Cooling Towers: OverviewTopicsIntroductionKey DefinitionsOperating ConditionsSlide 6Basic ComponentsSlide 8Parallel vs. Series FlowClassification of CTsInduced Draft, Cross Flow CTAtmospheric Cooling Tower (Natural Draft)Hyperbolic Cooling TowerForced Draft Cooling TowerInduced Draft Cooling TowersTroubleshootingProblems Faced by OperatorsWater Composition ControlWater Composition Control (Solutions)Slide 20Water Testing (by Operators)Humidity MeasurementsHumidity -- BackgroundDefinitionsSlide 25Slide 26Sling PsychrometerHumidity DeterminationEND LECTURE!1Cooling Towers: OverviewCM4120 Spring 20082TopicsIntroductionDefinitionsOperating ConditionsBasic ComponentsWater Cooling SystemsTypes3IntroductionBoxed shaped collection of multilayered wooden slatsAir flow breaks up water as it fallsDesign ensures good contact between water and airUsed to remove heat from water4Key DefinitionsWet-bulb temperature = air temperature measured by a wet-bulb thermometer–simulates effect of evaporative coolingDry-bulb temperature = air temperature measured by a dry-bulb thermometerApproach = difference in wet-bulb temperatures between inlet and outlet called “the approach to the tower”Latent heat = heat associated with change in state of matter (e.g., liquid to gas phase)5Operating Conditions10-20% of heat (sensible heat) removed from contact between water and air80-90% of heat removed following evaporationEvaporation is most critical factor affecting tower efficiency!6Operating ConditionsFactors which affect cooling tower performance:–relative humidity–temperature–wind velocity–tower design–water contamination–equipment problems (pump failure)Let’s discuss these!7Basic ComponentsWater distribution system = includes header which distributes (sprays) water from top of tower over splash barsFan = induced and forced draft towers use fans to push or pull airAir intake louvers = louvers on side of towers which direct air into tower (fixed or movable)Water basin = collects water at bottom of tower prior to discharge8Basic Componentsfill = material inside a tower which redirects air flow and watercolumn = wooden or metal post which supports towerstack = hyperbolic towers and chimney towers have huge stacks located at topmake-up water = water which is added due to evaporation and blowdownsplash bars = used to redirect the downward flow of water9Parallel vs. Series Flow10Classification of CTsBy direction of air flow–crossflow (airflow is horizontal )–counterflow (airflow is vertical) designs By how the air flow is produced –naturally (hyperbolic or chimney towers)–mechanically (forced draft or induced draft)11Induced Draft, Cross Flow CT12Atmospheric Cooling Tower (Natural Draft)Use natural forces (wind) to move air through CTAir flows in through the sides, and out the topDrift eliminators on the top13Hyperbolic Cooling TowerAlso called chimney CTOften seen at power plantsVery high flowratesAir flows up, creating a draft14Forced Draft Cooling TowerFans used to create a draftAir forced in the bottom, and flows out the topTypically solid sidesSome recirculation of air possible, harming efficiency15Induced Draft Cooling TowersFans located at the top of the CTLifts air out of the CT, preventing recirculationProbably the most common type used in chemical plants and refineries16TroubleshootingWater dissolves many things (especially hot water!)Water is cooled and results in deposits in towerSolids concentrate in cooling tower basinTrivia Question: Are Cooling Towers equipped with automatic sprinklers?17Problems Faced by OperatorsScale formation - suspended solids form depositsCorrosion - electrochemical reactions with metal surfacesFouling - due to silt, debris, algaeWood decay - fungi18Water Composition ControlSuspended solids levels checked by operators (ppm)Measured values compared to make-up water concentrationsProblem controlled by “blowdown” (i.e., old water replaced with new)Note: 100 ppm = 100 lbs. suspended solids/1,000,000 lb water19Water Composition Control (Solutions)Scale formation–remove scale forming solids with softening agents–prevent scale forming materials by addition of chemicals–precipitate scale for removal20Water Composition Control (Solutions)Corrosion–add chemical inhibitors (adds thin film to metal)Fouling–use filtering devices–use dispersants with filtering devicesWood decay–use biocides (chlorine or bromine)21Water Testing (by Operators)pH of watertotal dissolved solids (TDS)inhibitor concentrationchlorine or bromine concentrationprecipitant concentrationfilter and screen checkstemperature and humidity22Humidity Measurements23Humidity -- BackgroundHumidity is the amount of water vapor in the air Humidity is described in different ways–"relative humidity," which is the term used most often in weather information meant for the public–Relative humidity is the amount of water vapor in the air compared with the amount of vapor needed to make the air saturated at the air's current temperatureDewpoint temperature gives a much better estimate of the amount of moisture actually present in the air–very important in determining precipitation amounts and even how comfortable you feel24DefinitionsAbsolute humidity: Mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter)Dewpoint: Temperature air would have to be cooled to in order for saturation to occur (Assumes there is no change in air pressure or moisture content of the air).25DefinitionsWet bulb temperature: Lowest temperature that can be obtained by evaporating water into the air at constant pressure. 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. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.26DefinitionsRelative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can
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