| Natural ventilation depends on winds outside and | | | | precipitators. |
| convection currents inside a building. Winds raise | | | | Deep mines, underwater tunnels, and other |
| air pressure slightly on the windward side of a | | | | subterranean and submarine environments require |
| building and lower it slightly on the lee side. The | | | | elaborate mechanically operated systems for |
| pressure difference promotes circulation into the | | | | maintaining the air supply in a healthful condition. |
| building on the windward side and out of it on the | | | | The lives of those working in, or traveling through, |
| lee side. Convection currents are caused by the | | | | such areas depend upon a constant supply of |
| sinking of colder and therefore heavier air, which | | | | fresh air; not only must the systems used be |
| displaces the warmer air. A building may have a | | | | highly efficient, but there should be provision for |
| roof ventilator to allow the rising warm air to | | | | emergencies in case of failure of the apparatus in |
| escape. If there is an opening to the outside at | | | | operation. An outgrowth of studies of problems |
| the bottom of the building, fresh, cool air will be | | | | of ventilation is the development of methods of |
| drawn in. | | | | air conditioning heat pump is a reversible device |
| Proper ventilation requires that there be a | | | | that does mechanical work to extract heat from |
| movement or circulation of the air within the | | | | a cooler place and deliver heat to a warmer place. |
| space and that the temperature and humidity be | | | | The heat delivered to the warmer place is, |
| maintained within a range that allows adequate | | | | approximately, the sum of the original heat and |
| evaporation of perspiration from the skin. It was | | | | the work done. |
| formerly believed that the discomfort, headache, | | | | Such systems, unlike ordinary methods of |
| and lethargy commonly associated with poor | | | | ventilation, are independent of outdoor |
| ventilation were caused entirely by the increase in | | | | atmospheric conditions and can, therefore, |
| the amount of carbon dioxide and the decrease in | | | | maintain the indoor atmosphere at the most |
| the oxygen content of the air. There is evidence | | | | healthful temperature and humidity and can free |
| to show, however, that the deleterious effects | | | | the air of dust and other undesirable materials. |
| result largely from interference with the | | | | They accomplish this, however, at a considerable |
| heat-regulating mechanism of the body. Lack of | | | | cost in energy. |
| air currents and the increase in relative humidity | | | | Roof ventilators are another option. They work |
| and temperature (especially noticeable in crowded, | | | | in both summer and winter. In summer, roof |
| poorly ventilated places) prevent normal | | | | ventilators reduce the build up of heat in the |
| evaporation of perspiration and loss of heat from | | | | ceiling spaces, reducing the heat load on your |
| the surface of the skin. | | | | insulation, meaning that the insulation works more |
| A simple roof ventilator is essentially an opening in | | | | effectively. In turn, you will save on your |
| the roof with a cover to keep out rain and to | | | | electricity bills as the air-conditioning systems will |
| prevent winds from interfering with its functioning. | | | | not have to work as hard. In winter, it reduces |
| Natural convection is an appreciable aid to | | | | the amount of condensation that forms when |
| ventilation in a large building only if it contains | | | | warm, moist air from exhaust fans meets cold |
| sources of large amounts of heat. A further | | | | surfaces. Roof ventilators will also reduce the |
| useful adjunct is a fan fan, device for agitating air | | | | amount of corrosion and timber that occurs |
| or gases or moving them from one location to | | | | because of condensation. Roof ventilators |
| another. Mechanical fans with revolving blades are | | | | should be installed where there is sarking |
| used for ventilation, in manufacturing, in winnowing | | | | underneath tiled roofs, when metal deck roofing is |
| grain, to remove dust, cuttings, or other waste, | | | | used, and when there is evidence of condensation |
| or to provide draft for a fire. The addition of | | | | in the roof spaces or ceilings. |
| distribution ducts to the fan and a system for | | | | A reliable ventilation system that can operate |
| forcing air into the building provides greater | | | | during a fire is both vital and essential. It prevents |
| efficiency. Outlets are designed to attain | | | | smoke logging, allows people to escape quickly, |
| maximum mixing of air and to move large | | | | minimizes damages to property and contents and |
| amounts of air at low velocity so that | | | | enables firemen to attack the fire without |
| temperature layers are eliminated. Factories have | | | | hindrance. Such power roof ventilators have the |
| special suction hoods and enclosures to draw | | | | added advantage of providing normal day to day |
| away localized dust, fumes, and heat. Incoming air | | | | ventilation and in many cases can also be fitted |
| may be cleaned of dust by filters or electrostatic | | | | with duct work for use in multi storey buildings. |