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    PsychrometryBy

    Muhammad Adnan Tabassum

    2006-ag-1626MSc(Hons) Agriculture

    Department of Agronomy

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    Introduction to psychrometry

    Psychrometry is the science dealing with the

    physical laws of air water mixtures. Or The study of atmospheric air and its associated

    water vapour.

    the word Psychrometer is originally from theGreek, meaning cold + meter, i.e. literallymeaning a measurer of cold.

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    Introduction to psychrometry Its current meaning is two

    thermometers, mounted on the same

    scale, with indications of wet-bulb anddry-bulb temperatures. Hence the word Psychrometry is the

    determination of the degree of humidity

    in the atmosphere by means of apsychrometer and is therefore used todefine the study of atmospheric air andits associated water vapour.

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    When designing an air conditioning

    system, the temperature and moisture

    content of the air to be conditioned, and

    the same properties of the air needed to

    produce the desired air conditioning

    effect, must be known. Once these properties are known, the

    air conditioning task can be determined.

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    Air comprises a mixture of gases of whichnitrogen makes up 78%, oxygen 21% andcarbon dioxide and the inert gases (such asargon, neon, krypton, helium etc.) theremainder.

    These are known as the dry gases of theatmosphere

    In addition to these dry gases, the air also

    contains varying amounts of water vapour

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    In the air mixture, the dry gases andassociated water vapour behave

    according toDaltons law of Partial Pressures.

    That is they behave independently of

    one another and the pressure eachexerts combine to produce an overallatmospheric pressure.

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    Psychrometery Based on The Daltons law ofPartial Pressures. That is

    dry gases and associated water vapour behaveindependently of one another and the pressure

    each exerts combine to produce an overall

    atmospheric pressure

    and The First Law of Thermodynamics

    (Conservation) states that energy is always

    conserved, it cannot be created or destroyed.

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    Psychrometric Chart There is a fixed relationship between saturated

    vapour pressure and temperature, for example;

    and this relationship forms the basis for thepsychrometric chart which is essentially

    a plot of vapour pressure against air

    temperature.

    Temperature(C)

    10 20 30 40 50 60

    Pressure (Pa) 1.23 2.34 4.24 7.38 12.33 19.92

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    Psychrometeric Chart

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    Above chart graphically displaysseveral physical properties of air

    over a broad range of conditions

    Knowing the relationship of these

    air properties aids the task of airconditioning system design andanalysis.

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    The psychrometric chart contains five

    physical properties to describe the

    characteristics of air which are as

    follows:

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    Properties of Air Dry-bulb temperature

    Wet-bulb temperature

    Dew-point temperature

    Humidity ratio

    Relative humidity

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    Dry-bulb temperature (tdb) Normal air temperature can be

    measured by a normal mercury-in-glass

    thermometer.

    This is known as the Dry-bulb

    temperature (tdb) because the bulb of

    the thermometer is kept dry.

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    Wet-bulb temperatures are read from athermometer whose bulb is covered by a wet

    wick. The difference between the wet-bulb

    temperature and the dry-bulb temperature iscaused by the cooling effect produced by theevaporation of moisture from the wick. Thisevaporation effect reduces the temperature ofthe bulb and, therefore, the thermometer

    reading.

    Wet-bulb temperature (twb)

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    Thermometers

    Dry Bulb

    Wet Bulb

    Measured indegrees

    Fahrenheit

    (o

    F)

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    Dew-point temperature (tdp)

    Condensation

    occurs at the

    Dew PointTemperature

    (oF)

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    Saturation When the dry-bulb, wet-bulb, and dew-point

    temperatures are the same, the air is

    saturated. It can hold no more moisture.

    When air is at a saturated condition, moisture

    entering the air displaces moisture within the

    air. The displaced moisture leaves the air in

    the form of fine droplets. When this condition

    occurs in nature, it is called fog.

