COOLING TOWER BASICS

COOLING TOWER TERMINOLOGYCOOLING TOWER FUNCTIONCOOLING TOWER TYPES

COMPILED BYRUPESH G DESAIEXECUTIVE(ENGG SERVICES)

COOLING TOWER TERMINOLOGY

EVAPORATIVE COOLINGHEAT TRANSFER WHERE A LIQUID CONDENSES INTO ITS GASOUS STATE THEREBY GIVING UP ITS LATENT HEAT

RANGEIT IS THE DIFFERENCE BETWEEN THE WATER TEMPERATURE ENTERING THE COOLING TOWER HOT WATER DISTRIBUTION SYSTEM AND COLD WATER TEMP LEAVING THE SUMP OF COOLING TOWER.

The cooling tower efficiency can be expressed as: = (ti - to) 100 / (ti - twb) where = cooling tower efficiency - common range between 70 - 75%ti = inlet temperature of water to the tower (oC, oF)to = outlet temperature of water from the tower (oC, oF)twb = wet bulb temperature of air (oC, oF)

DRIFT Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same concentration of impurities as the water entering the tower. The drift rate is typically reduced by employing baffle-like devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower.

BLOW-DOWN The portion of the circulating water flow that is removed in order to maintain the amount of dissolved solids and other impurities at an acceptable level.

BLOW-OUT Water droplets blown out of the cooling tower by wind, generally at the air inlet openings. Water may also be lost, in the absence of wind, through splashing or misting. Devices such as wind screens, louvers, splash deflectors and water diverters are used to limit these losses.

Water Make-up Water losses include evaporation, drift (water entrained in discharge vapor), and blow down (water released to discard solids). Drift losses are estimated to be between 0.1 and 0.2% of water supply. Evaporation Loss = 0.00085 * water flow rate*(T1-T2) Blow down Loss =Evaporation Loss/(COC-1)where COC is the ratio of solids in the circulating water to the solids in the make-up water Total Losses = Drift Losses + Evaporation Losses + Blow down Losses

DRY BULB TEMPERATURETEMPERATURE OF AMBIENT AIR MEASURED IN REGULAR MANNER WITH CONVENTIONAL INSTRUMENTS

DEW POINTTEMPERATURE AT WHICH GIVEN MIXTURE OF AIR AND WATER WILL HAVE RELATIVE HUMIDITY OF 100% SATURATOIN

WET BULB TEMPERATUREThe lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place.

PLENUMTHE ENCLOSED SPACE BETWEEN THE DRIFT ELIMINATORS AND THE FAN STACK IN INDUCED DRAFT TOWERS OF THE ENCLOSED SPACE BETWEEN THE FAN AND FILLING IN THE FORCED DRAFT TOWER

PLUMEVISIBLE EXAUST FROM COOLING TOWER

FAN PITCHTHE ANGLE WHICH A FAN BLADE MAKES WITH THE PALNE OF ROTATON , DEGREES FROM HORIZONTAL

CROSS FLOW

FUNCTION OF COOLING TOWER

The primary task of a cooling tower is to reject heat into the atmosphere. This heat rejection is accomplished through the natural process of evaporation that takes place when air and water are brought into direct contact in the cooling tower. The evaporation is most efficient when the maximum water surface area is exposed to the maximum flow of air, for the longest possible period of time

FUNCTION OF COOLING TOWERThe primary task of a cooling tower is to reject heat into the atmosphere. This heat rejection is accomplished through the natural process of evaporation that takes place when air and water are brought into direct contact in the cooling tower. The evaporation is most efficient when the maximum water surface area is exposed to the maximum flow of air, for the longest possible period of time.Cooling towers are designed in two different configurations, counter flow and cross flow. The specific configuration indicates the direction of air flow through the tower relative to the direction of the water flow. Cooling tower water and air distribution systems are designed in concert, with each playing an equally important role in determining the efficiency and proper application of the cooling tower.

COOLING TOWER TYPES

ATMOSPHERIC MECHANICAL DRAFT a. FORCED DRAFT b. INDUCED DRAFT HYBRID DRAFT TYPED BY AIR FLOW a. COUNTERFLOW b. CROSSFLOW b.1 DOUBLE-FLOW b.2 SINGLE-FLOW c. SPRAY-FILLED TYPED BY CONSTRUCTION a. FIELD-ERECTED b. FACTORY-ASSEMBLED TYPED BY SHAPE a. RECTILINEAR b. ROUND MECHANICAL DRAFT (RMD) TYPED BY METHOD OF HEAT TRANSFER a. EVAPORATIVE b. DRY TOWER c. PLUME ABATEMENT

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:

ATMOSPHERIC COOLING TOWERS

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 cross flow or counter flow with respect to the falling water. Cross flow indicates that the airflow is horizontal in the filled portion of the tower while counter flow means the air flow is in the opposite direction of the falling water. The counter flow tower occupies less floor space than a cross flow tower but is taller for a given capacity. The principle advantages of the cross flow 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 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

FORCED DRAFT

FORCED DRAFT 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.

INDUCED DRAFT

HYBRID DRAFT

They are equipped with mechanical draft fans to augment airflow. Consequently, they are also referred to as fan-assisted natural draft towers. The intent of their design is to minimize the horsepower required for the air movement, but to do so with the least possible stack cost impact. Properly designed the fans may need to be operated only during periods at high ambient and peak loads.

HYBRID DRAFT

CHARACTERIZATION BY AIR FLOW COUNTERFLOW: IN the counter flow 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 use of low pressure gravity-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 counter flow tower also restricts exposure of the water to direct sunlight, thereby retarding the growth of the algae.

