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Filter design aids air compressor efficiency

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DESIGN of compressed air filter housings directly affects system pressure loss, which influences operating costs.

Typical compressed air filters are installed at 90° to the piping system due to installation restrictions and for ease of maintenance but this design creates what is known in aerodynamic terms, as an inefficient corner. These inefficient corners in compressed air filters can lead to turbulence, pressure loss and high running costs.

In an inefficient corner, the core of the approaching air stream is projected against the outside wall of the turn. Part of the flow is reversed and a vortex pair produced. The remaining air continues downstream in a slightly more uniform manner. The vortex pair has the effect of narrowing the flow path through the corner, throttling the air flow and increasing pressure losses.

To improve air flow and reduce turbulence, the corner should be made rounded and smooth. The rounded corner produces a different flow as air stream lines diverge near the outside wall of the turn and converge on the inside wall. Compressed air filters using this design reduce turbulence induced pressure losses compared with sharp cornered filters.

Using partitions (turning vanes) in the air flow further improves efficiency by separating corners into several more efficient corners.

The new Oil-X Evolution filters have been designed to minimise pressure losses. As the air stream enters the housing, a “bell mouth” inside the inlet port reduces turbulent entering the vessel. The air stream then enters the top endcap’s full flow inlet conduit where it is turned through 90° using a full radius blended bend.

Turning vanes within the inlet conduit split the 90° corner into smaller, more efficient corners. This channels the air stream, reducing turbulence and pressure loss.

Terminating the conduit directly after the 90° change in direction can cause additional areas of turbulence, increasing energy consumption and causing uneven usage of the filter media.

The Oil-X filter element employs a flow distributor to evenly distribute the air stream throughout the filter element. A conical flow diffuser is employed to prevent additional turbulence from air hitting the lower element endcap.

Filtration media

Correct filtration media selection and construction is key to achieving quality compressed air and minimising operating costs.

Poorly selected filter media could result in contamination being carried downstream and can increase the filter element’s maintenance requirements.

Typical compressed air filters ‘soak up’ oil and water and are said to run in a saturated state.

Saturated media temporarily blocks the compressed air’s path through the normally open filter structure. This increases pressure loss as the air has to force liquids through the filtration media.

Pressure loss increases through the element’s life as the filter media becomes blocked with particulate, reducing efficiency and increasing operating costs. Oil-X filter elements use borosilcate glass nanofibre filter media with 96% open area, or voids volume, providing a high dirt holding capacity.

A special oleophobic coating is also employed to actively repel oil and water, maximising the open area for dirt entrapment. Cartridge surface area is also maximised using ‘deep pleat’ media beds, which provide surface areas up to five times greater than standard wrapped construction filter elements.

High efficiency grade filter elements also use a graded density media pack consisting of two filter media grades within the pleat pack. A course pre-filter layer protects the fine layer from premature blockage.

Removing liquids

Liquids removed from the air flow must be prevented from re-entering clean air and being carried downstream.

Typically, the liquid coalesced by a filter forms a wet band around the bottom of the filter element. From there it drops into the filter bowl and is discharged by the drain. Designers usually incorporate a quiet zone into filter housings to ensure air does not flow through the wet band and cause re-entrainment.

The Oil-X filter uses an anti re-entrainment barrier to remove liquid contaminants before they can be carried downstream, and re-locates them under the filler element away from direct air flow.

* John Davis is business development manager for domnick hunter Australia .

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