Air filter technologies are helping to cool and protect electronics equipment enclosures found in process control applications.

Low-profile, low-air resistance filters meet thermal management and air filtration requirements for blowers and fans. They can be installed only millimeters away from the fan, where space is at a premium.

Electromagnetic interference (EMI) and thermal management problems are most often encountered in communications equipment. However, with the trend to pack more power and features into electronic equipment enclosures, these problems are increasingly affecting many process control applications. If practical thermal management and EMI shielding needs are not considered in the enclosure design, costly equipment downtime and even premature equipment failure can occur.

Fortunately, with some advanced planning and a basic understanding of air filtration technologies, plant personnel can develop an effective forced air thermal management program that can protect the equipmen

Figure 1. The fan curve will affect the cfm difference for the change in pressure drop. Click image for larger view.

Air Filter Considerations

Two key considerations engineers face as they develop or redesign equipment are the enclosure size and the density of the electronics and other components. Airflow losses occur any time air flows from one end of a system or duct to the other due to air resistance and friction. An empty enclosure usually reduces airflow by 5 to 20 percent, depending on the size and number of airflow bends and turns through the chassis. In comparison, a densely packed enclosure can restrict airflow by 60 percent or more. To overcome airflow restrictions, engineers must balance thermal management with limited space and an air filter pressure drop.

Air filter improvements such as low-profile and edge-to-edge designs can help engineers overcome this challenge. (See the “Filter Innovations” sidebar.) Low-profile air filters are typically less than 0.25" thick -- as much as 70 percent thinner than standard sized units -- and allow installation in limited-space environments. Edge-to-edge designs frame the filters with less intrusive channel housings and allow clean cooling air to move along interior chassis walls. Traditional U-channel air filters can be incorporated where space is not an issue. Standard filter frames range in thickness from 0.30 to 0.88", depending upon the media thickness selected (typically 0.25 to 0.50").

To protect against EMI/RFI (radio frequency interference) noise, dual-EMI honeycoamb air filters integrate an EMI shield directly into the air filter assembly unit. Honeycomb “cells” trap and absorb EMI noise while maintaining 95 to 99 percent openness.

Air filter initial resistance and dust-loading data, which is available in thermal analysis software from such companies as Flotherm, Icepak and Macroflow, can be used for electronics equipment applications requiring UL and CE compliance. When selecting air filters, engineers should verify that the product they choose meets UL 60950, UL 94, CE (EN 60950) and has been tested in accordance with MIL-STD-285 for EMI shielding and ASHRAE 52.1-1992 for particulate arrestance.

Air Movement

What volume of air must be moved through the system? What type of fan or blower will move the air, and what is its static pressure capability? The design engineer must answer all of these questions when developing a thermal management solution.

All of the components in the chassis -- including circuit cards, power supplies, Faraday cages, plenums and air filters -- contribute to the system's pressure drop. These factors must be considered when calculating the total static pressure and will help determine the maximum pressure drop an air filter can add to the system.

For example, if a practical means for cooling is needed, simple calculations cannot accurately account for the variables and airflow dynamics through the system. Instead, the pressure drop must be measured at the inlet duct to obtain an accurate sampling of airflow. ANSI/AMCA Standard 210 provides a good guideline for obtaining measurement readings based on duct dimensions.

The manufacturer's fan performance curve also can be used to optimize thermal management. This curve will be expressed in cubic feet per minute (cfm) of airflow vs. static pressure in inches water gage. The point on the curve at which the fan operates at maximum efficiency should match the air filter's initial resistance.

For example, most enclosure fans and blowers are forward-curve fans. Depending on the fan type, the curve might be steep or flat; either way, it will affect the cfm difference for the change in pressure drop (figure 1). Standard formulas for calculating volumetric airflow can be used to determine the proper fan size, enclosure hole dimensions and corresponding filter dimensions.

