This document discusses psychrometrics and air conditioning load estimation. It covers topics like: - The composition of air and properties of moist air like humidity ratio, enthalpy, specific volume. - Psychrometric chart which graphically represents thermodynamic properties of moist air. - Basic psychrometric processes like sensible heating/cooling, humidification, dehumidification. - Methods to achieve these processes like air washers, evaporative cooling, steam injection. - Factors affecting human comfort and the use of comfort charts. - Estimating cooling loads for air conditioning systems.

1. Refrigeration and Air
Conditioning
Unit IV
Psychrometry and Air
Conditioning Load Estimation
By
N.S. AHER
Assistant Professor
Department of Mechanical Engineering,
Sanjivani College of Engineering, Kopargaon

2. • 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
• Moist air is a mixture of various gases that
constitute air and water vapour
• As mentioned before the air to be processed in
air conditioning systems is a mixture of dry air
and water vapour. While the composition of dry
air is constant, the amount of water vapour
present in the air may vary from zero to a
maximum depending upon the temperature and
pressure of the mixture (dry air + water vapour).

3. Constituent Molecular weight
Oxygen 32.000
Nitrogen 28.016
Argon 39.944
Carbon dioxide 44.010
Composition of Standard Air
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.

4. • At a given temperature and pressure the dry
air can only hold a certain maximum amount
of moisture. When the moisture content is
maximum, then the air is known as saturated
air.
• 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. • For Moist air,
•
• where = total barometric pressure
• = partial pressure of dry air
• = partial pressure of water vapour

6. • Dry bulb temperature (DBT) is the temperature of the
moist air as measured by a standard thermometer or
other temperature measuring instruments
• Relative humidity (Φ) is defined as the ratio of the mole
fraction of water vapour in moist air to mole fraction of
water vapour in saturated air at the same temperature
and pressure. Using perfect gas equation we can show
that:
Psychrometric Properties
Relative humidity is normally expressed as a percentage.
When Φ is 100 percent, the air is saturated

7. • Humidity ratio (W): The humidity ratio (or specific
humidity) W is the mass of water associated with each
kilogram of dry air. Assuming both water vapour and dry
air to be perfect gases, the humidity ratio is given by:
Psychrometric Properties
Substituting the values of gas constants of water vapour
and air 𝑅 and 𝑅 in the above equation; the humidity ratio
is given by:

8. • 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:
Psychrometric Properties
where Φ is the relative humidity (in fraction). DBT & DPT
are in oC
The dew point temperature is the saturation temperature
corresponding to the vapour pressure of water vapour

9. • 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.,
Psychrometric Properties
Enthalpy: The enthalpy of moist air is the sum of the
enthalpy of the dry air and the enthalpy of the water
vapour

10. • The enthalpy of moist air is given by:
• where = specific heat of dry air at constant
pressure, kJ/kg K
• = specific heat of water vapor, kJ/kg K
• t = Dry-bulb temperature of air-vapor mixture, 0C
• W = Humidity ratio, kg of water vapor/kg of dry air
• = enthalpy of dry air at temperature t, kJ/kg
• = enthalpy of water vapor at temperature t, kJ/kg
• = latent heat of vaporization at 0C , kJ/kg
Psychrometric Properties

11. • Substituting the approximate values of and
, we obtain:
• Or
• Where = Latent heat of vaporization of
water corresponding to dew point
temperature.
Psychrometric Properties

12. • Humid specific heat:
• where = humid specific heat, kJ/kg K
• = specific heat of dry air, kJ/kg K
• = specific heat of water vapor, kJ/kg
• W = humidity ratio, kg of water vapor/kg of dry
air
• Since the second term in the above equation
(w. ) is very small compared to the first term,
for all practical purposes, the humid specific
heat of moist air, can be taken as 1.0216
kJ/kg dry air K
Psychrometric Properties

13. Specific volume: The specific volume is defined
as the number of cubic meters of moist air per
kilogram of dry air
Psychrometric Properties

14. Carriers Equation

15. • 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)
Psychrometric Chart

16. Psychrometric Chart

18. • Based on Gibbs’ phase rule, the thermodynamic
state of moist air is uniquely fixed if the
barometric pressure and two other independent
properties are known. This means that at a
given barometric pressure, the state of moist
air can be determined by measuring any two
independent properties. One of them could be
the dry-bulb temperature (DBT), as the
measurement of this temperature is fairly
simple and accurate.
• Compared to DPT, it is easier to measure the
wet-bulb temperature of moist air. Thus
knowing the dry-bulb and wet-bulb
temperatures from measurements, it is possible
to find the other properties of moist air
Measurement of psychrometric properties:

19. Dry bulb temperature lines
Specific humidity lines

20. Dew point temperature lines
Wet bulb temperature lines

21. Enthalpy lines
Specific volume lines

22. Relative humidity lines

23. Basic Psychrometric Processes

24. Basic Psychrometric Processes

30. • The heating of air, without any change in it’s specific
humidity, is known as sensible heating.
Sensible Heating

31. Sensible Heating

34. • The cooling of air, without any change in it’s
specific humidity, is known as sensible
Cooling.
Sensible Cooling

35. Sensible Cooling

38. By -pass Factor of Heating and Cooling coil

39. By -pass Factor of Heating and Cooling coil

40. Efficiency of Heating and Cooling coil

41. • Humidification: The addition of moisture to the air,
without change in its dry bulb temperature is known
as humidification.
• Dehumidification: The removal of moisture from the
air, without change in its dry bulb temperature is
known as dehumidification.
Humidification and DehumidificationHumidification and Dehumidification

42. • For humidification: Latent heat transfer
• Where, ℎ is the latent heat of vaporization at dry
bulb temperature 𝑡
• For dehumidification: Latent heat transfer
• LH = ℎ ℎ ℎ 𝑊 𝑊 )
Humidification and Dehumidification
LH = ℎ ℎ ℎ 𝑊 𝑊 )

43. Humidification and Dehumidification

44. Methods of obtaining humidification and
dehumidification
Air Washer

45.
• The ratio of the sensible heat to the latent heat is
known as sensible heat factor (SHF)
Sensible Heat Factor
Heat
Latent HeatSensible Heat

46. Cooling and Dehumidification

47. Cooling and Dehumidification

50. Cooling with adiabatic humidification

51. Cooling with adiabatic humidification

52. Cooling and humidification by water injection
(Evaporative Cooling)

53. Cooling and humidification by water injection
(Evaporative Cooling)

56. Heating and humidification by steam injection

57. Heating and humidification by steam injection

61. Heating and Dehumidification - Adiabatic
Chemical Dehumidification

64. Adiabatic mixing of two air streams

65. Adiabatic mixing of two air streams

66. Adiabatic mixing of two air streams

67. Adiabatic mixing of two air streams

70. Room Sensible Heat Factor (RSHF)
Grand Sensible Heat Factor (GSHF)

72. Effective Room Sensible Heat Factor (ERSHF)

74. • Human Comfort: Human comfort is that condition of
mind, which expresses satisfaction with the thermal
environment.
• A human body feels comfortable when the heat
produced by t metabolism of human body is equal to
the sum of heat dissipated to the surroundings and
the heat stored in human body by raising the
temperature of body tissues.
Human Comfort

75. Human Comfort

76. Factors Affecting Human Comfort
Effective Temperature

77. Comfort Chart

78. Cooling Load Estimation