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Duty, Honor and IntegrityChapter 6Air Conditioning System

Thermodynamics II (EMM3503)Chapter 6- Air Conditioning System

Thermodynamics II (EMM3503)Chapter 6- Air Conditioning SystemDelivered by: Ir Khalid

Duty, Honor and IntegritySplit Unit Air Conditioning

Duty, Honor andIntegrityFan MotorCondenserCollsHOT)Inside aBasic WindowAir ConditionerEvaporatorCoils(COLO)Expansionvalve



Thermodynamics II (EMM3503)

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Duty, Honor andIntegrity

CONDENSERFILTER DEHYDRATORBLOWtfiEVAPORATOR

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LOW PFF55TJSE COLD YAPOHEXPANSION VALVEWT-JOPsaCn LDW PR ESSLIRf ca L n LiOLMD-V APOFAir conditioning System for CarChapter 6- Air Conditioning System

Duty, Honor and IntegrityAir ConditioningIntroduction to Air ConditioningA system that keeps air cool and dry.Air conditioning may be required in buildings which have a high heat gain and as a result a high internal temperature.The heat gain may be from solar radiation and/or internal gains such as people, lights and business machines.The diagram below shows some typical heat gains in a room.

Solar Radiation: Up to 700 W/m2 glass area (U.K.)T^CTbif 1t*AHeat Gam through walls, roof, floor etc. due to temperature difference^Sensible Gain Office Machines:Up to 400 W per desk top computerOccupants (sedentary):Sensible gain 90-100 Watt per person Latent gain 40Watts per personSensible Gain Lights: G 5 to 25 W/m2 floor area.http://www.arca53.dsl.pipex.com/index files/ac1.htmIf the inside temperature of a space rises to about 25oC then air conditioning will probably be necessary to maintain comfort levels.This internal temperature (around 25oC) may change depending on some variables such as: type of building location of building duration of high internal temperature expected comfort conditions. degree of air movementhttp://www.engineeringtoolbox.com/air-change-rate-room-d 867.htmlAIRCHANGE RATE REFERENCE percentage saturationIn some buildings it may be possible to maintain a comfortable environment with mechanical ventilation but the air change rate (A measure of how quickly the air in an interior space is replace by outside (or conditioned) air by ventilation and infiltration) will tend to be high (above about 8 air changes per hour) which can in itself cause air distribution problems.Since air conditioning is both expensive to install and maintain, it is best avoided if possible.

Lower level ofDuty, Honor and IntegrityThis may possibly be achieved by careful building design and by uti window blinds or shading methods heat absorbing glass heat reflecting glass openable windows higher ceilings smaller windows on south facing facades alternative lighting schemes.

Thermodynamics II (EMM3503)Chapter #- Air Conditioning System

Thermodynamics II (EMM3503)

Thermodynamics II (EMM3503)The diagram shows some of these methods.

be reduced.Chapter 6- Air Conditioning SystemDuty, Honor andIntegrityIf air conditioning is the only answer to adequate comfort in a building then the main choice of system can be considered.Full comfort air conditioning can be used in summer to provide cool air (approx. 13oC to 18oC) in summer and warm air (approx. 28oC to 36oC) in winter.Also the air is cleaned by filters, dehumidified to remove moisture or humidified to add moisture.Air conditioning systems fall into three main categories, and are detailed in the following pages;1. Central plant systems.2. Room air conditioning units.3. Fan coil units.Central plant systems have one central source of conditioned air which is distributed in a network of ductwork.Room air conditioning units are self-contained package units which can be positioned in each room to provide cool air in summer or warm air in winter.Fan coil units are room units and incorporate heat exchangers piped with chilled water and a fan to provide cool air.http://www.arca53.dsl.pipex.com/index files/ac1.htm (introduction to Air Cond)