Proper thermal management planning also should consider the air filter's air-straightening capabilities. Many ventilation systems generate turbulent flow at the inlet, when air rapidly enters the enclosure while being sliced by fan blades and is forced to turn sharp corners. This turbulence creates noise, slows the airflow before exiting and reduces effective cooling. Air straightening decreases the vortices and input power, increases downstream velocity and maintains velocity over greater distances. These actions prevent large vortices from forming and distribute the pressure drop more uniformly across the chassis.

The air filter media can play a major role in straightening the air and keeping the airflow laminar, or evenly distributed, by “throttling-down” the system cooling air. As the fan tries to move a certain amount of airflow (cfm), high pressure builds on the upstream side of the filter. This pressure causes the airflow to spread evenly across the filter's surface area, and the cooling air exhausts in a laminar flow pattern, thereby ensuring that each interior component receives adequate air at the proper velocity.

The fan and air filter combination has a long, successful history of managing the thermal environment in equipment enclosures. While both components have been greatly refined, the basic strategy that should be used in designing optimized equipment enclosures remains the same. By selecting the optimal fan/filter combination for the system's volumetric airflow needs, and matching it to a properly sized opening, engineers can reduce EMI and thermal management problems in process control equipment. PCE

Filter Innovations

Air filter technology has experienced a technical revolution in recent years. A synopsis of major air filter design innovations for UL, CE and Telcordia NEBS-compliant applications includes:

  • Dual EMI air filters, which integrate a honeycomb EMI shield into the assembly, are easily cleaned and reinstalled, and meet stringent filtration performance and flammability requirements.

  • Edge-to-edge air filters, which trim away the frame flanges on the sides of full-framed filters to provide complete, edge-to-edge airflow.

  • Flex-frame air filters, which provide “bend to fit” flexibility designed for equipment that cannot accept a rigid air filter frame because of internal enclosure obstructions or the necessity to follow a curved track.

  • Low and super-low profile air filters, which are designed specifically for electronics enclosures with limited space and are as much as 70 percent thinner (less than 0.25” thick) than standard air filters.

  • Perforated panel air filters, which use a strong, one-piece hex pattern with a 50 to 85 percent open area to maximize airflow and extend the life of electronic components.

  • Plenum combination air filters, which direct the air and concentrate it on the exact components that need maximum cooling inside the enclosure while lowering air resistance.

  • Ruggedized air filters, which deliver reliable thermal management, EMI shielding and particulate protection for both indoor and outdoor environments.

  • Windowpane air filters, which have a frame-and-pane design that integrates an aluminum or stainless steel frame and a series of crossing, reinforcing intermediates that provide media support.

  • Wraparound air filters, which allow the filter media to be configured with extended surface area for minimal airflow resistance.

  • Zero-clearance air filters, which are fully framed, cleanable filters designed for large-scale equipment enclosures that incorporate retainers to prevent the filter media from being distorted or otherwise blown out of the frame and downstream.

8 Steps to Successful Filter Selection

Thermal design engineers typically use the following steps when designing an air filter:

  • Select the air filter dimensions (length, width and thickness).

  • Calculate the airflow velocity in linear feet per minute (fpm) using the air volume rating of the fan in cubic feet per minute (cfm) divided by surface area of air filter in square feet (e.g., 200 cfm/0.50 ft2 = 400 fpm)

  • Select disposable or reusable media.

  • Use media initial resistance vs. face velocity data/curves for preferred media. Typically, the initial resistance is 0.10" water gauge (wg) or lower.

  • Select a media color that will allow any dust accumulation to be easily visible or that will match the enclosure color or other aesthetic considerations.

  • Select the air filter model and a frame thickness that will accept the selected media thickness.

  • Determine whether any handles or other fixtures are necessary.

  • Prepare the air filter drawing.

To ensure the right air filter selection, design engineers should involve the filter vendor in the design phase as early as possible. An air filter expert can save the designer time and money by pointing out all the possibilities before potential hurdles or inefficiencies are locked in.