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Well what a year 2020 was, for all the wrong reasons. They do say though, that every cloud has a silver lining, out of darkness comes light, and so on. So perhaps the horrendous and rampant transmission of the Covid-19 virus could be the reality check that gets us all thinking about air quality and the effect that so many contaminants, not just viruses, have on our health and wellbeing. The benefits of good indoor air quality have been well documented for many years, whether that is benefits to our children’s education and learning1, or to the productivity and increased profitability of a company’s employees2, the importance of maintaining a good indoor climate is something that should have been on our radar long before Covid-19 forced us to consider it.
Whilst there are many differing opinions and still much unknown about the transmission of this particular virus and the best methods of preventing it, there are a number of areas where there seems to be a consensus of opinion and agreement, not just in relation to virus transmission, but to optimum indoor climate generally.
Ventilation. It now seems to be accepted without question that some of the best protection we can use is to ventilate. Historic focus on indoor climate has largely emphasised temperature control, which is of course important in terms of comfort, but ventilation is paramount in respect to our health and wellbeing – the higher the rate of ventilation, to give maximum dilution of contaminants, the better. This may suggest that ‘all-air’ systems are answer, but these will often be impractical or uneconomical in terms of the plant and services size necessary to deliver sufficient levels of cooling to the building. In such cases where secondary cooling systems will be required, there are choices; those that rely on a primary air supply to function would by default always ensure the designed ventilation rate is delivered, more so than recirculatory secondary cooling systems that can be run independently from the main ventilation system and so could be run without adequate ventilation being provided.
We need to be careful when increasing ventilation rates though, as it could come at a high cost if we simply increase our historic design figures by a given percentage. Larger ductwork and other services, leading to higher capital cost, less usable or lettable space, and of course higher energy usage. For these reasons Demand Controlled Ventilation (DCV) would offer a sensible solution; provide ventilation when and where it is necessary, based on the prevailing demand of occupancy or air quality, but don’t deliver air unnecessarily where there is little or no demand. As an office space typically has only a 40% occupancy3, a rented hotel room occupied 40% of the time4, and school classrooms for example only occupied 31% of the time5, this scenario would afford some considerable energy savings – with a change in duct pressure being the square of the reduction in air volume, and fan power being the square again, even operating at 70% of full air volume equates to 34.3% of the fan power, and at 31% of full air volume less than 3% of the fan power…food for thought…
We also need to be sure that the outdoor air we are bringing in to the building is not simply replacing one contaminant with another, so suitable levels of filtration to guard against pollutants such as pollen and NO2 from traffic being brought in from outdoors are a prerequisite. Secondary filtration inside the building on recirculatory cooling or heating devices is a more complicated situation, as these can have coarse filters that practically do not filter smaller particles but may still collect potentially contaminated particles which may then be released when fans start to operate6
As well as discomfort in terms of dry eyes and skin, humidity also seems to have some bearing on the spread of virus, too low a humidity can exacerbate its propensity to spread, so a relative humidity of between 40-60% seems to be accepted as optimal7
There are all these and many further considerations – there have been studies citing increased air velocities being responsible for intensified spread of viruses8, so effective ventilation without draught seems important – as well as ‘future proofing’ issues such as flexibility of product to maintain all our good intentions if/when there are changes of use or tenant in the space, accurate response to demand and feedback of information to the building user, all of which of course relies on constant measurement of what is actually happening within the occupied space
So what has any of this got to do with Active Chilled Beams, the headline of this article?
Well, active chilled beams rely on a primary outdoor air supply to function, so will always ensure design levels of ventilation whilst providing excellent thermal comfort. Active chilled beams are intended to operate with ‘dry coils’, i.e. using an elevated chilled water temperature above the dewpoint temperature of the room, so no condensation forms on the coil. As well as this giving the benefit of not having standing water on the coil or in a condensate tray or drainage system (not required with a dry coil system), this also means that active chilled beams do not need a secondary filter to protect the cooling coil, so therefore do not need any special consideration for ‘fan shock’ that may be necessary with other secondary cooling systems. The operation of active chilled beams above room dewpoint means that humidity control is important of course, to maintain a separation between room dew point and chilled water flow temperature. But this does not have to mean low levels of humidity; a typical active chilled beam operating with a chilled water flow temperature of 14degC in a 22-24degC space temperature equates to a requirement for 50-57% relative humidity being maintained, absolute ideal territory. And this conscious control of humidity gives us just that, conscious control of humidity, rather than alternative technologies operating with much lower coil temperatures that, by default, condense moisture from the room air and therefore reduce the humidity to uncontrolled levels. So active chilled beams inherently provide a great platform for good indoor air quality, as well as the known energy benefits of low system fan power from not having a secondary fan, and high chiller efficiencies gained and from utilising higher chilled water temperatures, which can be further enhanced with free cooling which becomes a viable consideration at these elevated chilled water temperatures. With the latest families of active chilled beams including modular units offering greater flexibility then in the past, inbuilt intelligence for occupancy and air quality monitoring, pressure independent operation entirely suited to DCV systems, and even wireless network communication, it really seems that the vision 2020 has given us is that of a bright future for active chilled beams.
References:
1 Wargocki, Pawel & Wyon, David. (2013). Providing better thermal and air quality conditions in school classrooms would be cost-effective. Building and Environment. 59. 581–589. DOI: 10.1016/j.buildenv.2012.10.007
2 Research: Stale Office Air Is Making You Less Productive, Joseph G. Allen, Harvard Business Review, March 21, 2017
3 Occupancy Pattern in Office Buildings, Johan Halvarsson, 2012
4 Hotel Energy Solutions, “Analysis on Energy Use by European Hotels: Online Survey and Desk Research”, Hotel Energy Solutions project publications, 2011
5 Dennis Johansson, D.Tech.: 2010 (building space occupancy rate - measurements and estimates)
6 REHVA COVID-19 guidance document, Paragraph 4.6, August 3, 2020
7 2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations To Reduce Transmission DOI: 10.1128/mSystems.00245-20
8 Evidence of Long-Distance Droplet Transmission of SARS-CoV-2 by Direct Air Flow in a Restaurant in Korea DOI: 10.3346/jkms.2020.35.e415