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    Saturation

    Fog DP, WB, DB same

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    Humidity Ratio Actual weight of water in an air water

    vapor mixture

    Pounds of moisture per pound of dry air

    7000 grains of water in a pound

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    Relative humidity (rh) The fourth property, relative humidity,

    is a comparison of the amount of

    moisture that a given amount of air is

    holding, to the amount of moisture that

    the same amount of air can hold, at the

    same dry-bulb temperature.

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    Relative Humidity

    Relative

    Humidity

    Amount of moisture that a given amount ofair

    is holding=

    Amount of moisture that a given amount ofaircan hold

    50% RH

    100% RH -Saturated

    (percentage)

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    PSYCHROMETERPSYCHROMETER

    PsychrometerPsychrometer isis usedused toto measuremeasure relativerelativehumidityhumidity (RH(RH%%)).. ItIt consistconsist ofof drydry bulbbulbthermometer,thermometer, wetwet bulbbulb thermometerthermometer andand aasmallsmall fanfan onon thethe toptop toto pushpush surroundingsurrounding airair totowetwet bulbbulb..

    A general Psychrometer consists of the

    following parts

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    DRYDRY BULBBULB THERMOMETERTHERMOMETER

    ItIt isis usedused toto measuremeasure thethe actualactual temperaturetemperature ofof

    thethe ii..ee.. mercurymercury inin glassglass thermometerthermometer.. ItItshouldshould bebe keptkept inin thethe StevensonStevenson screenscreen totoavoidavoid directdirect exposureexposure toto thethe sunrayssunrays.. ItIt shouldshouldbebe deptdept inin verticalvertical positionposition..

    AllAll liquidsliquids expandexpand oror contractcontract withwith increaseincrease orordecreasedecrease inin temperaturetemperature.. GenerallyGenerally mercurymercury isisusedused inin drydry bulbbulb thermometerthermometer..

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    Cont. MercuryMercury inin aa glassglass bulbbulb isis attachedattached toto aa

    graduatedgraduated capillarycapillary tubetube.. SoSo thatthat whenwhen

    mercurymercury expandsexpands withwith increaseincrease inintemperature,temperature, itsits valuevalue cancan bebe notednoted..

    MercuryMercury expandsexpands oror contractscontracts wheneverwhenevertherethere isis aa changechange inin thethe temperaturetemperature ofof

    thethe surroundingsurrounding airair.. TheThe changechange inin thethemercurymercury levellevel cancan bebe readread asas thethetemperaturetemperature ofof thethe ambientambient airair..

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    WETWET BULBBULB THERMOMETERTHERMOMETER ItIt isis usedused toto measuremeasure thethe wetwet bulbbulb

    temperaturetemperature.. EE..gg.. mercurymercury inin glassglass

    thermometerthermometer.. ItIt isis alsoalso placedplaced inin thetheStevensonStevenson screenscreen inin verticalvertical positionposition.. BulbBulb ofofthethe wetwet bulbbulb thermometerthermometer shouldshould bebe deptdeptcoveredcovered withwith aa muslinmuslin clothcloth andand itit shouldshould bebe

    deptdept wetwet byby aa threadthread.. OneOne endend ofof thethe threadthread ititfixedfixed toto thethe muslinmuslin andand otherother endend isis deptdept ininthethe distilleddistilled waterwater bottlebottle..

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    Cont.. ItsIts constructionconstruction isis samesame asas thatthat ofof drydry

    bulbbulb thermometer,thermometer, butbut itsits bulbbulb isis deptdept

    wetwet withwith waterwater usingusing muslinmuslin clothcloththroughthrough wickwick actionaction.. OneOne endend ofof thethemuslinmuslin clothcloth surroundssurrounds thethe thermometerthermometerbulbbulb.. WhereasWhereas thethe otherother endend inin thethe

    shapeshape ofof aa wickwick isis keptkept inin aa containercontainer ofofdistilleddistilled waterwater soso thatthat continuouscontinuous wettingwettingofof thethe clothcloth isis donedone..

