Chapter 2 Psychrometric Chart

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PSYCHROMETRIC CHART

DR HAMID YUSOFF

MEC 351

CHAPTER 2 PSYCHROMETRIC CHART

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2.1 Atmosphere presure and moisture

2.2 What is a Psychrometric Chart?

2.3 Psychrometric Processes: Sensible Cooling and Sensible Heating of Air

2.4 Psychrometric Processes: Heating & Humidification

2.5 Psychrometric Processes: Cooling with dehumidification2.6 Psychrometric Processes: Cooling with Adiabatic humidification (Water injection)

2.7Psychrometric Processes: Heating with Adiabatic dehumidification

(sorbent/desiccants)

2.8 Psychrometric Processes: mixing of two streams of air

CHAPTER 2 CONTENTS


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Air is a mixture of N2, O2and small amount of other

gasses.

Atmospheric air:Air in the atmosphere normally

contains some water vapor (a moisture) and isreferred to as atmospheric air.

Dry air:Air that contains no water vapor is called dry

air. The temperature of air in air-conditioning

application ranges between -10OC to 50OC.

2.1 Atmosphere presure and moisture
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Taking 0OC as reference temperature, enthalpy

and enthalpy change of dry air can be

determined from

In air-conditioning processes we are

concerned with the changes in the enthalpy

TCkgkJTCh OPairdry ./005.1 kgkJ

TCkgkJTCh OPairdry ./005.1

h


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The atmospheric air can be treated as a n ideal-gas mixture whose pressure is the sum of thepartial pressure of dry air Paand that of watervapor Pv.

where

p = total pressure of air (Pa, N/m2) pa=partial pressure dry air(Pa, N/m

2)

pw=partial pressure water vapor(Pa, N/m2)

Vapor Pressure:The partial pressure of watervapor is usually referred to as the vapor pressure.It is the pressure water vapor would exert if itexisted alone at the temperature and volume of

atmospheric air.

va PPP kPa

Daltons Law of Partial Pressures in Moist Air
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Absolute or specific humidity:

The mass of water vapor present in a unit

mass of dry air. [Note: Also called humidityratio]

(kg water vapor / kg dry air)a

v

m

m

a

v

aa

vv

aaa

vvv

a

v

P

P

RP

RP

TRVP

TRVP

m

m622.0

v

v

PP

P

662.0

For dry air: 0


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Saturated Air:

There is a limit on the amount of vaporthe

air can hold at a given temperature.

Air that is holding as much moisture as it can

at a given temperature is called saturated air.

Any moisture introduced into saturated air will

condense.


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Relative Humidity ():The amount of moisture the

air holds relative to the maximum amount of

moisture the air can hold at the same temperature.

where

The relative humidity ranges from 0 for dr y air to 1 for

saturated air.

See Example 13.1 (Text Book)

g

v

vg

vv

g

v

P

P

TRVP

TRVP

m

m

Tsatg PP @

gPP

622.0g

g

PP

P

622.0


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Total enthalpy of atmospheric air is the sum ofthe enthalpies of dry air and water vapor:

(kJ/kg dry air)

vvaava hmhmHHH

ga hhh


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Dry bulb temperature (Tdb):The ordinary

temperatureof the atmospheric air is

refereed to as the dry bulb temperature.

Dewpoint temperature (Tdp):The

temperature at which condensation begins

when the air is cooled at constant pressure.

vPsatdp TT @


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When the temperature of a cold drink is

below the dew-point temperature of the

surrounding air, it sweats.


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Relative humidity and Sp. Humidity of air can

be determined by measuring the adiabatic

saturation temperature of air, which is thetemperature air attains after flowing over

water in a long adiabatic channel until it is

saturated

21

212

1

fg

fP

hh

hTTCg

2

2

2

622.02

g

g

PP

P


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Wet bulb temperature

(Twb

):A thermometer

whose bulb is covered

with a cotton wick

saturated with water

and to blow air over thewick as shown in figure.

The temperature

measure in this manneris called wetbulb

temperature.


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Sling Psychrometer:The wet bulb

temperature can alsobe measured byplacing the wetwicked thermometer in

a holder attached to ahandle and rotatingthe holder rapidly. A

device that works onthis principle is calledSling Psychrometer.


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2.1 What is a Psychrometric Chart? Psychrometric charts are graphic representations of the

psychrometric properties of moist air. By using psychrometric

charts HVAC engineers can graphically analyzedifferent types of

psychrometric processes and find solution to many practicalproblems without having to carry out long and tedious

mathematical calculations.

