Psychrometry is the study of air-water vapor mixtures. Key terms include: - Dry bulb temperature (DBT) - measured by ordinary thermometer - Wet bulb temperature (WBT) - measured by thermometer wrapped in wet cloth - Dew point temperature (DPT) - temperature at which moisture condenses - Relative humidity (RH) - ratio of actual water vapor to maximum possible at given DBT An adiabatic saturator is used to measure the thermodynamic wet bulb temperature, which is the temperature air reaches when saturated adiabatically. Multiple inlet air conditions can result in the same thermodynamic wet bulb temperature.

1. PSYCHROMETRY
Prof. Harish Choudhary

2. PSYCHROMETRY
Psychrometry: is the study of the properties of mixtures of air and water
vapour.
 Atmospheric air is a mixture of many gases plus water vapour and a
number of pollutants.
 The amount of water vapour and pollutants vary from place to place. The
concentration of water vapour and pollutants decrease with altitude.
 Above an altitude of about 10 km, atmospheric air consists of only dry
air.
 The pollutants have to be filtered out before processing the air.
 Hence, what we process is essentially a mixture of various gases that
constitute air and water vapour. This mixture is known as moist air.

3.  The moist air can be thought of as a mixture of dry air and
moisture.
 For all practical purposes, the composition of dry air can be
considered as constant.
 In 1949, a standard composition of dry air was fixed by the
International Joint Committee on Psychrometric data.
Composition of standard air
Constituent Molecular weight Mol fraction
Oxygen 32.000 0.2095
Nitrogen 28.016 0.7809
Argon 39.944 0.0093
Carbon dioxide 44.010 0.0003

4. Based on the above composition the molecular weight of dry
air is found to be 28.966 and the gas constant R is
287.035 J/kg.K.
For calculation purposes, the molecular weight of water
vapour is taken as 18.015 and its gas constant is 461.52
J/kg.K.

5. Mixture of
permanent gases
(N2,O2,Ar,H2 Dust particles,
fumes etc
Water
vapour
Atmospheric Air
After filtration
Mixture of
permanent gases
(N2,O2,Ar,H2
Water
vapour
Moist air for conditioning

6. Psychrometric Properties
1) Dry bulb temperature t (DBT)
Dry bulb temperature is the temperature of the air, as measured by an
ordinary thermometer. The temperature of water vapor is the same as
that of the dry air in moist air. Such a thermometer is called a dry-bulb
thermometer in psychrometry, because its bulb is dry.
2) Wet bulb temperature tWB: (WBT)
Wet bulb temperature is thermodynamic adiabatic temperature in an
adiabatic saturation process, and measured by a wet bulb thermometer.

7. Psychrometric Properties
3) Dew point temperature tDP: When the unsaturated moist air is
cooled at constant vapor pressure or at constant humidity ratio, to
a temperature, the moist air becomes saturated and the
condensation of moisture starts, this temperature is called dew
point temperature of the moist air.
4) Relative humidity Ф: Relative humidity is defined as the ratio of
the mole fraction of the water vapor in a given moist air to the
mole fraction of water vapor in a saturated moist air at the same
temperature and the same atmospheric pressure.
Relative humidity is usually expressed in percentage (%).

8. Psychrometric Properties
From the ideal gas relations, relative humidity can be
expressed as
-- the mole fraction of the water vapor in moist air;
-- mole fraction of water vapor in a saturated moist air
at the same temperature;
sat
w
w
sat
w
w
P
P
x
x
,
,



w
x
sat
w
x ,

9. 5) Humidity ratio (Moisture Content) ω:
 The humidity ratio is the mass kg of water vapor interspersed in
each kg of dry air.
 It should be noted that the mass of water refers only to the
moisture in actual vapor state, and not to any moisture in the
liquid state, such as dew, frost, fog or rain.
 The humididy ratio, like other several properties to be studied-
enthalpy and specific volume-is based on 1kg of dry air.
T
R
V
P
R
V
P
Kg
Kg
a
a
v
v
/
/
air
dry
of
water
of




