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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)
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