Chapter 6- Air Conditioning SystemDuty, Honor andIntegrityFirst, let study the Psychrometry for air conditioning and discusses some properties of moist air.Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures (which deal with air and water vapor mixtures).Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilating, and air-conditioning (HVAC) and meteorology.In human terms, our comfort is in large part a consequence of, not just the temperature of the surrounding air, but (because we cool ourselves via perspiration) the extent to which that air is saturated with water vapor.The Ideal Gas Laws are used to determine psychrometric data for air so that the engineer can carry out calculations.To make life easier a chart has been compiled with all the relevant psychrometric data indicated.This is called the Psychrometric Chart.Note:Air is made up of five main gasesi. e. Nitrogen 78.03%, Oxygen 20.99%, Argon 0.94%, Carbon Dioxide 0.03%, and Hydrogen 0.01% by volumeThe Psychrometric ChartFor a given, fixed, total air-vapor pressure, the properties of the mixture are given in graphical form on a psychrometric chart.Temperalui&

EnthalpyDuty, Honor and IntegrityRelativeHumitfilyDewpointiTemperatureHumidity

The air-conditioning processes:Dry-Bulb *" Temperature!_SpecHicVolumeCooling (lowering temperature),Heating (raising temperature),Humidifying (adding moisture),Dehumidifying (removing moisture)tCORatio

Duty, Honor andIntegrityThe following is a brief description of each of the properties of air.1. Dry bulb temperatureThis is the air temperature measured by a mercury-in-glass thermometer.2. Wet bulb temperatureThis is the air temperature measured by a mercury-in-glass thermometer which has the mercury bulb wetted by gauze that is kept moist by a reservoir of water.When exposed to the environment the moisture evaporates from the wetted gauze, which gives a lower reading on the thermometer. This gives an indication of how dry or how moist the air is, since in dry air the water will evaporate quickly from the gauze, which depresses the thermometer reading.3. Specific Humidity (Moisture content)This is the amount of moisture in air given in kg of moisture per kg of dry air e.g. for room air at 21oC dry bulb and 15oC wet bulb, the moisture content is about 0.008 kg/kg d.a.This is a small mass of moisture ( 0.008 kg = 8 grams) per kg of dry air or 9.5 grams per cubic metre of air.4. Percentage saturationThe Percentage saturation is another indication of the amount of moisture in air.This is the ratio of the moisture content of moist air to the moisture content of saturated air at the same temperature.When air is saturated it is at 100% saturation and cannot hold any more moisture.5. Specific enthalpyThis is the amount of heat energy (kJ) in air per kg.If heat is added to the air at a heater battery for example, then the amount to be added can be determined from Specific enthalpy change.6. Specific volumeThis is the volume of moist air (dry air + water vapour) per unit mass.The units of measurement are m3 per kg.



ASHRAE Pshychrometnc Chart No. 1Also specific volume = 1 / density.

#

Refer: Fig A-31 page: 955#

Normal Temperature [Barometric Pressure: 101.325 kPa1992 American Society of Heating, C Refrigerating and Air-conditioning Engineers, Inc.SensibleTotal heatEnthalpy IHumidity ratio A WDry bulb temperature CPrepared by Center for Applied Thermodynamic Studies, University of Idaho

Duty, Honor andIntegrityPERCENTAGE SATURATION 90 80 70 60 50 403020WET-BULB TEMPERATURE 30,MOISTURE CONTENT Rg/kg da.SPECIFIC 50 ENTHALPY kJ/kfi /SPECIFIC _ VOLUME nf 0.35

SPECIFICENTHALPYkJ/koDRY-BULB TEMPERATURE C

Psychrometric ChartExample:http://www.arca53.dsl.pipex.com/index files/psy4.htm14-46 The pressure, temperature, and wet-bulb temperature of an are specified. The adiabatic saturation temperature is to be determined.Anafysis For an adiabatic saturation process, we obtained Eq. 14-14 in the text,

Watercp(T2-T{) + fi?2 hfsi

Tills requires a trial-error solution for the adiabatic saturation temperature, T2. The inlet state properties arec>[ = 0.0243 kgH^O / kg dry air hs\ hs ,r 32c = 1559.2 kJdcgAs a fnst estimate, let us take I": =29C (the mlet wet-bulb temperature). Also, at the exit. the relative humidity is 100% (2 = 0.025b kg H^O f kg dry anhr.= hf @ :

thermodynamics 2_chapter 6 - copy

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