Cooling System & Maintenance

After studying this chapter, student will beable to:List and describe the basic components of a cooling tower.Explain how cooling tower operate.Explain cooling tower standards and maintenance.

Cooling towers (figure 6.1)A heat transfer devices designed to cool water for reuse.They cool hot water by bringing it into direct contact with air, using countercurrent or crossflow patterns.Contains wood or plastic slats, called fill, that direct airflow and the flow of water falling from the top of tower.The downward flowing water coats the fill and forms a film, thereby increasing the surface area for contact between the cool air and hot water.

Hot water transfers heat to the cooler air it contact in the tower.This process results in both sensible heat loss and evaporation.Sensible heat is the heat that can be measured or felt.When water changes to vapor, the vapor takes heat energy with it, leaving behind the cooler liquid.

Evaporation, which accounts for 80% to 90% of the heat loss, is the most critical factor in cooling tower efficiency. It is affected by relative humidity (the amount of water in a given quantity of air at given temperature), temperature and wind velocity.Other factors that affect cooling tower efficiency :Tower designWater contaminationEquipment problem

The temperature difference (T) between the inlet air temperature (wet-bulb) and the outlet temperature is referred to as the approach to the tower.The temperature difference between the hot and cold water is referred to as the cooling range.

Two ways to measure temperatureDry-bulb temperature (DBT)Wet-bulb temperature (WBT)Wet-bulb temperature takes into account the relative humidity, whereas dry-bulb temperature does not.Wet-bulb temperature usually are lower than dry-bulb temperatures.

Water distribution system A pipe located at the top of most towers.As hot process water returns to the tower, it enter this part.Then, water is sprayed or allowed to fall down into the tower over the splash bar and fill.Splash barDirect the downward flow of water and increase the surface area available for air water contact.

Fill The material inside a tower that directs the flow of water and air.Can be arranged in patterns that produce either counterflow or crossflow.BasinPumps suction water from the water basin and discharge it into cooling water supply header.The supply header distributes water to process exchangers, where it absorbs heat and returns to the top of the cooling tower return header.Drift eliminatorsPrevent water from being blown or sucked out of the tower.

Drift lossWater loss in the process of blown or sucked out of the tower.Also called as windage loss.Blow- downMake up water is added to replace water that has been loss by evaporation or blow- down.Induced draftInduced draft cooling tower use fans to pull air out of the system.

Forced draftForced draft cooling tower use fans to push air into the system. Air intake louversSlats located on the side of the tower to direct airflow.Can be fixed or movable depending on the tower the tower design.

Cooling tower are classified by how they produce airflow and the direction the airflow takes in relation to the downward flow of water.The following are classification of cooling tower:Atmospheric Cooling TowerNatural- draft Cooling TowerForced- Draft Cooling TowerInduced draft cooling tower

Wind moves air into and out of the tower. Airflow rates are determined by wind velocity.The tower is designed so that winds blow in horizontally, so the air moves in a crossflow direction.Cool air enters the tower through the louvered sides and passes across the downward flowing hot water.As the air becomes heated by contact with the hot water, it travels up because hot air rises. This air is moving in a counterflow direction, opposite the direction of the falling water.

The wet-bulb temperature ( the single most important factor in cooling tower perfomance) can be described in several ways;The lowest theoretical temperature to which water can be cooled in the tower.The temperature of the air saturated with water (also referred as dewpoint of air)A theoretical temperature that cannot be reached, only approached.

The top of this tower has drift eliminators to stop water loss when wind velocity surges.Location of this tower is important because wind velocities of 4.5 to 6.5 mph are required in order for it to function properly.These tower are designed for water leaving the tower to be 4 or 5oF lower than the wet-bulb temperature of entering air.These tower have a 30 to 55% efficiency rating for cooling water.

It is very cost effective because the system does not require a fan.However, its efficiency can fluctuate greatly because it depends on an uncontrollable factor, wind velocity.Heat transfer drops significantly with the loss of airflow.

Draw a simple atmospheric cooling tower. List and describe seven components.

