2. Psychometrics
Definition: Graphical representation of thermodynamic properties of moist air.
Importance: design of systems such as air-conditioning equipment for storage of
fresh produce, dryers for drying cereal grains, and cooling towers in food
processing plants
Ideal gas laws are adequate for determining properties since the partial
pressure of water vapor is very small
Ideal gases are those that follow the law
pV=mRT
3. PROPERTIES OF DRY AIR
• Composition of Air
The apparent molecular weight of
standard dry air is 28.9645 kg/mole. The
gas constant for dry air, Ra, is computed as
8314.41/28.9645 =287.055 (m3 Pa)/(kg K)
11. Air vapour mixture and its properties
• Moist Air. It is a mixture of dry air and water vapour. The
amount of water vapour present in the air, depends upon the
absolute pressure and temperature of the mixture.
• Satuarted air:Air that contains the maximum amount of water
vapour that is possible at the given temperature and pressure,
i.e. air in which the relative humidity is 100%.
12. Pressure of Moist air
• Acc to Gibb’s Dalton Law
p = ρa 286.9 T + ρw 455 T (2)
where
ρa = density of air (kg/m3)
ρw = density of water vapor (kg/m3)
13. Dry Bulb Temperature (DBT)
• The temperature of the air
measured by the ordinary
thermometer is called as the dry
bulb temperature of air,
commonly referred as DBT. When
ordinary thermometer is exposed
to the atmosphere, it indicates
the dry bulb temperature, which
is nothing but the atmospheric
temperature.
14. Wet Bulb Temperature of Air (WBT):
• The Wet Bulb temperature is the adiabatic
saturation temperature. It indirectly
indicates the moisture content present in the
air.
• Temperature of the ordinary air measured by
the thermometer when it is covered by wet
cloth or wick is called as the wet bulb
temperature, commonly referred to as WBT.
• The wet bulb temperature of the air is
always less than the dry bulb temperature of
air. The difference between the DBT and WBT
is called as wet bulb depression.
15. Dew Point Temperature of Air (DPT)
• The temperature at which the water vapor within
the air at some temperature starts condensing is
called as the dew point temperature of the air or
DPT. When the dew is formed the air is said to be in
saturated condition.
• A glass filled with chilled water is an example.
• The dew point temperature of air depends upon
its mositure content. Higher the mositure content
of the air, higher is the dew point temperature. For
the saturated air, the dry bulb temperature, the
wet bulb temperature and dew point temperature
are same.
• Dew point depression = Dry bulb temperature - Dew
point temperature of air.
17. Degree of Saturation. It is the ratio of the
humidity ratio of moist air - to the humidity
ratio of saturated moist air at the same
temperature and pressure.
Absolute Humidity. It is mass of water vapour
present in 1 m3 of dry air, and is generally
expressed in terms of gm per cubic metre of
dry air. It is also expressed in terms of grams
per cubic metre of dry air. Mathematically, 1
Kg of water vapour is equal to 15,430 grains.
20. Enthalpy of Moist Air
• The enthalpy of moist and humid air
includes
• Enthalpy of the dry air -
the sensible heat
Enthalpy of the evaporated water in the
air - the latent heat i.e.
Total enthalpy:
h (KJ/kg)= 1.005 x DBT + w [( 2501.4 +
1.88 (DBT)]
21. Specific volume (m3/kg)
Specific volume is defined as the total
volume of dry air and water vapor mixture
per kg of dry air and water vapor (SI-
units). The specific volume can be
expressed as
22.
23. Construction of Psychrometry
• Saturation Line
• Plot Point corresponding to (x,y) i.e.
(DBT, w), then join to form saturated line
(RH=100%).
• WBT
DBT
w
saturation line
(RH=100%)
20°
20°
40°
40°
WBT
24. Construction of Psychrometry (to be cont….
Constant RH Line (ø)
• Let us assume, we need to draw RH=90%,
• 90/100 = Pw/(Pws)
• Pw = 0.9 x Pws
• Using below equation at DBT = 5, 10, 15°C …
• Calculate “w”
• Plot Point corresponding to (x,y) i.e. (DBT, w), then join to
form RH line (90, 80,70,60%......).
DBT
W
(RH=90%)
(RH= 80%)
25. Construction of Psychrometry (to be cont….
• Constant Enthalpy Line
h (KJ/kg)= 1.005 x DBT + w [( 2501.4 +
1.88 (DBT)]
Calculate w and Plot Point corresponding
to (x,y) i.e. (DBT, w), then join to form
enthalpy line (h).
20KJ/kg
40KJ/kg
26. Construction of Psychrometry (to be cont….
• Where Ra = 287 J/kg. K
• T = ambient air temperature (DBT)
• pa = total Pressure = pB - pw
• Assume V = 1.05 m3/kg, DBT = 70ºC and
find Pw
• Using equation,
Find w
Calculate and Plot Point corresponding to
(x,y) i.e. (DBT, w), then join to form
specific volume line (Va).
