Ventilation

Types of filters, Systems of ventilation (Natural and Artifical ventilation) and types of ventilation techniques.

1. NATURAL
&
ARTIFICIAL

2. Air filters
The purpose of an air filter is to free the air of as much of the airborne contaminants as
is practicable.
Filters will justify their cost by a reduction in the cost of cleaning and decorating the
building and the protection of the heating or air conditioning equipment. Filters are also
required for various dust-free industrial processes.
The main types of filters are:
1. Dry: In which the contaminants are collected in the filter medium.
2. Viscous or Impingement: In which the contaminants adhere to a special
type of oil.
3. Electrostatic: In which the contaminants are positively charged with
electricity and collected on negative earthed plate

3. Dry filters
Materials such as cotton wool, glass fibre, cotton fabric, treated
paper, foamed polyurethane as the cleaning medium.
The efficiency of the filter depends largely upon the area of medium
offered to the air stream, and for this reason the filter can be
arranged in a V formation which increases the area.
Figures 5.1, 5.2 and 5.3 show the arrangements of V type dry filter.
(NEXT SLIDE)
After a period of use (depending upon the atmospheric pollution) the
contaminars retained by the filter will increase and this will increase
the resistance to the now of air through the filter.
In the case of an automatic roller type filter when filter is dirty,
pressure switch will switch on an electric motor which will turn the
dirty spool
and allow clean fabric to enter the filter chamber.

5. •Figure 5.4 shows sections of two types of automatic roller type filter.
•Type A does not have as much filter area as type B.
•Figure 5.5 shows a view of an automatic roll type filter.
•With other types of dry filters an electrically operated pressure switch may
he installed, which switches on a warning light to draw attention to a dirty
filter.
•One type of dry filter is made up of a square cell of sizes from 254 by 254
mm to 600 by 600 mm.
•The filter medium, which is either 25 or 50 mm thick, is held in a metal or
cardboard frame.
•The cheaper type may be thrown away when dirty and the more
expensive, vacuum-cleaned two or three times before being discarded.
•Figure 5.6 shows a view of a disposable, or what is often called a
'throwaway dry cell type filter‘.

6. These are of dry fabric type and are very
efficient in moving even the smallest particle
from the air.
This high performance is obtained by close
packing of very large number of small
diameter fibers, but the unfortunately results
in a high resistance to air passing through
the filter.

7. Viscous filters
These have a large dust-holding capacity and are therefore often used in industrial
areas where there is a high degree of atmospheric pollution. The filter medium is coated
with a non-inflammable, non-toxic and odorless oil, which the contaminants adhere to as
they pass through the filter.
There are two types of viscous filters:
1. Cell type: which consists of a metal frame into which wire mesh. Industrial metal
swarf, metal stampings or a combination of these materials are inserted and coated with
the special oil The cells are placed across the air stream in a V formation. similar to the
dry cell type filter.fig 5.2 which will allow the maximum area of the filter to be in contact
with the air flow. When dirty, the cells are removed, washed in hot' water. allowed to dry
and re-coated with clean oil for further use. Figure 5.7 shows a view of a cell type
viscous filter.
2. Automatic types: one type uses a moving curtain. consisting of filter plates hung from
a pair of chains. The chains are mounted on sprockets located at the top and bottom of
the filter housing. so that the filter plates can be moved as a continuous curtain. up one
side and down the other ,side of the sprockets. The arrangement is such that at the
bottom the filter plates pass through a bath of special oil. which cleans the dirty oil from
the filter plates and re-coates them with cleaner oil.
Figure 5.8 shows a vertical section of an automatic revolving type viscous filter. An
electric motor is used to turn the sprocket and this may be arranged to move the curtain
continuously, or periodically, depending upon the degree of atmospheric pollution,
Another type has closely spaced corrugated metal plates.
which are continuously coated by oil from a sparge pipe' at the 'top of the filter.

8. The air passing through the plates has to take a torruous route and, in so doing,
the dust particles in the air which is washed down to an oil bath. The oil is
pumped from this bath through a filter to the sparge pipe, where it is discharged
over the filer plates thus recommencing the cycle.
Figure5.9 -shows-a view of a spray type viscous filter.

