Study of behavior of dry air and water vapour mixture under various sets of conditions

1. Aldel Education Trust’s
ST. JOHN COLLEGE OF ENGINEERING AND MANAGEMENT, PALGHAR
(ST. JOHN POLYTECHNIC)
DEPARTMENT OF MECHANICAL ENGINEERING
SUB: POWER ENGINEERING AND REFRIGERATION (PER)
22562
TOPIC:PSYCHOMETRY
PREPARED BY:-
Prof. Pranit Mehata
Lecturer, SJCEM
7972064172

2. INTRODUCTION
The branch of engineering
science which deals with the
study of moist air i.e. dry air
mixed with water vapour or
humidity.
It also includes the study of
behavior of dry air and water
vapour mixture under various sets
of conditions.

3. PSYCHOMETRIC TERMS
Dry Air: The pure dry air is a mixture of a number of gases such as nitrogen, oxygen,
carbon dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the
major portion of the combination.
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.
Saturated Air: It is a mixture of dry air and water vapour when the air has diffuses the
maximum amount of water vapour in it i.e. further addition of water vapour is not
possible by any method.

4. PSYCHOMETRIC TERMS
Degree of Saturation: It is the ratio of actual mass of water vapour in a unit mass of dry air
to the mass of water vapour in the same mass of dry air when it is saturated at the same
temperature.
Humidity: It is the mass of water vapour present in 1 kg of dry air and is generally
expressed in terms of gram per kg of dry air (𝑔/𝑘𝑔 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟). It is also called as specific
humidity or humidity ratio.
Absolute Humidity: The mass of water vapour present in 1𝑚3
of dry air. It is expressed in
terms of gram per cubic meter of dry air (𝑔 𝑚3
𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟 ).

5. PSYCHOMETRIC TERMS
Relative Humidity: The ratio of actual mass of water vapour in a given volume of moist air
to the mass of water vapour in the same volume of saturated air at the same temperature and
pressure. It is briefly written as RH.
Dry Bulb Temperature: The temperature of the air recorded by a thermometer when it is
not affected by the moisture present in the air. It is briefly written as DBT. It is generally
denoted by 𝑡𝑑 𝑜𝑟 𝑡𝑑𝑏.
Wet Bulb Temperature: The temperature of the air recorded by a thermometer when its
bulb is surrounded by a wet cloth exposed to the air. It is briefly written as WBT. It is
generally denoted by 𝑡𝑤 𝑜𝑟 𝑡𝑤𝑏.

6. PSYCHOMETRIC TERMS
Dew Point Temperature: The temperature of air recorded by a thermometer, when the
moisture present in it begins to condense. It is the saturation temperature corresponding to
the partial pressure of water vapour. It is denoted by 𝑡𝑑𝑝.
Dew Point Depression: The difference between the dry bulb temperature and dew point
temperature of air.

7. DALTON’S LAW OF PRESSURE
It states that, “ The total pressure exerted by he
mixture of air and water vapour is equal to the
sum of pressures which each constituents
would exerts , if it occupied the same space by
itself.”
Mathematically,
𝒑𝒃 = 𝒑𝒂 + 𝒑𝒗

8. PSYCHOMETRIC CHART
 It is graphical representation of the
various thermodynamic properties of
moist air.
 Useful for finding out the properties of
air and eliminate lot of calculations.
 Normally drawn for standard atmospheric
pressure of 760 mm of Hg (1.01325 bar)
 Dry Bulb Temperature is taken as
abscissa and specific humidity as
ordinates. Fig. Psychometric Chart

9. DRY BULB TEMPERATURE LINES
(DBT LINES)
The dry bulb temperature lines are vertical
i.e. Parallel to the ordinate and uniformly
spaced.
Generally the temperature range of these
lines on psychometric chart is from
− 6℃ 𝑡𝑜 45℃.
The dry bulb temperature lines are drawn
with difference of every 5℃ and upto the
saturation curve. Fig. DBT Lines

10. SPECIFIC HUMIDITY LINES
(MOISTURE CONTENT LINES)
The dry bulb temperature lines are horizontal
i.e. Parallel to the abscissa and uniformly
spaced.
Generally the temperature range of these
lines on psychometric chart is from
0 𝑡𝑜 30
𝑔
𝑘𝑔
𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟.
The dry bulb temperature lines are drawn
with difference of every 1 𝑔 (0.001 𝑘𝑔)and
upto the saturation curve. Fig. Specific Humidity Lines

11. DEW POINT TEMPERATURE LINES
(DPT LINES)
The dry bulb temperature lines are
horizontal i.e. Parallel to the abscissa and
non- uniformly spaced.
At any point on saturation curve the dry
bulb and dew point temperatures are equal.
The values of dew point temperatures are
generally given along the saturation curve
of the chart.
Fig. DPT Lines

