Psychrometric chart and process

BASICS OF AIRCONDITIONING

1. SANDEEP P S
Assistant professor
Mechanical Engineering Dept.
JCET

PSYCHROMETRY

2. Psychrometry
Psychrometry is the study of the thermodynamic
properties of moist air. It is used extensively to illustrate
and analyze the characteristics of various air
conditioning processes and cycles

3. Important Psychrometric Properties
1. Dry Bulb Temperature (oF or oC)
2. Wet Bulb Temperature (oF or oC)
3. Humidity ratio (g / kg or Grains / lb)
4. Relative humidity (%)
5. Dew-point Temperature (oF or oC)
6. Specific volume (m3/kg, Cu. Ft/lb)
7. Enthalpy (KJ/Kg or Btu/lb)

4. Dry Bulb Temperature (DBT)
Dry bulb temperature (DBT) is the temperature of the
moist air as measured by a standard thermometer or
other temperature measuring instruments

5. Wet Bulb Temperature
Wet bulb temperature is the lowest temperature that can
be reached by the evaporation of water only. It is the
temperature one feels when one's skin is wet and is
exposed to moving air

6. Dew-Point Temperature
This is the temperature at which condensation begins
when the air is cooled at constant pressure

7. Humidity Ratio / Specific Humidity
Specific humidity (SH) or humidity ratio is defined as
the ratio of the mass of water vapour to the mass of dry
air in a given volume of the mixture. It is denoted by
the symbol “ω”. Unit of specific humidity is “kg of
water vapor per kg of dry air”

8. Relative Humidity
Relative humidity (RH) is the ratio of the actual weight
of water vapor per given volume of air relative to the
weight of water vapor contained in same volume of
saturated air at the same temperature. It is expressed in
percentage

9. Specific Volume
 The specific volume is defined as the number of cubic meters
of moist air per kilogram of dry air.
Unit – m3/kg of dry air

10. Enthalpy
The enthalpy of moist air is the sum of the enthalpy of the dry
air (Sensible heat) and the enthalpy of the water vapour(Latent
Heat) .
Total Enthalpy = Sensible heat + Latent Heat
 Sensible heat = Enthalpy due to change in temp.
Latent heat = Enthalpy due to change in humidity

19. Psychrometric Processes
1. Sensible Heating
2. Sensible Cooling
3. Humidification
4. Dehumidification
5. Heating and Humidification
6. Cooling and Dehumidification
7. Cooling and Humidification
8. Heating and Dehumidification

20. Sensible Heating
It is addition of heat to moist air without the addition of
moisture. It follows a constant humidity ratio line on the
psychrometric chart
During this process, the moisture content of air remains
constant and its temperature increases as it flows over a
heating coil

21. Sensible Cooling
During this process, the moisture content of air remains
constant but its temperature decreases as it flows over a
cooling coil

22. Humidification
Humidification is the process of adding moisture at constant
temperature
Adding moisture at constant dry bulb temperature can be
represented by a vertical line on a psychometric chart.
This happens when water at the dry-bulb temperature of the
entering air is sprayed in the chamber whose temperature is
maintained at the air DB temperature.

Moisture added = w
2 – w
1 kg/kg dry air.

23. DEHUMIDIFICATION
The process in which the moisture or water vapor or
the humidity is removed from the air keeping its dry
bulb (DB) temperature constant is called as the
dehumidification process.
This process is represented by a straight vertical line
on the psychrometric chart starting from the initial
value of relative humidity, extending downwards and
ending at the final value of the relative humidity.

26. Basic Process in Air Conditioning

27. Adiabatic mixing of streams
The mixing of several streams of fluid is quite common in
engineering practice. The process can usually be assumed to
occur adiabatically. Mixing process is highly irreversible
because of eddying of fluid streams.

28. If the changes of kinetic energy are negligible, then the law
of conservation of energy gives:

??????
�1ℎ
1+??????
�2ℎ
2=??????
�3ℎ
3

Where ??????
�1+??????
�2=??????
�3

29. By-Pass factor
The inability of a coil to cool or heat the air to its
temperature is indicated by a factor called by-pass
factor (BPF) or Coil Bypass Factor.
This inability is due to the coil inefficiency and some
amount of air just bypassing the coil without getting
affected by it.
A coil with low BPF has better performance.

30. If Air is heated from temperature t1 to t2 by coils (ts)
then BPF is defines as:





Efficiency of Coil = 1 – BPF
For sensible heating, coil temperature (ts) should be
greater than DBT of air.
For sensible cooling, coil temperature should be less
than DBT but more than DPT.

31. Cooling coil

BPF =
�2−�??????
�1−�??????

32. Sensible heat factor
Heat added during a Psychometric process can be split in to
sensible heat and latent heat
?????????????????? =
????????????��??????�???????????? ℎ??????��
����?????? ℎ??????��
, ?????????????????? =
�??????��??????�???????????? ℎ??????��
�??????��??????�???????????? ℎ??????��+??????��??????�� ℎ??????��

33. Room sensible heat factor (RSHF)
The room sensible heat factor is the ratio of room sensible heat
to the summation of room sensible heat and room latent heat