PSYCHROMETRIC PROCESSES

1. Psychrometry: Properties and processes
B.Prabhu, T.Suresh & P.Selvan
Assistant Professor - Mechanical Engineering
Kamaraj College of Engineering of Technology
Virudhunagar

2. Psychrometry - Introduction
 Different gas-vapour mixtures are encountered in many engineering
applications and air-water vapour mixture is the most commonly encountered
gas-vapour mixture in practice.
air-conditioning of Air-water vapour mixtures play a dominant role in the
manufacturing plants and food preservation plants.
 A special field of study called ‘Psychrometry’ is evolved to study the
properties of air-water vapour mixtures .
 A common example of air-water vapour mixture is the moist atmospheric air.

3. Moist Air
 It is a mixture of dry air, water vapour and few other inert gases (such as argon, neon,
krypton, helium etc.) whose percentage is negligible.
 Hence air is regarded only as a mixture of dry air and water vapour.
 Dry air is considered to consist of 21% oxygen and 79% nitrogen by volume.
 The water vapour normally exists in a saturated or superheated condition in the
atmospheric air.

4. DRY AND ATMOSPHERIC AIR
Atmospheric air: Air in the atmospherecontaining
some water vapor (or moisture).
Dry air: Air that contains no water vapor.
Water vapor in the air plays a major role in human
comfort. Therefore, it is an important consideration in
air-conditioning applications.
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:
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%.

5. Moist air ….
 The amount of water vapour changes due to condensation by cooling or added to the
air by vaporization from a large water surface.
 Hence it forms a changing part in the moist air while the dry air forms a fixed part.
 The partial pressure of water vapour present in the atmospheric air is very low and at
such low pressures, the water vapour may be treated as an ideal gas.
 At 40C, the saturation pressure of water is 7.38 kPa at such low pressures the water
vapour can be treated as ideal gas with negligible error(below 0.2%).
 The dry air behaves almost as an ideal gas.
 Hence the atmospheric air can be treated as a mixture of ideal gases.

6. Psychometric properties
 The properties of air-water vapour mixture i.e. moist air are termed its psychometric
properties
 and the subject dealing with the same is named ‘Psychrometry’.
The following definitions are used in the study of moist air:

7. Vapour pressure and saturation vapour pressure
 Vapor pressure is the independent pressure exerted by the water vapor in the air.
 The vapor pressure is proportional to the humidity ratio.
 The natural tendency for pressures to equalize will cause moisture to migrate from an
area of high vapour pressure to an area of low vapor pressure.
 The saturation vapour pressure varies with temperature.
 Saturation vapor pressure (Psat) is the maximum possible vapor pressure for air at
some temperature (ignoring the possibility of super saturation).
 At any one time the actual vapor pressure Pw could be anything between 0 and Psat.

8. Specific humidity or absolute humidity or
humidity ratio
(denoted by ) Specific humidity is the ratio of the mass of water vapour present to the mass
of dry air in a given volume of the moist air at a given temperature.
=mV /ma Specific humidity of moist air 
Where
mV - mass of water vapour in a given volume of moist air at temperature T
ma - mass of dry air in the same volume of moist air at the same temperature
mV = pVV/RVT = pVV/ (RUT/MV)
where pV =Partial pressure of water vapour in the mixture
V = Volume of mixture
T = temperature

9. Specific humidity
 RV = gas constant for water vapour = RU/ MV = RU/18
 Mass of dry air = ma = paV/RaT = paV/ (RUT/Ma )
 where pa =Partial pressure of dry air in the mixture
 Ra = gas constant for dry air = RU/ Ma = RU/28.97

10. Specific humidity
specific humidity of moist air  =mV/ma
=  pVV/ (RUT/MV) (RUT/Ma )/ paV
=pV/paMv/Ma=pV/pa18/28.97
 = 0.622pV/pa = 0.622 pV/(p-pv)
Saturated air:
The air saturated with moisture.
For saturated air, the vapor
pressure is equal to the
saturation pressure of water.

