![]() ![]() The airflow direction may be visualized by smoke. In simple terms, the ventilation flow rate can be measured by measuring how quickly injected tracer gas is decayed in a room, or by measuring the air velocity through ventilation openings or air ducts, as well as the flow area. Ventilation effectiveness can be evaluated by either measurement or simulation ( Etheridge & Sandberg, 1996). The air exchange efficiency for displacement ventilation is somewhere in between, but for short-circuiting the air exchange efficiency is less than 50%. For piston-type ventilation, the air exchange efficiency is 100%, while for fully mixing ventilation the air exchange efficiency is 50%. ![]() The air exchange efficiency can be calculated from the air change per hour and the room mean age of air ( Etheridge & Sandberg, 1996). The age of air can be measured using tracer gas techniques ( Etheridge & Sandberg, 1996). Engineers define the local mean age of air as the average time that the air takes to arrive at the point it first enters the room, and the room mean age of air as the average of the age of air at all points in the room ( Etheridge & Sandberg, 1996). The air exchange efficiency indicates how efficiently the fresh air is being distributed in the room, while the ventilation effectiveness indicates how efficiently the airborne pollutant is being removed from the room. Two overall performance indices are often used. ![]() How efficient is the system in removing the airborne pollutants from each location in the room? If the environment in the room causes thermal discomfort spot cooling or heating systems and ceiling fans may be added.Īnother possibility is the installation of whirlybirds (whirligigs or wind turbines) that do not require electricity and provide a roof-exhaust system increasing airflow in a building (see Figure I.2 in Annex I). ![]() Problems associated with the use of exhaust fans include installation difficulties (especially for large fans), noise (particularly from high-power fans), increased or decreased temperature in the room and the requirement for non-stop electricity supply. The size and number of exhaust fans depends on the targeted ventilation rate, and must be measured and tested before use. The fans should be installed where room air can be exhausted directly to the outdoor environment through either a wall or the roof. However, this simple type of hybrid (mixed-mode) ventilation needs to be used with care. When natural ventilation alone is not suitable, exhaust fans (with adequate pre-testing and planning) can be installed to increase ventilation rates in rooms housing patients with airborne infection. It uses mechanical ventilation when the natural ventilation flow rate is too low ( Heiselberg &Bjørn, 2002). Hybrid (mixed-mode) ventilation relies on natural driving forces to provide the desired (design) flow rate. What is hybrid or mixed-mode ventilation? In an airborne precaution room for infection control, a minimum negative pressure of 2.5 Pa is often maintained relative to the corridor ( CDC, 2003).Ģ.1.3. For example, a slight negative room pressure is achieved by exhausting 10% more air than the supply in a cold climate to minimize the possibility of interstitial condensation. The room pressure may be maintained at either slightly positive or negative pressure, which is achieved by using slightly unequal supply or exhaust ventilation rates. A balanced mechanical ventilation system refers to the system where air supplies and exhausts have been tested and adjusted to meet design specifications. In a negative pressure system, the room is in negative pressure, and the room air is compensated by “sucking” air from outside. In a positive pressure system, the room is in positive pressure and the room air is leaked out through envelope leakages or other openings. For a room with locally generated pollutants, such as a bathroom, toilet or kitchen, the negative pressure system is often used. Conversely, in cold climates, exfiltration needs to be prevented to reduce interstitial condensation, and negative pressure ventilation is used. In these cases, a positive pressure mechanical ventilation system is often used. For example, in warm and humid climates, infiltration may need to be minimized or prevented to reduce interstitial condensation (which occurs when warm, moist air from inside a building penetrates a wall, roof or floor and meets a cold surface). The type of mechanical ventilation used depends on climate. Fans can either be installed directly in windows or walls, or installed in air ducts for supplying air into, or exhausting air from, a room. Mechanical fans drive mechanical ventilation. ![]()
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