Keeping Racks Cool

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Most engineers will be involved in at least one facility build at some point in their careers. This project will likely involve a rack room. It's always a challenge to maintain a balance between creature comforts (the people) and proper equipment cooling. A rack of equipment can create a great deal of heat, and it's no secret that heat is the enemy of electronics.

Most equipment rack installations I see are designed with passive thermal management techniques, but active thermal management is an option. Regardless of the technique, there are several common mistakes often made. These mistakes result in air flow hot spots, which can lead to premature equipment failure.

Figure 1. Ideal placement of cold air supply and hot returns in a rack room. (Click image to enlarge.)

Figure 1. Ideal placement of cold air supply and hot returns in a rack room. (Click image to enlarge.)

While investigating some information about proper thermal management I came across some information from Middle Atlantic. The company publishes a white paper that discusses thermal management in great detail. If you are designing a rack room, this is a good read. But there are some basic rules to keep in mind that can be applied in any situation.

Ideally, a rack room will be designed for proper air handling from the start. Middle Atlantic describes this as establish hot and cold aisles around equipment. This plan provides an air flow from cold to hot through and around the rack-mounted equipment.

Figure 2. Passive thermal management (Click image to enlarge.)

Figure 2. Passive thermal management (Click image to enlarge.)

Ideally, the cold air supply is delivered from the floor, which can be a raised floor or floor-mounted vents. The air return is in the ceiling. This establishes the basic of the air flow from cold below to heat above. The next step is to orient the racks so they alternate front and back. In Figure 1, there are three rack rows. Notice how the equipment is placed with the fronts facing in two rows and the back facing in the next row. The air intake for the rack is placed at the bottom of the rack front. Some equipment also has air intake vents on the front. The cold air will flow into the rack at the bottom and rise as it is heated by the equipment. The hot air returns are placed in the opposite aisles at the rear of the equipment.

This is only the first step in thermal management, however. Ensuring proper air flow within the rack creates the second step in the process. Too often, vented rack panels are placed in incorrect positions, resulting in localized convection loops that do not properly vent the heated air within the rack.

Figure 3. Active thermal management (Click image to enlarge.)

Figure 3. Active thermal management (Click image to enlarge.)

Figures 2 and 3 indicate the proper placement of vented panels to ensure optimal air flow. For passive cooling (Figure 2), a vented rear door is recommended. With active cooling, a solid rear door should be used. Both systems note that the air intake vent should be at the bottom of the rack.

Both methods rely on providing the most natural airflow to ensure the heated air is evacuated and not recirculated into the rack and equipment.

Diagrams courtesy of Middle Atlantic

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