Noise attenuation and onsite power systems
By Carol Brzozowski
On a noise level decibel scale, rustling leaves is 10 a-weighted decibels, or dB(A). A pneumatic riveter is 130 dB(A). The pain threshold is 140 dB(A). A generator set ranging from 50 kW to 1,500 kW produces noise at the level of 85 dB(A) to 103 dB(A). On a descriptive scale of “very quiet” to “uncomfortably loud,” gensets are classified as “very loud.” And, in order to comply with Occupational Safety and Health Administration (OSHA) regulations for workers exposed to genset noise above 80 dB(A) for any appreciable time and community noise ordinances—which are growing in number—gensets require noise attenuation.
New advances in noise attenuation combine it with emissions control. Dale Gremaux, sales and marketing manager for Harco Manufacturing, says combining silencing of onsite power with new emission products “are really going to change mufflers as we know them.”
“You can do both in the same housing or package,” adds Chad Kaderabek, marketing manager for the Universal Silence, which produces acoustic, emission, and filtration systems for natural gas turbines and diesel engines in stationary and portable power generation applications. “Otherwise, you’d have to have one component as an acoustics treatment and another as emissions treatment. It makes it a lot simpler for design as well as installation.”
There are now filters that take soot out of a diesel engine to keep it from spilling into the airshed.
“It looks like a silencer with a door,” says Gremaux. “It’s cleaning the air as well as taking the noise out of the exhaust. It’s two for the price of one. You’ll see a lot more people saying they have a particulate filter and muffler that will meet sound attenuation as well or like in California, that meets the California Air Resource Board Regulations for air quality.”
According to Dennis Aaberg, a principal acoustics technician for the Applied Technology Department at Cummins Power Generation Inc., the maximum permitted overall noise levels in North America range from 45 dB(A) to 72 dB(A), depending on location and zoning. Aaberg says there is a movement afoot to restrict community noise by frequency bands. He says strict ordinances exist in New Jersey, Illinois, and New York City, NY. Canada is also advocating legislation to restrict sound levels by frequency band, he adds.
“In the past, most community regulations just referred to overall dB(A) level, but many times low frequencies can be annoying at long distances, which usually isn’t accounted for in an overall dB(A) level because it’s a weighted spectrum,” he says. “That’s why communities have started to restrict it more by low frequencies, so there aren’t annoying low frequencies traveling long distances at night.”
Illinois’ Title 35 includes an environmental protection and noise requirement with strict sound level attenuations at about 65 dB(A), says Robin Moore, vice president of engineering and operations for Henning Enclosure Systems.
“They’re also requiring real dB(A) levels at all the various frequencies,” he says. “Extremely low and extremely high frequencies are very difficult to attenuate.”
While community noise ordinances come into play on outside genset installations, OSHA regulations generally regulate indoor noise. Michael Witkowski, vice president of sales and engineering for Pritchard Brown, points out that for the most part, standby gensets run an hour a week for testing and may run longer during a temporary power failure.
“If generator sets are being installed in a building’s equipment room, and also in that building is the electrical switch gear and other devices upon which there is regularly-scheduled maintenance, or if it is considered an occupied space, then the OSHA regulations come into play,” says Witkowski. “Typically, with an outdoor enclosure, OSHA basically says if it’s above 80 dB(A) and someone is going to be around it for more than eight hours, you need to start worrying about it.”
In a white paper on noise attenuation, Aaberg notes: “Since untreated generator set noise levels can approach 100 dB(A) or more, both the location of the generator set and noise mitigation take on great importance.”
Aaberg says generator set noise is produced by six major sources:
- Engine noise: Mainly caused by mechanical and combustion forces and typically ranges from 100 to 121 dB(A), measured at one meter, depending on the size of the engine
- Cooling fan noise: Results from the sound of air being moved at high speed across the engine and through the radiator. Levels range from 100 to 105 dB(A) at one meter. “Many times, the cooling system noise is louder than the engine noise itself,” says Aaberg. “Long duct lengths attenuate cooling system noise.”
