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 body heat generated/liberated by the occupants and of smoke.

7. Prevention of condensation of deposition of moisture on wall surfaces.

8. Prevention of suffocation conditions in conference rooms, committee halls, cinema hall, big rooms, etc

Functional requirements of ventilationsystem

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 the 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 if 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 (he 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.

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 :

Air space per person Fresh air supply per person

(m³) (m³/h)

5.5 28.5

8.5 20.5

11.0 arid upwards 17.0

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'С, 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 minion of sulphur dioxide is considered to be clean, and does not require further treatment. Air within the 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 substenance and improvement of health, for the perfect combustion of fuel and for the preservation of materials of which the building is constructed.

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 heat loss from the body etc. The common values of effective temperatures in winter and summer are 20°C and 22° C respectively.