Active Cooling
While many of the systems designed at Tracewell have been mechanically and electrically complex, usually that complexity is driven by power. Every watt that goes into a chassis has to come out somehow, somewhere.
The Problem
Most of the time, the majority of what goes in as system power comes out as heat. As the COTS initiative continues to spread through military systems, an increasing problem is the conflict between military and industrial component ratings. Sometimes, the air available for cooling is hotter than the part it is trying to cool. In a typical air-cooled system, the ambient air available for cooling might be 50°C, while the electronics must be kept below 70°C. With 20°C of headroom, we can cool a typical 500 W system with three fans drawing about 30 to 60 W. But what happens when the situation is reversed? What if the ambient air is 65°C, but there are delicate electronics in the system which must be kept below 60°C? In these cases, assuming a refrigerated liquid is not available, the only solution involves active cooling.
An active cooling system is like a heat pump. It requires much more energy than the usual fan power to move heat "uphill" from a cooler temperature to a warmer one. The problem is identical to that of home air conditioning, where a space is cooled to 72 degrees while surrounded by 90 degree air.
The Approach
In electronic systems, the most common method is to use Thermoelectric Coolers, or TEC's. A TEC is a specialized ceramic plate less than 1/4 inch thick, usually about 2 inches square. Based on the Peltier Effect, the temperatures of the two flat sides of the ceramic plate change when DC is pumped through the plate. One side becomes colder than the ambient air, and the other side becomes hotter. As the hot side is cooled down to ambient with a fan, the cold side becomes that much colder. Now some electronics can be pressed against the cold side, and the TEC will keep them below ambient.
However, TEC's are not very efficient. In a typical system, it takes 1 or 2 W of DC into the TEC for every watt of system heat being pumped through it from the cold side to the hot side. So cooling a 500 W system could take an additional 1000 W for the TEC's. For a recent design, we had to come up with a more efficient heat pump, and decided to use a phase change system. A phase change cooling system is the basis of most home air conditioners. A gas is compressed until it is a liquid, which raises its temperature. The hot liquid is cooled by ambient air, then routed to the evaporator. As it passes through the evaporator, it boils and expands, absorbing a lot of heat but staying at about the same temperature as it boils. The gas then returns to the compressor for another trip around the loop. Although this method is far more complex than using TEC's, it also is much more efficient. While a TEC design might pump 1/2 W for every watt consumed, a vapor change system can pump 3 or more watts for every watt consumed.
For a recent design, the parameters were as stated above - 500 W of system power, with come components that had to stay below the ambient air temperature. Added to this, the system had mostly conduction-cooled VME modules, but some convection-cooled boards. Also, the chassis had to be water-tight so it could be completely submerged, then operate while still dripping wet.
Usually this would be a liquid-cooled system, relying on an external supply of a cooled liquid. Inside the system, the liquid would be plumbed to the cold plates, where it would absorb heat (and increase in temperature), then exit the chassis to be cooled by some external heat exchanger. However, this system was to go in a HUMVEE, where liquid cooling was not available.
The Solution
A significant problem was finding a cooling system that would fit completely within a sealed chassis, extract heat to the outside air, and not break the power budget. We determined that 500 W was too much heat for a TEC design, but it seemed like too little for a PC system. 500 W equates to about 1/7 of a refrigeration ton. For perspective, a small window air conditioner is rated at 1/2 ton, and is larger than the entire chassis. Eventually we identified a vendor of a miniature vapor compressor. It was developed to cool fighter pilot flight suits. Working with this vendor, we designed a cooling system which would take up less than 20% of the available volume in the chassis, and consume only 300 W.
As indicated above, a common characteristic of liquid cooling is that the liquid heats up as it moved through the system. This means that all modules do not receive the same amount of cooling. The liquid might enter the system at 20°C, but increase to 50°C as it travels through the cold plate. This could be a significant problem if one of the "downstream" boards is sensitive to temperature.
The vapor change design has a significant advantage over liquid cooling in this situation. In a VC system, liquid enters the cold plate, but the exhaust is a gas. As the liquid passes through the cold plate, it picks up heat and starts to boil. With proper system design, the last of the liquid boils away just as it exits the cold plate. This means that the entire cold plate, from inlet to exhaust, is the same temperature - the boiling point of the refrigeration liquid. With constant "cooling power" available at all slots, the customer is free to arrange the board set based on signal and functional requirements, not bound by cooling system constraints.
An active cooling system is like a heat pump. It requires much more energy than the usual fan power to move heat "uphill" from a cooler temperature to a warmer one. The problem is identical to that of home air conditioning, where a space is cooled to 72 degrees while surrounded by 90 degree air.
