Making Air Systems Clean & Quiet

Most pneumatic systems are lubricated, despite the fact that oilless pneumatic systems have been available for some time. While lubricated systems require additional components, oil, and periodic maintenance, many users deem the total savings from lubricated air exceed the cost of installing and maintaining the lubricators.

Oil reduces friction between sliding surfaces, which improves efficiency and permits higher operating speeds. It also reduces wear, which ultimately means longer component life and less maintenance.

Moreover, lubrication in pneumatic systems can reduce both internal and external leakage around valve spools, cylinder rods and pistons, and air motor and rotary actuator vanes, rotors, and housings.

That said, a major drawback to lubricated systems is they discharge oil mist when the air exhausts. And that can bring the unwanted attention of regulatory authorities.

Fed regs
The Occupational Safety and Health Administration (OSHA) Air Contaminants Standard 1910.1000 mandates employee exposure limits to toxic and hazardous substances. For oil mist, compressed air exhaust may not contain more than 5 mg/m³ (about 4.3 ppm) on a timeweighted average basis. The standard also details how to calculate total risk when a worker’s exposure varies throughout the day.

Likewise, the National Institute for Occupational Safety and Health (NIOSH) recommends short-term exposure limits of no more than 10 mg/m3, defined as a 15-min exposure that should not be exceeded at any time during the workday.

In addition to oil-mist limits, regulations govern noise levels of compressed-air exhaust. OSHA Standard 1910.95 on occupational noise, in short, limits a worker’s average noise exposure in an 8-hr day to 90 dB, and less than 15 min at 115 dB.

Obviously, most manufacturers strive to keep their plants safe for workers. Failing to comply with the standards can result in considerable costs and headaches for a company. In addition to potential fines and penalties, operating outside recommended limits can bring requirements for respirators and hearing protection for employees, ongoing monitoring of noise levels and air quality, annual company-paid medical check-ups, and onerous paperwork and record keeping. And that’s not to mention the unpleasant prospect of a government inspector looking over your shoulder.

Coalescing mufflers
In a nutshell, exhausting noisy and oily compressed air into a plant creates headaches no manufacturer needs. Fortunately, there’s a simple solution to the problem — air mufflers that reduce noise and also limit emissions of oil and dirt. These devices go by a number of names, including coalescing mufflers, coalescing silencers, filter silencers, and reclassifiers.

Regardless of name, manufacturers claim they easily let pneumatics systems comply with government regulations. Internal geometry of the devices reduces air velocity while baffles for audio damping take care of noise and filtration takes care of the oil.

But not just any filter-muffler will work in all cases. A standard filter-muffler has a porous element that traps solids entrained in the compressed-air stream, but they are not designed to trap oil vapors. A coalescing muffler, on the other hand, operates the same as a coalescing filter. As air flows through the coalescing element, oil particles are captured using 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, a turbulent air stream throws oil particles against fibers, which trap the oil. And diffusion causes the smallest particles to vibrate and collide with each other — and eventually the element’s fibers — which trap the oil.

However, because coalescing mufflers are on the exhaust side of a pneumatic system they are exposed to sudden shock loading. This means oil-removal capabilities may not be as consistently good as those of a pure coalescing filter.

Nonetheless, a good coalescing muffler typically gives 2 ppm or less oil discharge in normal operation. (Without coalescing mufflers, contamination may exceed 50 ppm in a plant that is tightly closed for operation in cold weather.) Most also have a port for draining collected oil and elements that can be changed without having to replace the entire muffler.

Whatever the choice, mufflers that remove oil from exhaust air should be considered — especially if exhaust blows near workers. It is not uncommon for coalescing mufflers to remove 99.97% of entrained oil. Even if oil-free compressors are used, exhaust mufflers can reduce noise to OSHA-acceptable levels and ensure exhaust air is clean.

Product overview
A number of manufacturers provide coalescing mufflers under various monikers. For instance, the Atomuffler defogger/reclassifier muffler from Allied Witan reportedly reduces noise from air-operated devices and effectively remove entrained oil from exhausted air. It intercepts, extracts, and condenses oil to liquid droplets that gravitate into a sump for periodic draining. And it reduces objectionable noise and particulate contamination associated with pneumatic systems. The mufflers resist corrosion, come with disposable elements, and are available with male pipe thread connections (NPT) in sizes from 0.12 to 2 in.

Bosch Rexroth’s MU1 oil separator with silencer has a three-stage coalescing microfilter. Filter efficiency is reportedly 99.99% with remaining oil content in the discharge air as low as 0.01 mg/m3. The company recommends replacing the filter element annually. Noise reduction exceeds 40 dBA at 73 psi (5 bar) and nominal flow of 70 scfm (2,000 Nl/min). Nominal flow ratings are 88 to 124 scfm (2,500 to 3,500 lpm) depending on size, and pressure ratings are 0 to 145 psi (0 to 10 bar). Maximum temperature rating is 140°F.

The Numatics Reclassifier is an exhaust coalescing filter/ silencer said to remove 99% of the oil and reduce noise up to 25 dBA from exhaust ports on pneumatic systems — even at high flow rates — with low back pressures. It has replaceable, 1-μm coalescing filters and mounts to exhaust ports on valves or manifolds. Vertical mounting makes use of the oil-catching sump surrounding the filter element and simplifies draining. Maximum temperature rating is 125°F, maximum pressure is 100 psig, and flow ratings range to Cv = 10. Port sizes range from 0.5 to 1.25 NPT, and metric threads are available.

The Finite Exhaust Coalescing Silencer (ECS) from Parker Hannifin, Filtration and Separation Div., is said to be 99.97% efficient at removing oil aerosols from pneumatic valve, cylinder, and air-motor exhaust. The ECS also acts as a silencer with 25-dBA attenuation to lower exhaust noise below OSHA requirements.

Users can drain the sump manually or with an attached 0.25-in. plastic tube. Maximum ratings are 100 psig and 125°F. The standard units come in 0.5 and 1.0-in. NPT port sizes, and BSP (G) threads are available.

The ECS units are constructed from the same materials that make up the company’s standard coalescing filter elements. A seamless design ensures media conformity and strength, and provides high efficiency with low pressure drop. Cylindrical perforated steel retainers support the media inside and out. The retainers, galvanized for corrosion resistance, give the units high rupture strength in either flow direction. The ECS can also be used as a high-efficiency inlet or bypass filter for vacuum pumps, or breather elements to protect the air above critical process liquids.