9. Electrostatic filters
These types of filters have three main components:
ioniser, metal collector and electrostatic power pack.
The various air contaminants are given a positive electrostatic charge by an ioniser
screen which is the first part of the filter.
The screen consists of a series of fine wires possessing an electrostatic charge
produced by a direct current potential of 13 kV. The wires are spaced alternatively
with rods or tubes, which are at earth potential.
The air containing these positively charged contaminants then passes through a metal
collector, which consists of a series of parallel plates about 6 mm apart, arranged
alternatively so that one plate, which is earthed, is next to a plate which is charged with
a positive direct current potential of 6 kV. The positively charged air contaminants
passing through the collector are repelled by the plates of similar polarity (which are
positive) and arc attracted by the negative earthed plates, which are usually coated with
a water-soluble liquid to permit easy cleaning. When the plates require cleaning, they
are hosed down with warm water and it is sometimes found convenient to install sparge
pipes above the plates for this cleaning process.
With some types of electrostatic filters the plates may be removed and immersed in a
wash bath of warm water. Access to the filter is through air-tight doors with safety locks.
so that it is impossible to open the access door without first switching off the electricity
supply from the power pack. The resistance to the air flow through the filter is very low,
but some pre filteration is usually necessary .
Figure shows details of an electrostatic filter:

11. Ventilation
The purpose of “ventilation of buildings is to remove high concentration of
odours, carbon di oxide and water vapours and also to remove dust, fumes
and smoke' (which may be toxic) and excess heat.
The air in the room containing these contaminants is replaced by' fresh air and
this creates air movement inside a building, so that the occupants obtain a
feeling of freshness without draughts.
One the concentration of carbon dioxide was used as a criterion of good
ventilation; but even in very badly ventilated rooms the carbon dioxide rarely
rises to a harmful Level.
The absence of body odours. dust and fumes in the air is a better criterion of
good ventilation; also if the air movement is too low the air in the room wiII feel
'stuffy'.
An air velocity of between 0.15 and 0.5 m per second is acceptable to most
people under normal circumstances.
for example, Factories Act 1961, Offices, Shops and Railway Premises Act
1963.
A minimum ventilation rate of 28 m3 of fresh air per hour, per person, is also
required by most authorities for theatres, cinemas and dance haIls etc

12. VENTILATION:
DEFINITION AND necessity
Ventilation may be defined as supply of fresh outside air into an enclosed
space or the removal of inside air from the enclosed space. In other words,
ventilation is the removal of all vitiated air from a building and its replacement
with fresh air. Ventilation may be achieved either by natural or by artificial (or
mechanical) means.
Ventilation is necessary for the following reasons:
1. Creation of air movement.
2. Prevention of undue accumulation of carbon dioxide.
3. Prevention of flammable concentration of gas vapour.
4. Prevention of accumulation of dust and bacteria-carrying particles.
; 5. Prevention of odour caused by decomposition of building material.
6. Removal of smoke,odour and foul smell generated/liberated by the
occupants.
7. Removal of body heat generated/liberated by the occupants.
8. Prevention of condensation or deposition of moisture on wall surfaces.
9. Prevention of suffocation conditions in conference rooms, committee halls,
cinema hall, big rooms, etc.

13. FUNCTIONAL REQUIREMENTS OF VENTILATION SYSTEM
From the point of view of human comfort, ventilation system should meet the following
functional requirements:
1. Air changes or air movement.
2. Humidity.
3. Quality of air.
4. Temperature.
1. Air changes (or air movement)and rate of Supply of fresh air :
In an enclosed space, where people are working or living, air has to be moved or
changed to cause proper ventilation. The minimum rate of air change is one per hour,
while the maximum rate of air change is sixty per hour. Air change per hour is volume of
outside air allowed in the room or enclosed space per hour compared to the volume of
the room. If the rate of air change is less than one per hour, there will be no ventilation,
while the rate of air change is more than sixty per hour it will cause discomfort to, the
occupants because of high velocity of air.
Cross ventilation is provided to increase the rate of air movement in a naturally
ventilated building while fans etc. are used in case of mechanically ventilated buildings.
Since the amount of fresh air required to maintain the carbon dioxide concentration of air
within safe limits and to provide sufficient oxygen content to air for respiration is very
small and since the rate of ventilation to maintain satisfactory thermal environment for
A region varies from season to season, the minimum standards of ventilation are based
on control of body odour or the removal of products of combustion depending on the
requirements of each case,
,

