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70 HEATING, VENTILATION & SERVICES PICTURED


The effect of elbow throat on turbulence within a duct system. The sharp throat of (A) created more turbulence than the rounded throat of (B)


sound issues or it can attenuate them depending on the system. Branch ducting is the cheapest and therefore the most common type of ducting, but it’s also the least effective when it comes to attenuation. It’s almost impossible to clean and is the most difficult to hide given its size, especially if you need to include inline silencers which can be 250 mm diameter or more. Branch ducting tends to be full of 90° bends, which cause turbulence in the air, creating an unwanted low pitch hum, especially in the rooms closest to the MVHR plant where air velocity is greatest. Sharp bends also add static pressure to the system, meaning the fan unit needs to work harder to force air through the pipes. Another acoustic drawback with a branch system is that each room tee’s off a single branch pipe, which allows crosstalk between rooms. Fortunately, there is a solution and it’s called radial ducting. Radial ‘semi-rigid’ ducting has a much smaller diameter, typically 75 mm, so is easy to hide. Silencers are not necessary. Each room has its own dedicated pipe rather than teeing off a branch, thus preventing crosstalk. Each pipe connects to


a manifold normally located close to the MVHR unit, ideally an attenuating manifold. Turbulence is eliminated by the avoidance of sharp 90° bends, instead radial bends tend to sweep around corners. Air movement noise is avoided too because the air velocity in each pipe is much lower, just enough to serve the room at the end of the pipe and not the whole building. Static pressure on a radial system is typically around half the equivalent branch system, meaning the MVHR unit can more easily circulate air. Finally, the best radial ducting has an anti-static anti-bacterial lining to prevent dust and grime building up. As with anything, with ventilation you get what you pay for. There is certainly a big difference between a budget system designed to get through the regulations and a premier league system designed to get through the next 20 years, quietly without failure. You wouldn’t think about cutting corners on the aesthetics of a building, so why cut corners on something as important as air quality?


Eliot Warrington is managing director at Solarcrest


WWW.ARCHITECTSDATAFILE.CO.UK


ADF APRIL 2019


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