psychrometrics-120339802285862-4

Chatterjea - Psychrometrics & Air Conditioning Design

1

Psychrometrics


When a gas or gaseous mixture remains in contact with a liquid surface, it will acquire vapor from the liquid until partial pressure of the vapor in the Gas mixture equals the vapor pressure of the liquid at its existing temperature

When the vapor concentration reaches this equilibrium value the gas is said to be saturated with the vapor

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Partial Saturation

If a gas contains a vapor in such proportion that its partial pressure is less than the vapor pressure of the liquid at the existing temperature, the mixture is but partially saturated.

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Relative saturation

The relative saturation of such a mixture may be defined as the percentage ratio of the partial pressure of the vapor to the vapor pressure of the liquid at the existing temperature.

It is a function of both composition of the mixture and its temperature as well as of the nature of the vapor.

KChatterjea, 20084


Percentage saturation

Defined as the percentage ratio of the existing weight of vapor per unit weight of vapor free gas to the weight of vapor that would be exist per unit weight of vapor free gas if the mixture were saturated at the existing temperature and Pressure.

KChatterjea, 20085


Relative saturation

Relative saturation in %=

= Partial Pressure of vapor

= Vapor Pressure of Pure liquid.

KChatterjea, 20086

100*

s

vr p

py

vp

sp


Percentage Saturation

% Saturation=

= moles of vapor per mole of vapor free gas actually present

= moles of vapor per mole of vapor free gas at saturation

KChatterjea, 20087

ps

v ynn

100*

vn

sn


KChatterjea, 20088

v

vv

pppn

1 s

ss

pppn

1

v

s

s

v

s

v

pppp

pp

nn

v

srp pp

ppyy


KChatterjea, 20089

Introduction – Psychrometry-Introduction – Psychrometry-Humidity.Humidity.

The air conditioning processes are best understood by studying Psychrometry.

Psychrometry is the study of atmospheric air and its associated water vapour.

A psychrometric chart presents physical and thermal properties of moist air in a graphical form.


Humidity

Humidity or Absolute Humidity or Absolute saturation H : Defined as weight of water per unit weight of moisture free gas.

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gas )condesible-(non of Kg

vapourof KgH


Molal Humidity or Molal Saturation

Molal Humidity Hm : is the number of moles of water per mole of moisture free gas.

PA= Partial Pressure of Water Vapour, kpa. P=Total Pressure of the system.

KChatterjea, 200811

A

A

pPp

mH


KChatterjea, 200812

A

A

pPp

mH

A

A

pPp6207.0H

B

Am M

MHH

= Molecular weight of water=18

= Molecular weight of air=29

AM

BM


Saturation humidity

It is defined as the absolute humidity of vapour gas mixture at 100 % saturation.

KChatterjea, 200813

B

As M

MH

s

s

pPp

B

As M

MH

Bs

As

nn

s

s

pPp*6207.0Hs


If vapor is under consideration of water, The percentage saturation is termed as percentage Humidity

The relative saturation becomes the relative humidity.

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Percentage Humidity & Relative Humidity


Dew Point

The temperature at which condensation will first occur when the vapour- gas mixture is cooled at constant pressure.

KChatterjea, 200815


Relative Humidity or Percentage relative Humidity or Relative saturation

Relative saturation in %=

= Partial Pressure of Water vapor in air.

= Vapor Pressure of water liquid.

KChatterjea, 200816

100*%

s

A

ppRH

Ap

sp


Percentage Humidity or % absolute Humidity or % saturation

% Humidity=

=Absolute Humidity.

= saturation Humidity.

KChatterjea, 200817

100*

sHH

H

sH


Humid heat

It is Defined as the Heat capacity of 1 kg dry gas/air and vapour/moisture contained in it.

For air- Water system, The humid heat is given by:

Where 1.006 is the heat capacity of dry air in kJ/kg 0c

Where 1.846 is the heat capacity of water vapor in kJ/kg 0c

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K) air.dry of KJ/(kg 84.1006.1 HCs


Humid volume (VH)

The volume of a mixture of air and water vapour per kg of dry air at 101.325 kpa and the given temperature. It is also known as psychrometric volume.

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PDB

MMHV

BAH

325.101*273

*4136.22*1


Hygrometry or Psychrometry

The special application of wet and dry-bulb thermometry is termed hygrometry or psychrometry.

Humidity Cahrts have been developed which permit the determination of the humidites of air water vapor mixtures from wet- and dry-bulb temperature measurements.

KChatterjea, 200820


Humidity Chart

The chart covering a range of molal humidities from 0.00 to 0.34 and a range of dry-bulb temperatures from 80 to 210F.

The total pressure is taken at 29.92 i.Hg.

KChatterjea, 200821


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KChatterjea, 200823


KChatterjea, 200824


KChatterjea, 200825


KChatterjea, 200826

Psychrometry – Dry Bulb TempPsychrometry – Dry Bulb Temp

Dry-bulb temperature (DBT) is the commonly measured temperature from a thermometer.


KChatterjea, 200827

Psychrometry – Wet Bulb TempPsychrometry – Wet Bulb Temp

Wet Bulb Temperature (WBT) is determined when air is circulated past a wetted sensor tip.

Intersection of DBT and WBT provides the status point.


KChatterjea, 200828

Psychrometry – Saturation LinePsychrometry – Saturation Line

Saturation line is a temperature condition at which water will begin to condense out of moist air.

It is also called the dew point temperature line.


KChatterjea, 200829

Psychrometry – Relative HumidityPsychrometry – Relative Humidity

Relative Humidity (RH) is a ratio between actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature.

RH is affects human comfort significantly.


KChatterjea, 200830

Psychrometry – Specific VolumePsychrometry – Specific Volume

Specific Volume indicates the space occupied by air.

This is useful in converting volumetric flow to mass-flow rate calculation.


KChatterjea, 200831

Thank You!

psychrometrics-120339802285862-4

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