10. Substituting the values of gas constants of water vapour and air
Rv and Ra in the above equation; the humidity ratio is given by:
For a given barometric pressure Pt, given the DBT, we can find
the saturated vapour pressure Psat from the thermodynamic
property tables on steam. Then using the above equation, we can
find the humidity ratio at saturated conditions, ωsat.
It is to be noted that, ω is a function of both total barometric
pressure and vapor pressure of water.
v
t
v
P
P
P

 662
.
0


11. 6) Dew-Point Temperature: If unsaturated moist air is cooled at
constant pressure, then the temperature at which the moisture in
the air begins to condense is known as dew-point temperature
(DPT) of air.
An approximate equation for dew-point temperature is given by:
where Φ is the relative humidity (in fraction). DBT & DPT are in oC,
since from its definition, the dew point temperature is the saturation
temperature corresponding to the vapour pressure of water vapour, it
can be obtained from steam tables or using above eqn.
 
  235
ln
235
4030
235
4030






DBT
DBT
DPT

12. 7) Degree of saturation μ: The degree of saturation is the ratio of
the humidity ratio W to the humidity ratio of a saturated mixture
Ws at the same temperature and pressure, i.e.,
P
t
s
,


 

13. 8) Enthalpy: The enthalpy of moist air is the sum of the enthalpy
of the dry air and the enthalpy of the water vapour. Enthalpy
values are always based on some reference value. For moist air,
the enthalpy of dry air is given a zero value at 0oC, and for water
vapour the enthalpy of saturated water is taken as zero at 0oC.
The enthalpy of moist air is given by:
h = ha + ω hg = Cp t + ω (hfg + Cpw t)
where Cp = specific heat of dry air at constant pressure, kJ/kg.K
Cpw = specific heat of water vapor, kJ/kg.K
t = Dry-bulb temperature of air-vapor mixture, oC
ω = Humidity ratio, kg of water vapor/kg of dry air

14. ha = enthalpy of dry air at temperature t, kJ/kg
hg = enthalpy of water vapor at temperature t, kJ/kg
hfg = latent heat of vaporization at 0oC, kJ/kg
Substituting the approximate values of Cp and hg, we obtain:
h = 1.005 t + ω (2501 + 1.88 t)
9) Humid specific heat: From the equation for enthalpy of moist air,
the humid specific heat of moist air can be written as:
Cpm = Cp + ω Cpw
where cpm = humid specific heat, kJ/kg.K
Cp = specific heat of dry air, kJ/kg.K
Cpw = specific heat of water vapor, kJ/kg
ω = humidity ratio, kg of water vapor/kg of dry air

15. Since the second term in the above equation (ω.Cpw) is very
small compared to the first term, for all practical purposes, the
humid specific heat of moist air, cpm can be taken as 1.0216 kJ/kg
dry air.

16. PSYCHROMETRIC CHART
 A Psychrometric chart graphically represents the thermodynamic
properties of moist air. Standard psychrometric charts are
bounded by the dry-bulb temperature line (abscissa) and the
vapour pressure or humidity ratio (ordinate). The Left Hand Side
of the psychrometric chart is bounded by the saturation line.
Psychrometric charts are readily available for standard barometric
pressure of 101.325 kPa at sea level and for normal temperatures
(0-50oC). ASHRAE has also developed psychrometric charts for
other temperatures and barometric pressures (for low
temperatures: -40 to 10oC, high temperatures 10 to 120oC and
very high temperatures 100 to 120oC)

17. PSYCHROMETRIC CHART

18. Adiabatic Saturation And Thermodynamic Wet Bulb
Temperature:
Adiabatic saturation temperature is defined as that
temperature at which water, by evaporating into air, can
bring the air to saturation at the same temperature
adiabatically. An adiabatic saturator is a device using which
one can measure theoretically the adiabatic saturation
temperature of air.
As shown in Fig. an adiabatic saturator is a device in which
air flows through an infinitely long duct containing water.

19. Adiabatic Saturation Of Air

20. As the air comes in contact with water in the duct, there will be
heat and mass transfer between water and air. If the duct is
infinitely long, then at the exit, there would exist perfect
equilibrium between air and water at steady state. Air at the exit
would be fully saturated and its temperature is equal to that of
water temperature. The device is adiabatic as the walls of the
chamber are thermally insulated. In order to continue the process,
make-up water has to be provided to compensate for the amount
of water evaporated into the air. The temperature of the make-up
water is controlled so that it is the same as that in the duct.

