ventilation(redirected from Positive-Pressure Mechanical Ventilation)
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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 fanfan,
device for agitating air or gases or moving them from one location to another. Mechanical fans with revolving blades are used for ventilation, in manufacturing, in winnowing grain, to remove dust, cuttings, or other waste, or to provide draft for a fire.
..... Click the link for more information. 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 conditioningair conditioning,
mechanical process for controlling the humidity, temperature, cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and regulated to maintain the temperature-humidity ratio that is most comfortable and healthful.
..... Click the link for more information. . 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.
See F. Porges, Handbook of Heating, Ventilating, and Air Conditioning (1982).
the controlled exchange of air in an area; also, the equipment and installations creating such an exchange. Ventilation is designed to provide the necessary purity, temperature, humidity, and circulation of air. These requirements are determined by health standards: the presence of noxious substances in the air (gases, vapors, or dust) is restricted to permissible concentrations (those which are harmless to human health); and the temperature, humidity, and circulation of the air are determined in relation to the conditions required for optimum human comfort. For many industrial areas (shops for the assembly of precision instruments, radio-electronics equipment, and so on), air purity, temperature, and humidity are also determined by the special requirements of the technological processes involved. In a number of cases (museum storage facilities, archives, or warehouses) the air temperature and humidity in an area must correspond to the conditions required for the best preservation of objects and materials, equipment, or construction components contained in it.
In industrial areas, the principal source of emission of noxious substances, heat, and moisture is the technological process taking place within the area. In residential dwellings, these emissions occur primarily in lavatories and kitchens, especially during the use of gas ranges. The activity of the human organism is also accompanied by the emission of noxious substances (carbon dioxide), odors, heat, and moisture. The preparation of food, bathing, and the washing of dishes and clothing raise the temperature and humidity of room air and increase its dust content and bacterial contamination (particularly if sick persons are present). All of these substances may be removed by means of ventilation. In cases of pollution of the air basin (for example, by exhaust fumes from automobile traffic or industrial discharges), noxious substances may enter room areas with the outside air.
Solar radiation (chiefly through windows) and artificial lighting are sources of additional heat accumulation. The reduction of emissions of noxious substances and of excess heat and moisture is an important factor in improving the condition of the air in industrial environments, and it also creates more favorable conditions for the operation of ventilation. Nontoxic or low-toxicity materials are used for these purposes in industrial processes, production equipment and distribution supply lines are hermetically sealed, and surfaces that give off heat are covered with thermal insulation. The reduction of excess heat is achieved by the use of window-mounted sun protection devices, heat-absorbing glass, the use of forced heat-dissipation lamps for lighting, and other methods. Current health standards in the USSR specify that the air of living spaces must be as follows (during the winter season): temperature, 18°–22° C; relative humidity, 40-60 percent; circulation, 0.1-0.2 m/sec; and maximum C02 content, 0.1 percent. Noxious gas contaminants are not permitted in the air.
A distinction is made between forced, exhaust, suction-and-exhaust, general-exchange, local, natural, and mechanical types of ventilation.
Forced ventilation. Forced ventilation provides only a clean air supply to an area; the removal of air from the space occurs primarily through leakage in the surrounding structures and through open doors owing to the generation of excess pressure.
Exhaust ventilation. Exhaust ventilation is designed to remove air from a ventilated area and to create a negative pressure within it so that outside air and air from adjoining spaces can enter the area through doors and leaks in enclosures.
Suction-and-exhaust ventilation. Suction-and-exhaust ventilation provides simultaneous air supply and systematic air removal in an area; with this method, a positive or negative pressure can exist in the area depending on the ratio between the quantities of air being supplied and exhausted. The positive and negative air pressures in adjoining spaces prevent the infiltration of contaminated air from one space to another (for example, from a smoking room to a lobby, from a kitchen to a dining room, or from a galvanizing section to an assembly shop). For this method to be effective, it is necessary that the positive or negative pressure in the ventilated spaces be created and maintained by a continuous, intense air exchange. The indicator of the rate of air exchange, which can take place without ventilating equipment (through enclosure leaks or under the influence of wind or a difference between inside and outside air temperatures), is the frequency of air exchange—that is, the ratio of the volume of air entering or leaving over a period of 1 hour to the volume of the air space.