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    WorkingWorking ofof PsychrometerPsychrometer ItIt isis basedbased onon thethe principleprinciple thatthat coolingcooling isis

    causedcaused byby evaporationevaporation.. WhenWhen thethe waterwater

    evaporatesevaporates fromfrom thethe muslinmuslin clothcloth coveringcovering thethebulbbulb ofof thethe thermometer,thermometer, heatheat energyenergy isis takentakenfromfrom thethe bulbbulb.. ThisThis heatheat energyenergy isis latentlatent heatheatofof evaporationevaporation thethe lossloss ofof heatheat resultsresults inincoolingcooling ofof thethe bulbbulb causingcausing aa fallfall inin thethemercurymercury level,level, whichwhich isis readread asas wetwet bulbbulbtemperaturetemperature..

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    Cont. TheThe thermometerthermometer indicatesindicates thethe

    temperaturetemperature ofof thethe ambientambient airair underundersaturatedsaturated conditionsconditions.. TheThe differencedifferencebetweenbetween thethe readingsreadings ofof wetwet andand drydrybulbbulb thermometersthermometers isis calledcalled thethe wetwet bulbbulb

    depressiondepression.. RelativeRelative humidityhumidity cancan bebedetermineddetermined fromfrom thethe hygrometrichygrometric tabletableagainstagainst thethe readingsreadings ofof drydry bulbbulb andand wetwetbulbbulb temperaturetemperature..

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    Schematic Diagram of Sling

    Psychrometer

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    The Sling Wet-bulb

    The sling wet-bulb temperature readingobtained in moving air preferably above 2m/s. It can be obtained by whirling a slinghygrometer or by using the more

    cumberson, but more accurate Assmanhygrometer which is fan-assisted.

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    SlingPsychrometer

    Assman Psychrometer

    Aspirated Psychrometer

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    The Screen Wet-bulb

    The screen wet-bulb temperature is the readingobtained in still air. This is usually used for

    meteorological data and is obtained from a wet-bulb thermometer installed into a StevensonScreen (from which this type of measurementgets its name). This is a slatted enclosure,

    designed to screen direct radiation from theinstruments, but equipped with wet and dry-bulbthermometers and maximum and minimumthermometers.

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    Of the two methods of reading wet-bulb, thesling reading is considered to be more

    accurate.

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    Applications of psychrometery

    The principles of psychrometry apply to anyphysical system consisting of gas-vapor

    mixtures. The most common system of interest is the

    mixture of water vapor and air, because of itsapplication in heating, ventilating, and air-conditioning and meteorology.

    Psychrometry is a specialised area ofthermodynamics but obviosuly hasapplications in air conditioning.

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    Applications Practical Applications of Psychrometry

    Characteristics and use of psychrometric charts

    Software for psychrometric analysis

    Techniques of Psychrometric Analysis

    Psychrometrics and BioclimaticAnalysis

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    Application In Agricultural Meterology Psychrometers are

    used to measure the RH.

    Effect of Relative Humidity on CropProduction:

    Relative humidity (RH) directly influences the

    water relations of plant and indirectly affectsleaf growth, photosynthesis, pollination,occurrence of diseases and finally economicyield.

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    Refferences http://arch.hku.hk/~cmhui/teach/65156-7e.htm

    http://www.engineeringtoolbox.com/psychrome

    tric-terms-d_239.html

    http://www.uigi.com/psychrometry.html

    http://www.hku.hk/bse/MEBS6006/http://en.wik

    ipedia.org/wiki/Psychrometrics

    http://www.managemylife.com/mmh/questions/

    109579-heat-latent-heat

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    http://www.arch.hku.hk/~cmhui/teach/65156-7e.htm

    http://agritech.tnau.ac.in/agriculture/agri_agrometeorology_relativehumidity.html

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