The psychrometric chart looks complicated with vast numbers of

lines and curves in it, but is very easy to understand if you know

the basic properties of air. You will also understand its worth

when you actually use it considering the fact that you wont have

to use any formulae to find the properties of air in different

conditions, all you will have to know is two parameters of air and

the rest are easily found on the chart.
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Various Lines and Curves in the Psychrometric Chart

Dry-bulb temperature

It is temperature of the air measured with on ordinary thermometer.

Dew point temperature: The remperature at which water by evaporating into moist air at

given dry-bulb temperature and humidity ratio (w) can bring the air to saturation

adiabatically at the same temperature Twb while the pressure is maintained constant.

Relative humidity: Relative humidity is defined as the ratio of the partial pressure of the

water vapour in moist air at given temperature T, to the partial pressure of the watervapour in saturated air, at the same temperature T.

Specific humidity/humidity ratio: As the mass of water vapour in kg which is associated

with one kilogram of dry air water vapour mixture.

Percentage saturation: The ratio of the moisture content of moist air at a given

temperature, T, to the moisture content of saturated air at the same temperature T.

Specific volume: Is the volume in cubic metres of one kilogram of dry air together with the

mass of water vapour associated with it.

Sensible heat: Heat which produces a change of temperature in a body is known as sensible

heat.

Latent heat: Heat which is necessary to produce a change of state of a material at a constant

temperature


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The psychrometric chart

A AbsoluteHumidity Axis

B Specific VolumeCDry Bulb Axis

D

RelativeHumidity

D

Dew PointE

EnthalpyF

Wet Bulb &

Saturation CurveG


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For a saturated air, the drybulb, wetbulb

and dewpoint temperature are identical.


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AirConditioning Processes

Air-conditioning processes include:

Simple heating (raising the temperature),

Simple cooling (lowering the temperature)

Humidifying (adding moisture), and

Dehumidifying (removing moisture).


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2.3 Psychrometric Processes: Sensible Cooling

and Sensible Heating of Air The basic function of the air-conditioners is to cool the air absorbed from

the room or the atmosphere, which is at higher temperatures.

In HVAC systems, air is typically cooled by passing it over a cooling coil.

Most cooling coils are finned-tube heat exchangers in which cool water or

refrigerant flows through tubes, and the tubes have external fins to

increase heat transfer area. If the air is cooled below the dew point

temperature, water will condense and must be drained from the bottom of

the cooling coil.
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2.3 Psychrometric Processes: Sensible Cooling

Observable features of a sensible

cooling process

the dry bulb temperature decreases

the relative humidity increases

the enthalpy decreases the wet bulb temperature decreases

the specific volume decreases

the humidity ratio, vapor pressure

and dew point remains constant = =

=
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2.3 Psychrometric Processes: Sensible heating

In HVAC systems, air is typically heated by passing it over a heating coil or useof electrical strip heaters. A schematic of a cooling coil is shown below.

Observable features of a sensible

heating process

The dry bulb temperature increases

the relative humidity decreases

the enthalpy increases

the wet bulb temperature increases

the specific volume increases

the humidity ratio, vapor pressureand dew point remains constant

= =

=


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2.3 Psychrometric Processes:

Governing Equation for Sensible Heating and Cooling

The heat transfer is given by; =

= ( )

= 1.005 + 1.888 ( )

= 1.005 + 1.88

= 1.005 + (2501 + 1.88)

=

is the ratio of volume flow rate to the specific volume

=

= ()

= 1 ,

= 2 ,

= (

)

= is the enthalpy of stream 1, (kJ/kg)

= is the enthalpy of stream 2, (kJ/kg)

= specific heat of moist air
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2.5Psychrometric Processes: Heating & Humidification

Observable features of a humidification

process are:

1. Humidification adds moisture to the air

which increase the absolute humidity

2. Water is added in vapor form

3. Water is converted from liquid to gas4. There is an increase in the energy level

Sensible heat added =

Latent heat added

= (3 )

= 3

h1

h3h2
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2.6Psychrometric Processes: Cooling with dehumidification

=

Observable characteristics of a cooling anddehumidifying process are:

1. dry bulb temperature decreases

2. humidity ratio decreases

3. vapor pressure decreases

4. dew point temperature decreases

5. wet bulb temperature decreases

6. enthalpy decreases (there is a decrease in

the energy level and with the loss of

energy, condensation occurs)

7. relative humidity increases

= + ( )

= ( )

= ( )

= +

=

+
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2.7 Psychrometric Processes: Cooling with Adiabatic

humidification(Water injection)

Evaporative cooling is an adiabatic process,

that is, there is no net heat loss or gain.

