Rack Chassis Cooling and EMI Shielding: How to Specify Both Together

Airflow and shielding pull in opposite directions. Every hole that lets air through also lets electromagnetic energy out, so a rack chassis design is a negotiation between two requirements that are usually written by two different people. Treating them as one problem from the start is what keeps the design from being reworked at the first test.

Why Airflow and Shielding Conflict

A chassis is a container for two things that want opposite walls. The electronics want as much open area as possible for air; the electromagnetic environment wants as little as possible. Both requirements land on the same panel.

Because they compete, the specification should state the requirement of each and leave the reconciliation to the designer. A brief that names only a fan model has already removed most of the design freedom.

Specifying Airflow from the Requirement, Not the Fan

Start with the dissipation and the maximum inlet temperature, and derive the airflow from them. Only then choose the fan.

• State the heat load at worst case, not the nominal load.
• State the maximum inlet air temperature, which for a rack is often the temperature at the bottom of the cabinet rather than the room temperature.
• Allow for the pressure drop of the vent patterns and the filters; a chassis with a restrictive path delivers much less than the fan's free-air rating.
• Decide front-to-back, side-to-side or bottom-to-top early, because it decides where the vents go and how the internal layout is arranged.
• Provide a bypass path past tall components rather than forcing all the air through the tightest gap.

*Note* the difference between free-air delivery and system delivery is the single most common reason a chassis runs hotter than predicted.

Vent Patterns and Open Area

• Perforated panels are the general-purpose answer: good open area, predictable pressure drop, reasonable shielding above the aperture resonance.
• Honeycomb vents give high open area with strong attenuation at high frequency, at the cost of cost and cleanability.
• Louvres protect against drips and fingers but restrict airflow noticeably and shield poorly.
• Slot patterns are easy to produce but a long slot is a better antenna than the same area of small holes.
• Vent placement should avoid short-circuiting air straight from inlet to outlet without crossing the hot components.

Shielding, Apertures and Joints

Shielding effectiveness is limited by the largest aperture and by the quality of the joints, not by the thickness of the metal.

• Keep the largest dimension of any single aperture as small as the requirement allows.
• Use many small apertures rather than one large one for the same open area.
• Bring covers down onto a conductive frame, and space the fixings closely enough that the seam behaves as a continuous contact rather than a slot.
• Specify conductive gaskets at removable joints where the requirement is demanding.
• Keep seams away from the highest-current or highest-frequency loops.

Bonding and Grounding Continuity

A shielded chassis only works if all of its parts are at the same potential.

• Mask contact areas before coating so that joints are metal-to-metal.
• Use bonding straps across hinges, sliding covers and any joint that cannot be made conductive directly.
• Provide a defined chassis earth stud, sized and positioned on the drawing.
• Check continuity after finishing, because a good coat of powder is an excellent insulator.
• Where the chassis slides into a rack, bond it to the rack as well as to its own covers.

Internal Layout for Thermal and EMC Performance

The internal arrangement decides how much of the available airflow actually does useful work.

• Put the highest-dissipation components first in the air path, but downstream of any component with a temperature limit.
• Keep cabling away from the vent path where it would obstruct flow.
• Route high-frequency cabling away from apertures and along the chassis rather than across it.
• Leave space for a mid-plane or a divider that separates a noisy zone from a sensitive one.
• Plan the fan mounting so that the fan frame itself does not cover a large part of the vent it is drawing through.

Materials and the Shielding Consequence

Materials are rarely the limiting factor. The joints are.

Verification: What to Check and When

• At the drawing stage: confirm that the vent area, the fan curve and the pressure drop are consistent.
• At the first article: check vent open area, aperture sizes, joint contact, gasket compression and earth continuity.
• At thermal test: measure the actual temperature rise against the predicted case, and check the air temperature at the inlet of the hottest component.
• At EMC test: the enclosure is only one variable, so agree in advance who owns the cable routing and the filter placement.

A Short Brief That Gets a Good Design

1. Dissipation at worst case and the maximum inlet temperature. 2. Required airflow direction and any acoustic limit. 3. Shielding requirement: the standard, the frequency range and the attenuation target. 4. Environment and required finish. 5. Mounting standard, rail type and unit height. 6. Volumes and the expected design freeze point.

Our Approach to Cooling and Shielding

• We fabricate the vent patterns, honeycomb apertures, masks and bonding features that deliver both airflow and attenuation.
• We dimension the mounting and interface geometry so that fans, filters and shielded gaskets land where the drawing says they will.
• We will tell you when a requested vent area and a requested attenuation are mutually exclusive at the proposed aperture size.
• We do not perform EMC certification testing, and we do not design the electrical filtering; these are handled by the equipment designer and an independent test house.
• Every quotation states which operations are completed in our plant.

Send the dissipation figure, the shielding requirement and the rack standard, and the two constraints can be designed together rather than fought over later.

Frequently Asked Questions