Introduction to Engine/Compressor Building Containment
After engine exhaust and cooler fan noise, the mechanical noise radiating from an engine casing and attenuated by the building is usually the third environmental noise generator of concern. This is particularly true in summer when many operators leave building vent louvers (or even doors) open for ventilation. A typical double steel wall gas field engine/compressor building will attenuate casing noise by some 20 - 25 dB with all doors, windows and louvers closed. With open louvers or windows equating to as little as 2% of the total wall area, this attenuation drops by over 8 dB for a corresponding 8 dB increase from the casing radiated noise.
Manufacturers invariably quote compressor package noise levels with building doors closed. We recommend forced ventilation and no ridge vents for critical noise service. This is because circular and rectangular vents can easily be silenced where ridge vents cannot.
Introduction to Engine Exhaust Silencers
These are often loosely categorized as "residential", "critical", "hospital" etc. Technically, these terms give only a general impression of the performance, and are not adequate for engineering work. Exhaust silencers divide broadly into two types. The more common Absorptive Silencer is a straight-through pipe expansion with a perforated sound-absorbent liner. It performs quite well at mid and high frequencies and very poorly at low and very high frequencies.
The more expensive Reactive Silencer consists of series expansion chambers plus absorptive liners. It is not a see-through design; it results in much higher exhaust back pressure but exhibits much better low frequency noise attenuation. Figures 1 and 2 illustrate some typical Absorptive and Reactive Silencer designs and their typical acoustic performance by octave bands.
Silencer Specification
An engine exhaust silencer needs at least two sets of specification parameters. Regarding acoustics, the silencer Dynamic Insertion Loss (DIL) should be specified in octave bands from at least 63 Hz to 8000 Hz. This DIL is derived from the knowledge of the unsilenced engine exhaust spectrum and the maximum allowable exhaust sound power level. The latter value is determined from the maximum allowable sound level at the nearest residence; the distance to that residence; the terrain over that distance and the number and nature of noise sources at the operator's plant. For critical sound situations, the silencer shell radiated noise can also be significant and must be addressed.
Regarding process requirements, the silencer specification must include the operating flow, temperature and maximum back pressure. In the case of high back pressure Reactive Silencers, or more rarely, when two or more silencers in series are required to meet the noise targets, the operator should allow suppliers to install a pipe transition piece to increase the silencer diameter and reduce flow velocity and back pressure. As a default, we use 8" w.g. maximum particularly if the engine has a turbocharger.
Considering that silencer capital costs are typically less than 1/3% of project costs but that high back pressure may adversely impact engine fuel and horsepower by over 1% for the project life, the more expensive silencer is usually beneficial.