14. The volume of fresh air required for the removal of body odour is influenced by the air
space per person-the volume decreases as the air space per person increases. A rough
guidance can be taken from the following table:
Recommends values for residential buildings:
(i) Living rooms and bed rooms. In the case of living rooms and bedrooms, minimum of
three air changes per hour should be provided.
(ii) Kitchens. Large quantity of air are needed to remove the steam, heat, smell and
fumes generated in cooking and to prevent excessive, rise of temperature and humidity.
However, for the requirement of kitchen in which cooking is done for a family of not
more than five persons, minimum rate of ventilation of about three air changes per hour
should be provided.
(iii) Bath rooms and water closets. Considerable ventilation (bathrooms and water
closets is desirable after use, and the equivalent of three air changes per hour should
be provided.
(iv) Passages. The period of occupation of passages lobbies and the like is very short,
and as such no special consideration in designing their ventilation system.

15. Indian Standard has not made any recommendations for ventilation standards of public
buildings. However, guidance may be
RECOMMENDATIONS FOR VENTILATION IN PUBLIC BUILDINGS

16. 2. Humidity. Air contains certain amount of water vapour in it. Relative humidity is
defined as the ratio of amount of water vapour present in the air to the amount of water
vapour if the air were saturated at the same temperature. Thus, the relative humidity of
saturated air is 100 percent. Relative humidity within the range of 33 to 70 per cent at
the working of 21°C, is considered to be desirable. For higher temperatures, low
humidity and greater air movements are necessary for removing greater portion of heat
from the body.
3.Quality of air. The ventilating air should be free from impurities ,odours, organic
matter and inorganic dust. It should also be free from unhealthy fumes of gases, such as
carbon monoxide, carbon dioxide, sulphur dioxide etc. The ventilating air should not
come from the vicinity of chimneys, kitchens, latrines, urinals, stables etc. Air containing
less than 0.5 mg of suspended impurity per m3 and less than 0.5 part per million of
sulphur dioxide-is considered to be clean, and does not require further treatment. Air
within room containing. 0.06 percent of CO2 may be considered Vitiated but with 0.09 or
0.1 percent, it becomes stuffy and unbearable. Hence the air in habitable rooms should
never contain more than 0.06 percent of CO2. The air should be kept in this condition by
proper ventilation. Pure air in buildings is necessary for the sustenance and
improvement of health, for the perfect combustion of fuel and for the preservation of
materials of which the building is constructed.

17. 4. Effective temperature. It is desirable that the incoming Ventilating air should
be cool in summer and warm in winter, before it enters the room. The general
temperature difference between inside and outside is kept not more than 8’C. With
regard to human comfort the term effective temperature is more useful. It is an index
which combines into a single value, the effect of air movement, humidity and
temperature. Effective temperature indicates the temperature of air at which a person
will experience sensation of same degree of cold or warmth as in quite air fully saturated
(i.e. 100% humidity) at the same temperature. In other words, it is the effective
temperature which is more important than the actual temperature itself. If two rooms
have the same effective temperature, a person leaving one room and entering the other
will not experience any change of temperature though the actual temperatures in the two
rooms may be different.
The value of effective temperature, from human comfort point of view, depends upon the
type of activity, geographical conditions, age of occupants, amount of heatIoss from the
body etc. The common values of effective temperatures in winter and summer are 20’C
and 22 ‘ C respectively

18. SYSTEMS OF VENTILATION
Systems of ventilation may be divided into two categories
(i) Natural ventilation.
(ii) Mechanical ventilation or artificial ventilation.
Natural ventilationis the one in which ventilation is effected by the elaborated use of
doors, windows, ventilators and skylight
It is usually considered suitable for residential buildings and small houses. However, it is
not useful for big buildings, offices, conference halls, auditoriums, large factories etc. In
natural ventilation, cross ventilation is normally relied to secure air movement. It is
economical since no equipment is required for keeping the room ventilated .
Mechanical ventilationis the one in which some mechanical arrangements are
made to increase the rate of air flow. The system is more useful for large buildings,
assembly halls, factories, theaters etc. Though the system is more costly, it results in
considerable efficiency of the persons using the building.

