Google uses clear plastic curtains to prevent hot and cold air from mixing. The company also suggests using curtains to surround a bank of batteries for the uninterruptible power supply, to make it easier to keep them cool and stable. Why? What are the physical principles behind this?
In simplest terms, it's the same reason you close the refrigerator door or keep the doors and windows closed when you're trying to cool your house or car in the summer, or heat them in the winter. You don't want the warmer air getting into the refrigerator or into your house or car when you're trying to cool them down; nor do you want the cold air of winter to come into what you're trying to heat up.
The curtains create an enclosed space that keeps the air from going anywhere else. We call this containment. Assuming your data center cabinets are arranged in a hot aisle/cool aisle configuration, you can "contain" either the cold aisle or the hot aisle. Either way, it prevents the hot air discharged from the backs of your equipment from returning to the fronts of the cabinets and raising the temperature of the cool air produced by your air conditioners. If you contain the cold aisle -- or the batteries, as you mentioned -- you also create a smaller volume to keep cool than if you just let the air migrate out of the aisle and all over the room. This also prevents the cold air -- which is very expensive to produce -- from bypassing the equipment it is supposed to cool and from mixing with the hot air.
Whether we contain the hot aisle or the cold aisle, this accomplishes something else important. It keeps the air returning to the air conditioners at the highest possible temperature. Air conditioner performance charts will tell you that raising the return air temperature makes the cooling coils more efficient, which in turn delivers more cooling capacity from the units. So, containment not only improves our equipment-cooling effectiveness, but it also increases our energy efficiency.
I must note that for this to work right, you have to do more than just arrange your cabinets in a hot/cool aisle configuration. You also have to make sure that all unused rack spaces in cabinets are filled with blanking panels, and that all spaces between cabinets and all holes in raised floors have been closed. In other words, block those holes. Otherwise the air will find another path to where you don't want it, and you will have defeated the purpose of your containment.
I also have to caution that when you contain an aisle, you have to ensure that your fire protection system can still properly cover the space. The newest standards now prohibit "fusible links" sold by the containment curtain companies to drop the curtains out of the way, in the event of a fire.
About the author:
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC, with more than 35 years of experience. An expert in data center power and cooling, he helped pioneer building cable design and is a corresponding member of ASHRAE TC9.9. McFarlane also teaches at Marist College's Institute for Data Center Professionals.
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