Fluid-flow software, such as Flotherm, shows the streamlines through an enclosure to identify hot spots and dead air.

Packaging electrical and electronic medical devices includes sizing proper air filters. But ignore the task of specifying or building the filter till late in the design cycle and it can become challenging, costly, and time consuming. The factors often overlooked in fast-track designs include power consumed, heat load, EMI shielding, and ventilation. A few considerations at the right time make it easier to settle on a best air filter.

A trend to smaller medical enclosures has spurred innovative filter designs, such as low profile and edge-to-edge air filters. Low-profile filters are less than 0.25-in. thick and well suited for use in limited-space equipment. Low-profile filters are about two-thirds the size of standard filters. Edge-to-edge versions either cut back the framing that might run around the perimeter or eliminate it completely. Edge-to-edge filters let clean, cooler air pass along chassis' interior walls and across boards likely to carry heat-generating electronics.

Filter assemblies can have a perforated pattern or metal finish to serve as part of the enclosure exterior or a decorative bezel. External mounting also makes filters accessible for maintenance.

Filter media comes in a few colors but trim size and bracing are available as needed. One way to test a filter design is with a no-cost custom prototype. It ensures the air filter meets product specifications and industry standards.

Thermal-analysis software simulates airflow and heat transfer through enclosures and pinpoints hot spots and dead air zones. The software also lets users quickly and accurately predict pressure drops across filter elements. The trend to smaller medical enclosures on devices with more functions results in higher heat densities. Thermal analysis software helps factor in the effect of an air filter on the system pressure drop.

Dual EMI Honeycomb filters, from Universal Air Filter Co., facilitate uniform air diffusion and are easily removed for cleaning

In addition, filter companies have developed media to meet the stringent medical industry standards and requirements for flame safety. For instance, polyester and open-cell polyester polyurethane-foam media are rated UL 94 HF-1 for self-extinguishing flame safety. This rating meets CE, UL60950, and EN60950 for flame safety standards. In addition, these media offer low pressure drops, high-dust-capture rates, and are available in white, black, gray, or green.

EMI noise, another consideration, has been solved with recent filter innovations. Previously, honeycomb or perforated metal panels were specified separately and then mounted to shield against moderate EMI levels. The new technology, such as dual EMI honeycomb filters, provides filtration and EMI shielding in a single, cost-effective design that is compact and removable. Cells in the dual honeycomb filter trap and deflect EMI noise while maintaining 95 to 99% openness for minimal airflow restriction. Honeycomb media also facilitates uniform air diffusion.

The initial-resistance-versus-face-velocity chart shows pressure drops across the filter, information necessary for flow studies and fan sizing.