Subject:- Refrigeration and Air Conditioning

Topic:- Miscellaneous

Difficulty:- Medium

In the present scenario with the attempt to reduce energy use in HVAC systems, it is found that noise problems are on the rise. Increasing duct air velocities, lighter building structures and high speed equipment aggravate the acoustical problems. According to various HVAC handbooks, the noise from AC systems that reach the living/working quarters are made up of:

a) Low frequency fan noise

b) Mid-frequency airflow or turbulence generated noise

c) High-frequency damper and diffuser noises

The ductborne noise which is due to sound generated by fan and airflow in ducts can be controlled by selecting quiet fans based on sound power data and providing good fan outlet conditions according to ASHRAE guidelines.

Also variable frequency drives on air handling unit’s fans and pumps can be used to reduce noise. Care should be taken to not turn the air in the ‘wrong direction’ or the ducts will rumble.

Radiated equipment noise transmits through the wall or floor into the adjacent space and is generated by vibration of the fan casing and various motors of the AC system. This can be reduced by using flexible coupling between fan and duct and using vibration isolation footpads under the equipments.

Break-in noise is radiated equipment noise that enters the ductwork and propagates down the system. This can be avoided by replacing lightweight flexible ducts with heavier steel sheet ducting and avoiding the passage of ducts through noisy areas.

Duct break-out noise transmits through the duct wall impacting the adjacent space. To reduce this noise design low pressure-low velocity systems, make ducts stiffer by external bracing, use heavier materials and provide damping and acoustic silencers.

The addition of acoustic lagging results in significant attenuation of air conditioning sound from terminal devices like grilles, diffusers and vent covers.