Improving Cooling Techniques for Air-Cooled Mil/Aero Embedded Computing Systems

How to tackle the rigors of forced-air cooled Defense applications with solutions including hot-swappable redundant blowers.

Embedded computing systems for Mil/Aero applications are often conduction-cooled in an ATR or non-standard chassis. However, there are many designs that require 19” rackmount systems with forced-air cooling. As more processing performance is packed into tight spaces, often using the OpenVPX architecture, enclosures that provide advanced cooling options are increasingly in demand. Can we leverage the tools and lessons of cooling the 4-5 kW AdvancedTCA systems in Mil/Aero applications?  With a little modification, we certainly can.

Poor heat dissipation cannot only cause system failures, but significantly shorten component life. So it’s critical to cool a system properly. Air takes the path of least resistance, therefore pulling the air out is much more effective than pushing air through an enclosure. When pushing air, it can be challenging to have the proper bends to maximize cooling. Inevitably, there will be a backflow of the air and the backpressure will hamper the performance. Pulling the air alleviates these issues for much greater efficiency.

For many 19” rackmount systems, tube axial muffin fans can do the job. See Figure 1 for a rugged 4U OpenVPX rackmount chassis designed to meet MIL-STD-461F for EMI. The fans in this system can provide about 81 CFM each and pull the air directly out to the rear of the enclosure. While this can certainly do the job in many applications, systems with very high power processors may need another approach. Further, if rear I/O is required there could be challenges in removing rear modules efficiently and having enough chassis depth available.

Figure 1. Tube-axial fans cooling up to 50W/slot in an 18-slot 4U OpenVPX chassis from Pixus.

Figure 1. Tube-axial fans cooling up to 50W/slot in an 18-slot 4U OpenVPX chassis from Pixus.

Another design technique is using powerful air-pulling fans above the card cage. In some designs, this approach requires a lot of extra chassis height. But, there is a way to keep the chassis height to a minimum. One solution is to use fans that pull in one axis (to the top of the chassis) and blow the air out in another axis (through the rear of the chassis). Take the 13U AdvancedTCA system first designed in 2003. (See Figure 2)   It helps illustrate an excellent cooling solution even 13 years ago with a 1U section of RiCool II reverse impeller blowers providing 220 CFM system airflow at 70% efficiency (versus 20% when using typical 4.7” tube-axial muffin fans). As this hot-swappable cooling approach only takes up 1U of the space in the chassis, it is very space efficient as well. Even in those early days, this ATCA chassis cooled over 275W/slot. The result was the superior cooling design was chosen for the largest early programs and garnered over 15,000 installs globally.

Figure 2. The lessons for optimal cooling can be leveraged from this ATCA chassis with RiCool blowers into today’s forced-air cooled OpenVPX designs.

Figure 2. The lessons for optimal cooling can be leveraged from this ATCA chassis with RiCool blowers into today’s forced-air cooled OpenVPX designs.

The toughest cooling challenges have typically been in telecom systems using AdvancedTCA. The cooling requirements are approaching 375-400 Watts/slot. Most Mil/Aero requirements don’t tend to reach anywhere near those levels, but we can leverage many of the COTS components and design techniques for the more difficult cooling challenges. The blowers in the ATCA design can be leveraged in Mil/Aero OpenVPX or other architecture systems. Today, these hot-swappable blowers are even more powerful and efficient. Now, RiCool III blowers can generate 185 CFM of airflow with 71 mm (H20) of static pressure. Figure 3 shows an example of reverse impeller blowers in a 9U VPX chassis with front to rear airflow. This chassis design can dissipate over 2400W in redundant mode. Even if a fan went out, it could dissipate over 2.4 kW while the faulty fan was hot-swap replaced. Surprisingly, the weight of the newest generation of fans has decreased 25% as well.

Figure 3. Reverse impeller blower approach above the card cage cooling up to 120W/slot in a fully loaded 9U OpenVPX chassis.

Figure 3. Reverse impeller blower approach above the card cage cooling up to 120W/slot in a fully loaded 9U OpenVPX chassis.

Static Pressure

Static pressure gauges the resistance to the airflow and how equal the flow is maintained in all directions. It is particularly important to ensure the air can be moved through the tight spaces between the board slots. The reverse impeller approach using the new generation blowers creating 71 mm (H2O) of static pressure confirms that it is able to provide effective cooling in densely packed enclosures and subracks. By comparison, a typical 19” rackmount fan tray, consisting of (3) 4.7” x 4.7” 18W (110 CFM at free delivery) fans, generates only 0.22”-0.40” (H2O) of static pressure. The estimated (operating) static pressure point of a fan assembly mounted inside a fully loaded subrack is 0.3-0.5” (H2O). Under those conditions one reverse impeller blower assembly provides at least 70% higher airflow than a typical 19” rackmount fan tray with 3 tube-axial muffin fans.

Other Design Techniques

There are other design tricks-of-the-trade to enhance thermal management. Individual air flow management ensures targeted air routing and optimum heat dissipation. As described earlier, baffles to redirect airflow can help fine-tune the thermal management of a chassis platform. The guide rails and horizontal rails can feature a narrow design that is less of an impedance. Further, with a modular design the enclosure spacing between the slots (the pitch for example) and other parts of the chassis can be modified to address specific thermal concerns.

Acoustic noise is a concern in many applications. This design approach has low noise (48dBa at 3/4 speed and a life of approx. 60,000 hrs. at 40°C), making this blower a good fit in a wide range of applications.

Heat Dissipation and System Management

Utilizing a powerful reserve impeller blower approach, one can have effective cooling in densely packed systems where there is an average heat loss of nearly 115 Watts per PCB (at 21 slots). Most VPX systems are smaller, so at 12 slots for example, you have even higher levels of heat dissipation per slot.

Alarm control and system management are important issues in many applications. Very common in telecom, other applications are adopting similar approaches. In fact, the VITA 46.11 system management specification completely leverages the PICMG system management for AdvancedTCA and MicroTCA. The cooling approaches we have discussed employ all the required alarm, I2C, and IPMB output required for system management and alarm functions.

Cooling Extreme OpenVPX Systems

To cool the most demanding OpenVPX systems, we can leverage the tactics learned in telecom systems and apply industrial and MIL-grade components. Today’s reverse impeller blowers have options for 12V, 24V and 48V input requirements. With the latest generation of hot-swappable redundant blowers, we can meet even the most extreme challenges in forced-air cooled Defense applications.

Justin_Moll-ThumbJustin Moll is Vice President, US Market Development, Pixus Technologies.

Share and Enjoy:
  • Digg
  • Sphinn
  • Facebook
  • Mixx
  • Google
  • TwitThis


Extension Media websites place cookies on your device to give you the best user experience. By using our websites, you agree to placement of these cookies and to our Privacy Policy. Please click here to accept.