- Alternator noise: Caused by cooling air and brush friction and ranges from approximately 80 to 90 dB(A) at one meter and is one of the lesser noise sources, says Aaberg
- Induction noise: Caused by current fluctuations in the alternator windings that produce mechanical noise ranging from 80 to 90 dB(A) at one meter
- Engine exhaust: Without an exhaust silencer, this ranges from 120 to 130 dB(A) or more, and is usually reduced by a minimum of 15 dB(A) with a standard silencer.
- Structural/mechanical noise: Caused by mechanical vibration of various structural parts and components radiated as sound. “This tends to be more of an issue for building installation, where vibration can go through the generator set into the structure,” says Aaberg.
He adds that the total noise level from a generator set is the sum of all the individual sources, regardless of frequency. According to Aaberg, before a noise mitigation plan can be determined, accurate sound measurements of both the existing ambient noise and the noise contributed by the generator set are needed.
He cites seven strategies for reducing generator set noise:
- Reduce source sound levels
- Use acoustic barriers: Rigid materials with significant mass and stiffness that reduce sound transmission
- Employ acoustic insulation: Sound-absorbing materials that line air ducts and cover walls and ceilings
- Use isolation mounts: Flexible joints at connection points within the generator system to reduce noise transmission from vibrating equipment at connection points, such as skid anchors, radiator discharge air ducts, exhaust piping, coolant piping, fuel lines, and wiring conduit
- Attenuate cool airflow: Inlet and outlet air attenuation baffles can help reduce noise produced by the cooling air as it moves across the engine and through the radiator, which can be significant due to the required volume of about 20 cubic meters per second for a generator set with a 50-liter diesel engine
- Maximize distance: When there are no reflecting walls to magnify generator set noise, the noise level will decrease by 6 dB(A) every time the distance doubles. If the property line is within the near field of a generator set, however, the noise level may not be predictable.
- Use exhaust silencers: Also called mufflers, they are almost always part of the generator set equipment and are either chamber or spiral. While chamber can be more effective, spiral devices are more compact and may provide sufficient attenuation for a variety of applications. Stainless steel is preferred for outdoor use to guard against corrosion. Generally, the more effective a silencer is at reducing exhaust noise, the greater the restriction level on the engine exhaust. For long exhaust systems, piping provides some attenuation levels.
Aaberg also advises that steel and aluminum enclosures provide at least 10 dB(A) of attenuation for outdoor generator sets. Combined with an effective exhaust silencer, they may be sufficient to meet many local ordinances.
“When a greater amount of attenuation is needed to meet local noise ordinances or reduce impact on employees or neighbors, special sound-attenuating enclosures must be employed,” says Aaberg.
There are site-specific considerations with generator sets and noise, says Aaberg.
“If you install them indoors, you need a building surrounding it, plus all the airflow requirements,” he says. “You have to design exhaust systems that would go up and through the roof of the building. They can create backpressure, so you have to account for that. That can be very expensive because you need to build a building.”
Outside installations must account for community noise issues, especially at night. “You then have to design very expensive enclosures that, many times, end up being what we call ‘drop-over’ enclosures that allow for sufficient air to flow through, but have enough acoustic treatment to bring the generator set levels down to those outdoor requirements,” says Aaberg. “Usually, that involves some fairly substantial discharge and inlet duct lengths to handle the air flow—similar to a house furnace. You have to put duct liners in, which is like an acoustic absorption material, usually fiberglass.
|Photo: Harco Manufacturing
Diesel Particulate Filter replaces standard muffler on a Generator in Portland, OR.
“Determining what sound attenuation is required first requires knowledge of community regulations to know what the maximum-permitted sound levels are at the property line,” he adds. “Then you go to your product’s sound level to determine how much attenuation you need from the exhaust system and housing and design to meet that requirement.”
In mitigating sound, most lay people assume all noise comes from the manifold exhaust gas outlet on which they put a muffler, says Larry Hansen, a principal engineer with Engineered Aeroacoustics.
“That’s wrong,” he adds. “Although the exhaust gas on the manifold may be a loud point, it’s just one of several. There is some whining coming from the alternator. Most diesel engines are turbo-charged. You get about a 3,000-Hz tone coming off the turbocharger. Then, you’ve got the vortex shedding on the radiator, which is the air passing over the tubes on the heat exchanger.
