Air conditioning

PowerPoint Presentation

Refrigeration and Air Conditioning

NME:604Unit4

Presented byMr Naphis AhmadAssistant professorJETGI Barabanki

jahangirabad institute of technology12/31/20161

CONTENTSWhat is PSYCHROMETRY?Properties of PSYCHROMETRYPSYCHROMETRIC ProcessesPSYCHROMETRIC CHART12/31/2016jahangirabad institute of technology2

PsychrometryPsychrometry is the science dealing with the physical laws of air water vapourmixtures.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.In other words, we can say that Psychrometry is the study of MOIST AIR or mixture of dry air and water vapour.12/31/2016jahangirabad institute of technology3

PROPERTIESOF PSYCHROMETRYDEW POINT TEMPERATURERELATIVE HUMIDITYHUMIDITY RATIODRY-BULB TEMPERATUREWET BULB TEMPERATUREHUMIDITY Specific humidity Absolute humidityPSYCHROMETRIC RATIO

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Dew PointThe temperature at which moisture starts to condense out of the air is known as DEW POINT.

Dew point is also known as saturation temperature.

Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.12/31/2016jahangirabad institute of technology5

To remove moisture from the air, an air conditioner evaporator coil must operate below the dew point temperature.Moisture will collect on the cold surface of an evaporator coil and then drain through the condensate line.

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DRY -BULB TEMPERATUREThe dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature.


WET BULB TEMPERATURE

Wet bulb temperature is the temperature recorded by thermometer when the bulb is enveloped by cotton wick saturated with water.The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings.12/31/2016jahangirabad institute of technology7

HUMIDITY

Specific Humidity:Specific humidity is defined as the proportion of the mass of water vapour per unit mass of the moist air sample (dry air plus the water vapour); it is closely related to humidity ratio and always lower in value.

Absolute Humidity:The mass of water vapor per unit volume of air containing the water vapor. This quantity is also known as the water vapour density.


Psychometric RatioThepsychrometric ratiois the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation: r = hc / ky cswhere:r= Psychrometric ratio, dimensionlesshc = convective heat transfer coefficient, W m-2K-1ky= convective mass transfer coefficient, kg m-2s-1cs= humid heat, J kg-1K-1


Psychrometric Processes or Air conditioning Processes1. Sensible Heating : it is addition of heat to moist air without the addition of moisture. It follows a constant humidity ratio line on the psychrometric chart.

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Sensible heating


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2. Sensible Cooling: It is the removal of heat from moist air without the removal of moisture. It also follows a constant W on the psychrometric chart.

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Sensible Heat & Cooling Processes

12/31/2016jahangirabad institute of technology13Heating & Cooling process addition or removal of sensible heat without change in absolute moisture content.NOTE: RH changes as temperature changes.

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3. Cooling and Dehumidifying: It is the removal of heat and moisture from moist air, it involves sensible and latent heat transfer.

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Psychrometry - Dehumidification

12/31/2016jahangirabad institute of technology15Dehumidification by cooling in cooling coils temp of air reduces and the saturation point (dew point) is reached.Further cooling results in reduction of absolute humidity.

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Dehumidification by Cooling


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Heating and Humidifying: It is the addition of heat and moisture to moist air, it also involves sensible and latent heat transfer.

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5. HUMIDIFYING : It is the addition of moisture to moist air without the addition of heat.

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Removing Sensible Heat and Moisture12/31/2016jahangirabad institute of technology19

19Put all of these changes together on one chart and they show the direction the air condition will move when the dry-bulb temperature or moisture content is altered.

Adiabatic or evaporative coolingA psychrometric process which invovles the cooling without heat loss or gain. Sensible heat lost by air is converted to latent heat in the added water vapor.12/31/2016jahangirabad institute of technology20


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ADIABATIC MIXING OF MOIST AIR & STREAMA psychrometric process that invovles no net heat loss or gain during the mixing of two air streams.12/31/2016jahangirabad institute of technology22

Adiabatic mixingGoverning equation 12/31/2016jahangirabad institute of technology23

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Psychrometry Climate ClassificationClimate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart.


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12/31/2016jahangirabad institute of technology25Basic Factors That Affect Human Comfort In The Internal Environment

Thermal and air qualityWhat affects the surroundings you live in?Air quality is affected by how hot it is outside or inside your environmentWhat is humidity and what affects humidity?The amount of moisture that is present within the air will have an effect on humidity, which is linked to the amount of ventilation entering What is the normal temperature of a human being?Human temperature maintain an average core temperature of 37 depending on the metabolic rate


Nature of heatWhat is the measure of temperatureTemperature is measured in degrees celsius The lower is 0 fixed at a melting point of ice at a stand at atmospheric pressure of 101.32kN/m2The upper point is 100 degrees temperature of steam above the boiling pointWhat is the acceptable value of temperature taken at normal design?Normal design temperature are taken at 21 degrees inside and -1 degrees outside on average12/31/2016jahangirabad institute of technology27

Thermodynamic temperature scaleThis is another measure of temperature in degrees Kelvin0 degree celsius= 273.16 Kelvin (K)100 degree celsius = 317.16 KelvinThe unit of thermodynamic temperature is the fraction of the thermodynamic temperature at the triple point water (equilibrium point of the temperature and pressure at which three known phases of substance can exist i.e. liquid, water vapour and pure ice)12/31/2016jahangirabad institute of technology28

How do we measure the quantity of heat?Heat is measured in joules (J) which is a measure of work doneThe rate of expenditure of energy or doing work or of heat loss is measured in watts (W)1 watt is = 1 Joule per second1 W =1 J/s12/31/2016jahangirabad institute of technology29Quantity of heat

Heat transferName three ways heat is transferred from one mass to another, for instance a person sitting next to a radiator.ConductionConvectionRadiation


Thermal comfortIn high activity the temperature rises and the more heat you will give off. Several factors influences the level heat is generated (metabolic rate) including:Your surface areaAgeGenderLevel of activitye.g. Sleeping heat output 70W. Lifting 440W.12/31/2016jahangirabad institute of technology31

Typical heat output of an adult maleActivityExampleHeat outputImmobileSleeping70WSeatedWatching TV115WLight workOffice140WMedium workFactory Work265WHeavy workLifting440W


Heat losses from buildingsComfortable temperature for humans is provided by balancing the heat lost through conduction and ventilation through the fabric with similar heatOptimum temperature will depend on material used , type of construction, orientation of the building and degree of exposure to the rain and wind 12/31/2016jahangirabad institute of technology33

Room temperaturesWhat would you consider in design to maintain temperature in buildings?The resistance of a material to the passage of heat and the thermal conductivity of the material in passing the heat along are the basics of understanding of maintaining a steady temperature and a comfortable thermal indoor environmentIn order to maintain a comfortable room temperature the building must be provided with as much heat as is lost through ventilation12/31/2016jahangirabad institute of technology34

What will the loss of heat in buildings depend on?Materials usedType of constructionOrientation of the building in relation to the sunDegree of exposure to rain and wind


RELATIVE HUMIDITYHumans are used to a relative humidity of between 40 and 60%. Greater than this we start to describe air as being Humid.12/31/2016jahangirabad institute of technology36

HEAT LOSS DUE TO VENTILATIONNatural ventilation leads to the complete volume of air in a room changing a certain number of times in one hour

Type of roomAir changes in hrHalls1.0Bedrooms /lounges1.5WCs and bathrooms 2.0


Air conditioning

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