Air Pollution Control Equipment Services, Coal

Silence is Golden

Issue 11 and Volume 109.

Today’s stand-by gen-sets must be more powerful, more efficient, and produce fewer emissions than their predecessors. And in a growing number of applications, they must make less noise.

By: Dave Gries, E-A-R Specialty Composites

Makers of stand-by generators have always had to manage the inherently competing requirements of noise control and heat management. Handling one well tends to make handling the other more difficult. In recent years there have been significant improvements in the range and variety of materials available to control noise and thermal energy in gen-set enclosures. There are now better options than ever before to achieve improved noise reduction and thermal heat control when retrofitting an existing unit or specifying a new stand-alone power generator.

The extent to which noise will radiate is always a consideration when enclosing a gen-set. Typically, it is best to minimize enclosure openings and to incorporate tortuous paths to help attenuate noise where openings can’t be avoided. But eliminating enclosure openings can jeopardize generator cooling. The problem is exacerbated by recent trends toward hotter running engines in smaller profile gen-sets. Noise control materials are exposed to higher generator temperatures. This means that balancing thermal management with noise control continues to be a major design consideration for modern gen-sets. Enclosure designs must provide optimal noise control and thermal management.


Barrier-absorber composites with aluminized facings can provide solutions for both noise control and thermal management. They also allow a one-step installation.
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Some gen-set designs are better than others in this regard because designers have used appropriate elastomers and composites available for controlling unwanted acoustical and thermal energy. Designers have also combined the need to optimize thermal and acoustical management with related design considerations such as water resistance and durability. Leading gen-set manufacturers have asked companies like E-A-R Specialty Composites to analyze virtual models of generator designs long before tooling or component purchasing begins to suggest modifications best suited to tackle the unique acoustical signatures of different designs. Manufacturers have also asked noise control engineers to re-engineer noise and thermal solutions for enclosure designs in long-established models. A variety of solutions have been developed incorporating a range of acoustic and thermal control materials and the ways these materials are applied in specific gen-set models.

Noise Control Materials

There are two main methods for controlling airborne noise in a power generator: blocking airborne noise via a weighted barrier, or absorbing airborne noise via acoustical absorbing materials. To effectively use barriers and barrier composites, at least 90 percent of a gen-set enclosure should be lined with a weighted barrier or a decoupled weighted barrier (made of a composite of barrier over decoupling foam) and enclosure openings should be minimized to the greatest extent possible. The greater the coverage by barrier materials, and the smaller the percentage of openings in the enclosure, the greater the potential noise loss.

The performance of a barrier is quantified by transmission loss, or the level of sound blocked. For weighted barriers by themselves, the TL is governed by the Mass Law, represented by the equation:

TL = 20 Log 10 (SM x F)- 33.5 dB

Where

TL is transmission loss
SM is total surface mass in pounds per square foot
F is frequency.

By doubling either the mass of the barrier or the frequency of the sound, a theoretical 6 dB improvement in transmission loss may be realized, essentially doubling the attenuation of sound pressure.

Further improvements in transmission loss can be achieved by adding a decoupler between the weighted barrier and substrate, for example, in the enclosure housing. Decoupled barriers are typically comprised of acoustical foam sandwiched between the inner surface of a power generator enclosure and weighted barrier. The substrate-foam-barrier (double-wall) construction acts like a spring mass system, in which the composite has increased transmission loss exceeding that predicted by mass law above the resonant frequency or double wall frequency of the composite.

Decoupler thickness can be optimized to provide optimal sound transmission loss at specific frequencies. Specifically, thicker decoupler layers do not necessarily translate into better acoustical controls, meaning that the thickest decoupler layer applied to enclosures does not necessarily produce the quietest units. Rather, the thickness of the decoupler layer needed depends on the problem noise frequency in a particular design. Essentially, a thicker decoupler will result in a lower stiffness, which, in turn, results in a lower double wall frequency and higher transmission loss in lower frequencies. Conversely, the thicker decoupler with lower double wall frequency resonance can be detrimental to noise control at low frequencies because the transmission loss of the double wall frequency is dramatically reduced. Decoupler thicknesses in the best designs are optimized for maximum transmission loss for a particular unit’s problem noise frequencies.

Another avenue for controlling noise utilizes sound absorber materials that reduce airborne noise from mechanical sound energy by converting it into low-grade heat energy. With sound absorbers, as air is pushed into the absorbing material by the sound pressure, waveviscous forces dissipate the mechanical sound energy as heat.

