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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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