Designing in Hot Swap Fans for
Custom Rack Mount Systems

Rack mount enclosures dissipating 25 watts or more frequently need some form of force air convection cooling. Fans are often regarded as the low MTBF item in a system enclosure. Fans as FRUs are frequently required in a high-performance and mission-critical applications. Electronic Enclosures, Inc. (EEi) designs single rotor or dual-rotor hot-swap fans, beginning with 40mm axials for 1U enclosure and greater.

Typical fixed mounted fans require a scheduled maintenance shutdown. This may not be allowable in essential applications. Our hot-swap fans may be changed in a manner of 10 seconds allowing the system to be maintained in a constant power-up processing state.

hot swap fans single dual rotor

Single vs. Dual Rotor Fans

When there are multiple single rotor fans, which is common to many rackmount enclosures, and a fan fails the airflow direction reverses through the failed fan. If all fans were removing air from the enclosure, then the failed fan position now becomes an intake. This reduces overall system cooling by more than the loss of one fan resulting in a lower percentage of airflow through the entire enclosure. The failed fan circulates air to the fans adjacent and not through the chassis – reducing system performance. The percentage of through chassis loss is based upon the pressure characteristic unique to the enclosure assembly. One might consider an N+1 approach, however, it’s not a true N+1 when a fan fails.

When using dual rotor fans there are two independently powered and monitored rotors (if you select that feature) within a single fan frame. When one rotor fails the other rotor continues to spin. Thus airflow continues. There is generally some loss of overall system performance. Thermal monitoring on the system board may spin up the fans to compensate. Replacement of the hot-swap fan module may be scheduled while the system continues to operate normally.

Hot-swap fans may be designed to dock directly with your motherboard or through an interposer board. Two, three, or four wire fans may be implemented giving your system board control and feedback on fan performance or failure. Fans are selected based on rack mount enclosure thermal dissipation requirements as well as size, availability, voltage, watts, CFM, static pressure and noise. Our fan solutions can encompass any axial fan you choose.

Hot-swap fans are produced specific to your rack mount enclosure application to match your desired electrical characteristics. The docking connector is configured to match the position and elevation of the mating connector on your system board or our interposer board.

hot swap fans grill comparison

Fan Grill vs. Wire Finger Guard

The hot-swap fan modules may have either one of two different grill or guard types. Each has Pros and Cons. So here they are:

Perforated Fan Grill

  • Appearance
  • Improved EMI containment for higher clock speeds.
  • Increased noise from turbulence through the grill.
  • Some reduction of open grill area:
  • Reduces airflow
  • Increased noise from turbulence
  • Increase in static back pressure on fan (also affected by a closer proximity of grill to the fan blades).
  • More frequent service for dust removal.

Wire Finger Guards

  • Best airflow performance with the least air restriction and effect on static pressure.
  • Lowest noise with greater open area and distance to fan blades
  • Collects less dust for less frequent service.
  • EMI containment at lower clock speeds.
  • Appearance

Every fan option, regardless of size (40mm, 80mm & 92mm), single or dual rotor, 2, 3 or 4 wire fan can be produced with either the perforation or wire finger guard with little cost differential between the two options.

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