Ventilation (redirected from spontaneous ventilation)

ventilation, process of supplying fresh air to an enclosed space and removing from it air contaminated by odors, gases, or smoke.

Proper ventilation requires also that there be a movement or circulation of the air within the space and that the temperature and humidity be maintained within a range that allows adequate evaporation of perspiration from the skin. It was formerly believed that the discomfort, headache, and lethargy commonly associated with poor ventilation were caused entirely by the increase in the amount of carbon dioxide and the decrease in the oxygen content of the air. There is evidence to show, however, that the deleterious effects result largely from interference with the heat-regulating mechanism of the body. Lack of air currents and the increase in relative humidity and temperature (especially noticeable in crowded, poorly ventilated places) prevent normal evaporation of perspiration and loss of heat from the surface of the skin.

Natural ventilation depends on winds outside and convection currents inside a building. Winds raise air pressure slightly on the windward side of a building and lower it slightly on the lee side. The pressure difference promotes circulation into the building on the windward side and out of it on the lee side. Convection currents are caused by the sinking of colder and therefore heavier air, which displaces the warmer air. A building may have a roof ventilator to allow the rising warm air to escape. If there is an opening to the outside at the bottom of the building, fresh, cool air will be drawn in.

A simple roof ventilator is essentially an opening in the roof with a cover to keep out rain and to prevent winds from interfering with its functioning. Natural convection is an appreciable aid to ventilation in a large building only if it contains sources of large amounts of heat. A further useful adjunct is a fan in the roof ventilator. The addition of distribution ducts to the fan and a system for forcing air into the building provides greater efficiency. Outlets are designed to attain maximum mixing of air and to move large amounts of air at low velocity so that temperature layers are eliminated. Factories have special suction hoods and enclosures to draw away localized dust, fumes, and heat. Incoming air may be cleaned of dust by filters or electrostatic precipitators.

Deep mines, underwater tunnels, and other subterranean and submarine environments require elaborate mechanically operated systems for maintaining the air supply in a healthful condition. The lives of those working in, or traveling through, such areas depend upon a constant supply of fresh air; not only must the systems used be highly efficient, but there should be provision for emergencies in case of failure of the apparatus in operation. An outgrowth of studies of problems of ventilation is the development of methods of air conditioning. Such systems, unlike ordinary methods of ventilation, are independent of outdoor atmospheric conditions and can, therefore, maintain the indoor atmosphere at the most healthful temperature and humidity and can free the air of dust and other undesirable materials. They accomplish this, however, at a considerable cost in energy.

Bibliography

See F. Porges, Handbook of Heating, Ventilating, and Air Conditioning (1982).

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Ventilation

The supplying of air motion in a space by circulation or by moving air through the space. Ventilation may be produced by any combination of natural or mechanical supply and exhaust. Such systems may include partial treatment such as heating, humidity control, filtering or purification, and, in some cases, evaporative cooling. More complete treatment of the air is generally called air conditioning. See Air conditioning

Natural ventilation may be provided by wind force, convection, or a combination of the two. Although largely supplanted by mechanical ventilation and air conditioning, natural ventilation still is widely used in homes, schools, and commercial and industrial buildings.

Mechanical supply ventilation may be of the central type consisting of a central fan system with distributing ducts serving a large space or a number of spaces, or of the unitary type with little or no ductwork, serving a single space or a portion of large space. Outside air connections are generally provided for all ducted systems. Outside air is needed in controlled quantities to remove odors and to replace air exhausted from the various building spaces and equipment.

Exhaust ventilation is required to remove odors, fumes, dust, and heat from an enclosed occupied space. Such exhaust may be of the natural variety or may be mechanical by means of roof or wall exhaust fans or mechanical exhaust systems. The mechanical systems may have minimal ductwork or none at all, or may be provided with extensive ductwork which is used to collect localized hot air, gases, fumes, or dust from process operations. Where it is possible to do so, the process operations are enclosed or hooded to provide maximum collection efficiency with the minimum requirement of exhaust air.