Mastering The “Black Art” Of Noise Control : Selecting the right silencer for generator set installations requires balancing acoustic requirements, size and backpressure.
by chad kaderabek Chad Kaderabek is marketing manager at Universal Acoustic and Emission Technologies, Stoughton, Wisconsin, U.S.A.
Generator set design is complicated. Location, power output, fuel source and engine type are only a few factors involved. Add in noise level restrictions and the design phase can become even more complicated. Yet the process can be made simpler by focusing on a few key aspects. When it comes to noise control in gen-sets, the “black art” is balancing the level of attenuation required with silencer sizing and system backpressure considerations. Typically, two of the three can be maximized, but only at the expense of the third.
In addressing noise, the first step is determining the level of attenuation necessary, which is dependent on site and property site-line requirements. OSHA outlines permissible noise exposures to employees before requiring personal protective equipment. The permissible noise exposure takes into account both the exposure time in hours, as well as the level of sound, measured in dB(A). An accompanying table shows OSHA’s Permissible Noise Exposure Standards. If the facility where a gen-set will be installed does not want to require protective hearing, the gen-set will need to include an outside source to control mechanical noise.
Another consideration for the level of attenuation required is property line noise level. National, state and local regulations, as well as construction codes for noise pollution, need to be researched. If the project is a new facility or a major refurbishment, the project architect is a good source to help establish the maximum property site-line noise levels.
Most local regulations and codes have different night and day maximum noise levels. As the nighttime level requirements will be less than the daytime level, it should be determined if the gen-set will be operating during the more stringent night noise levels. If the gen-set is an emergency or backup system, it may need to operate on a 24-hour basis. In these instances, the gen-set should be designed to meet the more restrictive nighttime noise levels. Once the site and property site-line requirements are determined, the mechanical noise of the gen-set must be established. An operating gen-set has many sources of noise, with the two largest being the mechanical noise of the engine and the cooling fan. The manufacturers of each component will be able to provide a detailed sound attenuation chart.
If the mechanical noise of the genset is greater than the maximum site allowances, additional noise control measures are needed. Site features that have an acoustic impact may also need to be considered. trees and buildings can absorb or redirect sound waves before they meet the property line. Also, if the gen-set is specified with a sound absorptive enclosure, it will add additional noise control.
As the engine’s exhaust system is typically the largest source of noise, there are several types of silencers available. Each is designed for certain applications. The most common types are reactive, absorptive and combination. Reactive silencers are ideal for controlling low-frequency sound waves, which are common on low-speed, high-horsepower engines. Reactive silencers use expansion chambers and/or pass-through tubes to reduce exhaust noise. These are the most common style of silencer and are also some of the least expensive.
Absorptive silencers are used to treat high-frequency sound waves. High-frequency sound waves are more common in high-speed, lowhorsepower engines. The pulsation of the exhaust stream is treated by using insulating material such as fiberglass in the shell of the silencer.
- There are three types of silencers used on generator sets. A traditional reactive design (left) uses pass-through tubes and expansion chambers. The absorptive type (center) incorporates packing material in the shell of the silencer, while the combination silencer (right) incorporates both reactive and absorptive technologies.
The combination silencer incorporates both reactive and absorptive characteristics. If an engine produces both high and low frequencies, the combination silencer should be selected. The drawback of the combo silencer is that it tends to be more expensive than other types. In fully enclosed gen-sets, space inside the package is at a premium. Some sites allow the mounting of the silencer on the top or farther away from the package. Locations like hospitals and office buildings generally generally prefer the silencer to be mounted internally, as it provides a more streamlined and aesthetically pleasing design. As no two gen-set designs are ever alike, it’s rare to locate the silencer in the same location. The best option is to place the silencer in a corner, roof or center location near a point that can provide some air movement for cooling.
Several pieces of information are needed when selecting the silencer. They include:
• the engine exhaust flow;
• the engine exhaust connection size;
• the engine exhaust gas temperature;
• the maximum allowable engine backpressure;
• the required attenuation level reduction;
• the inlet/outlet configuration; and
• the operating environment.
The engine information is usually readily available from the engine manufacturer or distributor. Historically, engine manufacturers typically undersize their engine connection sizes. This produces additional engine backpressure when applying a silencer. Adapters are available to provide a larger connection size. If engine exhaust temperatures approach or exceed 677°C, a stainless-steel construction option should be considered.
Some silencer manufacturers offer online programs to make the sizing process easier. The selection process is carried out by using the information mentioned earlier or by selecting the type of gen-set manufacturer. There are four grades of attenuation levels available: Industrial Grade (12 to 18 dB(A) reduction); Residential Grade (18 to 25 dB(A) reduction); Critical Grade (25 to 35 dB(A) reduction); and Hospital Grade (35 to 42 dB(A) reduction).
Higher grades of attenuation create higher levels of backpressure due to increased exhaust flow restriction. The engine manufacturer can provide the maximum level of backpressure allowed. The higher the backpressure on the engine, the less efficient it becomes, so the difficulty for any package designer is to minimize engine backpressure while still maintaining allowable attenuation levels.
There are several types of inlet/outlet configurations available for silencers that allow it to be positioned in various locations within the gen-set enclosure, depending on space allowances. However, utilizing different configuration types can create additional backpressure as the exhaust flow is redirected in 90° or more pathways. An accompanying illustration outlines the common configurations.
Type 1, also called end-in/end-out or straight through, is the most common design. It provides the least restriction of the styles as it does not redirect the flow path. Type 2, also known as high side-in/end-out, creates the most restriction as the exhaust flow path is redirected twice. Type 3, also referred to as side-in/end-out, is also a common design. Its restriction levels are less than Type 2 and 4, but more than Type 1. Type 4, referred to as middle side-in/end-out creates similar restriction levels as Type 2, as it too redirects the exhaust flow twice.
Special designs are also available through some silencer manufacturers, though they generally cost more and will also require longer delivery times. Unfortunately, there are many variables that exist in calculating sound emissions. The “black art” of silencer sizing is open to oversight and miscalculations. Therefore, operating environment must also be considered in the silencer sizing. Is the application near a residential neighborhood? Does the application require an independent testing agency to verify sound levels before commissioning? A gen-set packager needs to consider the risk to their brand image as well as financial risk. Applications that carry additional risks need to be considered for additional attenuation.
An example is a new US$400 million manufacturing facility that cannot start production until it passes a sound level test at the site line. If the gen-set package contributes to the facility not meeting the sound level requirements, the penalties could be severe. In this instance, an extra level of attenuation should be incorporated. If the data determines a critical grade silencer, a hospital silencer should be used. The extra attenuation level may increase package cost and reduce engine efficiency, but may also be irrelevant when compared to high penalty risks.
The “black art” of silencer sizing for gen-sets can be managed effectively by understanding requirements of the installation and balancing attenuation level, silencer size and backpressure considerations. Consulting acoustics professionals can simplify the process. Although the costs of employing acoustics specialists and selecting specific silencer types may not be insignificant, it’s often money well spent because nobody in proximity of the generator set will complain about its efficiency. But they will complain about its noise.