VIBRATION AND NOISE
Vibration Isolation: All motor-driven HVAC equipment, along with ductwork and piping connected to it, must be installed with vibration isolators, with or without vibration bases, to prevent transmission of vibration to the building. Generally, the following guidelines apply to design of vibration bases (where required) and vibration isolators:
Component / Slab-on-GradeInstallation / Roof or Upper Floor
(20-30 feet span) Installation /
Base Type / Isolator Type / Min. Deflection (in) / Base Type / Isolator Type / Min. Deflection (in) /
Chiller, Water-Cooled
Centrifugal, Scroll Compressor
Helical Screw Compressor / None
None / I
I / 0.25
1.50 / None
None / 1
1 / 0.75
1.50
Chiller, Air-Cooled
Scroll Compressor
Helical Screw Compressor / None
None / I
I / 0.25
1.00 / None
None / 1
1 / 0.25
1.00
Pumps, Base-Mounted / CF / II / 0.75 / CF / 3 / 2.50
Cooling Towers, ≤300 tons
301-500 tons
>500 tons / Cooling towers are located on structural bases outdoors with no vibration isolation. / None / 1 / 89
64
19
Fans:
Inline, Suspended
Inline, Base-Mounted
Centrifugal (≤50 hp)
Centrifugal (>50 hp) / None
SR
SR
CF / IV
III
III
III / 0.75
0.75
1.50
2.50 / None
SR
SR
CF / 3
3
3
3 / 0.75
0.75
1.50
2.50
Air-Handling Units (≤15 hp)
(>15 hp) / None
None / II or III
II or III / 0.75
1.50 / None
None / 3
3 / 0.75
1.50
Packaged Air-Conditioning Units / None / III / 0.25 / VC / 3 / 0.75*
*If structural span exceeds 20 feet, select isolator for deflection 10 times the design roof or floor deflection based on information provided by the structural engineer or architect.
Base Types: CF Concrete-filled steel frame
SR Structural vibration rails
VC Spring isolator vibration roof curb
Isolator types defined in the table above are as follows:
Type I - Resilient pad type mountings consisting of any one of the following constructions:
1. Two layers of ribbed or waffled neoprene pads bonded to a 16 gauge galvanized steel separator plate. Bolting not required. Pads should be sized for approximately 20 to 40 psi load.
2. Pre-compressed fiberglass properly sized for 5 to 60 psi loading depending on density with steel plates bonded to top of isolator.
3. Two layers of ribbed or waffled neoprene pads bonded to vibration cork sized for 10 to 60 psi loading.
Type II - Elastomeric mountings having steel baseplate with mounting holes and a threaded insert at top of the mounting for attaching equipment. All metal parts should be completely embedded in the elastomeric material. Mountings should be loaded so that deflection does not exceed 15% of the free height of the mounting.
Type III - Adjustable, freestanding, open-spring mountings with combination leveling bolt and equipment fastening bolt. Spring (or springs) should be rigidly attached to mounting baseplate and to the spring compression plate. A neoprene pad having a minimum thickness of 1/4 inch should be bonded to the baseplate.
Type IV - Spring hangers consisting of a rectangular steel box, elastomeric element, coil spring, spring cups, neoprene impregnated fabric washer, and steel washer. The design should be such as to prevent metal-to-metal contact between the hanger rod and the top of the hanger box. The elastomeric element should meet the design requirements for Type II mountings. The hanger box should be capable of supporting a load of 200% of rated load without noticeable deformation or failure.
Noise: The noise level in a space can be effectively described with a single-number rating called the noise criteria (NC) rating. The NC rating is determined by measuring the sound pressure level of the noise in each octave band, plotting these levels on a graph, and then comparing the results to the defined NC curves. The lowest NC curve not exceeded by the plotted noise spectrum is the NC rating of the sound. The threshold of hearing of a continuous sound for a young, healthy person is essentially NC 0.
