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.