19. NATURAL VENTILATION
In this system, ventilation is effected by doors, windows, ventilators, skylights
and other openings in the enclosed space. The rate of ventilation depends on
two effects :
(a) Wind effect
(b) Stack effect.
(a)Ventilation due to wind effect
The rate of ventilation depends upon the direction and velocity of wind outside
and sizes and positions of openings.
such an effect is known as 'ventilations due to wind action' When wind blows
at right angles to one face of a building, pressure differences are created-
positive pressure is produced on windward face and negative pressure (or
suction) is produced on the leeward face. If the wind Direction is at 45°to
one of the faces, positive pressure will be
Produced on two windward faces and negative pressure on the two leeward
faces .Fig.27.1shows the movement of wind through buildings.

20. MOVEMENT OF WIND THROUGH BUILDINGS.

21. In designing a system of natural ventilation, the aim should be to make effective use of
wind forces. Since these are not constant, being dependent on the speed and direction
of wind, it is obvious that the ventilation is likely to be variable in quantity.
For design purposes, the wind may be assumed to come from any direction within 45°
of the direction of prevailing wind.
In the case of pitched roof, the pressure will' depend upon the pitch of the roof, as shown
in Fig. 27.2. It is seen that the roof pressures in general are negative, except on the
windward side of the roof with shape greater than 30". Wind will blow from windward
side to the other side if there is an opening.

22. Ventilation due to stack effect
In this, the rate of ventilation is affected by the convection effects arising from
temperature or vapour pressure difference (or both) between inside and outside of the
room and the difference in the height between the outlet and inlet openings. Ventilation
Due to stack effect is illustrated in Fig. 27.4. When air temperature inside is higher than
the outside, warmer air rises and passes through opening located in the upper part of
the room, whereas in cool air enters from the lower openings.

23. The rate of air flow in stack effect:
The rate of air flow arising from temperature difference between outside and inside is
given by

24. Ventilation due to both the effects
When both wind and stack pressures are acting, it is proper to calculate each pressure
acting independently under conditions ideal to it and then apply a percentage. However,
ventilation in residential buildings due to stack pressure both in hot-arid region and in
hot-humid region appears to be insignificant and at any rate may be neglected,
as when both wind pressure and stack pressure are acting, the wind pressure effect
may be assumed to be predominant.
General rules of natural ventilation. Indian Standard Code IS : 3362-1965 lays down the
following general rules of natural ventilation
1. Inlet openings in the buildings should be well-distributed and should be located on the
windward side at a low level; and outlet openings should be located on the leeward side
near the top so that incoming air stream is passed over the occupants. Inlet and outlet
openings at high levels only may clear top air at that level
without producing air movement at the level of occupancy.
2. Inlet openings should not as far as possible be obstructed by adjoining buildings,
trees, sign boards, or other obstructions or by' partitions inside in the path of air' flow. .
3. Greatest flow per unit area of opening is obtained by using inlet and outlet openings
of nearly equal areas.
4. Where direction of wind is quite constant and dependable, the openings can be
readily arranged to take full advantage of the force. of the wind. When the wind direction
is quite variable, the openings may be 'arranged so that, as far as possible there is
approximately equal area on all sides. Thus no matter what the wind direction is there
are always some openings directly exposed to wind
‘pressure and others to air suction and effective movement through building is assured.