“You end up with a very unique situation on the radiator discharge side, in that you’re combining a negative slope of the mechanical noise,” he continues. “That means that as the sound goes up in frequency, it goes down in intensity, and then you combine that with the vortex shedding off the radiator which is called positive slope. And with increasing frequency, you get increasing sound intensity.”
There’s a crossover point where the block becomes minimal at high frequency and the radiator generated noise becomes maximum, and at low frequency the block radiation becomes maximum and the radiator radiation is minimal, says Hansen. It is less costly to incorporate proper noise attenuation during the design phase of a genset system as opposed to a retrofit, which may equal the cost of the generator set.
“A lot of these gensets go into somewhat-tight spaces,” says Moore. “If you don’t have the space available, it can be very difficult. You might be able to direct the air up on top of the unit and build something on top, but is the enclosure able to handle more weight up on the top?
“It’s going to get real expensive,” he adds. “Plus, now you’re doing it out onsite where, when we’re in a controlled environment in the factory, we can do things a lot less expensively.”
There are added costs in needing to reroute extra distribution piping, Kaderabek says. “Also, you’ve got to look at the overall system, because the backpressure requirements need to be evaluated upfront on what that’s going to cost to your engine. Putting on a silencer after it may affect engine performance and efficiency.”
Noise control is intimately attached to aerodynamics, Hansen says. “It doesn’t do any good to attenuate a generator that’s going to go over temperature and shut down because you’ve got too much backpressure. The dynamics of a site’s topography and climatic conditions are also a factor. All this has to be addressed during the noise control design phase.
“Even if you are retrofitting, all of that has to be a consideration, because the basic elements that have to be addressed in sizing a generator design are also integral to the noise control issue. It is aeroacoustics, the combination of acoustics and aerodynamics, because they go hand-in-hand.”
Footprint issues also will affect noise attenuation. Gremaux points out there are a lot of challenges involved in trying to put a large backup generator in a small space and then realize it’s got to be muffled.
“If you put a muffler on the roof and pipe it so many floors, that pipe adds backpressure or stress to the engine so it will break,” he says, adding it’s important that be considered in the design process.
The key issue is exhaust noise versus engine noise, says Gremaux, whose company manufacturers products used in inside and outside applications.
“Silencers are built to muffle the exhaust noise,” he says. “That doesn’t mean the engine can be silent. People are surprised: ‘Shouldn’t the sound be what you told me it was going to be?’ It is, but you hear the engine noise rather than the flow of exhaust.”
|Photo: Pritchard Brown
Multiple units are assembled under an existing helipad at a medical center to form one large, ultra-quiet generator-set room in previously unused space
More often that not, it makes more sense for gensets to be installed in an outdoor enclosure, says Witkowski. “Normally, an outside standby or onsite generator set is going in an enclosure because there is no dedicated space in the building. In a data center, that’s because the square footage they can get renting that space out to a client is so high that the criticality of maximizing every square foot of space for generating income makes a lot of sense for putting the genset on the roof or in the parking lot where it is not eating up real estate and money.”
Existing hospitals and university campuses can get landlocked, and in adding more rooms, need more standby emergency power capacity yet cannot put more into existing utility space, so the genset is installed outside. To address footprint issues, Engineered Aeroacoustics came up with an “over-under” design, in which some requirements, such as telecommunications, is a prime consideration.
“In that, elevation is sacrificed for the sake of maintaining the absolute minimal footprint,” says Hansen. “At some power generation sites that’s a prime concern because real estate costs money.”
In the past, buffer zones were used to address acoustical issues on projects going up on empty spaces.
“At this point, it’s a much wiser financial decision to provide sound attenuation than to invest in a lot of real estate,” says Hansen.
It’s important that mechanical and design engineers consider all variables during the design process, Gremaux says. “Sometimes you get a situation where somebody is trying to come in under budget, and it just gets forgotten,” he says. “Sometimes, they bought the engine with a silencer on it. Sometimes it is a silencer, but it’s not a silencer for the application the engine is going into, so it won’t meet the sound requirements.”
For the design, Moore says in order to obtain a good estimate of what is going to happen in the field, there needs to be a good estimate of what the source noise is going to be.
