available in Ch. 18.