Air Mixing Upper-room UVGI systems rely on air movement between the lower portion of the room where droplet nuclei are generated and the upper irradiated portion of the room. Once in the upper portion, droplet nuclei containing M. tuberculosis may be exposed to a sufficient dose of UVGI to kill or inactivate them. The efficacy of upper-room UVGI systems is better in well-mixed rooms than in poorly mixed rooms [First et al. 2007; Xu et al. 2000].
Although 100% vertical air mixing between the lower and upper portions of a room is probably impossible to achieve, in most rooms the air is sufficiently well mixed [Beggs and Sleigh 2002]. A number of factors including the following may affect the vertical mixing of air between the upper irradiated portion of the room and lower occupied portion of the room:
• Temperature differential between the supply air and room air.
• Location of the supply diffusers and exhaust grills.
• Mechanical ventilation rate.
• Location of furniture and movement of people.
• Changing environmental conditions
• Use of fans.
In a time of pandemic every one needs to place emotional and technical trust into apparatus that they believe protects them, but many companies embellish their technical data reports grossly misrepresenting capability in real world applications. The data on air purifiers is always quoted on empty room statistics and sold as a soft breeze machine that extracts all particles and pathogens from all corners of a room. This just isn't the case and many £1000s are being wasted on placebo effect of hearing the fans and assuming the best results. The floor standing unit in this simulation is a branded product and the input data is based on their own published technical information and is running at the slowest speed of 200cfm, but at top speed 350cfm the UVC dosage would be highly dubious and the noise intolerable. The red marker particles show the stagnant air not being drawn into circulation or purification thus potentially toxic or carrying viruses and other pathogens. The sales pitch is always positioning them in the middle of empty rooms that is simply not viable.
Effectiveness of air cleaning devices depends on multiple parameters including the underlying technology, the design of the device, the in-room location of the device, the environment that it is used in and the maintenance of the device. The performance of most devices is based on data measured in idealised controlled environments, and is likely to be different and often lower in a real-world setting (high confidence). Caution should be used when considering idealised performance data stated by a manufacturer.
This CFD show 40,000 marker particles discharged from an AP emitter to then trace the air circulation. The blue particles are moving / velocity and the red particles are the stagnant, particles that cannot be recirculated and DO NOT re enter the air purifier thus holding contaminants and pathogens in the space.
In this case the AP unit like many, have air intake at floor level and emit out of the top.
This room is 72 Sqm but the unit is sold for functionality in 120 Sqm and the branded unit is running at 200 cfm.
Coughs and sneezes act like 'mini atomic bombs' and often exceed two metres, study warns.
Researchers from Loughborough University have warned that coughs and sneezes act like ‘mini atomic bombs’ and regularly exceed two metres.
In their study, the team created a mathematical model which showed that some droplets can fly more than 3.5 metres due to a phenomenon known as buoyant vortex - the turbulent motion of hot, dense air that we eject together with the droplets when we cough or sneeze.
Dr Emiliano Renzi, who led the study, said: “In the majority of our analyses, the predictions made by our model suggest that the largest droplets consistently exceed the horizontal ranges of two metres from the source before settling to the ground.
“In some cases, the droplets are propelled in excess of 3.5 metres by the buoyant vortex, which acts like a mini atomic bomb. (Mirror News paper Shivali Best 09/12/20)