Keeping pneumatics quiet
Exhaust mufflers have taken on greater importance with the recent enactment of OSHA (Occupational Safety and Health Administration) Standard 1910.1000. In part, the standard mandates that noise may not exceed 90 dBA during an 8-hr day of a 40-hr week. In addition,compressed air exhaust may not exceed 4.32 ppm of oil mist contamination in any 8-hr work shift of a 40-hr work week. In addition, OSHA regulations spell out specific noise and emissions limits and time periods in which these limits are allowed. For example, exhaust noise from a pneumatic system as high as 115 dBA is acceptable. However, workers may only be exposed to this noise for 15 min within an 8-hr shift. So if workers will be near the exhaust of a pneumatic system for an entire 8-hr shift, noise to only 90 dBA is allowed.
For decades, air exhaust mufflers have been used to reduce noise and emissions of compressed air exhausts. Now, however, specific guidelines exist. Internal geometry to reduce air velocity and baffles for audio damping take care of noise; filtration takes care of the oil. But not just any filter-muffler will do.
A standard filter-muffler has a porous element to trap any solids that may have been entrained in the com-pressed air stream. Porous elements, however, are not designed to trap va-pors or liquids, such as oil. So unless the pneumatic system uses an oil-free air compressor and no lubricators, exhaust air should be routed through a coalescing muffler.
A coalescing muffler operates on the same principles as a coalescing filter. As air flows through the coalescing element, oil particles are captured by three different mechanisms: direct interception, inertial impaction, and diffusion. In direct interception, oil particles simply collide with and are trapped by filter fibers. With inertial impaction, the element's turbulent air stream throws oil particles against fibers, which trap the oil. Diffusion causes the smallest particles to vibrate and collide with each other - and eventually the element's fibers — which traps the oil.