TOPIC 6 - Cooling System (new added)

8/7/2019 TOPIC 6 - Cooling System (new added)

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TOPIC 6

CHE 324 PLANT OPERATION & MAINTENANCE

6.1 Cooling system option6.2 Cooling tower operation6.3 Cooling tower standards and maintenance


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COOLING WATER SYSTEMCOOLING WATER SYSTEMCOOLING WATER SYSTEMCOOLING WATER SYSTEMCooling water (CW) system used for cooling the variouspieces of equipment in the facility in a closed cooling system2 different options: (1) once-through basis & (2) reuse basisOnce-through basis system operates using one way circuitwhereby the cooling water cooled the equipments only oncebefore drainageReuse basis system allows the cooling water to berecycle/reuse between cooling tower & process equipmentsMost process plant use reuse basis cooling system

CW system is designed for a supply (fresh) temp of 90F &maximum return (recycle) temp of 120FA cascade arrangement is used to minimize the amount of circulation water while maintaining the design temperatureriseThis cascade arrangement takes advantage of process usersthat cool enough to maintain a temperature rise between 5and 10 FCooling water is supplied from cooling tower to the user


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CT heat transfer devices designed to cool water for

reuseCool hot water by bringing it into direct contact with air,using counter current or crossflow patternsContains wood or plastic slats ( f ill) that direct airflow &the flow of water falling from the top of the tower The downward-flowing water coats the fill & form a film -increasing the surface area for contact between cool air & hot water Hot water transfer heat to the cooler air it contacts in the

tower This process results in both sensible heat loss (can bemeasured or felt) & evaporationWhen the water changes to vapor, the vapor takes heatenergy with it, leaving behind the cooler water

COOLING TOWERCOOLING TOWERCOOLING TOWERCOOLING TOWER


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COOLING TOWERCOOLING TOWERCOOLING TOWERCOOLING TOWERTemperature in C.T. are closely controlledTemp difference (T) bet. the inlet air temp (wet-bulb) &the outlet water temp is referred to as the approach tothe cooling tower Cooling range Temp difference bet. hot & cold water

Temperature measurement :D ry-bulb temperature ( DB T) take into account the relativehumidity & lower than dry-bulb temp.Wet-bulb temperature (W B T) does not take into account therelative humidity


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EQUIPMENT

PROB

LEMS

WATERCONTAMINATION

TOWERD ESIGN

WINDV ELOCITY

TEMPERATURE

RELATI V E

HUMID

ITY

EV APORATION

C.T.EFFICIENCY

FACTOR THAT AFFECT C.T.FACTOR THAT AFFECT C.T.EFFICIENCYEFFICIENCY

FACTOR THAT AFFECT C.T.FACTOR THAT AFFECT C.T.EFFICIENCYEFFICIENCY


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Water distribution header - a pipe that evenly dispersehot water over the fill of a cooling tower Splash bar & f ill - a device used in a cooling tower todirect the flow of falling water and increase surface areafor air-water contactWater basin - storage compartment for water at thebottom of a cooling tower D ri f t eliminators - devices in cooling tower to keepwater from blowing out

Make up water line - add water that loss during coolingprocessSome C.T. have air intake louvers slats located onthe side of the tower to direct airflow

B ASIC COMPONENTS OF C. T.B ASIC COMPONENTS OF C. T.B ASIC COMPONENTS OF C. T.B ASIC COMPONENTS OF C. T.


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COOLING TOWERCLASSIFICATION

FORCED-DRAFT

COOLING TOWER

ATMOSPHERIC

COOLING TOWER

NATURAL-DRAFT

COOLING TOWER

INDUCED-DRAFT

COOLING TOWER


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U se wind blows as a cooling medium

(plant near to wind source)Winds blow horizontally air moves in acrossflow directionCool air enters the tower through thelouvered sides & passes across thedownward-flowing hot water As air is heated by contact with the hotwater, it travels up because hot air rises(density difference)Air is moving in a counterflow direction,opposite with the falling water

Water leaving the tower to be 4/5Flower than wet-bulb temp. of entering air Has 30-55% efficiency of cooling & costeffective because does not require a fanBut efficiency can fluctuate depend on

wind velocity

ATMOSPHERIC C. T.ATMOSPHERIC C. T.ATMOSPHERIC C. T.ATMOSPHERIC C. T.


