Air Lecture

 Air is a mechanical mixture of gases. The
normal composition of external air by volume
is approximately as follows:
 Nitrogen - 78.1 per cent;
 Oxygen - 20.93 per cent;
 Carbon dioxide - 0.03 per cent.
 The balance is made up of other gases which
occur in traces, e.g., argon, neon, krypton,
xenon and helium. In addition to these gases,
air also contains water vapour, traces of
ammonia and suspended matter such as dust,
bacteria, spores and vegetable debris.

 Air is rendered impure by·
( 1) Respiration of men and animals
(2) Combustion of coal, gas, oil, etc.
(3) Decomposition of organic matter
and
(4) Trade, traffic and manufacturing
processes which give off dust, fumes,
vapours and gases

 This is brought about by certain self-cleansing
mechanisms which operate in nature
(1) Wind: Wind dilutes and sweeps away the
impurities by its movement. Because of wind
movement, impurities do-not accumulate in any
one place;
(2) Sunlight : The atmospheric temperature and
sunlight play their own part by oxidizing
impurities, and killing bacteria;
(3) Rain : It cleanses the atmosphere by
removing the suspended and gaseous impurities;
4) Plant life : The green plants utilize the
carbon dioxide and generate oxygen; this process
is reversed during the night time. When the rate of

 Human occupancy and activity vitiate air in
occupied rooms and give a sense of
discomfort to the occupants. The changes in
air that take place in confined places are
both chemical and physical.
(a) CHEMICAL CHANGES
(b) PHYSICAL CHANGES

 The air becomes progressively contaminated
by carbon dioxide and the oxygen content
decreases due to metabolic processes. An
average person at rest gives off 0. 7 c.ft. of
carbon dioxide per hour; this may increase
up to 2 c.ft. during physical activity. In a
mixed gathering comprising all age groups,
the per capita output of carbon dioxide is
taken as 0.6 c.ft. per hour.

 (i) Rise in temperature
 (ii) Increase of humidity
 (iii) Decrease in air movement
 (iv) Body odours
(v) Bacterial pollution
Unless the vitiated air is replaced by fresh air, it
may adversely affect the comfort, health and
efficiency of the occupants. It is known that a
feeling of suffocation or discomfort is experienced
by the occupants in insufficiently ventilated rooms
and also· complaints of headache, drowsiness and
inability to concentrate. There is also the risk of
droplet infection and lowered resistance to disease
(on prolonged exposure).

 Discomfort is a subjective sensation which
people experience in ill-ventilated and crowded
rooms.
 For a Jong time it was believed to be due to
increased carbon dioxide and decreased oxygen,
resulting from respiration. This theory has since
been refuted .
 Studies have shown that the oxygen content
may be reduced to 18 per cent and the carbon
dioxide content may be raised to over 5 per cent
, without adverse effects, provided the
temperature and humidity are kept satisfactory.

 In the 'Black Hole of Kolkata', 146 prisoners were
imprisoned in a room, 18 x 14 x 10 out of whom only
23 survived. There were two small windows which
were adequate to supply all the oxygen needs even
then only 23 survived.
 It was concluded that the deaths were due to
changes in the physical condition of the air, leading
to 'heat retention'. It is now established that the
causes of discomfort are not due to chemical changes
but physical changes. These are temperature,
humidity, air movement and heat radiation. These
factors determine the "cooling power" of the air with
respectto the human body. It has been so well said by
Professor Lee that "The problems of ventilation are
physical, not chemical; cutaneous not respiratory.

