Chassis Thermal issues
An equipment chassis normally draws air in through the bottom front of the chassis just below the card cage. The exhaust air is pushed out the rear of the chassis. Fans may be placed under the card cage pulling in the air, or at the rear of the chassis pushing the air out of the chassis. The number and placement of the fans has a determination as to the size of the chassis. Fan Manufacturers produce a number of different sizes and capabilities. So the number of required fans is determined by the fan size, and the amount of tolerated heat rise within the electronic chassis, or the amount of air flow required to keep the system below some maximum value.
Design Hint; The type and number of fans can greatly effect the MTBF of the system, so be sure to check out the next section on Fan Selection.
In addition to the use of chassis fans, Rack fan trays may also be used to reduce the inlet air temperature to the chassis, by moving air through the rack faster. Fan trays move air within the rack reducing its temperature; how ever fan trays may also be placed directly under a chassis to support air flow. Vendors that produce them are listed on the Fan Tray Manufacturers page.
Design Hint; A Fan Tray will consume a minimum of 1U of rack space.
Slot By-pass cards may be used to force air next to circuit cards when one or more chassis slots are empty. Of course Front Panels and Filler Panels are always required to control the flow of air within the chassis, insuring that the forced air passes over components requiring cooling.
System Engineering;
1. Determine the heat generated by the boards and components within the chassis.
2. Determine the amount of air flow required to keep the card temperature and components at the required level.
3. Design in the number of required fans which will deliver the amount of air flow required.
4. Determine the heat generated by the equipment within the rack.
5. Factor in the chassis inlet air temperature from the rack, and add an additional fan or fan tray as required.
Temperature Sensor Vendors
Design Hint; The ambient air temperature within an enclosed rack is not the same as the ambient air temperature within the room. By convention ambient air is defined as 250C, the air temperature inside a rack may be higher or lower than 25C.
PC Tower Thermal Design
Next section; Chassis Fan Selection
How To Specify a Equipment Chassis
Design Hint;
Lower component MTBF or FIT by desinging for maximum temperature conditions; Derating Electronic Devices
