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CHAPTER 7 : COOLING TOWER - Coggle Diagram
CHAPTER 7 : COOLING TOWER
APPLICATION & THEORY OF OPERATION
Cooling towers are heat transfer device designing to cool water for reuse.
They cool hot water by bringing it into direct contact with air using countercurrent & cross flow current.
A cooling tower contains wood / plastic static, called fill, that direct flow of water falling from the top of the tower.
Hot water transfer heat to the cooler air it contact in the towers.
When the water changes to vapour, the vapour takes heat energy with it, leaving behind the cooler liquid.
Hot water from heat exchangers is used to the cooling towers.
Make up water source is used to replace water lost to evaporation.
Water exits the cooling tower and is sent back to the exchangers / to other units for further cooling.
Evaporation, which accounts 80% to 90% of the heat loss, is the most critical factor in cooling tower efficiency.
Factors that affect cooling tower efficiency:
Wind velocity
Tower design
Temperature
Water contamination
Relative humidity
Equipment problem
Temperature in a cooling tower are closely controlled.
Temperature difference betweem hot & cold water in coolingtower is reffered to cooling range.
2 ways to measure temperature:
1) Dry-bulb temperature (DBT)
2) Wet Bulb Temperature
A theoretical temperature that cannot be reached, only approached.
The lowest theoretical temperature to which water can be cooled in the tower.
The temperature of the air saturated with water.
COUNTERCURRENT / COUNTERFLOW PATTERNS
Air flow is directly opposite to the water flow.
Air flow first enters, and is then drawn up vertically.
The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow.
Advantages:
Spray water distribution makes the tower more freeze-resistant.
Breakup of water in spray heat transfer more efficient.
Disadvantages:
Typically higher initial & long-term cost, primarily due to pump requirements.
Difficult to use variable water flow, as spray characteristics may be negatively affected.
CROSSFLOW PATTERNS
Air flow is directed perpendicular to the water flow.
Air flow enters one / more vertical faces of the cooling tower to meet the fill material.
Water flows (perpendicular to the air) through the fill by gravity.
The air continues through the fill & thus past the water flow into an open plenum volume.
Lastly, a fan forces the air out into the atmosphere
Advantages:
Gravity water distribution allows smaller pumps & maintenance while in use.
Non-pressurized spray simplifies variable flow.
Typically lower initial & long-term cost, mostly due to pump requirements.
Disadvantages:
More prone to freezing than counterflow designs.
Variable flow is useless is some conditions
TYPES OF COOLING TOWER
Natural Draft cooling tower (Naturally)
They are design for flows excess 500 000 GPM
Its can have fill patterns either cross flow / counter flow
Hyperbolic / chimney tower are natural draft towers that have a large stack.
The fill and water distribution system are located below the chimney.
Hot water is pumped to distribution system that is much lower than would be found in an atmospheric tower.
Airflow is produced by temperature induce density differences inside & outside the stack.
During operation a natural draft tower, air enters the cooling tower at the base and is directed into the internal fill pattern.
As hot water drops through the fill is exposed to the cooler air, density changes inside the chimney create the required upward draft.
Heat is removed through the chimney
Force Draft cooling tower (Mechanically)
Forced-draft cooling towers force air in mechanically by the use of fans on the lower side of the tower.
FDC towers usually have solid sides slides without louvers. Fan push in 100% of the process air.
Flow directions is counter-flow, the fans push air upward against the downward flow of water.
FDC towers have much higher heat transfer rates than atmospheric & natural-draft cooling tower.
They are significantly lower in height. This type of cooling tower is less efficient than induced-draft towers because some hot air is re-circulated.
Atmospheric cooling tower (Atmospherically)
Airflow rates are determined by wind velocity.
Designed so that winds blow in horizontally, so the air moves in a cross-flow direction.
Cool air enters the tower through the louvered sides & passes across the downward flowing hot water.
Atmospheric cooling tower have a 30% to 55% efficiency rating for cooling water.
Efficiency an fluctuate greatly because it depends on an uncontrollable factor, wind velocity.
Wind moves air into & out of the tower
Since this equipment does not require a fan, it is very cost effective
Heat transfer drops significantly with the loss of airflow
No mechanical fan to create air flow through the tower.
Induce draft cooling tower (Mechanically)
Produces airflow mechanically.
It differs from forced-draft cooling tower in that it pulls air out of tower rather than forcing it in.
Airflow in an induced-draft tower is slower than in a forced-draft tower, but heat transfer through evaporation is more efficient.
The tower fan, located on top of the tower produces discharge rates strong enough to lift the hot air above the tower, so hot air is not re-circulated into the tower.
Induced-draft towers can circulate airflow horizontally (cross-flow) / vertically (counter-flow)
BASIC COMPONENTS OF COOLING TOWER
Pump suction water from the water basin and discharge it into the cooling water supply header.
The supply header distributes water to process exchangers, where it absorbs heat & returns to the top of the cooling tower through the cooling water return through.
Fill can be arranged in patterns that produce either counterflow / cross-flow.
Most towers develop significant draft. Drift eliminators prevent water from being blown / sucked out of the tower.
From there, water is sprayed / allowed to fall down into the tower over the splash bars and fill.
This type of water loss is called drift loss / wind age loss. Makeup water is added to replace water that has been lost by evaporation / blown-down.
As the hot process water returns to the tower, it enters the water distribution header, which is a pipe located at the top of most towers.
Induced-draft cooling tower use fans to pull air out of the system. Some cooling towers have air intake louvers located on the side of the tower to direct airflow.
Modern cooling tower are built with treated wood, cedar, crpess, redwood / plastic - resistant to the bad effects of water
These louvers can be fixed or movable depending on the tower design. A hyperbolic / chimney, tower has a stack above the fill & water distribution system.
WATER COOLING SYSTEM
Cool water is pumped from the tower to a heat exchanger, the hotter process fluid transfers heat to the cooling water.
The water, in turn, removes heat from the process fluid. The process flow continues on to the next step, whereas the hot water is returned to the tower to be cooled.
Heat exchanger & cooling tower works hand to hand to create a water cooling system.
Heat exchangers can be connected to cooling towers in variety of ways. The 2 most common are parallel & series.
WATER PROBLEMS
Scaling
Problem:
Hot water dissolves solids faster than cold water. By the time the hot water returns to the tower, it is full of suspended solids. When hot water enters the cooling tower laced with suspended solids, it undergoes evaporation. This process removes water & leaves the solids. In addition, suspended solids can accumulate in the water & form deposits (scale).
Preventive measurement:
Suspended solid can be controlled by a process called blow-down ( a certain amount of water is removed & replaced with fresh makeup water), while cale can be removed with softening agents / adding chemicals.
Corrosion
Problem:
Caused by electrochemical reaction with metal surface
Preventive measurement:
Addition of chemical inhibitors, which form a film that protects metal.
Wood decay
Problem:
Caused by fungi & bacteria
Preventive measurement:
Addition of biocides (such as chlorine / bromine)
MAINTENANCE
Periodic draining & cleaning of wetted surfaces & areas of alternate wetting & drying to prevent the accumulations of dirt, scale / biological organism such as algae & slime, in which bacteria may develop.
Proper treatment of circulating water for biological control & corrosion, in accordance with accepted industry practice.
Periodic inspection of mechanical equipment, fill, and both hot water & cold water basins to ensure that they are maintained a good state of repair.
Systematic documentation of operating & maintenance functions. This is extremely important because without it, no policing can be done to determine whether an individual has actually adhered to a maintenance policy.