The following figure illustrates the NC curves:
A better criteria that is used is the room criteria (RC) rating, sometimes called the preferred noise criteria (PNC), as shown by the following figure:
The RC curves are designed to address the "quality" of sound, in addition to its quantity. Historically, spaces designed on the basis of NC values may allow background noises, particularly generated by mechanical systems, that, while meeting the loudness levels required, are considered to be too "rumbly" or too "hissy" by the space occupants. Thus, design should be based on RC levels rather than NC levels.
The noise from all HVAC systems and components can be held to acceptable limits by careful attention to equipment selection, vibration isolation, and piping, ductwork, and air distribution design.
Water-Cooled Chillers: Water-cooled chillers are located indoors, which means that noise and vibration created by the chiller and its associated pumps may become a problem throughout the building. Therefore, careful attention to location and installation of water-cooled chillers is required.
Water chillers have broadband sound levels with characteristic frequency ranges depending on the type of compressor (reciprocating, centrifugal, or screw). And, because some centrifugal chillers produce more noise under part load operation, chiller noise must analyzed for the entire load range. Chillers are tested for noise level in accordance with ARI Standard 570. This data can be used to evaluate chiller selection and installation requirements to minimize noise impact. Noise levels as high as 80-90 dBA can be produced by a chiller during part load operation and the chiller room must be designed to contain this noise. And, while vibration by a rotary compressor chiller is very low, it must also be considered and addressed.
To prevent the transfer of sound and vibration from water chillers into occupied spaces, the following recommendations should be implemented:
1. Ideally, locate the chiller(s) in a mechanical equipment building separate from the occupied building(s). If the chiller room is integrated into the building, the walls and ceiling must be designed to prevent noise transfer. Masonry construction and, perhaps, interior acoustical lining are required.
2. All piping and conduit penetrations must be sealed with flexible materials to stop noise and prevent the transfer of vibration to the building walls and floors.
3. Mount all equipment on vibration isolators. If the chiller is installed in a ground floor or basement location, it can be isolated with neoprene or other resilient pads designed to support the weight of the chiller without fully compressing. If located in the upper floors of a building, the chiller should be installed with spring vibration isolators.
4. Make sure that all piping and electrical connections to equipment are made with flexible connectors.
5. For critical applications, chiller noise can be reduced by a sound-absorbing enclosure. Constructed of movable panels, these enclosures consist of lightweight panels with 1”-4” of glass fiber sound insulation installed around and over the chiller. To allow air movement and access for servicing, the panels can be arranged with minimum 3’-0” overlaps to eliminate straight line noise paths.
Air-Cooled Chillers: Air-cooled chillers have noise emission as high as 85-90 dBA. The noise control design measures recommended for "Cooling Towers" in the following section also apply to air-cooled chillers.
Cooling Towers (and Evaporative Coolers): Typical cooling tower noise emission is in the range of 65-85 dBA measured at (typically) 5 feet from the tower, and each tower manufacturer can provide specific sound emission data for the required operating conditions. For most propeller fan towers, the manufacturers have developed “low noise” options, such as adding extra fan blades so the fan can be operated at lower speed, etc. Centrifugal fans are quieter than propeller fans (though far less energy efficient) and may be an acceptable option if community noise is expected to be a problem.
There are two caveats relative to manufacturer-provided sound data:
1. Cataloged noise data is typically based on a single tower or tower cell. Where multiple towers or tower cells are utilized, the manufacturer must be queried for cumulative effect sound data. In the field, fan speeds on multiple towers or cells must as nearly identical as possible to prevent annoying “out of phase” sound differences.
2. The noise emission from the tower may not be uniform in all directions. For crossflow towers, the inlet side(s) will be noisier than the enclosed side(s). The manufacturer data must define the tower orientation for each noise emission value.
The basic sources of cooling tower noise include fans, air movement and turbulence through the tower, motor and/or drive, and structural or casing vibration.
Fan noise is related to the fan design: centrifugal fans are inherently quieter than axial flow fans. Axial fan noise can be reduced by (1) reducing the fan speed and/or (2) increasing the number of fan blades. Some tower manufacturers are now offering special fan blades designed to reduce noise generation.
The tower location and orientation can have a pronounced effect on the sound condition. If any sound source is located within 3 feet of a reflective surface, such as a wall, the source effectively radiates a greater amount of sound…the sound increase is 3 dBA. When located in a corner, the sound increase will be about 9 dBA.
