Cooling blade servers

Expert Robert McFarlane explains what's needed to cool blade servers.

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How much air movement does each blade server typically need for cooling?

When all manufacturers follow the ASHRAE Guidelines for listing power and cooling requirements, this will be an easy answer for anyone to obtain. I suggest you check with the manufacturer of your servers before going by "rules of thumb". However, since you asked for "typical requirements", the following may help.

Air quantity (Cubic Feet per Minute, or "CFM") is determined by three things: the Wattage draw of the equipment; the entering air temperature; and the heat rise you are willing to accept as the air goes through the equipment. Without getting into the technicalities of air conditioning, wattage is converted to heat at the rate of 3.4 BTU per watt. (BTU=watts x 3.4). The heat rise is the temperature differential between the air entering and the air leaving the equipment, and is simply called "TD". The formula for air quantity is:

CFM = BTU / TD x 1.08 (The 1.08 correction factor can be ignored for small calculations if you want.)

Common numbers for Blade Servers would be:

Blade Center with 14 Blades = 4,000 Watts = 13,600 BTU

Entering Air = 55° F

Exiting Air = 75° F

TD = 20°

Air Quantity = 630 CFM

This is based on a good under-floor air system with the blade center near the bottom of the cabinet, where the cold air comes up from the floor. It also assumes that all spaces above and below it are filled with other equipment or blank panels to prevent warm "bypass air" from getting from the rear of the equipment back to the front and raising the incoming air temperature. If the blade center is mounted toward the top of the cabinet, the entering air temperature will have risen by the time it gets up there, perhaps to 75°F. In this case, either the equipment will tolerate a 95°F discharge temperature (20° TD), or you'll need more air to cool it.

 

Entering Air = 75° F

Exiting Air = 90° F

TD = 15°

Air Quantity = 840 CFM

Of course, this assumes that the tiny fans in the servers can actually move 840 CFM of air through the equipment. This is one of the reasons some manufacturers are specifying higher operating temperatures, and allowing 25° to 30° TD's. However, even if the specs say higher temps are OK, you can be sure the life of the hardware will be shortened, and that unexplained errors may very well be temperature related. And remember, a standard 25% open perforated tile, under normal "good" conditions (0.1" static pressure), can only pass about 675 CFM of air. Usually you'll get more like 500 CFM. Its not just the capacity of the air conditioners that counts – it's the air that actually gets to your technology.

Hopefully, this will help you determine what you need, whether or not your data center can provide it, and will also keep those expensive servers from burning up or shutting down at inopportune times.

This was first published in April 2006

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