Pyschrometric Chart

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Psychrometry and Air Processes

Palak Desai

Mechanical Engineering Department

C G Patel Institute of TechnologyUka Tarsadia University

Bardoli


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Composition and thermal propertiesof moist air


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1. Atmospheric air

Atmospheric air is not completely dry but a mixture of dry air and

water vapor.

In atmospheric air, the content water vapor varies from 0 to 3% by

mass.

The processes of air-conditioning and food refrigeration often involve

removing water from the air (dehumidifying), and adding water to the

air (humidifying).


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2. The thermal parameters of moist air

(1) Dry bulb temperaturet Dry bulb temperature is the temperature of the air, as measured by an

ordinary thermometer.

The temperature of water vapor is the same as that of the dry air in moistair.

Such a thermometer is called a dry-bulb thermometer in psychrometry,because its bulb is dry.

(2) Wet bulb temperaturetWB:

Wet bulb temperature is thermodynamic adiabatic temperature in anadiabatic saturation process, and measured by a wet bulb thermometer.


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(3) Dew point temperature tDP: When the unsaturated moist air is cooled at constant vapor pressure or at constant

humidity ratio, to a temperature, the moist air becomes saturated and the condensationof moisture starts, this temperature is called dew point temperature of the moist air.

(4) Relative humidity: Relative humidity is defined as the ratio of the mole fraction of the water vapor in a

given moist air to the mole fraction of water vapor in a saturated moist air at the sametemperature and the same atmospheric pressure.

Relative humidity is usually expressed in percentage (%).

From the ideal gas relations, relative humidity can be expressed as

---- the mole fraction of the water vapor in moist air;

---- mole fraction of water vapor in a saturated moist air at the sametemperature;

satw

w

satw

w

P

P

x

x

,,

wx

satwx ,


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(7) Specific Volume/Moist Volume v: Specific volume of moist airv , m3/kgdry is defined as the total volume of

the moist air (dry air and water vapor mixture) per kgof dry air.

(8) Specific Enthalpy: Specific enthalpy of moist airh (kJ/kgdry) is defined as the total enthalpy

of the dry air and water vapor mixture per kg of dry air.

Enthalpy values are always based on some datum plane. Usually the zero value of the dry air is chosen as air at 0, and the zero

value of the water vapor is the saturated liquid water at 0.


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Adiabatic saturation process and wet

bulb temperature


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1. Adiabatic saturation process An unsturated air, which has dry bulb temperature t1, humidity ratio w1

and enthalpy h1, flows through a spray of water, as shown in Fig.

The spray can provide enoughsurface area so that the air leavesthe spray chamber in equilibriumwith the water, with respect to boththe temperature and the vapor

pressue.

In order to perpetuate the process, itis necessary to provide makeupwater (w2-w1) to compensate for theamount of water evaporated into theair.

The temperature of the makeup

water is the same as that in the sump. If the device is adiabatic, then the

process is called adiabatic saturationprocess.

inlet

outlet

pumpsump

12

make up water

(w -w )

thermometer

(thermodynamic

wet-bulb temperature)

unsaturated air

t ,w ,h1 1 1

222

saturated air

t ,w ,h


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2.Thermodynamic wet bulb temperature

In an adiabatic saturation process, the temperature of the water

in the sump is called the thermodynamic wet bulb temperature

tWB of the air , or simply the wet bulb temperature of the air.

An adiabatic saturation process is a constant wet bulb

temperature process, the wet bulb temperatures of air at the

inlet and the outlet are same, and they are equal to tWB.

That is 21

WBWB

tt


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3.Measurement of wet bulb temperature

A simple wet bulb thermometer used to measure the wet bulbtemperature is based on the principle of the adiabaticsaturation process.

It is an ordinary thermometer being wrapt with a cloth sleeveof wool or flannel, around its bulb.

The cloth sleeve should be clean and free of oil and thoroughlywet with clean fresh water.

The water in the cloth sleeve evaporates as the air flows at a

high velocity (2.5m/sec). The evaporation, which takes the heat from the thermometer

bulb, lowers the temperature of the bulb.

