VENTILATION
1. MEANING
Ventilation is the process by which clean air (normally outdoor air) is intentionally provided
to a space and stale air is removed.
2. REQUIREMENT OF A GOOD VENTILATING SYSTEM
Following are considered to be the desired requirements of a good ventilating system:
(a) It should be so designed that the required quantity of fresh air is admitted in the premises
and that the vitiated air is extracted from the premises.
(b) The value of desired relative humidity should be maintained.
(c) The effective temperature should be properly maintained with regard to the human
comfort.
(d) The air movements should be uniform and it should be seen that pockets of stagnant air
are not formed.
(e) The incoming air should be free from impurities such as dust, odour, etc.
(f) The ventilating system should be such that it results in overall satisfactory performance.
3. TYPES OF VENTILATION
The systems of ventilation may broadly be divided into the following TWO categories:
(a) Natural Ventilation
(b) Artificial Ventilation
a) Natural Ventilation: In this system of ventilation, the use is made of doors, windows, ventilators
and skylights to make the room properly ventilated. The system is useful for small buildings and it
cannot be adopted for big offices, theatres, auditoriums, etc. The only advantages of this system are
that it is economical in the sense that no special equipment is necessary for making the room
adequately ventilated and that it affords living under natural conditions. The important points to be
remembered in connection with natural ventilating system are:
(i) The location, size and type of windows play a great role in imparting natural ventilation to
the room. The windows also supply light and afford protection against weather. All these
functional requirements should be properly correlated while deciding the location of windows
in a room.
(ii) The efficiency of roof ventilators depends on their location, wind direction and height of
building.
(iii) It is found that the window ventilation with a combination of radiator, deflector and
exhaust (as shown in Fig. 11-1 below) can give better results. The radiators are situated below
the sill level of the windows and they extend for the full length of the window. The windows,
deflectors and radiators should be properly manipulated for achieving the desired effects. The
exhaust duct is provided near the ceiling of the opposite wall and it is taken out of the roof to
act more or less like a chimney. The windows open from bottom and the deflectors may be of
curved vanes.
(iv) If gas or fuel is to burn in the room, the quantity of air to be supplied by natural ventilation
should be enough to fulfil two purposes, namely, to burn the gas or fuel and to ventilate the
room.
(v) The velocity of wind creates pressure differences between the inside and outside surfaces
of a room. Hence, if the velocity of wind is more, there will be more pressure difference and
this will result in the increase of rate of air charge in the room.
(vi) The rate of air change in a room mainly depends on the design of the opening, location of
inlet and outlet and the difference in temperature between the inside and outside air.
Generally, the outside air is cooler than the inside air. Hence, the cooler air enters from the
bottom and when it becomes hot during its stay in the room, it leaves from the top. (Fig. 11-
2 below) shows the natural ventilation when inlets and outlets are situated one above the
other. (Fig. 11-3 below) shows the natural ventilation when inlets are at the bottom and roof
ventilator is at the top.
(vii) The term cross ventilation is sometimes used to indicate the position of outlets just
opposite to inlets as (shown in Fig.11-4 below).
(viii) This type of ventilation depends on the direction of wind and it is very difficult to control
the entry of air containing smoke, dust, etc. To keep control over the quantity, velocity and
temperature of the incoming air is also not very easy.
b) Artificial Ventilation: In this system of ventilation, some mechanical arrangement is adopted to
provide enough ventilation to the room. This system has become popular due to recent change in
notion regarding ventilation. At present the ventilation is required not merely to furnish warm air or
cool air. But the ventilation system should provide air of such qualities regarding humidity,
temperature, etc. as to make the room comfortable at all times during the year. The system is costly,
but it results in considerable increase in the efficiency of the persons under the command of the
system. This system is adopted for big offices, banks, industrial plants, theatres, auditoriums, etc.
Following are the FIVE methods of the artificial ventilation:
(i) Exhaust System
(ii) Supply System
(iii) Combination of Exhaust and Supply Systems
(iv) Plenum Process
(v) Air-Conditioning
(i) Exhaust System: In this system, the partial vacuum is created inside of the room by
exhausting the vitiated inside air by fans or blowers. The extraction of air from inside sets up
the current of fresh air from outside to inside and thus, it becomes possible to provide fresh
air to the room through doors and windows. The fans or blowers are installed at suitable
places in the outside walls and they are connected to different rooms through duct system.
This system is useful for removing smoke, dust, odours, etc. from kitchen, industrial plants,
etc. The ducts are placed near the place of formation of smoke, dust, odours, etc.
(ii) Supply System: This system is just the reverse of the above system and it consists in
supplying fresh air to the room by installing input fans in outside walls. This system is used for
ventilating rooms where unusual quantity of heat or odours is not produced.
(iii) Combination of Exhaust and Supply System: This system is the combination of the above
two systems. The exhaust fans and input fans are installed at suitable places in the outside
walls so as to cause a current of fresh air from outside to inside of the room. The combination
of both the systems gives better results. But on the whole, either exhaust or supply system of
ventilation is not reliable for better ventilation throughout the year.
(iv) Plenum Process: in this process, the fresh air is forced in the room and the vitiated air is
allowed to leave the room by itself or is extracted by outlet fans. The incoming air may be
allowed to pass through screens or filters or stream of water at the point of entry. Also, the
disinfection of incoming air can be achieved by adding ozone at the point of inlet. Thus, it is
possible to control the quality, humidity and temperature of incoming air, when this process
is adopted.
The plenum process may be employed for the downward ventilation or upward ventilation. In
the former case, 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
floor level. In the latter case, the reverse procedure is adopted. The fresh air is allowed to
enter at the floor level and the outlet is provided at the ceiling height.
The ducts of a plenum process should be designed carefully to prevent draughts. The separate
dampers may therefore be required for each room. The process is thus likely to be more
expensive.
The process is used for theatres, big-offices, etc. and also for supplying air to the air-
conditioned buildings.
(v) Air-Conditioning: The term air-conditioning is used to indicate the science of controlling or
conditioning air with respect to humidity, temperature, movement of air, odour, bacteria
content, dust content, etc. so as to make the air to suit the physiological requirements of the
human body or to the needs of industrial processes.