CHILLED BEAMS


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 fi gures 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 off er 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 offi ce space typically has only a 40%


occupancy, a rented hotel room occupied 40% of the time, and school classrooms for example only occupied 31% of the time, this scenario would aff ord 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. 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 fi ltration to guard against pollutants such as pollen and NO2


from


traffi c being brought in from outdoors are a prerequisite. Secondary fi ltration inside the building on recirculatory cooling or heating devices is a more complicated situation, as these can have coarse fi lters that practically do not fi lter smaller particles but may still collect potentially contaminated particles which may then be released when fans start to operate. 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 optimal. There are all these and many further


considerations – there have been studies citing increased air velocities being responsible for intensifi ed spread of viruses, so eff ective ventilation without draught seems important – as well as ‘future proofi ng’ issues such as fl exibility 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


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user, all of which of course relies on constant measurement of what is actually happening within the occupied space. 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’ – 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 benefi t 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 fi lter 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 fl ow temperature. But this does not have to mean low levels of humidity; a typical active chilled beam operating with a chilled water fl ow temperature of 14°C in a 22-24°C 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 benefi ts of low system fan power from not having a secondary fan, and high chiller effi ciencies 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 off ering greater fl exibility 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.


February 2021 21


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