Ventilation: 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.
Requirement of air: Usually, the standards of ventilation have been based on the efficiency of ventilation in removing body odor.
1. Cubic space – Standards for the minimal fresh air that is advocated by different workers range from 300 to 3,000 cubic ft. per hour per person.
2. Air change – It is recommended that in living room there should be 2 or 3 air changes in one hour, in work rooms and assemblies 4 to 6 air changes. If the air is changed more frequently that is more than 6 times in one hour it is likely to produce a drought which should be avoided. Based on this concept it is now considered that a space of 1,000 to 1,200 cubic ft. per person is quite sufficient: The number of air changes per hour is calculated by dividing the total hourly air supply to the room by the cubic capacity of the room.
3. Floor space – Floor space per person is even more important than cubic space. Heights in excess of 10 to 12 feet are ineffective from the point of view of ventilation as the products of respiration tend to accumulate in the lower level. The optimum floor space requirement per person, vary from 50 to 100 sq. ft.
Disadvantages of bad ventilation – Defective ventilation and overcrowding cause an increase of temperature, excessive humidity and stagnation of air. These factors lower the vitality of the people in the room and so they become early prey to infection. If some persons in the crowd are suffering from contagious diseases like scabies, ringworm or respiratory diseases, viz. influenza, tuberculosis, smallpox, chicken pox measles, etc. then there is great risk to other people contacting these infections. Excess of carbon dioxide produces ill effects on the health such as headache, inability to concentrate, drowsiness, lassitude etc. It may even cause death especially in a closed room where coal is burning.
Natural method of ventilation – Fresh air enters the room through windows and doors and replaces the foal air. This is due to:
1. The wind – 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 house does not permit cross ventilation and therefore their construction is not allowed.
2. Diffusion of gases – Air passes through the smallest opening of spaces by diffusion. This means mixing of two different types of gases. This is a slow process and is therefore not relied upon.
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 out-side air which is cooler and more dense will enter the room through inlets placed low. The greater the temperature difference between the outside and inside air, the greater the velocity of the incoming air.
Artificial (mechanical) ventilation: This maybe of the following types:
(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 fan so as to create a positive pressure and displace the vitiated air. Plenum or propulsion system is used for supplying air to air-conditioned buildings and factories.
(3) Balanced ventilation -This is a combination of the exhaust and plenum system of ventilation. The blowing fan must balance the exhaust fan.
(4) Air conditioning – It is defined as the simultaneous control of all or at least the first three of those factors affecting both the physical and chemical conditions of the atmosphere within any structure. This controls temperature, humidity, air movement, distribution, dust, bacteria, odors and toxic gases. This is becoming more popular these days and is one of the very ideal methods of ventilation.
Ventilated or poorly ventilated Area
Generally, when a person enters a room from the open air should not perceive any smell or stuffiness. But when the CO2 of the air of a room exceeded the percentage in the out side air by more than 0.02%, the feeling of stuffiness become perceptible to the sense. So by the sense of smell and stuffiness one can detect the impurity of air in a room or places. Also estimation of temperature, humidity can give a better indication of proper ventilation.
(1) For temperature of the air use thermometer.
(2) For humidity use any type of Hygrometer or Dry and Wet Bubble Thermometer.
(3) For finding out the cooling power of air use Thermometer.