Filters and cutaway

Tips for Selecting Filters for Pneumatics

Feb. 12, 2020
A clean air supply is vital to equipment running on compressed air.

Compressed air is a necessity in many of today’s factories, from packaging and automobile production to high-end electronics and semiconductor manufacturing. It is critical for industrial air supply lines in these industrial sites stay free of water, oil, particulate and other contaminants. However, it’s common for supply lines to deliver a certain level of contaminants along with the air they carry. These present a challenge as they reduce efficiency, create maintenance hassles, and shorten the life of pneumatic components.

Installing a filtration subsystem can make it simple to ensure a clean air supply. These subsystems employ one or more filters to remove contaminants before they reach the pneumatic equipment. Selecting the most appropriate filtration system for a given application, however, can be difficult.

The difficulty stems in part from the different filtration requirements across industries. For example, semiconductor, automotive, and food and beverage plants each have different filtration needs based on their specific pneumatics and operating environments.

In addition, there are several different types of filters to choose from, and they are not all created equal. Choosing the wrong filter or applying the right filter incorrectly can do greater harm than good.

To identify the right filtration for an application, it’s important to understand the capabilities of different filter elements and how to combine them in series.

Filter Types

The most common types of industrial filters:

Water separators. Water or moisture can quickly damage pneumatic components, causing valves and cylinders to stick. Water separators rely on centrifugal forces generated by an internal spinning mechanism to remove water and waterborne contaminants. In addition, they protect other filter elements from liquid that could shorten filter life.

Particulate filters. Large particles, such as rust, debris and desiccant dust, can cause premature failures in pneumatic components. These contaminants are often generated by old carbon-steel pipes, compressor intakes, and desiccant air dryers. The most effective particulate filters use pleated designs, maximizing the surface area that traps particles. Particulate filters often achieve three-micron particle removal in dry systems.

Coalescing filters. At the next stage of air filtration these filters remove water, oil, rust and other contaminants from air supplies. The design of coalescing filters may vary, depending on the manufacturer. For example, some higher-quality coalescing filters consist of a porous network of borosilicate glass fibers. Air flows from the inside to the outside of the filter via progressively larger openings in the media. As contaminants move through the element, solid particles are trapped and liquids formed into large droplets that drain away. Then, as air exits the filter, surface tension holds the liquids, letting them drain to the bottom. Coalescing filters are available in different grades.

Typically, coarse coalescers are used for mainline plant filtration while fine coalescers are used for applications such as pneumatic tools, robotics and paint spraying. Ultra-fine coalescers are usually point-of-use filters for processes such as semiconductor packaging and instrumentation.

Adsorbing filters. These filters are often the final step in removing oil and hydrocarbon vapor from compressed air. Adsorbing filters are usually used immediately downstream from a coalescing element. Adsorption is best done at lower temperatures, so it’s a good move to install them as close to the point of use as possible. Adsorbing filters are often used to treat breathing air, well as food and drug applications in which the product contacts the exhaust air.

Design and Construction

Common filter types are sometimes referred to generically, but there are substantial quality and performance differences between filters from different manufacturers, even if they have the same nominal filtration specification. Filter design and manufacturing methods are often at the root of these differences.

Coalescing filters show the importance of design and manufacturing. Commodity coalescing elements often use a mechanically wound filter media. Higher-quality coalescing filters, however, employ a vacuum-formed filter media composed of glass fibers and an epoxy binder.

This difference makes a significant impact. The vacuum-formed design traps contaminants through the filter’s entire cross-section. This gives it a higher capacity and lower pressure drop than a comparable wrapped filter that mainly traps contaminants on the surface, quickly clogging as a result. It’s best to choose filters with an initial pressure drop of just 1.5 psi in a dry condition at rated flow, which is far lower than a typically wrapped filter. Lower pressure drops decrease energy consumption.

In addition, the vacuum-formed design lets the filter be customized to meet specific filtration levels. Some filter manufacturers can adjust the vacuum-forming process to form coalescing elements ranging from 0.01 micron to 1.0 micron.

Construction variations affects other filter types, too. Some higher-performance adsorbing filters, for example, consist of fine activated charcoal impregnated on polyester. These carbon particles have a strong affinity to vapor and are highly efficient due to their extensive surface.

Combining Filters

Proper filtration often requires a series of filter to remove the full range of particulate sizes and compositions that can harm downstream components. The specific combination of filters varies depending on the application’s air quality requirements.

For example, semiconductor or food and beverage applications may require three different filters, including ones that remove submicron particles. For others, such as main-line plant filtration, a coarse particulate filter may be enough. Examples of filter combinations used in various applications include: • For process air: one-micron coarse coalescer, 0.01-micron fine coalescer and a vapor adsorber. • For blow molding: three-micron particulate filter, 0.01-micron fine coalescer and a vapor adsorber. • For pneumatics controls: three-micron particulate filter and a 0.01-micron fine coalescer. • For food packaging: three-micron particulate filter, 0.01-micron fine coalescer and a vapor adsorber. • Electronics: three-micron particulate filter, one-micron coarse coalescer and a  0.01-micron ultra-fine coalescer. • Semiconductor packaging: three-micron particulate filter, 0.01-micron fine coalescer and a vapor adsorber.

Filter combinations do not always require separate products. In some cases, different filter elements are combined to simplify ordering and save space. For example, some coalescing filters feature a pleated three-micron prefilter option which eliminates the need for a separate coarse particulate filter. This two-in-one approach saves space and money because the prefilter and the coalescing filter share the same housing.

Maintenance and IIoT

Filter elements in a plant’s pneumatic system do not always get replaced at the intervals required to maintain peak performance. This can lead to inefficient machine performance and possible downtime. Often operators don’t know filters need to be replaced, so a proactive preventative maintenance programs is encouraged.

Some FRLs have visual or electronic ∆P indicators or system pressure switches to alert maintenance personnel that filters need to be looked at. In addition, today’s FRLs use modular components that simplify maintenance.

The latest machine automation trends have air preparation units playing an increasing role in IIoT. Some FRLs come with flow meters, pressure sensors and other sensors. They let users monitor machine performance by measuring air flow use and pressure.

For example, a machine records higher flow in the second shift. This might indicate that the first-shift operator changed pressure setting on certain regulators, or perhaps there a leak developed. These FRL units also let users compare the current flow curve of a machine’s complete cycle to the flow curve of a new machine. With proper data analytics, users can understand which factors are affecting performance and act accordingly.

A Systems Approach

Unfortunately, machine builders and installers often neglect to adequately consider air filtration, or they may treat it as an afterthought. The biggest benefits of proper filtration are realized when the filter and pneumatic components work together. Choosing the right filter type and construction based on the application is the first step. Combining filters appropriately based on air quality requirements completes the design. Only then will the filter offer the maximum protection against contaminants and safeguard against decreases in pneumatic performance.

Jeff Disbrow is a product marketing manager at Emerson Automation Solutions.

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