To avoid a community noise problem, locate the tower as far from adjacent property lines as possible. Since HVAC cooling towers have sound levels of 65-85+ dBA, the WHO criteria will generally require that cooling towers not be located within 50-100 feet of property lines without taking additional attenuation measures:
1. Select the quietest cooling tower possible for the application. Use centrifugal fans or select low rpm propeller fans for the application and/or take advantage of manufacturer options for reducing tower noise.
2. Orient a quiet side of the equipment toward any sound reflecting wall or side of a building and toward adjacent property. Locate cooling towers at least 3 feet and, if possible as much as 20 feet, away from any sound reflecting surface, no matter how oriented.
3. Reduce radiated noise with a solid wall sound barrier between the equipment and the adjacent property. However, take care not to create a tower performance problem.
4. Only as a last resort should sound attenuators on the tower inlet or outlet be utilized. These devices are expensive, seriously impact the performance of the tower (requiring, often, that a larger size be used), and create significant maintenance problems.
5. Cooling tower vibration results when fan balance and/or drive alignment is not correct. This is more often a problem with propeller fans and/or gear drives. Propeller fan blades, due to their relatively long length (large length-to-width ratio) tend to flex under air loading. Even if the blades are perfectly balanced by weight, dynamic balancing is required to eliminate unequal blade movement and resulting vibration. Shaft alignment into and out of gear drives is also critical to avoid vibration. Also, gear drives must be operated at speeds high enough to avoid “gear chatter”.
Centrifugal fans, unless they have very long shafts, have less vibration potential because of their construction. However, many cooling towers using centrifugal fans will be constructed with two or more fan wheels on a common shaft, driven by a single motor. To prevent vibration, the shaft must be carefully aligned and supported by bearing blocks at each end and between each fan wheel. The shaft must be rigid enough to avoid deformation, and the resulting vibration, due to the fan wheel(s) weight.
All propeller fan and/or gear drive cooling towers, and towers with multiple fan wheels on a common shaft, should be equipped with a vibration cutout switch.
Boilers: Small hot water and low pressure steam boilers with atmospheric burners typically have low noise emission and simply locating them in boilers rooms away from learning spaces will typically suffice to limit their noise impact. However, larger boilers using forced draft burners will have broadband noise emission caused by the fan(s) and the combustion process, typically in the 65-85 dBA range. For these boilers, the location and design of the boiler room is the key to containing the boiler noise. Note, too, that most installations consist of multiple boilers and the sound emissions are logarithmically additive.
The combustion process in some boilers, particularly large boilers (150 boiler horsepower or greater) or pulse combustion condensing boilers, produces strong low frequency noise that is carried through the flow system that radiates within the boiler room and, more significantly, into the atmosphere. The flue outlet should be located away from any noise sensitive areas to avoid having to deal with the low, rumbly discharge as an exterior noise source.
Fans and Air-Handling Units: The improper selection, application, and installation of fans and AHU's are the causes of most HVAC system noise problems. The number and extent of the problems can be minimized by selecting efficient air-moving equipment that delivers air to a duct system that is designed for the lowest practical pressure loss.
Generally, the noise from fans and air-handling units can be minimized if the following guidelines are followed:
1. Select the most efficient type and size of fan or AHU for the application. This will also, in most cases, be the least noisy. Typically, backward-inclined airfoil blade fans are much quieter than backward-inclined flat blade fans, which are much quieter than forward-curved blade fans. Low discharge velocity is also preferred.
2. Select the fan to operate on the right side of the fan curve, safely away from the stall region.
3. Blow-through AHU's are quieter than draw-through units and this is the preferred configuration.
4. Allow a clearance around the fan or AHU of at least 1 fan wheel diameter at all non-ducted inlets and 1.5 wheel diameters at all non-ducted outlets.
5. Attach ductwork to fans and AHU's with a canvas or elastomeric flexible connector. However, they are not completely effective because they become rigid under pressure, allowing vibration transfer. To maintain a slack position of the flexible duct connections, thrust restraints may be required (see "Vibration" section of this chapter).