2.5 HVAC SYSTEM SELECTION ISSUES
2.5.3 Noise and Vibration Control
Noise and vibration from HVAC&R equipment can be reduced by proper selection of equipment, vibration control, and the interpo-sition of sound attenuators and barriers. Residual sound may still be objectionable, however, when equipment is located near occupied areas. HVAC&R and architectural design decisions must go hand-in-hand since acoustical control may involve equipment location, floor and wall assemblies, and room function. HVAC&R equipment loca-tions may influence spatial planning—so that areas requiring a low-noise environment are not located next to major or noisy equipment.
The HVAC engineer should alert other members of the building design team regarding the location of noisy equipment. On large, acoustically sensitive buildings (e.g., theaters, museums), an acous-tical consultant should be part of the design team.
Noise and vibration transmission to an occupied space by sys-tem components will be an important consideration in syssys-tem selec-tion and design. Even after a system has been selected, component selection will significantly affect system acoustical performance.
Noise can be transmitted to occupied spaces from central station equipment along several airborne paths, through air or water flows, along the walls of ducts or pipes, or through the building structure.
If central station pumps or fans are used, each of these paths must be analyzed and the transmission of sound and vibration reduced to an acceptable level. Supply air outlets (and other terminal devices) must be selected to provide appropriate acoustical performance.
An initial step in noise control is to establish noise criteria for all spaces. These criteria should be communicated to the client early in the design process. All noise-generating sources within the air-conditioning system must be identified. Then, the effects of those sources can be controlled by
• equipment selection,
• air and water distribution system design,
• structural/architectural containment,
• dissipative absorption along the sound path, and
• isolation from the occupied space, where feasible.
These control techniques and information on how to apply them provided in the ASHRAE Handbook—Fundamentals and the ASHRAE Handbook—HVAC Applications can usually lead to a design procedure that can achieve the desired acoustical design intent and criteria. The acoustical design of systems and the build-ings they serve is, however, frequently quite complex and is often the proper province of specialists known as acousticians. This is especially true for spaces with exacting requirements, such as audi-toriums, or where noise-generating components must be located adjacent to occupied areas. Fan and other equipment noise emanat-ing from coolemanat-ing towers and air exhaust or intake points may affect the ambient noise level of the neighborhood surrounding a building and require evaluation and/or mitigation. Many municipalities have codes governing equipment noise.
The major paths that govern the sound transmission character-istics of an all-air distribution system are shown in Figure 2-3. It is absolutely critical to distinguish between airborne sound transmis-sion (where barriers are easily applied), duct-borne transmistransmis-sion (where other mitigation techniques must be used), and noise gener-ated by terminal devices. Occupied spaces on the floors directly above or below a room housing an air-handling unit may also be affected by equipment noise and vibration. Most acoustical barriers,
Figure 2-3. Noise propagation paths from HVAC equipment.
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such as floors and ceilings, while in themselves potentially effec-tive, frequently “leak” due to penetrations for pipes, electrical con-duits, and similar items. Structural components do not constitute effective sound barriers unless all penetrations are carefully sealed.
Air distribution systems, particularly high-pressure (high velocity) systems, must be examined during all stages of design and installation to ensure that they are quiet systems. The principal sources of noise in an air system are the fans, the duct distribution system itself, and terminal devices. Most fan manufacturers can readily provide a sound power spectrum for a particular fan operat-ing under a specific set of conditions. With this information, the designer can select an acoustical treatment to reduce this sound energy to an acceptable level. The fan noises most difficult to remove are those in the lower octave bands. Thus, sound attenua-tion in those bands is an important objective for acoustical treat-ment of fans with low-frequency characteristics (such as centrifugal fans). Sounds in the higher octave bands will normally be absorbed in the duct distribution system, particularly if the ducts are lined.
For quiet operation, fans should be selected for maximum static (or total) efficiency. In variable-air-volume systems, sound pressure levels should also be checked at minimum system flow condition if dampers, inlet vanes, or blade pitch fan control schemes are used.
In general, large fans at high static pressure conditions produce the highest noise levels.
