The introduction of rack-dense servers, and now blade servers, has brought an increased demand for additional power and cooling within the data centers where they are deployed. With the variety of options and configurations available, confusion often abounds as to what the best solution might be. The fact remains that the best solution for one company may not meet the needs of another. In this article, we’ll take a look at some of the solutions available and provide some helpful configuration suggestions.

Vendors have addressed the cooling problem with an array of ventilation options and additional cooling features. Rack cabinets can be purchased with perforated tops, doors, or side panels. Fan kits are also available to mount on the top or bottom of the cabinet.

Believe it or not, racks are designed to cool properly when the cabinet is full. Thus, to facilitate those who planned for growth, some vendors have also developed “blanks” designed to allow for proper airflow when racks are not full. Sensors are also available to monitor the temperature and humidity levels within the cabinet, complete with software to take necessary actions or send alerts when your preset thresholds are exceeded.

General cooling requirements

Environmental limits should be set 10 units from the equipment’s tolerance to allow time to take action once the threshold is reached and an alert is sent to an administrator. For example:

If typical temperature ranges are 50° to 95°, set minimum threshold at 60° and maximum at 85°.

If typical relative humidity tolerances are 8 percent to 90 percent, set minimum to 18 percent and maximum to 80 percent.

Rack power options
Racks can be powered via generator, UPS, or standard outlets; they can even be hard-wired. It is important to know your needs before ordering your rack system. If your company runs the entire computer room off of a room’s UPS, hard-wiring is a good solution for you. However, in many cases, the circuits within your computer room are hard-wired to the room’s UPS, which means that you can order your rack system with standard cords.

For added protection, you can use a twist-lock outlet and cord. These special outlets lock the cord into place to protect you from the dangers of accidental disconnection. Don’t forget to order the correct cords for your international sites.

Planning electrical circuits
When designing a new computer room, it is best to limit the full load amps of a given circuit to 75 percent of capacity. Of course, actually getting to design your computer room represents a perfect world that most of us can only dream of. So it’s nice to know that electrical circuits are a little more forgiving than you might expect and can usually handle a load 1.5 times its designed amperage rate.

Thus, a 20-amp breaker can easily carry 30 amps worth of equipment because we don’t draw full load power (in amps) on any given piece of equipment Rather, we draw watts at approximately 60 percent of the volt-amps. Most equipment has a full load amperage rating provided on the labeling. If there is no amperage rating, other information will be listed that enables you to deduce the amperage rating for that piece of equipment.

To calculate load so you can determine the power requirements for a given rack, add the amp ratings for all of the equipment you want to place in the rack to get a total full load amp rating. Don’t forget to calculate for those dual power supplies. You can also use watts and volts to calculate amps:

Amps = Watts/Volts

If your company generates its own power, it may also be prudent to purchase isolation transformers to provide conditioned “clean” power to your data center. This protects you from spikes and brownouts by controlling the voltage of the power as it leaves the transformer. Your engineering department will have information regarding these devices and can tell you whether they are necessary. If you’re curious and interested in looking up the details, check out Sola’s site.

Planning for UPS
Backup power is critical to the operation of your computer room, and UPS and generator sizing are imperative to the success of your backup power plans. The above formula can also be used for calculating load when sizing a UPS or backup generator for your computer room. Most components have a full-load rating of 200-1200 watts of power. When sizing a UPS, it is also advisable to add another 20-40 percent to the total watts in order to facilitate future growth or to add precious minutes to server uptime on UPS power. When sizing a UPS or generator, this volt-amp calculation should never exceed the UPS/generator rating.

Watts vs. volt-amps

Some UPS vendors use these terms interchangeably, but it is important to note that watts and volt-amps are not the same. Watts are the power actually drawn by a piece of equipment, while volt-amps, as defined in the equation above, are used for capacity planning. The wattage consumed by a piece of equipment is generally 60 percent of its volt-amps. For the purpose of sizing UPS or generator components, the volt-amps total should be less than the watts of the system selected. A useful article on the difference between watts and volt-amps is available on the APC Answers site.

Rack-based UPS systems are now available from many vendors. APC, Tripp Lite, and Belkin are the leaders in computer UPS systems, but many major rack and server manufacturers also have their own supported solutions. They offer broad load rating ranges, varying numbers of available outlets, hot-swappable batteries and backup batteries, as well as up to three load segments enabling you to prioritize your servers by selecting the sequence in which they are shut down—or started up. These systems cost a little more than the leading third-party brands, but don’t be surprised if, for example, your Compaq rack-based UPS has Deltek in the fine print on the label.

These UPS units take up 2U to 3U of rack space, so don’t forget to save room when configuring your rack. For your safety, never plug a UPS into another UPS. Many server-vendor UPS solutions also offer manageability software, which can help you to monitor the status of your power and notify you via page or e-mail in the event of an outage.

Now you’re ready to configure your own rack power and cooling to meet your needs. Here are the key points to keep in mind:

  • If your racks aren’t full, install the appropriate blanks to allow for proper flow.
  • Monitor your temperature and humidity levels.
  • Plan rack space for monitoring and UPS units.
  • Order the right power cords for your configuration: hard-wired, standard, twist-lock, international, etc.
  • Be sure that watts are < 3,000 per 20-amp circuit (or amps < 30).
  • Remember that internal power generation may require isolation transformers.
  • Make certain that watts are always < UPS rating.
  • Don’t connect one UPS to another.

What tips do you have for managing rack server power and cooling?

We look forward to getting your input and hearing about your experiences regarding this topic. Join the discussion below or send the editor an e-mail.