# Cooling towers

COOLING TOWERS INTRODUCTION

The machines and processes of industry, as well as those devoted to human comfort and well being generated tremendous amounts of heat, which must be continuously, dissipated if those machines and processes are to continue to operate efficiency. Although this heat is usually transferred to a cool, flowing volume of water, final rejection is always to the atmosphere and, invariably, is accomplished by some form of heat exchanger.

The natural process of evaporation makes them very effective heat transfer mediums, although somewhat inefficient due to their limited surface area and their total dependence upon random winds.

TYPES OF COOLING TOWERS

Cooling towers are designed and manufactured in several types:

1. ATMOSPHERIC
2. MECHANICAL DRAFT

a. FORCED DRAFT

b. INDUCED DRAFT

1. HYBRID DRAFT
2. TYPED BY AIR FLOW

a. COUNTERFLOW

b. CROSSFLOW

a.1 DOUBLE-FLOW

a.2 SINGLE-FLOW

c. SPRAY-FILLED

1. TYPED BY CONSTRUCTION

a. FIELD-ERECTED

b. FACTORY-ASSEMBLED

1. TYPED BY SHAPE

a. RECTILINEAR

b. ROUND MECHANICAL DRAFT (RMD)

1. TYPED BY METHOD OF HEAT TRANSFER

a. EVAPORATIVE

b. DRY TOWER

c. PLUME ABATEMENT

1. ATMOSPHERIC

The atmospheric cooling towers utilize no mechanical fan to create air flow through the tower, its air is derived from a natural induction flow provided by a pressure spray.

We can see it in the following picture:

1. MECHANICAL DRAFT

Mechanical draft towers uses fans (one or more) to move large quantities of air through the tower. They are two different classes:

• Forced draft cooling towers
• Induced draft cooling towers

The air flow in either class may be crossflow or counterflow with respect to the falling water. Crossflow indicates that the airflow is horizontal in the filled portion of the tower while counterflow means the air flow is in the opposite direction of the falling water.

The counterflow tower occupies less floor space than a crossflow tower but is taller for a given capacity. The principle advantages of the crossflow tower are the low pressure drop in relation to its capacity and lower fan power requirement leading to lower energy costs.

All mechanical towers must be located so that the discharge air diffuses freely without recirculation through the tower, and so that air intakes are not restricted. Cooling towers should be located as near as possible to the refrigeration systems they serve, but should never be located below them so as to allow the condenser water to drain out of the system through the tower basin when the system is shut down.

FORCED DRAFT

The forced draft tower, shown in the picture, has the fan, basin, and piping located within the tower structure. In this model, the fan is located at the base. There are no louvered exterior walls. Instead, the structural steel or wood framing is covered with paneling made of aluminum, galvanized steel, or asbestos cement boards.

During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. The drift eliminators located at the top of the tower remove water entrained in the air. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems.

INDUCED DRAFT

The induced draft tower show in the following picture has one or more fans, located at the top of the tower, that draw air upwards against the downward flow of water passing around the wooden decking or packing. Since the airflow is counter to the water flow, the coolest water at the bottom is in contact with the driest air while the warmest water at the top is in contact with the moist air, resulting in increased heat transfer efficiency.

1. HYBRID DRAFT

Tgey are equiped with mechanical draft fans to augment airflow. Consequenly, they are also referred to us fan-assisted natural draft towers. The intent of their desing is to minimize the horsepower required for the air movement, but to do so with the least possible stack cost impact. Properly desogned the fans may need to be operated only during pereiods ao high ambientsand peak loads.

1. CHARACTERIZATION BY AIR FLOW

The cooling towers by the relative flow are divided in several groups :

COUNTERFLOW:

IN the counterflow towers, the air moves vertically upward through the fill, counter to the downward fall of water. Because of the need for extended intake and discharge plenums; the use of high pressure spray systems; and the typically higher air pressure losses, some of the smaller counter flow towers are physically higher; require more pump head; and utilize more fan power than their cross flow counterparts. In a larger counter flow towers, however, the us of low pressure grativity-related distribution systems, plus the availability of generous intake areas and plenum spaces for the air management, is tending to equalize, or even reverse, this situation. The enclosed nature of a counterflow tower also restricts exposure of the water to direct sunlight, thereby retarding the growth of the algae.

CROSSFLOW:

The crossflow towers have a fill configuration throught, which the air flows horizontally, across the downward fall of water. Water to be cooled is delivered to hot water inlet basins located atop the fill areas, and is distributed to the fill by gravity throught metering orifices in the floor of those basins.

The crossflow towers can be divided in:

# DOUBLE-FLOW:

In this kind of towers the fan is inducting air through two inlets and across two banks of fill.

SINGLE-FLOW:

This kind of towers only has one air inlet and one fill bank, the remaining three sides of the towers being cased. Single-flow towers are customarily used in locations where are unrestricted air path to the tower is available from only one direction.

1. SPRAY – FILLED

This kind of towers has not a heat transfer surface, depending only upon the water break-up af-forded by the distribution system to promote maximum water-to-air

characterization by construction

we can see two different kinds of cooling towers by construction:

• Field-erected
• Factory-assembled

Field-erected:

The field-erected cooling towers are those on which the primary construction activity takes place at the site of ultimate use. All large towers, and many of the smaller towers, are prefabricated, piece-market and shipped to the site for the cooling towers manufacturer usually provides final assembly.

FACTORY-ASSEMBLED:

The factory-assembled cooling towers undergo virtually complete assembly at their point of manufacture, whereupon there are shipped to the site in as a few sections as mode of transportation will permit.

1. TYPED BY SHAPE

There are two different types:

RECTILINEAR:

These towers are constructed in cellular fashion, increasing linearly to the length and numbers of cells necessary to accomplish a special thermal performance.

ROUND MECHANICAL DRAFT:

Are towers as the name implies, are essentially round in plan configuration, with fans clustered as close practicable around the center point of the tower. Multi-faceted towers, such as the octagonal mechanical draft (OMD) also fall in the general classification of “round” towers.

1. TYPED BY METHOD OF HEAT TRANSFER

All of the cooling towers described here are evaporative type towers, in that they derive their primary cooling effect from the evaporation that takes place when air and water are brought into the direct contact. At the other end os the spectrum is the Dry tower, where by full utilization of dry surface coil sections, no direct contact (and no evaporation) occurs between air and water. Hence sensible heat transfer cools the water totally.

IN between these extremes are the plume abatement and water conservation towers, wherein progressively greater portions of dry surface coil sections are introduced into the overall heat transfer system to alleviate specific problems or to accomplish specific requirements.