Movement of Air

Natural ventilation is caused by naturally produced pressure differences due to wind outside the building and/or differences in air temperature inside the building. Natural ventilation is achieved by infiltration and/ or by allowing air to flow in and out of the building by opening windows and doors. The term "infiltration" is used to describe the flow of outdoor air through leakage paths in the building envelope.

Air movement through buildings result from the difference in pressure indoors and outdoors , which may be created either by natural forces (wind induced pressure difference and stack effect e.g. pressure difference induced by temperature gradients between the inside and outside of the building) or mechanical power (fan). Air flow patterns are the result of differences in the pressure distribution around and within the building. Air moves from high pressure regions to low pressure ones.

Forced air movement inside the building must be used when the natural driving forces are inadequate or when an unacceptable noise or security problem is generated by opening the windows. Box, oscillating and ceiling fans could increase the interior air velocities and convection exchange improving the sense of comfort. Increased air movement in a room may create comfortable conditions with 2°C increase in allowable space temperature.

Successful design of naturally ventilated building requires a good understanding of the air flow patterns around it and the effect of the neighboring buildings. The objective is to ventilate the largest possible part of the indoor space. Fulfillment of this objective depends on window location, interior design and wind characteristics.

New buildings generally allow for 0.2 to 0.5 air changes per hour (ACH) by infiltration, while with the windows wide open during summer, it is possible to achieve 15- 20 ACH. Even larger air changes, around 30 ACH, can be achieved by natural means, but this requires a large number of window openings carefully placed within the space.

Wind induced pressure differences

Positive pressure is created on the building sides that face the wind (windward sides) whereas suction regions are formed on the opposite sides (leeward sides) and on the side walls. This results in negative pressure inside the building, which is sufficient to introduce large flows through the building openings. In a general case, n airflow of air is induced on the windward side and an outflow on the leeward side. Airflow through an external opening is mainly attributed to a wind induced pressure difference across it.

The figure above illustrates the wind flow pattern distribution around a building with no openings. As the wind flows past the building, a positive pressure is created on the windward fašade. The wind is diverted and a negative pressure is created along the side walls due to the high speed of the flow along them. A large, slow- moving eddy on the leeward facade produces a smaller suction.

Stack Effect

Stack effect relies on thermal forces set up by the density difference (caused by temperature differences) between the indoor and the outdoor air. It can occur simply through an open window (when the air is still); the warmer and lighter indoor air will flow out at the top, and the cooler, denser outdoor air will flow in at the bottom.

Stack effect can be used as a special provision in ventilation shafts. Higher the shaft, larger cross sectional area, greater the T, greater the motive force, the more air will be moved.

F= P * A Stack pressure, Ps (N/m²)= 0.042 (N/m3degC) * h(m) * T (deg C)

This effect is utilized in multistory apartments for ventilation of the shafts of toilets, bathrooms, kitchen, etc. Stack effect occurs in tall buildings, particularly at places with vertical passages such as stairwells, elevators or shafts. The stack pressure decreases with height.

Air flow through large openings is usually bi- directional. In the general case, cold air flows in at the lower part of the opening, while warmer air flows out from the upper part. In general, the air velocity decrease and becomes zero at a height of hn from the bottom of the opening. At that level, the so called neutral level, the pressure difference across the opening is zero.

stack stack stack stack