Page 208 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

206 Chapter 7 Industrial cooling systems

Classification by build
Packaged Towers: These are common facilities for low heat rejection requirements, such as food
processing plants, textile plants, hospitals, hotels, malls, chemical processing plants, automotive
factories, etc. The preassembled towers are transported and fixed at an appropriate location. Due to
their intensive use in domestic areas, sound level control is an important issue.
Field erected towers: These are larger compared to the packaged type and common in petroleum
refineries, petrochemical complexes, power plants, steel plants, fertilizer plants and other process
industries.

Classification based on air draft
Atmospheric Towers: These are rectangular chambers with fills having two opposite louvered walls for
entry of atmospheric air driven by its own velocity. The air inside is moist and is also warmer than the
ambient air due to contact with hot water. The presence of water vapor (molecular weight-18) makes
moist air lighter than dry air (molecular weight-28.8) at the same temperature. This creates an updraft
in the tower due to the buoyancy of moist and warmer air. As hot air moves upwards through the tower,
fresh cool air is drawn in. These towers are cheap but inefficient and are not economical for high
capacities. Its performance depends upon the direction and velocity of wind.
Natural Draft Tower: Natural draft cooling towers are tall, up to 200 m and have a hyperbolic
shape. The draft is generated by the difference in density between the ambient air and the moist,
warmer air inside the tower. Atmospheric air flowing at an altitude across the top of these tall towers
creates an additional draft. Fresh cool air is drawn in through the air inlet at the bottom. These towers
use very large concrete chimneys to direct air through the fill media. The natural draft created is
sufficient and no fan is required. There is almost no recirculation of hot air that could affect the
performance in these tall towers.
Natural draft towers can be crossflow type or countercurrent flow type. In crossflow configuration,
the packing is external to the tower and the ambient air drawn through the fill is in crosscurrent flow to the
falling water. In counterflow towers, the fill is inside and an updraft of air flows through it. Concrete is
used for the tower shell supported on a set of reinforced concrete columns. The hyperbolic shape allows
more packing to be fitted in the bigger area at the bottom of the shell. This shape also provides greater
structural strength and also directs the air to flow smoothly towards the center, thus aiding the upward
draft. The pressure drop across the tower is low and the air velocity above the packing may vary from 1 to
1.5 m/s. These cooling towers are for large heat duties and are generally used for water flow rates
3
above 45,000 m /hr because large concrete structures are expensive. Natural draft cooling towers are
usually economical for substantially large cooling requirements in industries like steel plants.
Because of their large size, construction difficulties and cost, natural draft towers have been
replaced by mechanical draft towers in many installations when economics favor such an option.
Mechanical Draft Towers: These employ large fans to force or draw air through the water stream.
The water falls downwards over fill surfaces, which increases the contact between the water and the air.
Cooling rates of mechanical draft towers depend upon various parameters, such as fan diameter and
speed, fill characteristics, etc.
The two different classes of mechanical draft cooling towers are e
Forced draft: These have one or more fans located at the air entry to force air into the tower. The
vertical fans force air at a low velocity horizontally through the packing and then vertically against the
downflow of water. The drift eliminators located at the tower top disengages the water droplets
entrained in the air. The fans handle mostly dry air, significantly reducing erosion and water
condensation problems.