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Ventilation

Ventilation is replacement of vitiated indoor air by fresh supply of outdoor air. Modern concept of ventilation includes quality of incoming air in terms of air temperature, humidity and purity of air with a view to provide an environment that is comfortable and free from risk of infection.

To decide the standard of ventilation is a tough job. Standard of ventilation takes into account certain things like cubic space of the occupied room , air change and floor space. Cubic space: requirement of a minimum cubic space per person was considered important but modern ventilation standard gives more importance to air change.

Air change: it is the most important factor in the modern standard of ventilation. It is recommended that in the living rooms there should be 2 to 3 air changes in one hour, that is if cubic capacity of a room is 1,000 cubic feet than 2,000 to 3,000 cubic feet of fresh outdoor air should enter the room every hour. In work rooms there should be 4 to 6 air changes in one hour. If air change is more than 6 times per hour it may produce drought, which should be avoided. Based on this principle it is now considered that 1,000 to 1,200 cubic feet per person is sufficient.

Floor space: Floor space is more important than cubic space. Heights more than 12 feet are not important from ventilation point of view due to the fact that products of respiration accumulates in the lower levels. The optimum floor space requirement per person is 50 to 100 per head. While calculating cubic space for ventilation, heights above 12 feet are not taken into consideration.

Types of ventilation:

Natural ventilation: this is the simplest way of ventilation and used in schools, houses and small offices. Natural ventilation is due to certain forces that play in nature. The wind: it is the most active force in ventilation. When wind blows through a room it is called perflation. When wind meets an obstruction it bypasses the obstruction and causes a suction action at its tail end, called aspiration. Doors and windows facing each other provides cross ventilation. Ideally every house should be constructed in such a way that it provides enough cross ventilation. Back to back houses do not provide cross ventilation, so this type of houses are not allowed. Diffusion: this can occur through smallest of the openings, but it can not be relied upon as only source of ventilation, because this process is very slow. Temperature difference: hot air is lighter than cold air and air flows from higher density (heavier) to lower density (lighter). When temperature of rooms rises the air becomes light and escapes through openings and cooler air enters. The greater the temperature difference the higher the velocity of the incoming air. Due to higher outside temperature in the tropics the above process can reverse and cooler room air may go out and hotter outside air may come in. the biggest flaw of natural ventilation is that incoming air, temperature and humidity can not be regulated.

Mechanical ventilations are of four types, exhaust ventilation, plenum ventilation, balanced ventilation and air conditioning.

  1. Exhaust ventilation: in this system, air is driven out by exhaust fans. As air is removed a vacuum is created and fresh air enters inside through doors, windows and other inlets. This type of ventilation is used for large rooms and halls to remove vitiated air. Ventilation may be adjusted by adjusting the speed of the fans. This is widely used in industry to remove dust, fumes and other contaminants.
  2. Plenum ventilation: in this system fresh outside air is blown into the rooms and vitiated air is replaced. The air is delivered through inlets at desired points. This is of limited use.
  3. Balanced ventilation: it is combination of both exhaust and plenum ventilation.
  4. Air conditioning: It is simultaneous control of those factors affecting both physical and chemical condition of air within a confined space. These factors are temperature, humidity, air movement, distribution, dust, bacteria, odor and toxic gases most of which affect in greater or lesser extent human health and comfort. Air conditioning is popular in large institutions, hospitals and dwellings. It is used in operation theaters in hospitals and of particular value in control of pathogenic bacteria. Air is filtered when drawn into the air conditioning system. Excess humidity is removed and the air is circulated back into the room after cooling or heating it so that the temperature of the room come to comfort zone. Mixing of fresh air with re circulated air is controlled.

When temperature difference is large between outside and air conditioned room, some times transition room is provided which maintain temperature between the two and prevent sudden exposure to high or low temperature.

Air conditioning is very costly. It uses certain chemicals like CFC (chlorofluorocarbon) which causes depletion of ozone layer. Ozone layer is the layer of ozone high in atmosphere which prevent harmful cosmic rays and gamma rays from entering the atmosphere. Cosmic rays and gamma rays can give rise to skin cancer and can raise the temperature of the world (global warming).

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