Within any form of fluid transportation there is always some pressure drop off in the system due to acombination of two factors: •Friction of the fluid moving across the walls of the duct– which depends on flow type,internal face’s geometry and the friction coefficient. •Dynamic drop due to variations in ductgeometry and/or the direction of the air flow.
In order to achieve correctperfusion,ducts musthave a correctly defined section,with a balanced average air velocity and defined static pressure.
Inadequate ductnetwork design or defects in material installation will change the operating conditions,giving rise to thermal loads in the targetareas and insufficientair replenishment(with a decline in perceived comfortfor the building’s occupants).
d) Noise in ductnetwork and acoustic attenuation
Another aspectindependentof material which plays a fundamental role in a ductis acoustic attenuation ofthe noise produced by the HVAC system itself (air handling units,airflow in the ducts,diffusers etc.…),as well as from ‘transmission noise’produced in the building and thattransmitted to neighbouring buildings via the ductsystem.As noise is considered ‘unwanted sound’,itis evidentthatreducing noise results in better indoor air quality & comfort.To achieve the desired level of attenuation,ducts made from or insulated using a material with high acoustic absorption properties,such as glass wool need to be installed. The most effective solution would be based on glass wool ductboards. Alternatively,the application of glass wool ductliners mightalso be recommended.If ULTIMATE mineral wool is to be used,due to fire resistance requirements,italso significantly attenuates noise produced in the installation.The following table shows the results of the studies carried outby NAIMA on ductnoise attenuation for differentducts solutions:
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