Keeping compressed air clean in pneumatic circuits is a must. Using the right filters ensures equipment runs efficiently and helps minimize breakdowns. Neglecting filtration is a recipe for wasting energy, more-frequent component replacement, and higher operating costs. 

In a recent blog, “How do you select an air filter?” we discussed two important parameters for selecting general-purpose pneumatic filters: the micron rating of filter elements; and size based on the required flow and pressure drop across the filter. Here are several additional parameters engineers should consider when specifying air filters for pneumatic circuits.

Pressure. Ensure the filter under consideration can handle the available system pressure. Standard filters are typically made to withstand pressures as high as 250 psi.

Ports and piping. Filters come with various port types and thread styles to handle lines as small as 0.12 in. and as large as 2.0 in., in typical industrial settings. Other sizes are available as needed.

Bowls. Well-operating filters remove dirt and water from the air stream, and liquids and sludge collect in the filter bowl. Manufacturers offer plastic (often polycarbonate) versions, plastic protected by a metal guard, and metal bowls made of materials such as aluminum, steel, and stainless steel. Select a suitable construction based on ruggedness and ability to withstand corrosives – internal and external – that would otherwise attack the housing.

Drains. Drains are needed to remove water and sludge that collects in the housing bowl. Low-cost manual drains are available, but they require constant attention by maintenance personnel. Better options include automatic float-type and electronically controlled internal drains that flush out contaminants as needed; as well as external drains that route accumulated fluid away to a suitable collection area.

Temperature. Don’t overlook temperature ratings. For instance, plastic constructions may not be suitable in high-temperature settings.

Differential-pressure gages. New filters usually exhibit a pressure drop of 2 to 5 psi between inlet and outlet ports. Note that this pressure drop increases as the filter element becomes loaded with contaminants. A 10-psi pressure drop across the filter is considered excessive. Users should replace contaminated filter elements to avoid restricting flow and limiting filtration performance.

To avoid the guesswork on the part of the user, many manufacturers offer pressure-drop indicators mounted on general-purpose filters. They come in various versions but, in general, they point to or otherwise indicate a green field on a gage when the filter is new. As the filter collects dirt and a pressure differential develops across the element, an internal spring-loaded device pushes the indicator toward a red “danger” region. The red indication shows that pressure differential exceeds 10 psi and the element should be replaced.

Quality and efficiency. Filters, by their nature, restrict flow to a certain extent and that consumes energy. A well-designed filter not only removes water and particles efficiently, it has a low pressure drop at a given flow. Filter quality can vary widely from one manufacturer to another. The efficiency of filters with the same micron and flow rating can vary with the type of construction and media. In general, lower-quality filters have a higher pressure drop, clog more quickly, don’t retain contaminants as well and require more-frequent replacement.

Higher-performance filters also conserve energy. Here, the housing and filter element are designed for low flow resistance, and they have a better ability to trap and hold contaminants without creating a high pressure drop.

As a final note of caution: Not all manufacturers quote performance under the same conditions. If stated flows or pressure drops are not the same for different products, users often must resort to testing them side-by-side under the same conditions to compare performance.