The thermometer indicates approximately the wet bulb

temperature tWB.


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The difference between the dry-bulb and wet-bulb temperaturesis called the wet-bulb depression.

If the air is saturated, evaporation cannot take place, and thewet-bulb temperature is the same as the dry-bulb temperature;and the wet-bulb depression equals zero.


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Psychrometric chart


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A psychrometric chart graphically represents the thermodynamicproperties of moist air

It is very useful in presenting the air conditioning processes

The psychrometric chart is bounded by two perpendicular axes and a

curved line:

1) The horizontal ordinate axis represents the dry bulb

temperature line t, in ;

2) The vertical ordinate axis represents the humidity ratio line w ,

in kgw/kgdry.air

3) The curved line shows the saturated air, it is corresponding to

the relative humidity =100% .


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The psychrometric chart incorporates seven parametersand properties.

They are

Dry bulb temperature t

Relative humidity

Wet bulb temperature tWB

Dew point temperature tDP

Specific volume v

Humidity ratio w

Enthalpy h


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Dry-bulb temperature t is shown along the bottom axis of thepsychrometric chart. The vertical lines extending upward fromthis axis are constant-temperature lines


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Relative humidity lines are shown on the chart as curved linesthat move upward to the left in 10% increments. The linerepresenting saturated air ( = 100% ) is the uppermost curvedline on the chart. And the line of = 0% is a horizontal ordinate

axis itself.


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Wet-bulb temperature tWB: On the chart, the constant wet-

bulb lines slope a little upward to the left, and the wet bulb

temperature is read following a constant wet-bulb line from

the state-point to the saturation line.


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Dew point temperature tDP

: This temperature is read by

following a horizontal line from the state-point to the

saturation line.


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Specific volume v: It is shown from the constant-volume lines

slanting upward to the left.


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Humidity ratio w: it is indicated along the right-handaxis of the chart.


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Enthalpy h: It is read from where the constant enthalpyline crosses the diagonal scale above the saturationcurve. The constant enthalpy lines, being slanted lines,

are almost coincidental as the constant wet-bulbtemperature lines.


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Only two properties are needed to

characterize the moist air because the pointof intersection of any two properties linesdefines the state-point of air on a

psychrometric chart.

Once this point is located on the chart, the

other air properties can be read directly


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2 M i i h di d th i


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2. Main air handing processes and theirvariations in properties

(1) Sensible cooling along a cooling coil, or sensible heatingalong a heating coil

The sensible cooling happens when the air is cooled without altering the specifichumidity.

The process is shown on the psychrometic chart by line 1a in Fig.

During this process, the relative humidity of the air will increase.

The sensible cooling can only take place under the condition when thetemperature of the cooling coil is not below the dew point temperature of the air

being processed.

The sensible heating is similar to sensible cooling, but with the dry bulbtemperature increasing (line 1b in Fig.).

It should be noted that there should be no water within the heating system becausethe evaporation of the water will increase the specific humidity of the air.


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WB

Dp

humi

dity

rel

ativ

e=100

%

3

2b

2a

1b1a w

hum

idityratio

t wet-bulbtemp

t

dry-bulb tempdew-point temp

(2) Adiabatic humidification and dehumidification using a


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(2) Adiabatic humidification and dehumidification using ahumidifier or chemical dehumidifier

The adiabatic humidification occurs when water vapor, of which temperatureis near the wet bulb temperature of the moist air, is added to the air (line 2a in

Fig.9-4).

A humidifier performs this function by supplying the water vapor.

During the adiabatic humidification process along the constant wet bulbtemperature line, the specific humidity of air will increase.

Reduction in dry bulb temperature will happen as the evaporated water willabsorb heat .

The adiabatic dehumidification is a reverse process of the humidification, asshown by line 2b in Fig.9-4.

This dehumidification is usually achieved by use of chemical agents, likeabsorbents or adsorbents.

In this process, the specific humidity decreases along the constant wet bulb

temperature line whilst the dry bulb temperature will increase as a result ofh i l ti

Pyschrometric Chart

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