How can fans that are quiet affect indoor environmental quality (IEQ)

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The majority of people today spend most of their time indoors. Indoor environmental quality (IEQ), has become increasingly important. IEQ is often referred to as temperature, CO2 level, humidity. It has been proven that sound quality is a key factor in indoor comfort. Not only can noise directly cause hearing loss and tinnitus, but it can also indirectly damage your health through irritation, sleep disturbances and stress. These can all affect performance and well-being.

Research has shown that noise can have a negative effect on writing and reading, and that children’s cognitive development is affected by prolonged exposure to noise.

How do we measure sound quality?

The noise level inside a building is controlled by today’s requirements. A normal value for this setting is 30 dB(A). How does 30 dB(A), sound? Tonal frequencies are what determine the sound characteristics. This means you can have tonal sounds with low frequency or high frequency as well as broadband noise that meets the 30 dB(A). The subject was investigated in a RISE project. Different heating, ventilation and air conditioning systems (HVACs) were measured and set up with the requirement that they each meet the 30 dB(A).

You can hear 3 different sets with different sound characteristics.

Sound 1 Sound 2 Sound 3

All of them meet the 30 dB(A), but they sound very different. It may be time to modify the requirements to include sound characteristics. You can find out more information here

Noise-related diseases have a negative impact on the health of millions. Lifestyle diseases are usually related to stress, unhealthy eating habits, and a nonactive lifestyle. However, a WHO study has shown that prolonged exposure to tonal sound can cause damage to the body’s autonomous and hormonal systems. This can lead to symptoms like cardiac arrest, high blood pressure and hearing loss as well as mental disorders.

Reduce fan noise

Modern buildings have excellent noise reduction capabilities. Despite significant improvements in the last few years, it is still difficult to isolate the noise coming from HVAC systems. Silencers are commonly installed in order to reduce noise, but this can increase energy consumption and CO2 emissions. Noise is usually tonal and changes with fan speed. Variable air volume (VAV), systems where the fan speed is constantly changing to meet demand, means that the tonal noise fluctuates continuously across a wide frequency range. Silencers can absorb broadband noise, but not tonal. You can design a silencer to match a specific tone frequency, but not to cover a broad range of tones.

Swegon collaborated with Chalmers University of Technology to sponsor my industrial Ph.D. “Numerical methods for predicting and minimising fan tonal noise”. This research aims to reduce the noise from centrifugal fans by identifying and decreasing their sources.

Now, the sources have been identified and the results have been published in Physics of Fluids. The article was selected as Editors Pick.

The article can be found here.

What can we learn from this study?

There must be a gap between the fixed inlet tube and the rotating impeller. Air is forced through the gap due to the pressure difference. Turbulence occurs when the flow through the gap meets main flow into fan impeller (green rectangle, Figure A). This turbulence, colored in blue in Figure A, flows along the fan shroud and is swept downstream to intersect with the leading edge the fan blade (orange rectangle in Figure B). This interaction causes uneven and unstable surface pressure distribution between the blades. Tonal noise is produced when the surface fluctuates in these areas. This indicates that fan noise is coming from a source (see Figure C, red area).

Why is this important?

Using the information gained from the research, we can develop fans further and introduce new products to market. These products will have better sound characteristics and help improve indoor environment quality. This creates comfortable and healthy indoor environments. We cancel the noise directly at source. This saves fan energy and CO2 emissions.

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