11. Relative humidity(Ø)
 Ratio of the mass of water vapor actually present in the air to the mass of
water vapour that would be present if the air is saturated at the same temperature and
pressure.
 Relative humidity(Ø)= pv/ps
Ø =(mvRT/V)/(mgRT/V) = mv/mg
The difference between specific
and relative humidity

12. Relative humidity(Ø)
 When the air is saturated its relative
humidity is 100% .
 It shows that the air can not hold
any more moisture.
 When the air is purely dry, it has no
moisture content in it and the
relative humidity is 0% and partial
pressure of water vapour ( pv) is
zero.

13. Dry bulb temperature
(DBT) Tdb andSaturated vapour pressure (psat)
 Dry bulb temperature is the temperature of the air measured with an ordinary
thermometer with the bulb of the thermometer open to the atmospheric air.
 The bulb of the thermometer that is making the measurement has no moisture on it.
 Saturated vapour pressure (psat) is the saturated partial pressure of water vapour at
the dry bulb temperature.
 This is readily available in thermodynamic tables and charts.

14. Wet bulb temperature (WBT) Twb
 Wet bulb temperature is the temperature measured by a thermometer whose bulb is
covered with a moistened wet wick.
 When the water evaporates the temperature of the bulb is lowered. So the wet bulb
temperature depends on the moisture in the air.
 When the atmospheric air is saturated with water (that is, it has 100% relative
humidity), no water can evaporate from the wet bulb.
 Hence the wet bulb temperature will be same as the dry bulb temperature.
 This temperature is therefore also referred to as the saturation temperature.
 The difference between the dry bulb and wet bulb temperatures is known as wet bulb
depression.
 Larger the wet bulb depression, lower is specific humidity of air.

15. The temperature measured is
the wet-bulb temperature
Twb and it is commonly used
in A/C applications. A simple arrangement to measure
the wet-bulb temperature.
For air–water vapor mixtures at atmospheric
pressure, Twb is approximately equal to the
adiabatic saturation temperature.
Sling psychrometer
Wet-bulb temperature measurement
Todetermine the absolute
and relative humidity of air, a
more practical approach is to
use a thermometer whose
bulb is covered with a cotton
wick saturated with water and
to blow air over the wick.

16. other•The sling psychrometer is widely used for measurements involving room air or
applications where the air velocity inside the room is small.
•The sling psychrometer consists of two thermometers mounted side by side and fitted in a
Wet-bulb temperature measurement ….
• frame with a handle for whirling the device through air.
• •The required air circulation (≈ 3 to 5 m/s) over the sensing
bulbs is obtained by whirling the psychrometer (≈ 300 RPM).
Readings are taken when both the thermometers show steady-
state readings.
• •In the aspirated psychrometer, the thermometers remain
stationary, and a small fan, blower or syringe moves the air
across the thermometer bulbs.
• •The function of the wick on the wet-bulb thermometer is to
provide a thin film of water on the sensing bulb.

17. DEW-POINT TEMPERATURE
Dew-point temperature Tdp:
The temperature at which condensation ofwater
vapour in the moist air begins when the air is
cooled at constant pressure (i.e., the saturation
temperature of water corresponding to the
vapor pressure.)
The normal thermodynamic state 1 (as shown in the
Figure) of moist air is considered as unsaturated air.
The water vapour existing at temperature T1 of the
mixture and partial pressure pv of the vapour in the
mixture is normally in a superheated state.
When the
temperature of a
cold drink is below
the dew-point
temperature of the
surrounding air, it
“sweats.”

18. Dew point temperature (DPT) Tdp
 When a bottle of cool water is taken
out of the refrigerator it can be seen
that water condenses on the outside of
the bottle.
 This means that the temperature of the
bottle is below the dew point
temperature of air.
 When the relative humidity of the air is
100% i.e. the air is saturated, the dew
point temperature equals the wet bulb
temperature, which is also equal to
the dry bulb temperature.

19. For saturated air,
the dry-bulb,
wet-bulb, and
dew-point temperatures
are identical.

20. *State 1: water vapour in the super heated
thermodynamic state - unsaturated moist air
* the water vapour exists at the dry bulb
temperature T of the mixture and partial
pressure pv more water vapour is added in
this control volume V at temperature T itself.
*The partial pressure pv will go on increasing
with the addition of water vapour until it
reaches a value ps corresponding to state 2
* after which it cannot increase further as ps is
the saturation pressure or maximum possible of
water at temperature T.
* the thermodynamic state of water vapour is
now saturated at point 2.
The air containing moisture in such a state is
called saturated air. In this state the air is holding
the maximum amount of water vapour( the
specific humidity being ωs, corresponding to the
partial pressure ps ) at temperature T of the
mixture.
Degree of saturation ()

