New Strategies for Noise Mitigation

More stringent emission requirements, coupled with greater energy requirements at large facilities, where owners and operators seek more power density, have become a driving factor in current sound attenuation designs.

“The biggest challenge we’re facing is dealing with the airflow—the increased cooling and combustion airflow that needs to be addressed,” notes Mike Witkowski, chief operating officer with Pritchard Brown.

The company provides solutions for projects with stringent sound requirements or a combination of stringent sound requirements and site constraints.

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Jeremy Off, engineering manager for Robinson Custom Enclosures, agrees.

Off notes that one of the latest trends is end-users updating equipment to meet the Tier IV engine standards with part of the consideration being acoustical attenuation through balancing out airflow and heat rejection.

Off notes the impact equipment choice has on acoustical output.

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Equipment that is improperly sized can “lead to a poorly performing package in terms of acoustics and how that structure-borne noise can actually translate into audible noise, affecting the overall acoustic performance of the package,” he says.

While Off is not seeing an increase in noise regulations, he is noting an increase in awareness of them among specifying engineers.

“Those laws are there to protect against noise pollution more or less and to make sure that everybody has a suitable environment acoustically,” he points out.

Given that, approaching sound attenuation is “really a matter of how you’re going to deal with sound waves,” notes Dale Gremaux, sales and marketing manager for Harco Manufacturing, “a lot of times, the trick is to tire them out. Some companies use space to tire them out—the longer the better. We try to do it with a maze.”

Gremaux points out there are some materials new to the market that, while they are “great for sound attenuation,” they get hung up with the heat in exhaust.

Most of the highly sound attenuative products are “critical grade or super-critical grade,” says Gremaux.

In choosing an acoustics or emissions partner for a project, understanding the site’s environmental regulations and acoustical impact of single and multiple engine installations is key, notes Nick Detor, division sales manager of power generation at Miratech.

“It’s important to understand the complete system—including intake and exhaust—as well as the potential for breakout sound and the effect layout has on system back pressure,” he says.

Also important: “Understanding the cumulative effect of multiple engines or finding unique ways to deal with low-frequency sound or engineering for seismic and wind load for example,” says Detor.

Any given facility can be operating under a myriad of noise regulations, from the federal to local level.

In choosing the appropriate enclosure, a primary consideration for energy professionals is the site.

“Routinely what happens is that they’re trying to minimize the amount of real estate that is dedicated to the onsite power system,” says Witkowski of facility owners. “But a very large generator set requires more cooling air—we routinely see more than 100,000 cfms of cooling air.

“When that much air needs to be moved, there are all sorts of fluid dynamic properties that need to be taken into consideration. It’s not just a matter of covering this thing that’s going to absorb and block the sound. Now you need to move an incredible amount of air through the system efficiently while still maintaining the sound specification.”

As such, his company is often asked to design a system to direct the air in a particular direction based on the site.

“If a unit is being placed right outside the building where people will be walking nearby or there are building ventilation systems nearby or ordinances, this hot air that’s being rejected from the radiator on the onsite power equipment needs to be directed appropriately, and that needs to be part of the sound attenuation solution,” points out Witkowski.

Space availability is another consideration. “We’re trying to get facility owners to work with engineers on the front end to make sure they’ve allowed enough space and have thought through all of these things because once a unit is onsite, it’s very difficult to modify it appropriately if it’s not doing what it’s supposed to do,” he adds. For instance, egress and how the units interface with the building and other equipment onsite is a prime consideration, says Witkowski.

Thus, it is critical in a facility’s planning stages that a facility manager’s needs for adding onsite power generation take noise ordinances and emissions equipment into account from the very beginning “so that the enclosure design can be consistent with making sure it gets things quiet enough as well as provide the proper environment for the engine to run,” he adds.

Witkowski also notes another design phenomenon taking place as emissions requirements change given the temperatures at which the units are running have increased. “The exhaust gas temperature and some of the heat ejected into the room and the enclosure has gotten much greater with much higher temperatures,” says Witkowski. “Sound attenuation materials need to be chosen to make sure they’re consistent with the higher temperatures of the equipment.”

In the past, certain insulation materials would have been sufficient for 800-degree exhaust temperatures, but with temperatures now up to 1,400 degrees, “the materials need to be chosen accordingly to make sure it’s safe and functioning,” he adds.

