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11aPage 1a11bPage Chapter 1 Introduction to Psychrometry
Learning OutcomesLearning Outcomes:
When you have studied this chapter you should be able to:
1 Explain what is meant by the term Psychrometry.2. Relate Daltons law of Partial Pressures to the term
Atmospheric Pressure.3. Explain what is meant by the term Saturated Vapour
Pressure.4. Use a Psychrometric Chart to find:
a. A saturated vapour pressure for a giventemperature.
Find, for a given air sample, the following: b. The moisture content c. The percentage saturation d. The relative humidity.
5. Explain what is meant by the wet-bulb temperature andits use in the psychrometric equation.
6. Show how the Psychrometric Chart is used to determine: a. Dew-point temperature b. Specific Enthalpy
Suggested Study Time:(a) For study of chapter material;
(i) Initial on-screen study 1 hour(ii) Printing of notes and subsequent
in-depth study 2 hours(b) For completion of the quick revision study guide
Total estimated study time 3 hours
Chapter 1Introduction toPsychrometry
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Chapter ContentsItem page
Learning Outcomes 1-1bIntroduction to Psychrometry 1-3a
The Atmosphere 1-3aWater Vapour 1-4aSaturated Vapour Pressure 1-5a
Psychrometric Chart (Theory) 1-5bMoisture content 1-6aRelative humidity 1-6bPercentage saturation 1-7aRelationship between g, m and rh 1-7bComparision of percentage saturation & rh 1-8aWet-bulb temperature 1-8b
1. The Sling Wet-bulb 1-8b2. The Screen Wet-bulb 1-8b
The Psychrometric Equation 1-9aDew-point temperature 1-10bSpecific enthalpy 1-11aSpecific volume 1-12aDensity 1-12a
Psychrometric chart 1-13Quick Revision Study Guide 1-14aChapter Notes 1-15a
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Introduction to PsychrometryThe Atmosphere
Psychrometry is the study of atmospheric air and its asso-ciated water vapour. Air comprises a mixture of gases1.1 of whichnitrogen makes up 78%, oxygen 21% and carbon dioxide andthe inert gases (such as argon, neon, krypton, helium etc.) theremainder. These are known as the dry gases of the atmosphere.
Principle Dry Atmospheric Gases
Gas A atpartial pressure
Gas B atpartial pressure
Gas A + B attotal pressure
PA + PB
Daltons Law, illustrates that if two gases are combinedinto the same volume, the total pressure is the sum of theindividual partial gas pressures.
In a vessel open to theatmosphere, it is the weightof the atmospheric gasesabove the earths surfacethat produce the pressure1.2of the atmosphere.
- 60C- 60C+ 20C
No Clouds,No Winds
The Earths Atmosphere
In the air mixture, the dry gases and associated water va-pour behave according to Daltons law of Partial Pressures. Thatis they behave independently of one another and the pressure eachexerts combine to produce an overall atmospheric pressure.
Nitrogen Oxygen Carbon Dioxide
Molecular Weight % Atmosphere
Notes1-1: Air is a mixture containing a group of gases of nearly constant proportions (% of atmosphere):Nitrogen (N2) 78.804Methane (CH4)0.0002Oxygen (O2) 20.964Krypton (Kr)0.000114Argon (Ar)0.934Hydrogen (H2)0.00005Neon (Ne)0.0018Nitrous Oxide (N2O)0.00005Helium (He)0.000524Xenon (Xe)0.0000087and a group of gases present in proportions, variable both in time and space (% of atmosphere):Water Vapour (H2O)0 7Carbon Dioxide (CO2)0.01 0.1 (Average about 0.032)Ozone (O3)0 0.01Sulphur Dioxide (SO2)0 0.0001Nitrogen Dioxide (NO2)0 0.000002(Ref Encylopaedia Britannica CD-ROM, 1998 Edition).
