chuka_lis (chuka_lis) wrote,

Вентиляция и коронавирус в классе

Препринт, в котором довольно подробно (и наглядно) разобраны различные модели поведения воздушных потоков, содержащих вирус, в обычном школьном классе.
Авторы рассматривают дистанцию в 6 футов, в то время как СДС ее уже сократило до 3х.
Однако, и 6 футов могут не спасти- если в классе будет больной, даже если открывать окна или двери и "вытягивать" воздух. Часть людей неизбежно окажется по дороге воздушного потока, и воздух будет содержать вирус. Потому что выдыхаемый воздух успешно перемешивается с окружающим и распространяется по комнате. Помочь может то, что все будут в правильно одетых масках, и если будет кондиционер с вытяжкой вверх- чтоб выдыхаемый воздух, не успевая отойти далеко от выдыхающего, поднимался к потолку и фильтровался. Так что надо бы вытяжки,  фильтры и высокий уровень воздухообмена.
Конечно, это касается и вообще любых патогенов, переносимых воздушно-капельным путем.
Although current industry guidelines to control the spread of SARS-CoV-2 (COVID-19) have adopted a six-foot (~1.8m) spacing between individuals indoors, recent evidence suggests that longer range spread is also responsible for infections in public spaces. The vehicle for long-range spread is smaller droplets or particles, termed bio-aerosols, or aerosols for short, which have a large surface area to volume ratio such that aerodynamic drag is much larger than gravity forces. The aerosols remain suspended in air for extended time periods and they essentially move with air currents. Prediction of the danger to occupants in a closed room when exposed to an infected individual requires knowledge of the period of exposure and the concentration level of aerosols in the breathing zone of an occupant. To obtain an estimate of the concentration level, a common assumption is well-mixed conditions within an interior space. This is obtained from a mass balance between the level of aerosol produced by an infected individual along with the airflow rate into and out of the entire space. In this work, we use computational fluid dynamics, verified by experimental results, to explore the aerosol concentration distribution in a typical classroom for several common conditions and compare these results to the well-mixed assumption.
COVID-19 is an aerosolized virus. With such small particle sizes, the established six-foot (~1.8 m) spacing guidelines and heightened air change rates are an incomplete picture. The results from this paper help reinforce the concept that exhaled particles of modest velocity (such as a person with a maskor face shield) are entrained in the thermal plume of the body heat and rise toward the ceiling. However, higher horizontal velocities (such as a person without a mask or face shield) can escape the thermal plume and can linger within the breathing plane of others. These thermal plumes from people and heated objects continue to have an impact on air flow within the room, along with influences from cold or heated surfaces, mechanical HVAC systems, and outside air ventilation. Two HVAC systems have been examined to show that, even with industry standard air change rates, the well-mixed assumption as applied to a conventional HVAC system made up of ceiling diffusers may underpredict the contaminant levels inhaled by the occupants by half. Even with the window open, the perceived safety of the well-mixed assumption is too conservative and underpredicts the potential exposure by 60%. Aside from air change rate changes, other strategies can be employed to limit potential exposure of non-infected individuals. For situations where a cold outside surface (i.e. an uninsulated window) may exist, contaminant levels can be reduced by an order of 20% by the addition of a convective heater below the window, the use of curtains or blinds, or replacement of the windows to improve their insulative value. Care should be taken as a similar case is presented by the plastic enclosures used by many eateries where cold outside surfaces allow for recirculation of contaminants.While open windows may give the impression of ventilation, they also create their own problems as the influx of air near the breathing plane carries contaminants horizontally from an infected person near the window to other occupants. Staggering seating arrangements and redirecting window air to the ground may remove this issue as it allows for the buoyant plumes to redevelop and bring contaminants out of the breathing plane. Regarding flushing out contaminants in the absence of any occupants, the well-mixed assumption is valid for this application and a generalized air change model may be used.Future work should include other HVAC systems such as displacement ventilation for improvements. For properly operated displacement ventilation systems with low speed floor inlets and ceiling exhaust, concentration at breathing levels should be below well mixed values. However, if occupants have loosely fitted masks, the performance could degrade if breathing exhaust escapes the thermal plumes around the individual. Cold windows could also degrade performance Based on this work, air change rate increases above the current recommendations are necessary to maintain exposure limits equivalent to the current well-mixed assumption for all occupants. Such an increase in air change rate will require a commensurate increase in energy requirements, to which local and zonal air filtration may be helpful in reducing contaminant levels near the source.
Tags: коронавирус, окружающая среда, статьи, эпидемия

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