1. Constant wet bulb temperature

2. Decreasing dry bulb temperature

3. Increasing humidity ratio

4. Increasing dew point temperature

5. Increasing vapor pressure

6. Essentially constant enthalpy

7. Decreasing specific volume

8. There is no loss or gain of heat within thesystem because the amount of sensible

heat removed equals latent heat added to the

water.
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2.8Psychrometric Processes: Heating with Adiabatic

dehumidification(sorbent/desiccants)

Observable characteristics of a heating and

dehumidification process are:

1. dry bulb temperature increases2. humidity ratio decreases

3. dew point temperature decreases

4. vapor pressure decreases

5. relative humidity decreases

6. enthalpy remains essentially constant

7. wet bulb temperature remains essentially constant

When air is brought into contact with a desiccant

or sorption material the moisture will be

adsorbedand the air will become "drier". As this

occurs, the heat of condensation will enter the air

stream, thereby increasing its temperature. The

process is essentially adiabatic or isenthalpic. Aircan be dehumidified by passing it over

sorbents/desiccants that have affinity for

moisture. Examples; silica gel, activated alumina.
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2.9 Psychrometric Processes: mixing of two streams of air


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Psychrometric Processes: mixing of two streams of air


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Example

Solved Problem 1 -Assume

that the outside air

temperature is 32C with a

relative humidity = 60%.

Use the psychrometric chart

to determine the specifichumidity [18 gm-

moisture/kg-air], the

enthalpy h [78 kJ/kg-air], the

wet-bulb temperature Twb

[25.5C], the dew-point

temperature Tdp[23C], andthe specific volume of the dry

air v [0.89m3/kg]. Indicate all

the values determined on the

chart.


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Solved Problem 2:

Assume that the

outside air

temperature is 8C.

If the air in a room

is at 25C with a

relative humidity

= 40%, use the

psychrometric chart

to determine if the

windows of thatroom which are in

contact with the

outside will become

foggy.

The air in contact with the windows will become colder until the dew point is reached. Notice

that under the conditions of 25C and 40% relative humidity the dew point temperature is

slightly higher than 10C, At that point the water vapor condenses as the temperature

approaches 8C along the saturation line, and the windows will become foggy.


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One of the major applications

of the Psychrometric Chart is in

air conditioning, and we find

that most humans feel

comfortable when the

temperature is between 22Cand 27C, and the relative

humidity between 40% and

60%. This defines the "comfort

zone" which is portrayed on the

Psychrometric Chart as shown

below. Thus with the aid of the

chart we either heat or cool,

add moisture or dehumidify as

required in order to bring the

air into the comfort zone.

Solved Problem 3:


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Solved Problem 4:Outside air at 35C and 60%

relative humidity is to be

conditioned by cooling and

heating so as to bring the

air to within the "comfort

zone". Using the

Psychrometric Chart neatly

plot the required air

conditioning process and

estimate (a) the amount of

moisture removed [11.5g-

H20/kg-dry-air], (b) the heat

removed [(1)-(2), qcool=

48kJ/kg-dry-air], and (c) the

amount of heat added [(2)-

(3), qheat= 10kJ/kg-dry-air].


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Solved Problem 5:

Hot dry air at 40C and 10%

relative humidity passes

through an evaporative cooler.

Water is added as the air

passes through a series ofwicks and the mixture exits at

27C. Using the psychrometric

chart determine (a) the outlet

relative humidity [45%], (b) the

amount of water added [5.4g-

H20/kg-dry-air], and (c) thelowest temperature that could

be realized [18.5C].


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Solved Problem 6: Heating


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Solved Problem 7: Heating

Determine the load (kW) required to raise 0.5 m3/s of air at 10 degree C and 80% RH to 30 deg C.

What would be the final condition of air?


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Solved Problem 8: Cooling


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Solved Problem 9: Cooling with dehumidification


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Solved Problem 10: Cooling with humidification


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Solved Problem 11: Cooling with adiabatic humidifier


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/ /

Solved Problem 12: Heating with dehumidifier adiabatic process

Documents

Chapter 2 Psychrometric Chart