Notes1-2: Pressure; force per unit area, i.e. the force spread over a particular area, is measured in newtons and area in square metres. So pressure is measured in newtons per square metre but it can also be measured in pascal (symbol Pa, where 1 Pa = 1 N/sq.m).Traditionally, it has also been common to express atmospheric pressure in units of bar or millibar, where 1bar = 100,000 Pa, (Bar: from the Greek meaning weight, thus a barometer [a word again derived from the Greek meaning weight + measure], is an instrument to measure the weight or pressure of the atmosphere).
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Since at normal temperatures and pressures we are unableto condense these gases out from the atmosphere, for the purposeof psychrometry, this mixture of gases can be treated as if it werea single ideal or perfect gas behaving as a single element knownas dry air.
Note: Psychrometry is a specialised area of thermodynam-ics but obviosuly has applications in air conditioning. None theless, in order to fully appreciate the material within this courseyou need to be familiar with the gas laws, the first law of thermo-dynamics and the concepts of internal energy, enthalpy and othergas properties. For those of you who lack this experience I haveincluded within the extra notes - Thermodynamics Refresher, abrief set of notes covering these topics which you are advised tostudy before continuing.
A consequence to Daltons law of Partial Pressure is thatthe total enthalpy of a mixture of gases will equal the sum of theenthalpies of each component part, i.e.:
h = m1 .h1 + m2 . h2 + m3 .h3 + .... etc.Water Vapour
In addition to these dry gases, the air also contains varyingamounts of water vapour. At normal temperatures and pressures,water is able to exist in both a liquid and a gaseous (or vapour)form, but it cannot be treated in the same way as the other gasesof the atmosphere because its quantity, and hence proportion arecontinually varying.
Water vapour is said to be associated with dry air and themore vapour associated with the dry gases, the more humid theair. At sea level, atmospheric pressure is normally within therange 95 to 105 Pa, but this depends upon weather conditions.The agreed international standard atmosphere has a pressure of101.325 kPa (1013.25 mbar) and this reduces at the rate0.013 kPa per metre of height above sea level and increases atthe same rate below sea level.
The water vapour is completely independent of the dryatmospheric gases, and its behaviour is not affected by their pres-ence of absence, but for any given temperature there is a maxi-mum amount of vapour that can be absorbed or associated with agiven volume of air. In addition, although the actual quantity ofwater is quite small, it can have a dramatic effect on our percep-tion of comfort.
(Under certain conditions the mass of water vapour willchange due to condensation or evaporation (known as dehu-midification and humidification respectively), but the mass ofdry air will remain constant. It is therefore convenient to relateall properties of the mixture to the mass of dry air rather than tothe combined mass of air and water vapour).Xtra
Particles in a solid normally haveno motion relative to each otherexcept vibration about a fixedposition. If we increase the energycontent, particles can break thebonds which bind them and thesolid melts
Particles in a liquid do not occupyfixed positions, but they are notcompletely free. The particles canslide over one another allowing theliquid to flow.
Energy supplied to a liquidallows the particles to breakfree of each other andbecome a gas. If energy isremoved from the gas it willagain condense to a liquid
The States ofMatter
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Saturated Vapour Pressure (PSS)1.3Heat is a form of internal energy. This is a thermodynamic
property1.4 and represents the internal energy of the molecules.With increase temperature there is an increase in molecular activ-ity and thus more water can escape from the liquid into the gas aswater vapour or steam. After a while however, even at this in-creased temperature, the air will become fully saturated with watervapour so that no more water can evaporate unless we again in-crease the temperature.
The pressure produced by the water vapour in this fullysaturated condition is known as the saturated vapour pressure(pss) and since at a given temperature the air cannot absorb morewater than its saturated condition, the saturated vapour pressureis the maximum pressure of water vapour that can occur at anygiven temperature.
Eventually of course, if we continue to supply heat thewater will boil, and this will occur when the saturated vapourpressure is equal to the atmospheric pressure.
Psychrometric ChartThere is a fixed relationship1.5 between saturated vapour
pressure and temperature, for example;
Normal air temperature, as you might expect, can be meas-ured by a normal mercury-in-glass thermometer
Temperature (C) 10.0 20.0 30.0 40.0 50.0 60.0Pressure (Pa) 1.23 2.34 4.24 7.38 12.33 19.92
and this r