5.19 Vibration Isolation, Acoustical Isolation, and Seismic Design for Mechanical Systems
Noise and Vibration Isolation. Refer to and incorporate the basic design techniques as described in ASHRAE Applications Handbook, Sound and Vibration Control. Isolate all moving equipment in the building.
Mechanical Room Isolation. Floating isolation floors should be considered for major mechanical rooms located in penthouses or at intermediate levels in mid-rise and high-rise construction. See Chapter 3: Architectural and Interior Design, Special Design Considerations, Acoustics, Design Criteria for Building Spaces, Class X Spaces.
Mechanical Shafts and Chases. Mechanical shafts and chases should be closed at top and bottom, as well as the entrance to the mechanical room. Any piping and ductwork should be isolated as it enters the shaft to prevent propagation of vibration to the building structure. All openings for ducts and piping must be sealed. Shafts dedicated to gas piping must be ventilated.
Acoustical criteria for all building spaces are described in Chapter 3: Architectural and Interior Design, Special Design Considerations, Acoustics. For HVAC noise levels refer to Table 3-4, “Design Guidelines for HVAC-Related Background Sound in Rooms.”
Also, for design criteria, refer to “Selection Guide for Vibration Isolation,” ASHRAE 99 Application Handbook, Chapter 46.
Isolators. Isolators should be specified by type and by deflection, not by isolation efficiency. See ASHRAE Guide for Selection of Vibration Isolators and Application Handbook for types and minimum deflections. Specifications should be worded so that isolation performance becomes the responsibility of the equipment supplier.
Concrete Inertia Bases. Inertia bases should be provided for reciprocating and centrifugal chillers, air compressors, all pumps, axial fans above 300 RPM, and centrifugal fans above 37 kW (50 HP).
Ductwork. Reduce fan-generated noise immediately outside any mechanical room wall by acoustically coating or wrapping the duct. The ductwork design shall appropriately consider and address airborne equipment noise, equipment vibration, duct borne fan noise, duct breakout noise, airflow generated noise and duct borne crosstalk noise. All ductwork connections to equipment having motors or rotating components shall be made with 6-inch length of flexible connecters. All ductwork within the mechanical room or serving courtrooms shall be supported with isolation hangers.
Piping Hangers and Isolation. Isolation hangers should be used for all piping in mechanical rooms and adjacent spaces, up to a 15 m (50-foot) distance from vibrating equipment. The pipe hangers closest to the equipment should have the same deflection characteristics as the equipment isolators. Other hangers should be spring hangers with 20 mm (.75 inch) deflection. Positioning hangers should be specified for all piping 200 mm (8 inches) and larger throughout the building. Spring and rubber isolators are recommended for piping 50 mm (2 inches) and larger hung below noise sensitive spaces.
Floor supports for piping may be designed with spring mounts or rubber pad mounts. For pipes subject to large amounts of thermal movement, plates of Teflon or graphite should be installed above the isolator to permit horizontal sliding.
Anchors and guides for vertical pipe risers usually must be attached rigidly to the structure to control pipe movement. Flexible pipe connectors should be designed into the piping before it reaches the riser.
Noise Control in VAV Systems. System sound levels at maximum flow must be carefully evaluated to ensure acoustic levels required in Chapter 3. Inlet guide vanes should be evaluated for noise in their most restricted position. Duct noise control should be achieved by controlling air velocity, by the use of sound attenuators, by the use of double wall ductwork with insulation in between (only on courtroom return air transfer grilles) and by not over sizing terminal units. Terminal units should be selected so that design air volume is approximately three-quarters of the terminal box’s maximum capacity. Volume dampers in terminal units should be located at least 1.8 m (6 feet) from the closest diffuser and the use of grille mounted balance dampers should be restricted except for those applications with accessibility problems.
Noise Transmission Attenuation (Courthouses).
Attenuate noise trans-mission to and from courtrooms, judges’ chambers, jury rooms, prisoner consulting rooms and from prisoner detention areas.