How to Improve Your PUE Through Cable Management
Cable management can have a significant impact on airflow management (AFM) in a data center, which in turn has a significant impact on the efficiency of a data center’s mechanical plant. As we all know, the efficiency of a data center’s mechanical plant has a significant impact on PUE. Therefore, cable management can have a significant impact on a data center’s PUE.
The relationship between cable management and efficiency plays out in two primary ways. First, if cable management becomes worse over time, then a higher flow rate of conditioned air and/or colder conditioned air will be required to prevent hot spots from forming. Increasing densities in the computer room may mask the contribution of poor cable management to this problem. Second, and more likely, is the situation where a data center is making an effort to improve efficiency and reduce its PUE by increasing temperature set points and reducing cooling unit fan speeds. In this instance, the data center is constrained by how far it can push these limits. However, what many sites don’t recognize is that if cable management was improved, cooling unit fan speeds could be reduced further and temperature controls set points could be increased; both of which would further reduce a site’s PUE.
Let’s examine a few common trouble areas and some best practices to help deal with them.
Cable management through the raised floor
Any air escaping from cable openings becomes bypass air, which is air that cooling unit fans are using energy to blow around, but does not actually contribute to cooling the IT equipment. Poor management of raised floor openings results in approximately half of the total conditioned airflow in data centers escaping through unsealed cable openings and misplaced perforated tiles. When a data center has common openings like these, it obviously requires running more fans to provide vital conditioned air to the heat load. So management of raised floor open area, particularly cable openings, is an obvious area where cable management intersects AFM and PUE.
Tips for improving cable management through the raised floor:
- Seal all cable openings under racks. While challenging in some situations this is extremely important to do well.
- Seal all openings under power equipment such as power distribution units (PDUs). The openings under PDUs are often very large, and can release as much as 1/3 of the capacity of a cooling unit.
- Relocate or replace all supply tiles not in front of IT equipment with solid tiles. This also improves the flow rate of conditioned air through supply tiles.
Cable management under the raised floor
Cable management under the floor is important for two reasons. First, cables can create obstructions that affect the airflow velocities and static pressure under the floor. Air dams created by poor cable management limit the amount of air that can get past, reducing the static pressure and the amount of IT equipment that can effectively be cooled beyond the air dam.
Tips for improving cable management in the raised floor:
- Place cable trays in hot aisles. This prevents cable trays from blocking the flow through supply tiles.
- Place cable management trays as high as possible, allowing air to flow underneath them. This is particularly important when running cable trays close to or in front of cooling units where most of the airflow movement is close to the floor. If done incorrectly, the distance the conditioned air can travel will be impacted.
- Place cable trays at a consistent height as much as possible. This allows conditioned air to flow in a straight path.
- Enforce a strict policy of removing all unused cables.
Cable management in the rack
With increasing densities comes an increase in the individual component count (i.e. the number of components in the cabinet), which increases the number of power and data cords. The more cords you have, the more likely you are to block the airflow or exhaust from IT equipment.
Cables inside the cabinet can impede exhausting hot air out of the rear of equipment cabinets, resulting in re-circulated hot air that increases the equipment inlet air temperature, possibly creating hot spots or widening the variation of inlet temperatures throughout the data center. These issues in turn can drive down set points and thus create energy expense penalties from less efficient chillers, reduced access to free cooling hours, and likely increased fan energy. These types of obstructions can also affect the performance of server fans.
Tips for improving cable management in the rack:
- Use wider cabinets with cable management built into the side and not directly behind the exhaust ports.
- Use deeper cabinets that allow the air more room to escape vertically.
- Use blanking panels. When cables increase the pressure within the cabinet, blanking panels become especially important.
- Do not block the exhaust from servers, particularly ones with high volume and high velocity fans.
Cable management overhead
Finally, moving cable distribution from under the floor to overhead can help with underfloor air distribution; however, it is important to consider where overhead cable trays are located so they don’t create undesirable consequences.
Tips for improving cable management overhead:
- Do not place cable management trays high above the cabinets. In rooms without a ceiling plenum return, it forces hot air returning to cooling units to go under the cable trays and closer to IT intakes, which can cause hot spots.
- Place cable management trays within a few inches of the top of an IT cabinet so that all exhaust air flows to the top of the room and over the top of cable trays. This can actually improve the AFM in the room.
- If your room has no containment, consider the type of containment that may be installed so that the cost and effectiveness of future containment is not compromised.
Cable management falls squarely under the responsibility of IT management of the data center, even if in larger organizations there is a network administration group that is still within the IT organization. Keeping network cables and power cables neatly dressed and labeled has become more and more critical for the effective management of moves, adds and changes (MACs) as density and application complexity have proliferated.
By following these basic guidelines for cable management, significant improvements can be made to AFM, positively impacting your capacity, reliability, operating cost, and ultimately your PUE.
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