psychrometry - sctce.ac.in€¦ · psychrometrics or psychrometry or hygrometry • they are terms...
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PSYCHROMETRY
GS, AP, ME Dept.
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Psychrometrics or psychrometry or hygrometry
• They are terms used to describe or determine the physical andthermodynamic properties of gas-vapor mixtures.
• The term derives from the Greek psuchron meaning"cold"and metron meaning "means of measurement”
• the principles of psychrometry apply to any physical system consisting ofgas-vapor mixtures,
• But the most common system of interest is the mixture of water vaporand air, because of its application in heating, ventilating, and air-conditioning
GS, AP, ME Dept.
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PSYCHROMETRIC TERMS
➢ Dry air
• Pure dry air is a mixture of various gases
• theoretical sample of air that has no water vapor
• Pure dry air doesn't exist in nature
➢ Moist Air
• Mixture of dry air and water vapour
➢ Saturated Air
• Air that contains the maximum amount of water vapour that is possible
at the given temperature and pressure
GS, AP, ME Dept.
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PSYCHROMETRIC PROPERTIES
➢ Dry-bulb temperature (DBT)(tdb)
• The dry-bulb temperature is the temperature indicated by a thermometerexposed to the air in a place sheltered from direct solar radiation.
• In psychrometrics the word temperature by itself without a prefix usuallymeans dry-bulb temperature.
➢ Wet-bulb temperature (WBT)(twb)
• It is the temperature a parcel of air would have if it were cooled to saturationby the evaporation of water into it
• Wet Bulb temperature can be measured by using a thermometer with the bulbwrapped in wet muslin.
• A wet bulb temperature taken with air moving at about 1–2 m/s is referred toas a screen temperature,
• a temperature taken with air moving about 3.5 m/s or more is referred toas sling temperature.
➢ Wet bulb depression
• Difference between dry and wet bulb temperature
GS, AP, ME Dept.
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➢ psychrometer
• is a device that includes both a dry-bulb and a wet-bulb thermometer.
• A sling psychrometer requires manual operation to create the airflowover the bulbs
• a powered psychrometer includes a fan for this function.
• Knowing both the dry-bulb temperature (DBT) and wet-bulbtemperature (WBT), one can determine the relative humidity (RH) fromthe psychrometric chart appropriate to the air pressure.
GS, AP, ME Dept.
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➢ Dew point temperature(tdp)
• The saturation temperature of the moisture present in the sample of air
• it can also be defined as the temperature at which the vapour changes intoliquid (condensation).
• is the temperature at which a moist air sample at the same pressure wouldreach water vapor "saturation."
➢ Dew point Depression
• Difference between tdb and tdp
➢ Humidity
• Humidity is the amount of water vapor in the air.
• There are three main measurements of humidity:
❑ absolute,
❑ relative and
❑ specific.
➢ Absolute humidity
• The mass of water vapor per unit volume of air containing the water vapor at agiven temperature and pressure.
• This quantity is also known as the water vapor density
• It is expressed in gram per cubic metre.
GS, AP, ME Dept.
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➢ Relative humidity
• is the ratio of actual mass of water vapour present in a given volume of moistair to the mass of water vapour in the same volume of moist air when it issaturated at the same temperature and pressure
• RH is dimensionless, and is usually expressed as a percentage.
➢ Specific humidity
• is a ratio of the water vapor content of the mixture to the total air content on amass basis in the moist air sample (dry air plus the water vapor)
• sometimes referred to as the humidity ratio
• It is also known as the moisture content or mixing ratio.
• Specific Humidity is defined as:
SH=m of {v}/m of {a}
➢ Degree of Saturation (Percentage Humidity)
• Degree of Saturation is the ratio of the humidity ratio of moist air - to thehumidity ratio of saturated moist air at the same temperature and pressure.
GS, AP, ME Dept.
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➢ Specific enthalpy
• is the sum of the internal (heat) energy of the moist air in question
• In psychrometrics, the term quantifies the total energy of both the dryair and water vapour per kilogram of dry air.
➢ Specific volume
• is the volume of the mixture (dry air plus the water vapor) containingone unit of mass of "dry air".
• The SI units are cubic meters per kilogram of dry air
➢ Psychrometric ratio
• It is the ratio of the heat transfer coefficient to the product of masstransfer coefficient and humid heat at a wetted surface.
➢ Adiabatic Saturation Temperature.
• Also called The thermodynamic wet-bulb temperature
• It is the lowest temperature which may be achieved by evaporativecooling of a water-wetted (or even ice-covered), ventilated surface.
GS, AP, ME Dept.
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GS, AP, ME Dept.
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Terminology
• A psychrometric chart is a graph of the thermodynamic parameters ofmoist air at a constant pressure, often equated to an elevation relativeto sea level.
➢ Dry-bulb temperature
➢ Wet-bulb temperature
➢ Dew point temperature
• At this point further removal of heat would result in water vaporcondensing into liquid water fog or, if below freezing point, solidhoarfrost.
• Relative humidity
• Humidity ratio
• It is typically plotted as the ordinate (vertical axis) of the graph.
• For a given DBT there will be a particular humidity ratio for which the airsample is at 100% relative humidity
➢ Specific enthalpy
➢ Specific volume GS, AP, ME Dept.
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Staturation Line
Saturation Line
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Dry Bulb Temperature
Constant Dry Bulb Temperature
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Humidity Ratio
Constant Humidity Ratio
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Humidity Ratio
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Relative Humidity
Constant Relative Humidity
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Specific Volume
Constant Specific Volume
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Web Bulb Temperature
Constant Wet Bulb Temperature
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
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Constant Enthalpy
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Linric Company Psychrometric Chart, www.linric.com
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Linric Company Psychrometric Chart, www.linric.com
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Typical Chart Without Enthalpy Lines
GS, AP, ME Dept.
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GS, AP, ME Dept.
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➢ Specific Humidity(w)
➢ Degree of Saturation
➢ Relative Humidity
GS, AP, ME Dept.
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➢ Pressure of water vapour
➢ Vapour density or Absolute Humidity
GS, AP, ME Dept.
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GS, AP, ME Dept.
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Sensible heating
Sensible cooling
Humidification & dehumidification
Cooling and adiabatic humidification
Cooling and humidification by water injection
Heating and humidification
Humidification by steam injection
Adiabatic chemical dehumidification
Adiabatic mixing of air streams
GS, AP, ME Dept.
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Heating of air without changing specific humidity.
GS, AP, ME Dept.
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Cooling of air without changing specific humidity.
GS, AP, ME Dept.
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▪ Addition of moisture to air without change in its dry bulb temperature is known as humidification.
▪ Removal of moisture from air without change in its dry bulb temperature is known as dehumidification.
GS, AP, ME Dept.
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Basic Concepts