Air in the atmosphere normally contains some water vapor ...aast-notes.weebly.com/uploads/3/9/0/5/3905160/thermodynamicsii... · Slide Nr. 0 of 18 Slides Introduction Air in the atmosphere

Slide Nr. 0 of 18 Slides

Introduction

� Air in the atmosphere normally contains some water

vapor (or moisture) and is referred to as atmospheric

air.

� By contrast, air that contains no water vapor is called

dry air.

Mon 2:04:28 PM

Chapter 2:

Psychrometry and Air-Conditioning

Dr. Eng. Sameh Shaaban

31 October 2009

dry air.

� It is often convenient to treat air as a mixture of water

vapor and dry air since the composition of dry air

remains relatively constant.

� The amount of water vapor changes as a result of

condensation and evaporation from oceans, lakes,

rivers, showers, and even the human body.


Saturation Temperature and Saturation Pressure

Mon 2:04:28 PM

At a given pressure, the temperature atwhich a pure substance changes phase iscalled the saturation temperature Tsat.

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31 October 2009

sat

At a given temperature, the pressure atwhich a pure substance changes phase iscalled the saturation pressure Psat.


Psychrometric Mixtures

Mon 2:20:10 PM

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.

� Consider atmospheric air at 1.013 bar and 15 °C. The saturationpressure of water vapor corresponding to 15 °C is 0.01704 bar.

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31 October 2009

pressure of water vapor corresponding to 15 °C is 0.01704 bar.

� Unless the water vapor is in contact with its liquid it will not besaturated, and its pressure will be below the saturation value of0.01704 bar.

� In normal applications the atmosphere is well removed from thesaturated condition. At such low vapor pressures the vapor canbe considered to act as a perfect gas, and the properties of themixture can be found using the Gibbs-Dalton law.



Mon 2:20:10 PM

as ppp +=

Assume that in a quantity of atmospheric air the vapour pressure is 0.01 barat 15°C and the total pressure is 1.013 bar

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31 October 2009

288 K

280 K

bar003.101.0013.1pa =−=

The saturation temperaturecorresponding to 0.01 bar is 7°C,

hence the vapor in atmospheric air

under these conditions has a degree

of superheat of ( 15 - 7) = 8 K.(point 1)



Mon 2:20:10 PM

The dew-point temperature Tdp is defined as the temperature at which

condensation begins when the air is cooled at constant pressure. In otherwords, Tdp is the saturation temperature of water corresponding to the

vapor pressure.

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31 October 2009

Cooling


Specific humidity, relative humidity, and percentage saturation

Mon 2:20:10 PM

The specific humidity (or moisture content or humidity ratio) isthe ratio of the mass of water vapour to the mass of dry air in agiven volume of the mixture

s

m

m=ω

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31 October 2009

am=ω

TR~

m~Vpmand

TR~

m~Vpm aa

sss

s ==

a

s

a

s

a

s

a

s

aa

ss

p

p622.0

p

p

96.28

18

p

p

m~m~

m~Vp

TR~

TR~

m~Vp=×=×=×=ω



Mon 2:20:10 PM

a

s

m

m=ω

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31 October 2009

(the total pressure p is usually the barometric pressure).

s

s

a

s

pp

p622.0

p

p622.0

−==ω



Mon 2:20:10 PM

The relative humidity of the atmosphere is the ratio of theactual mass of the water vapour in a given volume to thatwhich it would have if it were saturated at the sametemperature (maximum amount of moisture the air can hold atthe same temperature)

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31 October 2009

( )sats

s

m

m=φ

( )TR

Vpmand

TR

Vpm

s

g

sats

s

ss ==

where pg is the saturation pressure at the temperature of the mixture.



Mon 2:20:10 PM

sp=φ

( )TR

Vpmand

TR

Vpm

s

g

sats

s

ss ==

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31 October 2009

g

s

p=φ



Mon 2:20:10 PM

The term percentage saturation is also used, defined as theratio of the specific humidity of a mixture to the specifichumidity of the mixture when saturated at the sametemperature, expressed as a percentage

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31 October 2009

gω

ω=ψ

s

g

pp

pp100

−

−φ=ψ

where pg is the saturation pressure at the temperature of the mixture.


Examples

Thu 8:40:02 AM

Ex

am

ple

Chapter 2:



31 October 2009


Examples

Thu 8:40:02 AM

Ex

am

ple

Chapter 2:



31 October 2009


Examples

Thu 8:40:02 AM

Ex

am

ple

Chapter 2:



31 October 2009


Wet Bulb Temperature

Mon 2:20:10 PM

The temperature measured by athermometer whose bulb iscovered with a cotton wick

Chapter 2:



31 October 2009

covered with a cotton wicksaturated with water and air ismoved over the wick


Measurement of relative humidity

Mon 2:20:10 PM

An instrument used to measure relative humidityis called a psychrometer, or a hygrometer

sling psychrometer

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31 October 2009

� Some of the water evaporates and this produces a cooling

effect at the bulb.

� the wet bulb indicates a lower temperature than the drybulb.

� If the relative humidity is low, then the rate of evaporation

� at the wick is high, and hence the wet bulb depression is

high.


THE PSYCHROMETRIC CHART

Mon 2:20:10 PM

The state of the atmospheric air at a specified pressure iscompletely specified by two independent intensive properties.

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31 October 2009

Slide Nr. 16 of 18 SlidesMon 2:20:10 PM

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31 October 2009

Slide Nr. 17 of 18 SlidesMon 2:20:10 PM

0.0144 kg H2o/kg dray air

73.5 kJ/kg

24.2°C

0.895 m3/kg

Chapter 2:



31 October 2009

0.0144 kg H2o/kg dray air19.8°C

Slide Nr. 18 of 18 SlidesThu 8:46:56 AM

Chapter 2:



31 October 2009

Air in the atmosphere normally contains some water vapor ...aast-notes.weebly.com/uploads/3/9/0/5/3905160/thermodynamicsii... · Slide Nr. 0 of 18 Slides Introduction Air in the atmosphere

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