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THERMODYNAMICS 1 Psychrometrics and Air conditioning (Ch 14) 2 Psychrometrics and Air conditioning (Ch 14) 3 Psychrometrics and Air conditioning (Ch 14) 4 Thermodynamic Property Relations (Ch. 12) 5 Thermodynamic Property Relations (Ch. 12) 6 Thermodynamic Property Relations (Ch. 12) “Thermodynamics: An Engineering Approach” 7 th edition, Cengel and Boles 1

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THERMODYNAMICS

1 Psychrometrics and Air conditioning (Ch 14)

2 Psychrometrics and Air conditioning (Ch 14)

3 Psychrometrics and Air conditioning (Ch 14)

4 Thermodynamic Property Relations (Ch. 12)

5 Thermodynamic Property Relations (Ch. 12)

6 Thermodynamic Property Relations (Ch. 12)

“Thermodynamics: An Engineering Approach” 7th edition, Cengel and Boles

1

Objectives• Why air conditioning?

• What are physical systems that

enable air conditioning

• Differentiate between dry air and

atmospheric air.

• Air as a mixture of dry air and water

vapor.

HUMAN COMFORT

AND AIR-

CONDITIONING

We cannot change the weather, but

we can change the climate in a

confined space by air-conditioning.

WHY AIR CONDITIONING

A body feels comfortable when it can freely

dissipate its waste heat, and no more.

Today, modern air-conditioning systems can

condition the air to peoples’ desires.

The rate of heat generation by human body (average

adult male): ~ 87 W when sleeping ~115 W when

resting or doing office work, and ~ 440 W when

doing heavy physical work.

Body heat is too quickly dissipated – we feel cold

Body heat is too slowly dissipated – we feel warm

A comfortable environment.

•The relative humidity affects the

amount of heat a body can dissipate

through evaporation. Most people

prefer a relative humidity of 40 to 60%.

•Air motion removes the warm, moist air

that builds up around the body and

replaces it with fresh air. Air motion

should be strong enough to remove

heat and moisture from the vicinity of

the body, but gentle enough to be

unnoticed.

•An important factor that affects human

comfort is heat transfer by radiation

between the body and the surrounding

surfaces such as walls and windows.

•Other factors that affect comfort are air

cleanliness, odor, and noise.• Temperature

• Humidity

• Air Motion (wind-chill factor)

AIR CONDITIONERS

1 – compressor

2 – condenser

3 – expansion valve

4 - evaporator

Dry warm

air in

Liquid water

Spray

Cool moist air

out

EVAPORATIVE COOLERS AND

HUMIDIFIERS

Liquid water

Cool moist air

outDry warm

air in

Internal mass and energy transfers result in cool, moist air exiting the duct

MIXTURES OF GASSES AND VAPOUR/LIQUID

The cp of air can be assumed to be constant at 1.005 kJ/kg · °C in the temperature

range -10 to 50°C with an error under 0.2%.

DRY AND ATMOSPHERIC AIR

Atmospheric air: Air in the atmosphere containing some water vapor (or moisture).

Dry air: Air that contains no water vapor.

Atmospheric air = Dry Air + Water Vapor

MIXTURES OF GASSES AND VAPOUR/LIQUID

Air conditioning applications: Between the temperature range of -10 to 50 oC

In this region, water vapor in air behaves as if it existed alone and obeys the ideal-

gas relation Pv = RT.

Then the atmospheric air can be treated as an ideal-gas mixture:

Pa Partial pressure of dry air

Pv Partial pressure of vapor (vapor pressure)

Dalton’s law of additive pressures for a mixture of two ideal gases.

Dalton’s law of additive pressures: The pressure of a

gas mixture is equal to the sum of the pressures each

gas would exert if it existed alone at the mixture

temperature and volume.

MIXTURES OF GASSES AND VAPOUR/LIQUID

Air conditioning applications: Between the temperature range of -10 to 50 oC

In this region the cp , h, and u can be evaluated from table or using:

For water vapor

hg = 2500.9 kJ/kg at 0°C

cp,avg = 1.82 kJ/kg · °C at -10 to 50°C range

Additional Info:

Subcooling a Liquid = To cool the liquid below its

saturation temperature at a given P

Superheating a vapour = To increase the

temperature of the vapour at constant P

Degree of Superheat = (T-Tsat) for a given pressure

MIXTURES OF GASSES AND VAPOUR/LIQUID

Water is a common component in mixtures, and therefore requires special treatment.

E.g.: In a mixture of ideal gasses, the increase of pressure or reduction of

temperature will cause one component of the mixture to a state of saturation and then

condensation.

This can be seen for a gas mixture containing a vapour as shown below:

Mixture at constant T but P is increased

2 = saturated vapour

3 = saturated liquid

Mixture at constant P but T is decreased

2 = saturated vapour

3 = saturated liquid

Mixture

2

1

3

vapour

P

V

2

1

3

vapourT

S

Mixture Mixture

P = ConstantT = Constant

PHYCHROMETRY

Study of air and water vapour mixtures relevant to air conditioning plant, and water

cooling tower analysis.

Some special terms that will be used to analyse these system are defined as follows:

Specific Humidity (w)

This is the ratio of masses of water vapour to air in a given volume V:

Ra= 0.287 kJ/kg.K

Rv= 8.314/(16+2) kJ/kg.K

PHYCHROMETRYRelative Humidity

Saturated air: The air saturated with moisture.

Relative humidity: The ratio of the amount of moisture the air holds (mv) to the

maximum amount of moisture the air can hold at the same temperature (mg).

Relationship between absolute and relative humidity

In most practical applications, the

amount of dry air in the air–water-

vapor mixture remains constant,

but the amount of water vapor

changes.

Therefore, the enthalpy of

atmospheric air is expressed per

unit mass of dry air.

The enthalpy of moist (atmospheric) air is

expressed per unit mass of dry air, not per

unit mass of moist air.Dry-bulb temperature:

The ordinary temperature

of atmospheric air. Lecture 4,5&6/ MEC 3454

Lecture 4,5&6/ MEC 3454

EXAMPLE

Atmospheric air at 30oC, 100 kPa, has a dew point of 21.3oC.

Find the relative humidity?

2.5480.6 60%

4.247v

g

P kPaor

P kPa

For saturated air, the vapor pressure is equal to the saturation pressure of

water.

•Relative humidity ranges from 0 to 1

•Relative humidity changes with

temperature, although specific humidity

may remain constant

Lecture 4,5&6/ MEC 3454

Additional Info: Self Learning

The difference between specific and relative humidity.

Lecture 4,5&6/ MEC 3454

Additional Info: Self Learning