Air Conditioning System Lecture
Air Conditioning System Lecture
Air Conditioning System Lecture
Babylon University
College of Engineering
Department of Mechanical Engineering
LECTURE-1
Air-Conditioning Systems
Introduction
Air-conditioning is a process that simultaneously conditions air; distributes
it combined with the outdoor air to the conditioned space; and at the same time
controls and maintains the required spaces temperature, humidity, air
movement, air cleanliness, sound level, and pressure differential within
predetermined limits for the health and comfort of the occupants, for product
processing, or both.
The
HVAC&R
is
the
heating,
ventilating,
air-conditioning,
and
Importance of air
The human need to 1.2 kg. of water , 2.7 kg. of food and 16 kg. of air per day .
The healthy requirements is :
Moving the air by 2.5 L/s
AIR-CONDITIONING SYSTEM
A third of the feeding air must be fresh air and the 2 thirds is return air
after purifying
the entire air in the air conditioned areas must be moved at a velocity
between 0.15 to 0.25 m/s . This velocity doesnt cause a noise
Humans comfort
The human
properties treatment : 1. Dry bulb temp.(d.b.t) : It is needed to cool or heat the air before flowing
2. The humidity ratio () : The air must be humidified or dehumidified before
flowing.
3. The air movement : the flow rate of air and its distribution must be changed
to be homogeneous at the conditioned areas to make everyone feel the same
conditions (the static air cause a temperature difference between the
breath level and the ceiling level about 8-16 co)
AIR-CONDITIONING SYSTEM
4. Air purifying : By using filters to prevent dust and killing the bacteria
5. Ventilation : It is required to use fresh air to provide the needed oxygen for
the conditioned areas
between the
conditioned
space
and
surroundingswithin
predetermined limits.
AIR-CONDITIONING SYSTEM
1.
houses, and private homes. Many systems serving the lodging industry and
apartment houses are operated continuously, on a 24-hour, 7-day-a-week
schedule, since they can be occupied at any time.
4.
convalescent care facilities. Special air filters are generally used in hospitals to
remove bacteria and particulates of sub-micrometer size from areas such as
operating rooms, nurseries, and intensive care units. The relative humidity in a
general clinical area is often maintained at a minimum of 30 percent in winter.
5.
Psychrometric Relations
Though there are many Psychrometric terms, yet the following Psychrometric
relations are important from the subject point of view :
1. Dry Air: The pure dry air is mixture of a number of gases such nitrogen, oxygen, carbon
dioxide, hydrogen, argon, helium etc. The composition of dry air varies slightly at
AIR-CONDITIONING SYSTEM
2. 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 mixture.
3. Saturated Air: It is a mixture of dry air and water vapour, when the air has diffused the
maximum amount of water vapour into it.
4. Specific Humidity, Humidity Ratio or Moister Content (): It is the ratio of the mass of the water vapour to the mass of the dry air
contained in the mixture of the moist air, in (kg/kg).
The equation of state for an ideal gas that describes the relationship between its
thermodynamic properties for dry air is:
= ..1
= ..2
AIR-CONDITIONING SYSTEM
Also
V a=V v
and
Ta=Tv=Td
...3
=
=
4
Substituting Ra=0.287 kJ/kg.K for dry air and Rv=0.461 kJ/kg.K for water
vapour , get
= . = .
Where:
AIR-CONDITIONING SYSTEM
5. Dew Point Temperature: It is the temperature of saturated moist air of the moist air sample having
the same humidity ratio at the same atmospheric pressure. Two moist air
samples of similar dew points tdp at the same atmospheric pressure have the
same humidity ratio (W) and the same partial pressure of water vapour (pw).
Pv
td= tsup
tdp= tsat
Dew Point Temperature
6. Degree of Saturation (): It is defined as the ratio of the humidity ratio of moist air, W, to the
humidity ratio of saturated moist air, Ws, at the same temperature and pressure:
=W/ Ws =
.....6
Notes: The partial pressure of saturated air (Ps) is obtained from the steam tables
corresponding to dry bulb temperature (tdb).
AIR-CONDITIONING SYSTEM
If the relative humidity is equal zero, then the humidity ratio W=0, and for
dry air =0
If the relative humidity is equal one (1), then the humidity ratio W=Ws,
and for dry air =1 thus varies between 0 and 1.
7. Absolute Humidity(vapour density): It is the mass water vapour in 1 m3 of dry air or expressed in term of
gram per cubic meter of dry air(g/ m3). From perfect gas for water vapour and
dry air the mass flow rate ratio is:
..7
=
= ...8
From equation of state equation 1 for (1 kg) mass flow rate and equation
8, the vapour density is:
=
=
9
8. Relative Humidity(): It is the ratio of actual mass of water vapour in a given volume of moist
air to the mass of vapour in the same volume of saturated air at the same
temperature and pressure.
AIR-CONDITIONING SYSTEM
10
= .11
Hence, relative humidity is:
= ...12
Note: For saturated air, the relative humidity is 100%. = 1
For dry air, the relative humidity is 0%. = 0