Noise and vibration can also be generated within and when exit-ing the distribution system by the movement of air or water. These problems can be controlled by velocity limitations, appropriate dis-tribution layout, use of attenuators, and equipment selection. (For piping design, see Section 5.7; for duct design, see Section 5.8.) Several noise sources can exist within an air distribution system. In general, components with higher pressure drops will produce higher sound levels. Some of the sound-generating elements in ducts include abrupt transitions, turbulent conditions caused by poor duct fittings or improper duct taps, partially closed dampers used for bal-ancing purposes, improperly located dampers in duct shafts, sound traps improperly selected (with more than 2000 fpm [10 m/s] face velocity), or installed too close to fans or fittings, sharp bends, crimping of flexible connections, and duct leakage. A pres-sure-reducing device, damper, or pressure regulator located in a ter-minal unit may generate noise as the energy expended in pressure reduction is converted to sound. This is why oversizing of terminal air devices is undesirable. Large-volume terminals (>2000 cfm
[950 L/s]) and those with higher pressure differentials produce greater noise levels. Pressure-reducing devices should be installed in the duct system with sufficient downstream ductwork to absorb the sound generated by the device. Large terminal units with pres-sure-reducing devices should not be installed in occupied spaces without considering acoustical treatment downstream and in the radiated sound path from the terminal to the room.
Sound can travel through ductwork from one room to another.
For example, an air-conditioning system that serves a series of music practice rooms will require ductwork with sound baffles between rooms, lined ducts, or ample duct turns to attenuate noise.
Noise control will influence duct configuration, size, and system static pressure.
The sound produced by room terminal equipment cannot be easily reduced. Control of this potential problem starts with system selection and entails careful equipment selection and sizing to achieve the noise criteria for a given conditioned space. The more moving parts in a terminal, the noisier it will be. Air-cooled unitary terminal equipment is likely to be near the high end of the noise scale. Wacooled terminals, including wasource unitary ter-minals, can be significantly quieter. Air terminal equipment, in ascending order of noisiness, include air diffusers, variable-air-vol-ume boxes, fan-coil units, high-induction-ratio terminals, and pack-aged terminal air conditioners. Continuous terminal noise is usually less annoying than intermittent or alternating noise. Thus, ON/OFF
control of terminal refrigeration equipment and air-circulating fans may produce annoyance even though the sound pressure level dur-ing equipment operation is within acceptable limits.
Terminal equipment, because of its location, provides the few-est options for acoustical mitigation. The solution is essentially in the selection of the equipment itself. Greater opportunities for noise control through attenuation (e.g., duct lining) and barriers (e.g., solid ceilings) are possible when terminal equipment can be located outside the occupied space (for example, placing air-mixing boxes above a ceiling with low-velocity ducts connecting to diffusers). Air ducts passing through adjacent rooms can be transmission channels for cross-talk, as can unsealed openings around ducts or pipes.
Cross-talk through such paths can be controlled through building design. Occasionally, partitioning will be located so as to divide a room terminal or outlet. This creates a virtually uncontrollable path for sound transmission between rooms.
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Even inherently noisy terminal equipment can be engineered to meet acceptable noise levels for most applications. Different prod-ucts vary in their acoustical performance. Often such equipment is not acoustically rated, at least not on a basis that permits compari-son with other equipment using catalog data. When in doubt, con-sider visiting operating installations or arranging for prototype testing to ensure that the design objectives can be met.
Vibration from fans, pumps, refrigeration compressors, and other moving equipment must be kept within tolerable levels. As in the case of sound, degrees of satisfaction vary depending upon the function of an occupied space. Extraordinary precautions must be taken to protect sensitive areas, such as those housing electron microscopes or research animal colonies.
Vibration from imbalanced forces produced by a fan wheel and drive, unless suitably isolated, will pass undiminished into the structure and be transmitted to occupied spaces, where less stiff building members (centerpoints of structural spans, windowpanes, a chandelier in a ballroom) may respond with noticeable secondary vibrations. The ASHRAE Handbook—HVAC Applications contains valuable guidance regarding the isolation of moving equipment.
Every member of the building design team must contribute toward achieving a satisfactory acoustical (including sound and vibration) environment. It is up to the HVAC&R design engineer to alert the other parties to their role in this endeavor relative to HVAC&R equipment.