21. Degree of saturation () ….

22. Calculation of vapour pressure

24. Solved examplesP1. On a particular day the weather forecast states that the dry bulb temperature is 37 °C, while the
relative humidity is 50% and the barometric pressure is 101.325 kPa. Find (i) specific humidity, (ii) dew
point temperature and (iii) enthalpy of moist air on this day.
Answer:
At 37 °C the saturation pressure (ps) of water vapour is obtained from steam tables as 6.2795
kPa.
Since the relative humidity is 50%, the vapour pressure of water in air (pv) is:
pv = 0.5 x ps = 0.5 x 6.2795 = 3.13975 kPa
(i) the specific humidity or humidity ratio ω is givenby:
ω = 0.622 x pv/(pt−pv) = 0.622 x 3.13975/(101.325−3.13975) = 0.01989 kg of water/kgof
dry air
(ii) Dew point temperature Tdp (Tsat at pv = 3.13975 kPa) = 24.8 °C
(iii) The enthalpy of air (h) is given by the equation:
h = 1.005t + ω (2501+1.88t) = 1.005 x 37+0.01989 (2501+1.88 x 37) = 88.31 kJ/kg of dryair

25. P2. Will the moisture in the above air condense when it comes in contact with a cold
surface whose surface temperature is 24 °C?
Answer: Moisture will condense when it is cooled below its dew point temperature.
The dew point temperature of the air at 37 °C and 50 % relative humidity is equal to
the saturation temperature of water at a vapour pressure of 3.13975 kPa.
From steam tables, the saturation temperature of water at 3.13975 Kpa is 24.8 °C,
hence moisture in air will condense when it comes in contact with the cold surface
whose temperature, 24 °C is lower than the dew point temperature, 24.8 °C .

26. P3. Moist air at 1 atm. pressure has a dry bulb temperature of 32 °C and a wet bulb temperature
of 26 °C. Calculate a) the partial pressure of water vapour, b) humidity ratio, c) relative humidity,
d) dew point temperature, e) density of dry air in the mixture, f) density of water vapour in the
mixture and g) enthalpy of moist air using perfect gas law model and psychometric equations.
Answer:
a)Using modified Apjohn eqn. and the values of DBT,WBT and barometric pressure, the vapour
pressure is found to be:
pv = 2.956 kPa
b) The specific humidity or humidity ratio W is given by:
W = 0.622 x 2.956/(101.325-2.956) = 0.0187 kg of water/kg of dry air
c) Relative humidity RH is given by:
RH = (pv/ps) x 100 = (pv/saturation pressure at 32 °C) x 100
From steam tables, the saturation pressure of water at 32 °C is 4.7552 kPa, hence,
RH = (2.956/4.7552) x 100 = 62.16%

27. d) Dew point temperature is the saturation temperature of steam at pv=2.956 kPa.
Hence using steam tables we find that:
DPT = Tsat(2.956 kPa) = 23.8 °C
e) Density of dry air and water vapour
Applying perfect gas law to dry air:
Density of dry air ρa =(pa/RaT)=(pt−pv)/RaT = (101.325−2.956)/(287.035 x 305)x103
= 1.1236 kg/m3 of dry air
f) Similarly the density of water vapour in air is obtained using perfect gas law as:
Density of water vapour ρv = (pv/RvT) = 2.956 x 103/(461.52 x 305) = 0.021 kg/m3

28. Psychrometric Chart
 All data essential for the complete thermodynamic and psychrometric analysis
of air-conditioning processes can be summarized in a psychometric chart.
 At present, many forms of psychrometric charts are in use.
 The chart which is most commonly used is the ω-t chart, i.e. a chart which
has specific humidity or water vapour pressure along the ordinate and the dry
bulb temperature along the abscissa.
 The chart is normally constructed for a standard atmospheric pressure of
760 mm Hg or 1.01325 bar, corresponding to the pressure at the mean sea level.

30. Figure: Constant Property Lines on a Psychrometric Chart
The psychrometric chart serves as a valuable aid in visualizing the
A/C processes such as heating, cooling, and humidification.