Hyperbolic, or chimney , towers are natural- draft towers that have a large stack, or chimney.Usually associated with power plant operation.Commercial towers are typically around 310 feet high with a lower diameter of 210 feet and throat around 120 feet that gradually widens to 134 feet at the top.Designed for flows in excess of 500 000 GPM ( gallons per minute).Airflow is produced by temperature induced density differences inside and outside the stack.

Hyperbolic towers can have fill patterns that are either crossflow or counterflow.Airflow rates are higher in a crossflow tower, but evaporative heat transfer is more efficient in a counterflow tower.The fill and water distribution system are located below the chimney or stack.Hot water is pumped to a distribution system that is much lower than would be found in an atmospheric tower.

During operation, a natural-draft tower resembles a large smokestack.Air enters the cooling tower at the base and is directed into the internal fill pattern.As hot water drops through the fill, it is exposed to the cooler air.Density changes inside the chimney create the required upward draft.Heat is removed through the chimney.Natural- draft efficiency is linked to the relative humidity and the temperature of the outside air.

Force air in mechanical by the use of fans. (fig. 8.5) on the lower side of the tower.Usually have solid sides without louvers.The fans push in 100% of the process air.Flow direction is counterflow; the fans push air upward against the downward flow of water.Air in this tower can have high velocities, but the exiting air slows so much that it is recirculated back into the tower, cutting efficiency by 20%.

Have much higher heat transfer rates than atmospheric and natural-draft cooling towers, and they are significantly lower in heightLess efficient than induced- draft tower because some hot air is recirculated.

Produces air- flow mechanically (fig 8.6)Differs from the forced-draft cooling tower in that it pulls air out of the tower rather than forcing it in.Airflow in this tower is slow than in a forced-draft tower, but heat transfer through evaporation is more efficient.The tower fan, located on top of the tower (fig 8.7 and 8.8) produces discharge rates strong enough to lift the hot air above the tower, so hot air is not recirculated into the tower.

It can circulate airflow horizontally (crossflow) or vertically (counterflow).During crossflow operation, drift eliminators are located in the center of the tower to reduce water loss.Counterflow operations force air vertically across a solid area of fill, and drift eliminators are located above the fill and water distribution header.

Discuss each of the cooling towers in terms of their working principle.

In the manufacturing environment, heat exchangers and cooling towers work in hand to create a water- cooling system.As cool water is pumped from the tower to a heat exchanger, the hotter process fluid transfers heat to the cooling tower.The water in turn, removes heat from the process fluid.The process flow continues on to the next step, whereas the hot wateris return to the tower to be cooled.

A centrifugal pump sends the hot water to the top of the tower, where it enters the water distribution header.The hot water is distributed evenly throughout the tower, where much of the heat it had gained is given up in evaporation.The cooled liquid is recirculated back to the heat exchanger and the process loop continues.

Hot water dissolves solids faster than cold water.Hot water dissolves a little bit of everything it comes into contact with. By the time the hot water return to the tower, it is full of suspended solids. When it undergoes evaporation, the remaining fluid concentrates in the basin.Hot water also has a tendency to become corrosive and to form deposited.

Materials used to construct a cooling tower must be durable and capable of withstanding wide temperature difference.Treated wood, cedar, cypress , redwood, and plastic usually are used as construction materials.Operaters face the following problem:Suspended solid can accumulate in the water and eventually form deposit (scale)Electrochemical reactions with metal surfaces cause corrosion to Silt, debris, and algae foul and plug exchanger tubes.Fungi and bacteria cause wood decay.

The problem of suspended solids is controlled by a process called blow-down.Scale- forming solid can be removed with softening agents or suspension of solids can be prevented by adding chemicals.Another approaches is to precipitate the scale so it can be removed (to precipitate is to get particles fall from suspension.Corrosion can be minimized by the addition of chemical inhibitors, which form a film that protects metal.

Fouling can be controlled by filtering devices, alone or with dispersant that prevent suspended solids from accumulating.Biocides (such as chlorine or bromine) can be used to prevent wood decay.

pH of waterTotal dissolves solids (TDS)Inhibitor concentrationChlorine or bromine concentrationPrecipitants concentrationsTower equipment checklistFilters and screenWet-bulb temperature and humidity

Explain the problems related with cooling tower and ways to overcome it.