General-exchange ventilation. With general-exchange ventilation, which is used in all residential and public buildings, noxious substances released in an air space are diluted to permissible concentrations by supplying clean air to the area; excess heat and moisture are dissipated by the air supply, which in this case must be of lower temperature and humidity.
Local forced ventilation. Local forced ventilation creates the necessary conditions for the air environment in confined areas of industrial installations by means of spot cooling units, air oases, and such. With local exhaust ventilation, noxious impurities are collected and removed from their points of origin by means of local exhaust systems such as laboratory fume hoods, furnace hoods, or side-slot hood exhausts. Processing equipment giving off noxious substances is equipped with built-in local exhaust units and housings, which are an integral part of the unit.
Natural ventilation. With natural ventilation, air enters and leaves an area owing to temperature differences (and, consequently, density differences between outside and inside air) and wind action. Nonsystematic natural ventilation occurs by infiltration and exfiltration of air through leaks in building enclosures, windows, doors, and such, and systematic natural ventilation is accomplished by supplying and exhausting air circulated through air ducts and adjustable vent openings in walls, windows, and skylights.
Mechanical ventilation. Mechanical ventilation (Figure 1) is performed primarily by electrically driven fans or blowers. In the supply systems, outside air is prepared by equipment that is usually located in an air-supply chamber near the intake. The air from the chamber is distributed through ducts to the areas to be ventilated and is then supplied to the spaces through louvers, perforated ceilings, decorative grilles, and other types of supply ventilation outlets. In general-exchange ventilation, air is removed through a network of ducts with grilles into an exhaust duct system, and in local ventilation it is removed through local exhausts, which are usually connected to separate exhaust systems. Air that is contaminated by particularly toxic substances or local exhausts must be cleaned. Air-cleaning equipment, such as dust collectors or filters, is installed for this purpose directly before the discharge of the contaminated air into the atmosphere. Equipment for exhaust ventilating systems is located in exhaust chambers; exposed installations are possible at industrial plants under favorable climatic conditions. Ventilating equipment may be installed directly in the area to be ventilated if the noise from the ventilating equipment does not significantly interfere with the work to be performed in a shop, warehouse, or other industrial area. Forced, heating, and exhaust ventilating units mounted on floors, columns, or walls are also used in these instances.
Exhaust ventilation with natural induction from kitchens and lavatories and with the intake of outside air through adjustable window vents, fortochki (small hinged panes), leaks in walls, is most widely used in residential dwellings. Mechanically induced suction-and-exhaust ventilation is widely used in modern industrial facilities and public buildings. In a number of cases, forced ventilation is combined with hot-air heating, for which the system is furnished with a more powerful air preheater that heats the air supply to a temperature higher than that in the area. In this way, the excess heat carried along by the forced air supply makes up for the heat loss of the area. Air conditioning is used if the air parameters in an area must be maintained constantly in accordance with strictly specified conditions (quality requirements). In this method, the prescribed air parameters are maintained in the areas by means of automatically controlled air-treatment processes within the air-conditioners that are regulated depending on the condition of the outside air and the loss of heat and moisture in the interior air spaces. The development and expansion of air-conditioning applications and improvements in the methods of its distribution to air spaces contribute to a further increase in ventilation efficiency.
REFERENCESLivehak, I. F. Ventiliatsiia mnogoetazhnykh zhilykh domov. Moscow, 1951.
Kamenev, P. N. Otoplenie i ventiliatsiia,2nd ed., part 2. Moscow, 1964.
Baturin, V. V. Osnovy promyshlennoi ventiliatsii,3rd ed. [Moscow] 1965.
Retter, E. I., and S. I. Strizhenov. Aerodinamika zdanii. Moscow, 1968.
Marzeev, A. N., and V. M. Zhabotinskii. Kommunal’naia gigiena, 3rd ed. Moscow, 1968.
I. F. LIVCHAK
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.
ii. Biologically, the aeration of the lungs and blood by breathing. The inhalation and exhalation of air in the process of respiration.