Food Science & Technology

1. Air-Water Mixture:
Psychrometric Charts and Their
Application.

2. Psychrometrics
 Psychrometrics is “The study of properties of mixtures of
air and water vapor”.
 Necessary for unit operations.
 Amount of water vapor present in air under various conditions.
 Thermal properties can changes in the heat and moisture
contents.
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3. Psychrometrics
Air–water vapor mixture present into
Heating,
Cooling,
Humidification,
Dehumidification, and
Mixing
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4. Wet Basis and Dry Basis Moisture
Contents
Moisture Content:-
 “The amount of free, absorbed and adsorb water in the food e.g.,
grain”.
Water in grain is present
 Inter granular spaces in the endosperm
 Seed Coat
 Bonded by weak hydrogen bonds to the molecules of starches and
protein bodies
 Bulk of the water as free water adsorbed or absorbed
 Surface
 Between the husk and kernels.
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5. Wet Basis and Dry Basis Moisture
Contents
Grain Moisture Content:-
 Normally moisture content denotes the quantity of water in a unit mass of
a grain sample.
 It is expressed in two different forms:
1) Moisture content wet basis
2) Moisture content dry basis
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6. 1. Moisture Content Wet Basis:-
 It is the mass of water present per unit mass of undried grain
sample .i.e
 Where,
 MWb= present moisture content wet basis, %
 mw = mass of water in grain sample, g
 md= mass of dry matter in grain sample, g
 mg = mass of grain sample = md - mw' g
100100 X
mm
m
X
m
m
M
wd
w
g
w
wb


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7. 100100 X
mm
m
X
m
m
M
wg
w
d
w
db


2. Moisture Content Dry Basis
 It is the mass of water present per unit mass of dry matter in a grain
sample.
 Where,
 Mdb= present moisture content dry basis, %
 mw = mass of water in grain sample, g
 md= mass of dry matter in grain sample, g
 mg = mass of grain sample = md - mw , g
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8. Relationship Between Wet Basis and Dry Basis Moisture
Content:-
 The conversion of moisture content from wet basis to dry basis and vice
versa is simple,and can be calculated from the given equations,
 Where,
 Mdb and MWb are respectively percent moisture content on dry basis and
wet basis.
db
db
wb
wb
wb
db
M
MX
M
M
MX
M




100
100
100
100
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9. Properties of Air–Water Vapor Mixture
Absolute Humidity:-
 “The kilograms of water vapor present in 1 kg of dry air
under a given set of conditions.”
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10. Therefore, H can be expressed mathematically as follows:
 When
 As per pw is small,
 Again, from Equation 2.1
 Rearranging Equation 2.4
P
H
H
p
pP
p
pP
p
H
p
H
kgkg
p
p
H
atmP
pP
p
H
w
w
w
w
w
w
w
w
w
w


















611.11
611.1
)(611.1)(
18
29
29
18
/
)1(29
18
1
)(29
18
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11. • Where,
• H = Absolute Humidity of air kg of water vapor/kg of dry air
• Pw = Partial Pressure of water vapor
• P = Total Pressure
• MWater = 18 kg/kmol
• MAir = 28.9 kg/kmol
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12. Percentage Humidity:-
• “The ratio of the weight of water present in 1 kg of dry air at any
temperature and pressure and the weight of water present in 1 kg of
dry air, which is saturated with water vapor at the same temperature
and pressure”.
H= Weight of water present in 1 kg of dry air
Hs= Satureted weight of water present in 1 kg of dry air
100=Humidity%tage X
H
H
S






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13. Relative Humidity:-
• “The ratio of the partial pressure of water vapor in the air to the
partial pressure of water vapor in saturated air at the same
temperature”.
• Where,
RH= Relative Humidity
pw = partial pressure of water vapor in air
ps = partial pressure of water vapor in saturated air
100X
p
p
RH
s
w







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14. Humid Heat:-
• “The number of kilocalories necessary to raise the temperature of 1
kg dry air and its accompanying water vapor through 1°C”.
• S= Humid Heat
• H= Absolute Humidity of air kg of water vapor/kg of dry air
)/(,88.1005.1
)/(,45.024.0
CkgkcalHS
CkgkcalHS




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15. )/(,)88.1005.1(h'
)]/(,)45.024.0(h'
)0(45.0[)0(24.0h'
CkgkcalHt
CkgkcalHtH
tHt








Enthalpy:-
• “The total heat content of air water mixture at the given temperature
and pressure”.
• Where,
• 0.24t = average heat content of dry air, kcal/kg of dry air
• 0.45 = specific heat of water vapour
• H = absolute humidity
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16.   
kgmtHv
H
tv
tt
v
/)2 7 3)(0 0 4 5 6.0 0 2 8 3.0(
1 82 9
1
2 7 3
2 7 3
4.2 2
2 7 3
2 7 3
1 8
4.2 2
2 7 3
2 7 3
2 9
4.2 2
3













 





 

Humid Volume:-
• “The total volume in cubic meter of 1 kg dry air and its
accompanying water vapor”.
• Where,
• V= Humid Volume
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17. Saturated Volume:-
• “The volume of 1 kg of dry air plus that of the water vapor necessary
to saturate it”.
Dew Point:-
• “The temperature at which condensation occurs when the air is
cooled at constant humidity ratio and constant atmospheric
pressure”.
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18. Introduction of Psychrometric Chart
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19. 1/27/2020 19

20. 1/27/2020 20

21. Psychrometry – Dry Bulb Temp
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22. Psychrometry – Wet Bulb Temp
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23. Psychrometry – Saturation Line
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24. Psychrometry – Relative Humidity
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25. Psychrometry – Humidity Ratio
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26. Psychrometry – Dew Point Lines
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27. Psychrometry – Specific Enthalpy
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28. Psychrometry – Specific Volume
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