Designing an efficient and durable HVAC system requires careful consideration of multiple factors, including acoustics, vibration isolation, thermal expansion, and seismic design. These elements play a crucial role in ensuring building safety, occupant comfort, and equipment longevity. This blog explores key principles, applicable standards, and best practices in these areas, providing a comprehensive guide for HVAC engineers, designers, and facility managers.
Seismic Restraint in HVAC Systems
What is Seismic Restraint and Why is it Important?
Seismic restraint ensures that non-structural components (HVAC equipment) move in sync with the building during an earthquake, preventing damage and maintaining functionality. This is crucial for occupant life safety and for reducing costly post-earthquake repairs. Without proper seismic considerations, HVAC systems can become hazards, causing system failures and endangering the life safety of the occupants.
Applicable Codes & Standards
Codes and standards governing seismic restraint include:
- National Building Codes (NBC 2020, OBC 2024) – Clause 4.1.8.18 Elements of Structures, Non-Structural Components and Equipment.
- CSA S832:14R2019 – Seismic risk reduction of operational and functional components (OFCs) of buildings
- ASHRAE – Practical Guide to Seismic Restraint
- SMACNA – Seismic restraint manual – guidelines for mechanical systems
- NFPA 13 – Standard for the Installation of Sprinkler Systems
Where and When is Seismic Restraint Required?
Seismic restraint requirements depend on factors such as geographic location, building importance, and seismic hazard classification. The 2020 National Building Code of Canada Seismic Hazard Tool helps determine if the requirements for seismic requirements are to be addressed.
General Restraint Rules
- Piping & Ductwork: Restraint required for pipes and ducts exceeding specific sizes and weights, ensuring they do not sway excessively during seismic event.
- Suspended Equipment: Equipment (rigidly attached) over 75 lbs and equipment with flexible connectors over 20lbs requires seismic restraints to ensure they do not sway excessively during seismic event
- Wall-Mounted Equipment: Equipment over 20 lbs must be securely anchored to walls.
- Base-Mounted & Roof-Mounted Equipment: These must be restrained unless meeting specific exemptions. Roof-mounted units also require additional wind load considerations.
Examples of Seismic Restraint Installation
Shop Drawings & Final Certification
Seismic restraint shop drawings should include equipment details, anchorage methods, and calculations and be stamped by a professional engineer. Review on site of the installation as project progresses is a must. At the conclusion of the project, a seismic restraint certification letter to be issued with a professional engineer stamp.
Vibration Isolation in HVAC Systems
Basic Terminology
- Vibration: A periodic back-and-forth motion caused by rotating or moving equipment.
- Isolation: The process of reducing transmitted vibration to prevent noise and structural damage.
- Static Deflection: The displacement experienced under a static load, crucial for isolator selection.
- Natural & Driving Frequency: Key factors in analyzing vibration behavior to prevent resonance issues.
Types of Vibration Isolators
- Fiberglass & Neoprene Pads: Cost-effective solutions for minimal vibration control.
- Neoprene-In-Shear Mounts & Spring Hangers: Offer medium to high deflection and reduce noise transmission.
- Restrained Spring Isolators: Ideal for seismic applications where stability is a concern.
- Thrust Restraints & Air Springs: Used for high-powered fans and pumps to absorb dynamic forces.
Flexible Connectors
Flexible connectors absorb vibrations and accommodate thermal movements:
- Rubber Arches: Absorb pulsations in pumps and chillers, reducing wear and tear.
- Braided Hoses: Withstand environmental conditions in outdoor applications, ensuring flexibility without failure.
Mechanical Equipment Isolation
- Base Type A - Direct Isolation: Suitable for rigid units like water-cooled chillers, reducing vibration transmission.
- Base Type B – Structural Steel Bases: Designed to support equipment that does not permit direct isolation or point loading (such as cooling towers).
- Base Type C - Concrete Inertia Bases & Curb-Mounted Bases: Stabilize HVAC equipment, minimizing movement and noise.
- Base Type D – Curb Mounted Bases: Specifically designed to support curb mounted rooftop units.
Lessons Learned in Vibration Isolation
- Cooling tower replacements require precise alignment with structural supports to prevent excessive vibration.
- Rooftop unit isolation must consider noise sensitivity and structural constraints to prevent airborne noise propagation.
- Isolator Combinations: AVOID Springs on springs stacking.
- Modular chillers: Common steel supports required.
- Installations:
- AVOID short circuiting
- Proper installation & adjustment
- Special attention to rooftop unit with cantilevered condensing section.
Vibration Isolation Schedules & Submittals
Project documentation should outline:
- Equipment type and isolator specifications.
- Static deflection and weight distribution for proper load management.
- Point loads and interference risks to avoid structural conflicts.
Thermal Expansion Compensation For Building Services
Thermal Expansion Principles
Pipes undergoing temperature changes will expand and contract according to the formula:
ΔL = αLΔT where:
- ΔL is thermal expansion
- α is the material’s coefficient of thermal expansion
- L is the length of the pipe
- ΔT is the change in temperature.
Codes & Standards
The code mandating the accommodation of thermal expansion and contraction is:
- National Building Codes (NBC 2020, OBC 2024) – Clause 6.2.9.1 and 7.3.3.9.
Design guidelines can be found in the following standards:
- ASME B31.9 (Building Services Piping) is the most applicable to hydronic piping.
- ASME B31.1 (Power Piping) focuses on power and heating plants.
- EJMA Standards focuses on design guidelines using expansion joints.