In choosing the correct sound attenuation product, facility managers need to know where they are going to put it, how they are going to measure it if they have sound requirements, and what is the starting point, says Gremaux.

“Sometimes people don’t give thought to the fact that they’re using a generator and are going to measure sound, but when the generator is running, what is the sound coming from it? Those are key questions we try to probe and ask engineers, the mechanical contractor, or a facilities manager when they have requirements,” he says.

Those tasked with choosing appropriate sound attenuation “should know that closer to a wall makes the sound worse—it turns into an echo chamber,” says Gremaux. “Most facilities managers put it out in the middle of a generator room or have a room dedicated to all of the load banks and gensets.”

Witkowski agrees that unit placement will dictate noise levels.

“If an enclosure is placed right next to a building wall, because of the nature of acoustics and how everything works exponentially, you can sometimes double the effective loudness by all of the noise that would be reflecting from the buildings,” he says. “That needs to be taken into account at the very beginning when you’re trying to determine how to specify what the system needs to do.”

Witkowski points to data center facilities, which have dozens of enclosures.

“If they’re in an enclosed area, you can have not only all of the sound—it’s an additive characteristic with all of the different sources—but you can also create a situation where you get standing waves,” he says, adding that the site’s geometry will enhance certain frequencies and can create a problem.

“That needs to be part of the initial evaluation when they are designing their site,” he says.

The unit’s material is another key design factor.

“Most enclosure manufacturers use steel panels and acoustically, mass is king. So building an enclosure out of steel makes the job easier acoustically because it is providing more of a barrier,” says Witkowski.

Alternately, steel is a material that will inherently corrode in the weather, so it’s important that the sound attenuation unit is painted or coated as to mitigate deterioration, he notes.

Many engineers now specify aluminum enclosures from a corrosion-resistant standpoint, notes Witkowski, adding that while it is preferred as a material that will stand up better over time, from an acoustic standpoint, they have to devise a way to add mass to the enclosure.

Robinson Custom Enclosures offers a variety of materials, including stainless steel, aluminum, carbon steel, and specialty alloys.

Speaking to the difference among the materials, Off notes that the end-users are the drivers in what materials are utilized.

“Some people are in it for the finish, some people are in it for the fit, some people are in it for more of a cost standpoint,” he says.

“You’re naturally going to get more of an insertion loss from a steel material than you would for a similar aluminum material, so we compensate by adjusting the wall structure and the components within our wall accordingly,” notes Off.

Miratech’s products are specified by engine owners for Best Available Control Technology exhaust emission solutions, which often include not only reduced exhaust emissions but also better noise abatement, notes Detor.

Included in Miratech’s product offerings are engine silencers ranging from 10 kW to 10,000 kW and beyond in a number of physical configurations and acoustic grades.

“High-performance silencers are becoming an increasingly important part of distributed energy plant design as locations begin to dictate stringent acoustic performance requirements,” notes Detor.

The Miratech Quad-Tuned Silencer exemplifies how the company applies standard customization to address challenging acoustic targets by incorporating a wide range of acoustic elements including quadruple-tuned reactive components that effectively control sound and allow for higher sound attenuation than multiple silencers in series while reducing the risk
of excessive back pressure, notes Detor.

The field-tested, base-mounted vertical silencers provide sound attenuation of up to 60 dB(A). The company also offers process silencers, including vent and blowdown as well as intake silencers and PD blower silencers.

Vent and blowdown silencers are used to vent high-pressure steam, air, or other gas to atmosphere, Detor points out. “These silencers typically consist of a diffuser inlet engineered and manufactured to pressure vessel code, combined with acoustically treated concentric rings, stacked to reduce high-frequency sound to an accepted level for surrounding receptors,” he says. “Intake silencers are used in many applications, including combustion air inlets where fresh air is brought into the plant for engine combustion or other processes.”

Miratech’s solutions are typically packaged in combination with a housing that has sound attenuating properties. Most are combinations of two or more of the technologies.

The company’s systems are designed to reduce regulated exhaust stream pollutants to failsafe, first-step compliance levels for any stationary reciprocating natural gas, diesel, or dual-fuel gas engine.

Those pollutants include Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hazardous Air Pollutants, Unburned Hydrocarbons (HC), Volatile Organic Compounds, and Particulate Matter (PM).