“There is a certain amount of testing done by the generator manufacturers and some lack of information also,” he says. “There is a de-ration that takes place in a gas-powered generator instead of a diesel-powered generator. You may be getting a 750-kilowatt generator, but it’s taking an equivalent of a 1,500-kilowatt engine to get it. Those sound levels will certainly be higher because the engine is quite a bit bigger. We’re able to tell someone the reduction we can give them, but if their source noise is off and it’s higher than what’s been estimated, they may not achieve that end result they wanted. You can calculate all you want, but the truth is you need to make measurements, find out what these things are going to do, and then set standards.”
Moore says there is some debate among manufacturers over sound transmission loss over sound absorption. “Foam is a very good sound absorber,” he says. “We use a lot of mineral wool-type product that absorbs sound pretty well. Transmission through a wall or otherwise is a big issue and is attenuated by having high densities in that wall.”
There is a balance that needs to be achieved in doing noise attenuation, says Moore. “You want a certain amount of absorption. You don’t want the sound bouncing around in your enclosure and continuing to hit the walls and vibrate the walls, so we want to absorb as much as we can. But also we want to have as much density in those walls as possible to keep the sound from going through to dampen that noise.”
|Photo: Pritchard Brown
With attention to landscaping and color selection, even large sound attenuated units can enhance the surroundings.
Witkowski points out that everything about noise behaves exponentially. “In order to provide sound attenuation on standby power generation—particularly a reciprocating engine that requires large amounts of cooling air flow, gives off a lot of heat and has a very loud noise signature—know that whatever you use to quiet that system is going to grow exponentially,” he says. “The size and cost of the sound attenuating enclosure also grows exponentially as more attenuation is required.”
Designing a proper noise attenuation system requires the input of consulting engineers, architects, and planners, he says.
“When there is a new facility that is going to require so many kilowatts of standby power, the very first thing they should be doing while they are looking into building and emission permits is what the local noise ordinances are and determine what existing noise is at the site, so they can work with a reputable manufacturer to determine how much space is needed for the system,” says Witkowski.
Moore, whose company—Henning Enclosure Systems—deals primarily with outside genset noise attenuation, says a lot of the company’s clients aren’t educated on noise, especially if they have a difficult sound specification.
“They will ask for sound attenuation of results that are less than ambient,” he says. “You will have people ask you to be in the 50 dB(A) range. Ambient is probably 60 to 65. They’re asking for something they don’t even need that will not have any effect on the environment.
“The other thing is you’re dealing with spaces where you may be near a building where you’re getting the sound waves bouncing off of a building and coming back, and it’s hard to get good readings,” adds Moore. “Sometimes, you’re dealing with issues where they may have offices above the enclosure that can create some challenges when you’re trying to run the sound up.”
The motorized fan that makes noise and the openings needed to accommodate cooling airflow becomes part of the challenge of attenuating noise, Moore points out.
The marketplace offers a number of choices.
Universal Silencer is working on combination silencers that use absorption technologies. “There’s also one that is reactive—they are engine exhaust silencers,” says Kaderabek. “Active silencers are more applicable to automotive applications, but the technology is too expensive and unreliable.”
The technology consists of a microphone and speaker that picks up different engine frequencies, which go through a processor and emits a sound wave that cancels the sound wave being emitted. The processor has to be constantly changing based on the performance of the engine.
Pritchard Brown custom builds sound-attenuated weatherproof enclosures of 10 to 45 dB(A) noise reductions. “In essence, the way you quiet down something that’s loud is you use transmission loss—you add mass around it to prevent the sound from getting through the walls,” says Witkowski. “Then you treat the cooling air and engine exhaust noise by inserting devices that have a high insertion loss, which allows air to pass through but prevents the noise from passing through.
“There are new insulating materials that will take higher temperatures, and new composites available to be used within the walls that allow you to add more mass without necessarily using steel or concrete—the traditional methods of making a wall heavier,” he says.
Aaberg says that while the industry is still using passive systems—a barrier between the sound source and ear—“there are active noise and vibration systems that can absorb certain frequencies. They tend to be very expensive, so they may be cost-prohibitive, but can be used at some point if it becomes necessary.”
Hansen says the industry has historically “erected barriers, put the generator in a pit either above or below grade, and tried to screen it visually, hoping that would also provide attenuation, but in most cases it aggravated the acoustic and aerodynamic issues. Such barriers would promote recirculation of aerodynamics, reverberation, and mechanical amplification of the sound.”
