NATURAL VENTILATION

Natural ventilation, or gravity ventilation, uses the natural forces of wind, stack effect, and breathing of structures caused by the interiorexterior temperature difference to induce air circulation and removal. Generally, air enters through openings at or near the floor level in a building and escapes through openings high in the walls or ventilators on the roof.

Natural ventilation is used only where the necessary quantity of ventilation can be induced by natural forces. Applications that require a continuous supply of outdoor air for human comfort, or the safe use of space (or process), should not be designed for natural ventilation. In such cases, natural ventilation is not reliable because of wide variations in the natural forces, such as wind velocity and direction and the insideoutside temperature difference.

For an installation using natural ventilation, you should consider the location and control of ventilation openings. Locate the air inlet openings on the side of the building facing directly into the prevailing winds. Locate the air outlets where prevailing winds movements would create low-pressure areas; that is, on the side directly opposite the prevailing wind direction. Outlets may also be placed on a roof in the form of individual gravity ventilators or ridge ventilators. Calculate the ventilation rate due to wind velocity and the stack effect as detailed in criteria established by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE). When natural ventilation is provided for temperature control, you should provide a means for closing the openings during the heating season.

The use of gravity ventilators is another method. A roof-mounted gravity ventilator may be a stationary, a pivoting winddirectional, or a rotating-turbine type of ventilator. You should select gravity ventilators based on the rating tables for the mounting height involved and a wind velocity of 4 miles per hour.

Natural ventilation has limited uses. In general, natural ventilation is inadequate for the following examples:

Offices having an open window area less than 5 percent of the floor area

Offices over 24 feet deep and without cross ventilation

Offices having cross ventilation but having occupied space more than 35 feet from a window or an air inlet

Dining rooms having a window area less than 6 percent of the floor area

In using natural ventilation, you should consider local building and safety codes and the minimum requirements of the Occupational Safety and Health Act (OSHA), part 1910.

MECHANICAL VENTILATION

Mechanical ventilation uses mechanical forces to induce air circulation within buildings or spaces. Air movement is created by fans or by fans combined with a supply air and/or exhaust air duct system.

You should provide mechanical ventilation equipment when the necessary quantity of outside air cannot be supplied continuously by natural forces. The quantity of air supplied should be kept to an acceptable minimum. You should install mechanical ventilation equipment in the following cases:

For a supply of outside air and the removal of bad air or air contaminated by smoke, body odors, and so forth, in areas having a high occupancy level (auditoriums, assembly halls, and cafeterias).

For processes giving off noxious or hazardous fumes, dust, or vapor, resulting in unsafe or unhygienic conditions (paint spray booths, electroplating plants, welding booths, and other similar applications).

For limited comfort of operators as in laundries, projection booths, and kitchens.

For spaces containing fumes and vapor with specific gravity higher than air, such as garages and some refrigeration rooms. In these cases, provide exhaust intakes at floor level.

For electronic or electric equipment installed in confined spaces where the operating temperatures of the equipment may exceed the safe limit.

For spaces having explosive vapors or dust, use explosionproof ventilation equipment regardless of the concentration of explosive substances.

For odor removal in bathrooms.

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