The company utilizes a variety of technologies to address the reduction of pollutants, with Diesel Oxidation Catalyst, Diesel Particulate Filters (DPF), and Selective Catalytic Reduction (SCR) systems being the most utilized.

Miratech offers active and passive DPF solutions, designed to meet application and operating requirements while packaging the system for minimal space and long service intervals.

For example, the Miratech Mira-Cube offers stationary diesel engine owners and operators new technology within a single housing assembly. It contains low-temperature regen DPF, vortex oxidation catalyst, and SCR NOx reduction systems assembled into a low-profile, integrated package that also provides sound attenuation.

The system is designed for diesel engines in the range of 1,000 kW to 4 MW. It can be retrofitted to existing engine applications and will control pollutants (NOx, CO, HC, and PM) to EPA Tier 4 Final or higher levels, says Detor.

Part of the Miratech offering is the Vaporphase ECXW Hot Water Bare Fire Tube Exhaust Waste Heat Recovery Silencer, a two-pass design permitting convenient inlet and outlet locations, vertical or horizontal orientation, and silencing even with the optional exhaust bypass valve in bypass.

As a key component in combined heat and power plants, Exhaust Gas Heat Recovery provides an additional level of efficiencies in carbon footprint reduction over traditional engine installations, notes Detor.

Firwin offers a sound blanket design to go over equipment to mitigate noise and sometimes partners with other sound attenuation companies to offer complete solutions.

“Noise usually requires a multi-pronged attack because the noise can spread very easily,” notes Brett Herman, vice president of sales and engineering for Firwin. “If you have a noise source, it’s very hard to capture all of the noise and this one literally encapsulates the whole piece of equipment.”

Facility owners and operators could use insulations as noise absorbers, but Firwin also can add sound barriers into the blanket, says Herman.

The company also makes panels that can be attached to the walls of an enclosure to block the noise emanating from it.

“Sometimes that is the preferred way to go because you are capturing all of the noise as opposed to blankets that would be on a particular part of the component but may not address every area where the noise is coming out,” says Herman.

“Sometimes it’s just too expensive to build a room or walls around it. If it’s a big piece of equipment, sometimes it’s not practical. It’s too difficult or will get in the way of functions of people moving around, so that’s why they would try to address a specific area of equipment.”

Typically, end-users approach Firwin for removable insulation blankets to deal with heat-related issues. Sometimes, the company also gets requests for help on sound attenuation. Its sound attenuation solutions are based on using combinations of materials to absorb sound, offer barriers to sound, and to close up places where there is sound leakage.

While the company offers a range of sound attenuation products, some applications call for the combination of sound attenuation products from within its removable insulation blankets product line.

Case in point: the company had been commissioned to develop and install such a combination blanket for a large newsprint manufacturer’s 10 CAT Natural Gas engines. Each engine had its own Miratech SCR after-treatment system.

Firwin designed and installed 4-inch thick removable insulation blankets as a combination heat abatement/noise attenuation for the SCR.

The company also incorporated a Noise Block sound barrier into the blanket design to arrive at the noise control goals over the client-specified frequency range. The blankets needed to be designed in a “multi-part, multi-layer” format due to the large size of the SCRs and the thickness of the insulation blankets.

The design also had to meet the challenge of ensuring that access ports to the SCRs were left open and the blanket joints and seams needed to be staggered to ensure a snug fit and optimal noise reduction.

While sound attenuation is primarily focused on stationary units, mobile generators used during power outages also require it.

“It becomes that much more challenging if the equipment is requiring more air because of the sound attenuation devices that are needed to address airflow in and out of the enclosure in a mobile application if the customer wants to be able to pick it up and move it over the road legally within an 8.5 foot wide footprint,” says Witkowski.

Mitigating that may entail using smaller generator sets and multiple trailers if more power is needed; the sets can be used in parallel, he adds.

“Or they can design it so it can fold up within itself for transport,” says Witkowski. “Once it gets to the location where you’re going to need the power system to operate it, there’s labor associated with installing or actuating whatever external air handling devices are going to be on there that will allow the unit to run quietly and still get the proper airflow.”

Typically, facility owners and operators bring in rental units through local dealer distributors, he notes.

“The facilities managers are normally not worried about specifying the design of a mobile unit, but they would need to have provisions for a mobile unit built into their facility and room for enough mobile units to give them the power they would need once they are brought onsite.” 

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