Engineered Aeroacoustics uses computational fluid dynamics (CFD) analyses as a design element to meet the needs of available static pressure of the cooling fan on the radiator and also meet topographical and climatic conditions in which the generator is being set. The company offers clients the ability to put together three-dimensional computer simulations that incorporate a site’s topographical map or show a view from a premium satellite service and scan the site with radar altimetry to obtain topographical data.
“Then you get the effect of the natural barriers—the hills, the berms, the valleys, the foliage,” says Hansen.
In some cases, there may be a fine line between applying noise attenuation and sacrificing generator set performance, Aaberg says, adding that sometimes performance must be sacrificed to meet strict noise requirements. Otherwise, a system must be engineered to meet that performance drop and still be efficient, he adds.
“For example, you may need a 500-kilowatt set, and, to reduce the noise sufficiently to meet the requirements, it might end up only producing 450 kilowatts,” says Aaberg. “You may have to go to the next size generator set and attenuate it to the point where you can still produce the power, but meet the property line requirements,” he says.
Hansen says silencing and backpressure need to be provided within the constraints of the engine manufacturer’s specifications.
“The lower the pressure, the less horsepower taken from the engine, which means the less fuel wasted on engine parts,” he says. “For an emergency power unit, you’re more concerned that the generator is going to be available for whatever length of time the emergency lasts. When you get into a situation where the engine’s being used as a prime source for gas compression or pumping, for example, then it has to provide that mechanical or electrical energy on an ongoing basis and it gets even more critical in terms of ongoing cost to the owner.”
The more static pressure inherent in the design, the more cost it is to the owner on an ongoing basis, Hansen says.
“The number one issue on prime power—be it mechanical or electrical—is assuring the end user is not being severely penalized on horsepower, because the cooling fan is going to take more horsepower out of the engine or it’s going to require more fuel on the compression site, and that’s less fuel that can be delivered to their customer.”
Witkowski points out if the genset does not get the proper amount of air, it can overheat and be unable to provide the amount of electrical capacity that the supplier promised it would in summer conditions. “If the air handling is not sized appropriately, it causes an excessive static pressure drop through the enclosure,” he says. “Essentially, it creates a negative pressure that not only can lead to problems with the unit drawing in weather, but cause safety concerns.”
His company has seen units with air handling so poorly designed that when the genset is running, the personnel door cannot be opened because of vacuum pressure created by the large fan trying to pull in its cooling air. “It can literally create an environment where if someone is in there working on it and the gensets running, and there’s a problem and they need to get out, they can’t because they can’t open the doors,” says Witkowski.
To minimize the effect of backpressure, a larger sound-attenuating air handling device can be designed to minimize restrictive flow distribution.
“Depending on the type of noise that needs to be treated, you can have different styles of silencers, such as reactive and absorptive,” says Witkowski.
Gremeaux’s company works from a database of genset engine models, and will not set up a silencer that will go over the line on how the silencer will put backpressure on the engine.
“Sometimes, the engine might have an outlet coming out that’s 12 inches,” he says. “We want the inlet of our silencer to be 14 inches, so we’ll make a transition piece that will meet all of the requirements and it doesn’t hurt the engine.”
Moore says his company allows for 100% load ratings. “We do it within maintaining the temperature level of the generator needed to keep it running,” he says. “There are sensors on these generators. If they get heated up over that level, they will shut down.”
Henning Enclosure Systems addresses the challenge by putting plenum on the ends of the incoming and outgoing air enclosures.
“Typically, on the intake side, that plenum will be open at the bottom,” says Moore. “It’s quieter on that end anyway, because you don’t have the fan or radiator on that end, so you have a better chance. The radiator end is where we’ll typically run the air up through a plenum, and it will be open at the top.”
Another approach is to use baffles, Moore says. “We make that air run through a tortuous path. Within that tortuous path is insulation that is absorbing energy,” he says. “Every time a sound wave gets turned it loses energy, so we make it bounce around in this tortuous path, and eventually it comes out into the plenum. We may have more baffles on that plenum, depending on what sound levels people are trying to attain.”
Author's bio: Carol Brzozowski writes on the topics of technology and industry.