25. 5. Natural ventilation occurs when the air inside a building is a different temperature
than the air outside. Thus in a heated building and in an ordinary building during summer
nights and during pre monsoon period when the inside temperature is higher than
outside, cool outside air will tend to enter through openings at low level and warm air will
tend to leave through openings at high level. It would, therefore, be advantageous to
provide ventilators as close to the ceiIing as possible. Ventilators can also be provided in
roofs, for example, cowl, vent pipe, covered roof and ridge vent.
6. Windows of living rooms, should either open directly to an open space or to an
unobstructed facing on open space. In places where building sites are restricted, open
space may have to be created in the buildings by providing adequate courtyards.

26. MECHANICAL (OR ARTIFICIAL) VENTILATION
Mechanical ventilation or artificial ventilation involve the use of
some mechanical equipment for effective air circulation .It is
provided In those circumstances where satisfactory standard of
ventilation in respect of air quantity, quality or controlability cannot
be obtained by natural means. This system is costly, but it results
in considerable Increase in the efficiency of persons under the
command of the system. There are following systems of artificial
ventilation:
(i) Extraction system.
(ii) Plenum system.
(iii) Extraction-Plenum system.
(iv) Air-conditioning.

27. Extraction system (or exhaust system)
This system is based on creation of vacuum in the room by
Exhausting the vitiated inside air by means of propeller type fans
(Exhaust fans). Air inlets are formed at a height of 1.2 to 1.8 m
through Tobin tubes, and the outlet is arranged within a quarter a
meter of the ceiling on the opposite side of the room from which
air enters. The extraction of air from the room permits the fresh
air to flow from outside to inside either through Tobin tubes or
'even through a window. This system is more useful in removing
smoke , dust, odours, etc from kitchen ,toilet ,industrial plants etc.

28. Plenum system (or supply system) ,
In this system, fresh air is forced into the room and the vitiated air is allowed to
leave through ventilators. The air inlet is selected on that side of the building
where purest air is available. The incoming air which is mechanically forced
into the room is passed through a fine gauge screen or filter. A constant stream
of water is kept flowing down the screen giving a fine mist of water through
which the air is drawn by means of blower fan. Thus, all the mechanical
impurities are removed from the air. In summer, this also results in cooling of
air. At this point air may be further disinfected by the introduction of ozone from
an ozonizing apparatus. In winter the air may be forced through a battery of hot
water tubes be heated before being forced into the room. In the case of big
properly formed sheet iron ducts with properly dimensioned branches. This
ventilation system is costly, but is used for factories, conferences, halls,
theatres, big offices, etc.
The ventilation by plenum, process may be either downward or upward. In the
downward ventilation, the incoming air is allowed to enter at the ceiling height
and while mixing with the vitiated air during its downward journey, it is taken out
through outlets situated at the floor level.
In the upward system, fresh air enters at the floor level and moves out at the
ceiling level.

29. Extraction-Plenum system
This is an extension of plenum system in which extraction
fans are used for the exit of the vitiated air from the room.
This system is adopted where the delivery of fresh air is
either sluggish or where it is desired to discharge vitiated air
containing obnoxious fumes as from kitchens, laterines, or
various manufacturing processes, in specially isolated
areas.

30. Air conditioning
This is the best system of artificial ventilation in which provision is made for
filtration, heating or cooling, humidifying or dehumidifying etc, thus creating
most comfortable working condition.
Air-conditioning may be defined as the process of treating air so as to control
simultaneously its temperature, humidity, purity and distribution to meet the
requirements of the conditioned space,
The various requirements of a conditioned space may be comfort and health of human
beings, needs of certain industrial processes, efficient working of commercial premises
etc.
Air-conditioning is resorted to for the following :
(i) It helps in preserving or maintaining health, comfort and convenience of occupants of
residential building.
(ii) It helps in improving the quality of products in certain industrial processes such as
artificial silk, cotton cloth. etc. In other cases of industries, it provides comfortable
working conditions for the workers, resulting in the increase of the production.
(iii) It helps in marking the commercial premises, such as banks, offices, restaurants
etc. more active and efficient.
(iv) It provides more comfortable entertainment in theatres
(v) In the case of air conditioned railway/roadways coaches, of air travel, journey
becomes more, comfortable.