31. Today, modern air-conditioning systems can
heat, cool, humidify, dehumidify, clean, and
even deodorize the air–in other words,
condition the air to peoples’ desires.
The rate of heat generation by human body
depends on the level of the activity. For an
average adult male, it is about 87 W when
sleeping, 115 W when resting or doing office
work, and 440 W when doing heavy physical
work.
When doing light work or walking slowly, about
half of the rejected body heat is dissipated
through perspiration as latent heat while the
other half is dissipated through convection
and radiation as sensible heat.
HUMAN COMFORT AND AIR-
CONDITIONING
A body feels comfortable
when it can freely dissipate
its waste heat, and no more.

32. We cannot change the weather, but we can
change the climate in a confined space by
air-conditioning.
In an environment at 10°C with 48 km/h winds feels as cold as an
environment at -7°C with 3 km/h winds as a result of the body-chilling
effect of the air motion.

33. The comfort of the human body depends primarily on three factors:
the (dry-bulb) temperature, relative humidity, and air motion.
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.
HUMAN COMFORT AND AIR-
CONDITIONING

34. AIR-CONDITIONING PROCESSES
Maintaining a living space or an industrial facility
at the desired temperature and humidity
requires some processes called air-conditioning
processes.
These processes include simple heating (raising
the temperature), simple cooling (lowering the
temperature), humidifying (adding moisture),
and dehumidifying (removing moisture).
Sometimes two or more of these processes are
needed to bring the air to a desired temperature
and humidity level.
Air is commonly heated and humidified in
winter and cooled and dehumidified in summer.
Various air-conditioning processes

35. Sensible Heating and Cooling ( = constant)• Many residential heating systems consist of a stove, a
heat pump, or an electric resistance heater.
• The air in these systems is heated by circulating it
through a duct that contains the tubing for the hot gases
or the electric resistance wires.
• Cooling can be accomplished by passing the air over
some coils through which a refrigerant or chilled water
flows.
• Addition or removal of sensible heat without change in
moisturecontent.
• Heating and cooling appear as a horizontal line since no
moisture is added to or removed from the air.
Dry air mass balance
Water mass balance
Energy balance

36. During simple cooling, specific humidity
remains constant, but relative humidity
increases and temperature decreases.
During simple heating, specific humidity
remains constant, but relative humidity
decreases and temperature increases.

40. Heating with HumidificationProblems with the low relative humidity
resulting from sensible heating can be
eliminated by humidifying the heated air.
This is accomplished by passing the air
first through a heating section and then
through a humidifying section.

41. An example for heating and humidification

42. Cooling with Dehumidification• The specific humidity of air remains constant during a sensible cooling process, but
its relative humidity increases.
• If the relative humidity reaches undesirably high levels, it may be necessary to
remove some moisture from the air, that is, to dehumidify it.
• This requires cooling the air below its dew-point temperature to condense vapour.

55. • Air can be brought to saturation state,
adiabatically, by the evaporation of water into
the flowing air.
• As the air passes through the chamber over a long
sheet of water, the water evaporates which is
carried with the flowing stream of air, and the
specific humidity of the air increases.
• Both the air and water are cooled as the
evaporation takes place.
• This process continues until the energy
transferred from the air to the water is equal to
the energy required to vaporize the water.
• When steady conditions are reached, the air
flowing at section 2 is saturated with water
vapour.
• The temperature of the saturated air at section
2 is known as thermodynamic wet bulb
temperature or adiabatic saturation
temperature.
Adiabatic saturation process

56. Evaporative coolers lower the temperature of air using
the principle of evaporative cooling, unlike typical air
conditioning systems which use VCR and VARS.
Evaporative cooling is the addition of water vapor into
air, which causes a lowering of the temperature of the
air.
The energy needed to evaporate the water is taken from
the air in the form of sensible heat, which affects the
temperature of the air, and converted into latent heat,
the energy present in the water vapor component of the
air, whilst the air remains at a constant enthalpy value.
This conversion of sensible heat to latent heat is known
as an adiabatic process because it occurs at a constant
enthalpy value.
Evaporative cooling therefore causes a drop in the
temperature of air proportional to the sensible heat drop
and an increase in humidity proportional to the latent
heat gain.
Evaporative cooler (also swamp cooler, desert cooler and wet air cooler)

57. Thank
You