- ASHRAE Handbook design recommendations from an HVAC perspective.
Ways to Address Thermal Expansion & Contraction
Thermal expansion and contraction can be addressed in the following ways:
- Piping Flexibility: Uses offsets and hard pipe loops with 90° bends to add flexibility. Avoid 45° bends due to stress accumulation.
- Flexible Loops: Allows for movement in three directions using flexible hose components.
- Expansion Joints: Allow for axial movements only. The components also add pressure thrust and spring forces to the anchors.
Structural Components of Thermal Expansion Systems
- Guides: Direct thermal expansion/contraction into the expansion compensator and prevent buckling in riser systems. Properly designed guides can also act as seismic restraints.
- Anchor: Act as the thermal origin to system. Loads on anchors should be designed and reviewed. They must be designed to handle loads such as weight, thermal loads, pressure thrust, expansion joint spring rates, seismic, and wind forces.
- Spring hangers and isolators: Distribute the weight of pipes or risers across building sections. Springs should prevent lifting, misalignment, buckling, or overstressing of pipes, with a recommended maximum load variation of 25%.
Acoustic Considerations in HVAC Design
Why Acoustics Matter
HVAC equipment contributes to interior and exterior noise levels. Improper acoustic control affects occupant comfort, work environments, and overall building functionality, and may lead to noise complaints from the building’s occupants and neighbours.
Key Acoustic Concepts
- Sound vs. Noise: Sound is a propagating disturbance in a solid or fluid. Noise is unwanted sound that disrupts environments.
- Noise Source, Receiver, and Sound Paths: A noise source gives off unwanted sound (e.g. an AHU fan), this noise then travels down a sound path which is the distance and route the sound energy travels to get to a given noise receiver, which is a given person or location where the noise is being perceived (e.g. boundary line of the property).
- Sound Power vs. Sound Pressure: Sound power is the rate at which acoustical energy is released by a noise source, while sound pressure is the average variation in atmospheric pressure at a given location caused by sound waves propagating from a noise source (i.e. the perceived sound level by a noise receiver)
Introduction to Silencers and Indoor Noise Attenuation
- Airborne Noise vs Breakout Noise: The two most common forms of indoor noise generated by HVAC equipment. Breakout noise refers to unwanted sound typically originating from air handling equipment emanating off the casing of the ductwork. Airborne noise refers to this same unwanted sound exiting the ductwork through air terminals such as grilles and diffusers.
- Silencers: Engineered ductwork with perforated baffles containing fiberglass media. Effective solution for attenuation duct borne HVAC noise. They are characterized by the following properties:
- Insertion Loss: Difference in sound power level of a system with noise attenuation installed.
- Pressure Drop: Airflow resistance caused by silencers resulting from creating an obstruction in the airstream.
- Generated Noise: Noise generated by the silencer as air flows through the passage.
- Silencers can be fine-tuned to target specific frequencies, can be custom designed to fit a given space (e.g. tee-elbow or transitional silencers), can come with media coverings or no-media configurations to suit lab and other environments, and come with heavy gauge HTL casing to contain breakout noise.
- System effects: Duct elements up and down stream of the silencer may cause an increase in installed pressure drop from the catalogued silencer pressure drop, this is referred to as system effects. Care must be taken when placing silencers on plans.
Environmental Noise Solutions:
For HVAC equipment such as chillers, condensing units, RTUs, cooling towers, generators, etc. that emit noise to the environment, it is important to understand how noise propagates to the environment as sound pressure decreases with increase in distance from noise source. Also, noise propagation is affected by surrounding structures (such as buildings, rooftops. walls, etc.) which may result in noise being magnified due to reflectivity.
The following definitions should be taken into account:
- dBA Level: The a-weighted sound pressure level that is heard at the property line and/or receptor. This is a filter used to adjust sound decibels for the frequency range that the human ear is capable of hearing which is from 500 Hz to 2000 Hz. The adjustments are added to each octave bandwidth and the sound levels are then added logarithmically to produce a single resulting dBA level.
- Directivity: Should pressure level or dBA level will vary dependent on which direction the noise source is in relation to the property line/receptor. Having the receptor right in front of the noise source, will result in maximum levels. These levels will decrease depending on the angle that the receptor is to the noise source. Larger the angle, lesser will be the amount of noise reaching the receptor. Similarly, elevation also plays a part in directivity.
- Divergence: The closer the noise source is to the receptor, the louder it will sound. Having more distance will result in lower sound pressure levels at the receptor.
Some other acoustical products besides silencers that are used for reducing propagating noise levels at the receptor, include:
- Acoustic Louvers & Panels: Dependent on the amount of noise radiating out into the environment, noise mitigation can be provided by containing the HVAC equipment with acoustical panels which may or may not have acoustical louvers on the walls and/or top for the intake and exhaust air distribution.
- Sound Barriers: For reducing noise propagation (to the neighbouring residences or public property) from HVAC equipment located outdoors such as chillers, condensing units, RTUs, cooling towers etc. acoustical barrier walls are placed in between the respective noise equipment and the target receptors. Purpose is to attenuate the radiating noise down to meet acceptable city noise bylaws as well as to enhance urban building acoustics.
Conclusion
Effective HVAC system design integrates seismic control, vibration isolation, thermal expansion management, and acoustic considerations. Adhering to industry standards and best practices enhances safety, performance, and occupant comfort. By understanding these principles, engineers and designers can create resilient and efficient HVAC solutions that stand the test of time.