12. WET BULB TEMPERATURE LINES
(WBT LINES)
The dry bulb temperature lines inclined
straight lines and non uniformly space.
At any point on saturation curve the dry bulb
and wet bulb temperatures are equal.
The values of wet bulb temperatures are
generally given along the saturation curve of
the chart.
Fig. DPT Lines

13. ENTHALPY LINES (TOTAL HEAT
LINES)
The enthalpy lines are inclined straight lines
and uniformly spaced.
These lines are parallel to the wet bulb
temperature lines and are drawn up to the
saturation curve.
Some of these lines coincide with the wet
bulb temperature lines also.
The values of total enthalpy are given on a
scale above the saturation curve. Fig. Enthalpy Lines

14. SPECIFIC VOLUME LINES
The enthalpy lines are obliquely inclined
straight lines and uniformly spaced.
These lines are drawn upto the saturation
curve.
The values of specific volume are
generally given at the base of the chart.
Fig. Specific Lines

15. VAPOUR PRESSURE LINES
The enthalpy lines are horizontal
straight lines and uniformly spaced.
These lines are not drawn in the main
chart.
A scale showing vapour pressure in
mm of Hg is given on the extreme left
side of the chart. Fig. Vapour Pressure Lines

16. RELATIVE HUMIDITY LINES
The relative humidity lines are curved
lines and follow the saturation curve.
These lines are drawn with values
10%, 20%, 30% etc. and upto 100%.
The saturation curve represents 100%
relative humidity.
Fig. Relative Humidity Lines

17. PSYCHOMETRIC PROCESSES
PSYCHOMETRIC PROCESSES
Sensible Heating
Sensible Cooling
Humidification and Dehumidification
Cooling with Adiabatic Humidification
Heating and Humidification
Heating and Dehumidification
Cooling with Dehumidification

18. SENSIBLE HEATING
The heating of air without any change in its specific
humidity is called as sensible heating.
The process of sensible heating on the psychometric
chart is shown by horizontal line 1-2. The point 3
represents the surface temperature of the heating
coil.
Specific humidity during sensible heating remains
constant. The dry bulb temperature increases and
relative humidity reduces.
The heat added during sensible heating is given as
𝑞 = ℎ2 − ℎ1
= 𝑐𝑝𝑠 𝑡𝑑2 − 𝑡𝑑1 + 𝑊𝑐𝑝𝑠 𝑡𝑑2 − 𝑡𝑑1
∴ 𝑞 = 𝑐𝑝𝑚(𝑡𝑑2 − 𝑡𝑑1)
𝒒 = 𝟏. 𝟎𝟐𝟐(𝒕𝒅𝟐 − 𝒕𝒅𝟏) 𝒌𝑱 𝒌𝒈
Fig. Sensible Heating

19. SENSIBLE COOLING
The cooling of air without any change in its specific
humidity is called as sensible heating.
The process of sensible heating on the psychometric
chart is shown by horizontal line 1-2. The point 3
represents the surface temperature of the cooling
coil.
Specific humidity during sensible heating remains
constant. The dry bulb temperature reduces and
relative humidity increases.
The heat added during sensible cooling is given as
𝑞 = ℎ1 − ℎ2
= 𝑐𝑝𝑠 𝑡𝑑1 − 𝑡𝑑2 + 𝑊𝑐𝑝𝑠 𝑡𝑑1 − 𝑡𝑑2
∴ 𝑞 = 𝑐𝑝𝑚(𝑡𝑑1 − 𝑡𝑑2)
*- 𝒒 = 𝟏. 𝟎𝟐𝟐(𝒕𝒅𝟏 − 𝒕𝒅𝟐) 𝒌𝑱 𝒌𝒈
Fig. Sensible Cooling

20. BY PASS FACTOR AND EFFICIENCY
Fig. By Pass Factor
By pass factor depends upon:
1. The number of fins provided in a unit length
2. The number of rows in a coil in the direction
of flow
3. The velocity of air.
By pass factor of cooling coil decreases with
decrease in fin spacing and increase in number
of rows.
𝑩𝑷𝑭𝑯𝒆𝒂𝒕𝒊𝒏𝒈 𝑪𝒐𝒊𝒍 =
𝒕𝒅𝟑 − 𝒕𝒅𝟐
𝒕𝒅𝟑 − 𝒕𝒅𝟏
𝑩𝑷𝑭𝑪𝒐𝒐𝒍𝒊𝒏𝒈 𝑪𝒐𝒊𝒍 =
𝒕𝒅𝟐 − 𝒕𝒅𝟑
𝒕𝒅𝟏 − 𝒕𝒅𝟑
𝜼𝑯 = 𝟏 − 𝑩𝑷𝑭 = 𝟏 −
𝒕𝒅𝟑−𝒕𝒅𝟐
𝒕𝒅𝟑−𝒕𝒅𝟏
=
𝒕𝒅𝟐−𝒕𝒅𝟏
𝒕𝒅𝟑−𝒕𝒅𝟏
𝜼𝑯 = 𝟏 − 𝑩𝑷𝑭 = 𝟏 −
𝒕𝒅𝟐−𝒕𝒅𝟑
𝒕𝒅𝟏−𝒕𝒅𝟑
=
𝒕𝒅𝟏−𝒕𝒅𝟐
𝒕𝒅𝟏−𝒕𝒅𝟑