21. After the adiabatic saturator has achieved a steady-state
condition, the temperature indicated by the thermometer
immersed in the water is the thermodynamic wet-bulb
temperature. The thermodynamic wet bulb temperature will be
less than the entering air DBT but greater than the dew point
temperature.
Certain combinations of air conditions will result in a given sump
temperature, and this can be defined by writing the energy
balance equation for the adiabatic saturator. Based on a unit
mass flow rate of dry air, this is given by:
h1 = h2 – (ω2 – ω1) hf

22. where hf is the enthalpy of saturated liquid at the sump or
thermodynamic wet-bulb temperature, h1 and h2 are the
enthalpies of air at the inlet and exit of the adiabatic saturator,
and W1 and W2 are the humidity ratio of air at the inlet and exit of
the adiabatic saturator, respectively.
It is to be observed that the thermodynamic wet-bulb temperature
is a thermodynamic property, and is independent of the path
taken by air. Assuming the humid specific heat to be constant,
from the enthalpy balance, the thermodynamic wet-bulb
temperature can be written as:
T2 = t1 – hfg2 (ω2 – ω1)
Cpm

23. where hfg2 is the latent heat of vaporization at the saturated
condition 2. Thus measuring the dry bulb (t1) and wet bulb
temperature (t2) one can find the inlet humidity ratio (ω1) from the
above expression as the outlet saturated humidity ratio (ω2) and
latent heat of vaporizations are functions of t2 alone (at fixed
barometric pressure).

24. On the psychrometric chart as
shown in Fig., point 1 lies below
the line of constant enthalpy
that passes through the
saturation point 2. t2 = f(t1,W1)
is not a unique function, in the
sense that there can be several
combinations of t1 and W1 which
can result in the same sump
temperature in the adiabatic
saturator.
Adiabatic Saturation Process 1 - 2

25. A line passing through all these points is a constant wet bulb
temperature line. Thus all inlet conditions that result in the same
sump temperature, for example point 1’ have the same wet bulb
temperature. The line is a straight line according to the straight-
line law. The straight-line joining 1 and 2 represents the path of
the air as it passes through the adiabatic saturator.
Normally lines of constant wet bulb temperature are shown on the
psychrometric chart. The difference between actual enthalpy and
the enthalpy obtained by following constant wet-bulb temperature
is equal to (ω2 - ω1)hf.

26. Example 1
Two and a half cubic meters of lumber is being dried at 60°C dry
bulb temperature and 52°C wet bulb temperature. The drying rate
of the lumber is 12.5 kg of water per hour. If outside air is at 27°C
dry bulb temperature and 80% relative humidity, how much
outside air is needed per minute to carry away the evaporated
moisture?

27. 92 g/kg d.a.
18 g/kg d.a.
27°C 60°C
52°C
0.87 m3/kg d.a.

28. ΔHR = (92.0 – 18.0) g/kg dry air
= 74.0 g/kg dry air
wa1 = drying rate/ΔHR
= (12.5 kg/hour)/(0.074 kg/kg dry air)
= 168.9 kg dry air/hour
VF1 =(wa1)(v1)
=(168.9 kg dry air/hour)(0.87 m3/kg dry air)
= 147 m3/hour = 2.45 m3/minute

29. Moist air at 50°C dry bulb temperature and 32% relative humidity
enters the cooling coil of a dehumidification kiln heat pump
system and is cooled to a temperature of 18°C. If the drying rate
of 6 m3 of red oak lumber is 4 kg/hour, determine the kW of
refrigeration required.
Example 2

30. 115.7 kJ/kg d.a.
25.2 g/kg d.a.
12.9 g/kg d.a.
1
2
50.8 kJ/kg d.a.
18°C 50°C
28.8°C

31. ΔHR = (25.2 – 12.9) g /kg dry air
= 12.3 g /kg dry air
wa1 = drying rate/ΔHR
= (4 kg/hour)/(0.0123 kg/kg dry air)
= 325.2 kg dry air/hour
Δh = (115.7 – 50.8) kJ/kg dry air
= 64.9 kJ/kg dry air
q = (Δh) (ωa)
q = 64.9 X 325.2
= 21105.7 KJ/h = 5.9 KW