 (1) AIR TEMPERATURE
(2) AIR TEMPERATURE AND HUMIDITY
(3) COOLING POWER
(4) EFFECTIVE TEMPERATURE
(5) CORRECTED EFFECTIVE TEMPERATURE

Corrected
effective temperature deg C
 1. Pleasant and cool -----------------20
2. Comfortable and cool ------------20-25
 3. Comfortable -----------------------25-27
 4. Hot and uncomfortable ----------27-28
 5. Extremely hot----------------------28+
 6. Intolerably hot -------------------30+

 Effective temperature 25-27degree c
 Relative humidity 30-65 %
 Dry kata 6 or above
 Wet kata 20 or above

 1.Acid rain
 2.Global warming causing –
-Increase green house effect
-Reduction of food
-Increase melting of polar
icecap
-Increase sea level –flood
-Smog formation
-Increase in cancer

 The methods employed are :
 (1) MECHANICALVENTILATION : This reduces
vitiated air and bacterial density.
 (2) ULTRAVIOLET RADIATION : This has been
found to be effective in special situations such
as operation theatres and infectious disease
wards. Since direct exposure to ultraviolet rays
is a danger to the eyes and skin, the ultraviolet
lamps are shaded and located in the upper
portion of the rooms near the inlet of air.
Ultraviolet rays have proved effective for
general use in public assembly and school rooms.

 (3) CHEMICAL MISTS : Triethylene glucol
vapours have been found to be effective air
bactericides, particularly against droplet
nuclei and dust.
 (4) DUST CONTROL : Application of oil to
floors of hospital wards reduces the bacterial
content of the air. Air disinfection is still in
the experimental stage

 The modern concept of ventilation implies
not only the replacement of vitiated air by a
supply of fresh outdoor air, but also control
of the quality of incoming air with regard to
its temperature, humidity and purity with a
view to provide a thermal environment that
is comfortable and free from risk of
infection.

 1. NATURAL VENTILATION (1) THE WIND : The
wind is an active force in ventilation. When it
blows through a room, it is called perflation.
When there is an obstruction, it bypasses and
exerts a suction action at its tail end - this is
called aspiration. Doors and windows facing each
other provide "cross-ventilation". Back to back
houses do not permit _cross ventilation and
therefore, their construction is not allowed.
 (2) DIFFUSION : Air passes through the smallest
openings or spaces by diffusion. This is a slow
process and therefore, is not relied upon as the
sole means of ventilation

 (3) INEQUALITY OF TEMPERATURE: Air flows from
high density to low density; it rises when slightly
heated and escapes from openings provided high up
in the room. The outside air which is cooler and more
dense will enter the room through inlets placed low.
The greater the temperature difference between
outside and inside air, the greater the velocity of the
incoming air. In the tropics the outside air may be
hotter than the inside and the reverse may take
place (2). These properties of air are utilized to best
advantage by the proper location of windows, doors,
ventilators and skylights. The chief drawback of
natural ventilation is that it is not possible to
regulate the velocity of the incoming air nor to
adjust its temperature or humidity

 2. MECHANICAL VENTILATION :
 Mechanical or artificial ventilation may be
of the following types : (1) Exhaust
ventilation.
 (2) Plenum ventilation.
 (3) Balanced ventilation.
 (4) Air conditioning

 (1) EXHAUST VENTILATION : In this system,
air is extracted or exhausted to the outside
by exhaust fans usually driven by electricity.
As air is exhausted, a vacuum is created
which induces fresh air to enter the room
through windows, doors and other inlets.
 . (2) PLENUM VENTILATION : In this system,
fresh air is blown into the room by
centrifugal fans so as to create a positive
pressure, and displace the vitiated air.

 (3) BALANCED VENTILATION: This is a
combination of the exhaust and plenum
systems of ventilation. The blowing fan must
balance the exhaust fan.

 Large institutions or hospitals often install
central airconditioning system for entire
building, instead of installing equipments for
individual rooms. Better controls and
economy is achieved in central
airconditioning.
 Where the temperature difference is large
between outside atmosphere and
airconditioned room, "transition room" is
sometimes provided, which maintains
temperature in between the two, so as to
prevent sudden exposure to high or low
temperature.

Air Lecture

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Air Lecture