How to prevent HVAC vibration

HVACs are comprised of so many components, both large and small. HVACs are machinery, and all machinery will have some vibration due to moving air, fluid and parts. That’s not a problem, except when it’s excessive, which is when HVAC noise becomes an issue and sound is transmitted all over the building, travelling through exposed HVAC ducts, non-visible ducts, pipes, framing members and even drywall.

Vibration tends to occur with either poor design or as system components begin to wear down. Be aware of these problems:

• Oversized compressor or chiller, fans and blowers

This is sometimes done to create extra capacity, but it leads to performance issues and high operation costs. For instance, a designer might think oversizing an air conditioner will cool the building in less time, but what’s really happening is that its components are wearing down faster, reducing its service life and leading to poor humidity control.

• Water pumps

Systems that use hydronic piping can reduce vibration with choosing adequate and damping pads.

• Inadequate supports

Consider adding neoprene pads for HVAC equipment that minimise the vibration channelled between moving parts and the underlying structure.

• Air ducts

When poorly designed, ducts vibrate excessively due to high pressure or airspeed, creating especially annoying air-duct noise.

• Misaligned rotating equipment

If the shaft is not aligned properly, moving equipment will vibrate. Shaft couplings between parts, such as an electric motor and centrifugal pump, are particularly vulnerable. The centre lines of both shafts should match.

Limiting vibration will significantly help with HVAC noise reduction. A quiet HVAC system is the result of addressing vibration at the design stage. Otherwise, components may malfunction, which in turn can lead to system failure.

Include a vibration-isolation system into your design

Vibration isolation is exactly what the name says. It’s the process of isolating vibrating parts from the rest of the system. This prevents vibration and noise spreading to other parts of the machine. Isolate the moving parts from the components that can conduct noise. An example of this would be isolating the cooling fans’ moving parts from the ducts, which effortlessly carry vibrations, and thus sound, to other areas of a building.

Duct layout is essential to reducing noise and vibration. Make it easy for air to flow freely. Limit:

• Angles
• Bottlenecks
• Branches

Also, avoid flexible ducting when possible. If you can lower the turbulence, you lower the risk of noise and vibration produced by duct fittings and dampers. Another benefit to this is reducing the stress on your system – it doesn’t have to work hard to provide the airflow required by the building.

Vibration isolation requires damping to some degree. The benefit of damping is that it dissipates mechanical energy from the system and attenuates vibrations faster. One way of isolating vibration is through the small components you use within the system. Excellent materials for this job include thermoplastic elastomer (TPE) and a synthetic rubber called neoprene. They control structural-born noise and reduce high-frequency vibration.

You should also vibration-isolate all equipment connected to a structure. Always use independent structural elements to separate the parts of the building that house the HVAC from quiet spaces. On top of that, use HVAC isolator pads, or as some people call them, noise cancelling pads. You might need to incorporate flexible piping connections and flexible electrical conduit connections for pipes and ducts connected to the HVAC.

Be sure to vibration-isolate ducts and pipes. This can be done using spring or neoprene hangers for at least the first 50 feet from vibration-isolated equipment.

Duct material for connectors

Some flexible duct connectors can help with damping noise as air travels through ducts. For a duct noise reducer, consider connectors made of fibreglass, PVC, polyurethane, neoprene and silicon. Insulation in air ducts with a sound proofing material can also reduce vibration.

Summary: strategies for your HVAC design

The current ASHRAE Handbook series, which includes specific guides to HVACs, is available online for purchase. What we can provide here is information based on 2007’s ASHRAE Handbook–HVAC Applications.

Fans:
Do	Why?
Minimise flow resistance and turbulence in your air-distribution system.	When flow resistance is high, you increase the fan pressure required. This results in the fan creating higher noise, especially at low frequencies. Turbulence also increases flow noise generated by duct fittings and dampers.
Choose a fan to operate between 70% and 80% of its rated peak efficiency when handling the required airflow and static pressure. Also, the fan you choose should generate the lowest possible noise at the design conditions needed.	Keeps noise levels down. Using an oversized or undersized fan that fails to operate at or near rated peak efficiency can substantially increase noise levels.
Locate fan-powered mixing boxes associated with variable-volume air-distribution systems away from noise-sensitive areas.	Avoids significant noise problems.
In ducts serving sound-sensitive spaces, keep airflow velocity as low as possible by increasing the duct size.	Minimises turbulence and flow-generated noise.

Ducts:
Do	Why?
For consistent and straight airflow, ducts should connect at both the fan inlet and outlet.	Turbulence at the fan inlet and outlet, and flow separation at the fan blades, can significantly increase fan-generated noise. Also, turning vanes near fan outlets can also inflate turbulence and noise, especially if airflow is inconsistent.
Select air duct noise silencers that do not significantly increase the required fan total static pressure.	Opting for silencers with static-pressure losses of 0.35 in. of water or less can decrease regenerated noise from silencer airflow.
Place elbows or duct-branch takeoffs at least 4 to 5 duct diameters from each other. For high-velocity systems, you may need to increase this to 10 duct diameters in critical noise areas. Use flow straighteners, or honeycomb grids, in the necks of short-length takeoffs leading to grilles, registers and diffusers. Place grilles, diffusers, and registers into occupied spaces as far as possible from elbows and branch takeoffs.	Minimises flow-generated noise and vibration.
Duct transitions should NOT exceed an included expansion angle of 15˚.	The flow separation may produce rumble noise.
Employ turning vanes in large 90˚ rectangular elbows and branch takeoffs.	Provides a smoother directional transition, which reduces turbulence.
Minimise use of volume dampers near grilles, diffusers, and registers in acoustically critical spaces.	Significantly lowers noise.

 

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