Requirements of Outside Air for Air conditioning

The air in the external environment in any air conditioning system should be similar or better than the expected normal leakage of air from the building. This leakage occurs through gaps near the windows and doors, flue, various outlets, and also in small quantity through walls. The transfer of air by leakage is due to the external wind pressure called infiltration, and also due to variation in density between internal and external temperatures, which is known as stack effect.

Infiltration determines the quantity of external air that must be delivered to ensure maintenance of an inner static air pressure, which should be sufficient to avoid infiltration by leakage. This air supply to balance infiltration is a separate requirement, which should be added to the external air.

In any air conditioning system, it is essential to replenish the oxygen consumed in external air. The new air required to meet oxygen demands is negligible. The inescapable infiltration factor in any type of structure will exceed the volume of external air required for adequate oxygen replenishment. The normal infiltration will generally be more than one air change in an hour.

All ventilation is fundamentally dilution, except for the smallest amount of air volume necessary to neutralize infiltration and to provide oxygen. This process of ventilation acts to change polluted indoor air with comparatively uncontaminated air, at a rate that will continuously maintain the desired level of air impurities. The standards of ventilation depend upon the type, and amount of internal air pollution. Inhabitants, substances, items and processes all are factors for air pollution. The larger the internal creation of impurities, the higher will be the rate of ventilation required. Ventilation can be obtained by providing outside air to the space, or by purifying and recirculating space air, at an appropriate rate. The ventilation necessary for a specific space, with predetermined occupancy and activity, has been established with experience and studies. Requirements of ventilation are mentioned as cubic feet per minute (cfm) or liters per second of fresh air per occupant. If smell and gas extraction equipment is used, the outside air requirement will be reduced.