21. HUMIDIFICATION AND
DEHUMIDIFICATION
The addition of moisture to the air, without change
in its dry bulb temperature is called as
Humidification.
The process of heat addition during humidification
is known as Latent Heating
The removal of moisture to the air, without change
in its dry bulb temperature is called as
Dehumidification.
The process of heat removal during
dehumidification is known as Latent Cooling.
In humidification, Relative humidity increases and
specific humidity also increases.
In dehumidification, relative humidity decreases and
specific humidity also decreases.
Fig. Humidification and
Dehumidification
𝑳𝑯 = 𝒉𝟐 − 𝒉𝟏 = 𝒉𝒇𝒈(𝑾𝟐 − 𝑾𝟏)
𝑳𝑯 = 𝒉𝟏 − 𝒉𝟐 = 𝒉𝒇𝒈(𝑾𝟏 − 𝑾𝟐)

22. COOLING WITH ADIABATIC
HUMIDIFIACTION (EVAPORATIVE
COOLING)
Fig. Cooling with Humidification
A adiabatic saturation of air in which cooling
is done by evaporating the water without
transfer of heat to or from the surrounding.
When the air is passed through an insulated
chamber having sprays of water maintained at
temperature ( 𝑡1) but lower than its DBT (𝑡𝑑1)
or equal to the WBT (𝑡𝑤1) then air is said to
be cooled and humidified.
Enthalpy remains constant during process and
process takes place along constant WBT.
The process is shown by line 1-3 on
psychometric chart.
𝜼𝑯 =
𝒕𝒅𝟏−𝒕𝒅𝟐
𝒕𝒅𝟏−𝒕𝒅𝟑
=
𝑾𝟐−𝑾𝟏
𝑾𝟑−𝑾𝟏

23. HEATING WITH HUMIDIFICATION
Fig. Heating with Humidification
This process is generally used in winter air
conditioning to warm and humidify the air.
It is the reverse process of cooling and
dehumidification.
When an air is passed through a humidifier
having spray water temperature higher than the
DBT the unsaturated air will reach the condition
of saturation and the air becomes hot.
The heat of vaporization of water is absorbed
from the spray water itself and hence it gets
cooled.
The process of heating and humidification is
shown by line 1-2.
In this DBT as well as specific humidity of air
increases

24. HEATING WITH
DEHUMIDIFICATION(ADIABATIC
CHEMICAL DEHUMIDIFICATION)
This process is mainly used in industrial air
conditioning and can also be used for some comfort
air conditioning installations requiring either a low
RH or low DPT in the room.
The air is passed over chemicals which have an
affinity for moisture. As the air comes in contact
with these chemicals the moisture gets condensed
out of the air and gives up its latent heat.
Due to condensation the specific humidity decreases
and DBT increases.
The process is reverse of adiabatic saturation process
shown by line 1-2.
The path followed during the process is along
constant WBT line or constant enthalpy line
Fig. Heating with Dehumidification

25. COOLING WITH DEHUMIDIFICATION
This process is generally used in summer air
conditioning to cool and dehumidify the air.
The air is passed over a cooling coil or through
the cold water spray.
In this process, DBT as well as specific
humidity of air decreases.
This process is only possible when the effective
surface temperature of the cooling coil (𝑡𝑑4) is
less than the DPT of the air entering the coil
(𝑡𝑑𝑝1)
The effectives surface temperature of the coil is
known as Apparatus Dew Point (ADP).
Fig. Cooling with Dehumidification
𝑩𝑷𝑭 =
𝒕𝒅𝟐 − 𝒕𝒅𝟒
𝒕𝒅𝟏 − 𝒕𝒅𝟒
=
𝒕𝒅𝟐 − 𝑨𝑫𝑷
𝒕𝒅𝟏 − 𝑨𝑫𝑷 𝑩𝑷𝑭 =
𝑾𝟐 − 𝑾𝟒
𝑾𝟏 − 𝑾𝟒
=
𝒉𝟐 − 𝒉𝟒
𝒉𝟏 − 𝒉𝟒

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