COUNTER FLOW

CROSSFLOW: The cross flow towers have a fill configuration through, 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 cross flow 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.

SINGLE FLOW

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

SPRAY FILLED

CHARACTERIZATION BY CONSTRUCTION

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.

TYPED BY SHAPE

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.

RECTILINEAR

ROUND MECHANICAL DRAFT

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 of 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.

HOT WATER DISTRIBUTION SYSTEMS

The overall efficiency of a cooling tower is directly related to the design of the tower's hot water distribution system. The primary consideration in selecting the type of hot water distribution system for a specific application is pump head. The pump head imposed by a cooling tower consists of the static lift (related to the height of the inlet) plus the pressure necessary to move the water through the distribution system and over the fill. The pump head varies according to the cooling tower configuration.Counter flow towers use a high pressure spray nozzle hot water distribution system to achieve water coverage of the fill. The nozzle spray pattern is sensitive to changes in water flow, and consequent change in nozzle pressure. The air movement is vertically upward through the fill, counter to the downward fall of the water (Figure 1). Counter flow towers typically have a smaller footprint than cross flow towers, but require additional height, static lift, and dynamic head to achieve the same cooling effect.

Cross flow towers utilize a distinctly different type of water distribution system. Hot water is distributed to the fill by gravity through metering orifices in the floor of the inlet basin. There is no pressure spray distribution system. The air movement is horizontally through the fill, across the downward fall of the water (Figure 2). In cross flow towers, the internal pressure component of pump head is insignificant because maximum flow is achieved by gravity.Compared to cross flow towers, counter flow towers may require up to five or six psig added pump head to achieve the proper spray distribution. The high counter flow pumping head requirement (tower height plus nozzle pressure) leads to a higher first cost pumping system and significantly higher annual pump energy consumption and operating costs. If the system condenser pumps are not properly sized, the additional pump head required in counter flow towers may result in inadequate hot water flow, reducing tower efficiency and performance

AIR FLOW DISTRIBUTION SYSTEMS Cooling tower performance is also related to the amount of air moving through the tower and coming into direct contact with the water. In counter flow towers the air movement is vertically upward through the fill, counter to the downward fall of the water. This configuration, along with the finer water droplet size available from pressurized spray nozzles, allows counter flow towers to make more efficient use of available air. However, the resistance to upward air travel against the falling water results in higher static pressure loss and greater fan horsepower than a cross flow system.Cross flow towers have a fill configuration through which air flows horizontally across the downward flow of the water. Cross flow towers utilize essentially the full tower height for inlet louvers, reducing air inlet velocity and minimizing recirculation and drift loss. The air inlet louvers in counter flow towers are restricted to the tower base, increasing inlet velocities and susceptibility to airborne trash and other debris

CROSS FLOW

CROSS FLOW ADVANTAGES OF CROSSFLOW cooling towers due to their gravity flow hot water distribution system:Low pumping head. Lower first cost pumping systems. Lower annual energy consumption and operating costs. Accepts larger variation in water flow without adverse effect on the water distribution pattern (flat plate heat exchanger operation in winter). Easy maintenance access to distribution nozzles.

DISADVANTAGES OF CROSSFLOW cooling towers due to their gravity flow hot water distribution system:Low pressure head on the distribution pan may encourage orifice clogging and less water breakup at spray nozzle. Exposure to air in the hot water basin may accelerate algae growth. Larger footprint.

COUNTERFLOW

COUNTERFLOWADVANTAGES OF COUNTERFLOW cooling towers due to their pressurized spray water distribution system:Increased tower height accommodates longer ranges and closer approaches. More efficient use of air due to finer droplet size from pressure sprays.

DISADVANTAGES OF COUNTERFLOW cooling towers due to their pressurized spray water distribution system:Increased system pumping head requirements. Increased energy consumption and operating costs. Distribution nozzles difficult to inspect and clean. Requires individual risers for each cell, increasing external piping costs.

CROSSFLOW

ADVANTAGES OF CROSSFLOW cooling towers due to their horizontal air distribution system:Low static pressure drop. Reduced drift. Reduced recirculation. More air per fan horsepower. Larger diameter fans can be used so that fewer cells are required for a given capacity. Lower energy and operating costs.

DISADVANTAGES OF CROSSFLOW cooling towers due to their horizontal air distribution system:Larger louver surface area makes icing more difficult to control.

COUNTERFLOW

ADVANTAGES OF COUNTERFLOW cooling towers due to their vertical air distribution system:The vertical air movement across the fill allows the coldest water to be in contact with the driest air maximizing tower performance.

DISADVANTAGES OF COUNTERFLOW cooling towers due to their vertical air distribution system:The resistance to upward air travel against the falling water results in higher static pressure loss and a greater fan horsepower than in cross flow towers. The restricted louver area at the base with high velocity of inlet air increases the fan horsepower. Tendency for uneven distribution of air through the fill with very little movement near the walls and center of the tower. High inlet velocities are liable to suck airborne trash and dirt into the tower.

CONCLUSIONS AND RECOMMENDATIONS

The air and water distribution systems for counter flow and cross flow cooling towers have advantages and disadvantages inherent in their respective designs. It cannot be said that one is better than the other. Rather, with the proper application, both configurations are cost effective and can serve the end user well.Cross flow cooling towers should be specified when the following criteria and limitations are important:To minimize pump head. To minimize pumping and piping first costs. To minimize operating costs. When condenser water flow variance is expected. When ease of maintenance is a concern. Counter flow cooling towers should be specified when the following criteria and limitations are important:When space (footprint) is restricted. When icing is of extreme concern. When pumping is designed for additional pressure drop.