Sound absorbers are useful as a lining for louvers or to create a tortuous path for airflows, where noise is absorbed before it escapes the enclosure. Because power generation equipment requires several openings in the metal enclosure. These sound absorbing materials are important because power generation equipment requires several openings in the metal enclosure for air intake, exhaust and heat release. The geometry and physical makeup of the absorber determines its sound absorption characteristics. For instance, increasing the thickness of an absorber increases the absorption at the lower frequencies – the ones most difficult to control.

There are a variety of protective facings used on sound absorbing foams, which help guard them from fluids and grease and also enhance the noise control performance at lower frequencies. Many of the high-quality new designs make use of aluminized polyester facings that are especially beneficial because they reflect radiant heat. They offer an excellent solution for the inherent thermal management trade-off issues. Foams with black facings, still common in many enclosure designs, can absorb radiant heat and often make thermal management more difficult.

Thus, multi-layer composites are often the noise control materials of choice because they provide the most cost-effective solutions for many gen-set designs. Barrier-foam or foam-barrier-foam constructions can both block and absorb the airborne sound. Such multi-layer composites can help achieve dramatic noise reductions over a large span of frequencies within relatively limited space constraints. These are often the most cost-effective solutions because, by combining materials with different properties for noise and thermal control, the amount of materials used can be decreased, thereby providing the lowest-cost noise reduction ratios (dB reduction per dollar). Moreover, such multifunction composites can also save money by decreasing installation time.

Other Gen-set Design Constraints

Flame resistant properties of noise control materials can be important considerations, especially in meeting UL 2200 requirements for stationary gen-sets. Manufacturers should be aware of UL 2200 requirements relative to noise and thermal control materials. Furthermore, facings on acoustical foams vary in their ability to protect against rain, moisture and other contaminants. Impervious foam facings can be an improvement over more traditional solutions, and some newer acoustic foam formulations are also ideal for direct water exposure applications. Only recently have these impervious foams been available for retrofitting existing units or use in new gen-set enclosure designs. Greater durability can also be achieved by using facings with reinforced grid designs that resist puncture propagation during operation or maintenance. Aluminized facings have the added advantage of providing reflective surfaces that can enhance visibility, simplifying gen-set maintenance.

Gen-sets With and Without Acoustical Controls

Materials described for noise control solutions can make dramatic differences in noise outputs. Figure 1 shows a solution created for a 12 kW, LP-fueled standby gen-set enclosure in order to meet market driven requirements. The enclosure had less than 10 percent total surface area open, but only a small space for additional acoustical insulation. The acoustical signature of this unit was at the lower frequencies from the engine and exhaust, and higher frequency noise from the airflow, fan and alternator. Due to the limited space inside the generator enclosure and due to low frequency noise problems, a multi-layer composite decoupled barrier and faced absorbing foam were used. By incorporating an aluminized polyester facing on the acoustical foam, the acoustical properties were tuned for the lower frequency absorption where they were most needed. Additionally, the design achieved thermal management and protection of the acoustical insulation at elevated temperatures close to the engine and muffler. To attenuate the higher frequency noise, 1-inch faced acoustical foam was used near the enclosure’s fan and air inlets. The result was a reduction in noise from 73.5 dBA to 67.0 dBA.

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Figure 2 shows a solution for a diesel-fueled, 17 kW stationary power generator intended for light industrial use and with a metal enclosure with more than 20 percent of its total surface area open. In this solution, faced acoustical absorbing foams of 1-inch and 2-inch thicknesses were selected. Low problem frequencies were addressed by incorporating acoustic foams of increased thickness and facing constructions specifically tuned for low frequency sound absorption.

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Here, too, aluminized polyester facings were used as heat reflectors in selected locations of the enclosure. In addition, two inches of a highly moisture-resistant, rigid acoustical foam were used to line end-cap baffles. This package realized a noise reduction from 78.1 dBA to 73.9 dBA.

Proper engineering and design of gen-set enclosures and use of the best acoustical and thermal materials are critical to optimizing sound attenuation in power generation equipment. Power engineers having difficulty meeting the challenges of ever more stringent community noise ordinances need to revisit the types of gen-sets they are offering. Properly designed, many of the best acoustical control solutions are also the most cost-effective ones, meaning that gen-set prices do not necessarily equate with acoustical control quality.

Author:

Dave Gries is an applications engineer specializing in noise, vibration and harshness serving industrial OEM markets for E-A-R Specialty Composites, maker of solutions for noise, vibration, thermal and other engineered solutions. He has played a leading role in helping power generator manufacturers develop next generation gen-sets.