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Have large stack or chimney associated with power plant operationAir flow is produced by temp-induced density differences inside & outsidethe stackFill patterns crossflow (higher airflow rate) or counterflow (more efficientevaporative heat transfer)

Fill & water distribution system located below the chimney or stackDuring operation, air enters the cooling tower at the base & directed intothe internal fill patternAs hot water drops through the fill, it exposed to the cooler air & density

changes (hot to cool air) create upward draft and remove heat via thechimneyThe efficiency is depend to relative humidity & temp of the outside air

NATURALNATURAL- -D RAFT C. T.D RAFT C. T.NATURALNATURAL- -D RAFT C. T.D RAFT C. T.


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Forced air in mechanically by the use

of fans on the lower side of the tower Have solid sides without louversFan push 100% of the process air upward against the downward flow of water

Flow direction is counterflowAir flow can have high velocities butthe exiting air slows so much that it isrecirculated back into the tower,cutting efficiency by 20%

Have higher heat transfer rates thanatmospheric & natural-draft C. T. &they are lower in heightBut Forced draft C.T. less efficientthan induced-draft because some hot

air is recirculated

FORCE DFORCE D --D RAFT C. T.D RAFT C. T.FORCE DFORCE D --D RAFT C. T.D RAFT C. T.


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This designs differ from the forced-draft cooling tower that it pulls air outof the tower rather than forcing it inAirflow is slower than forced-draft type but heat transfer throughevaporation is more efficient

The tower fan located top of the tower produce discharge rates strongenough to lift the hot air above the tower so hot air is not recirculated intothe tower This design can circulate airflow both crossflow (horizontally) or counterflow (vertically)Counterflow operation force air vertically across a solid area of fill & drifteliminator located above the fill & water distribution headers

IND UCE DIND UCE D --D RAFT C. T.D RAFT C. T.IND UCE DIND UCE D --D RAFT C. T.D RAFT C. T.


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Standard material used must be durable & capable of

withstanding wide temp differencesTreated wood, cedar, cypress, redwood & plastic areusually usedWood is able to handle the temp variations experienced incooling tower Hot water has tendency to become corrosive & to formdepositsMaintenance issues associated with cooling tower isregarding the impurities of water

Suspended solids can accumulate in the water formdeposits (scale)Electrochemical reactions with metal cause corrosion to theinternal components of cooling tower.Scale can also favor the algae and bacteria formation onthe wooden fill & lead to wood decay

C. T. STAN D AR D & MAINTENANCEC. T. STAN D AR D & MAINTENANCEC. T. STAN D AR D & MAINTENANCEC. T. STAN D AR D & MAINTENANCE


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MAINTENANCE APPROACHMAINTENANCE APPROACHOperator check the level of suspended solid

frequentlyControl suspended solids by a process of blowdown (removed certain amount of water &replaced by fresh markup water)

Scale-forming solid removed with softeningagentsSuspension of solid prevented by addingchemicals

Minimized corrosion by adding chemical inhibitors(form a film that protects metal)Fouling controlled by filtering devicesPrevent wood decay by using biocides (e.g

chlorine/bromine)


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Operators are responsible for testing & checking:pH of water

Total dissolved solids (TDS)Inhibitor concentrationChlorine or bromine concentrationPrecipitant concentration

Tower equipment checklistsFilters and screensWet-bulb temperature and humidity

MONITORING OF PRO B LEMS &MONITORING OF PRO B LEMS &

SOLUTIONSSOLUTIONS


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COOLING TOWER SYM B OLSCOOLING TOWER SYM B OLSCOOLING TOWER SYM B OLSCOOLING TOWER SYM B OLS

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TOPIC 6 - Cooling System (new added)