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Are Chimneys Needed on Every Cabinet if Each Rack Uses Different kW?

April 14, 2009

We often receive questions from folks in the industry about our products and solutions. Bill Watts, CPI Sr. Data Center Architect, responds to one.

Question: Is there a need to "balance" the air volume with the chimney solution when racks utilize different kW? Can I use chimneys on some cabinets and allow other cabinets to function without chimneys?

Answer: It is not necessary to balance the air regardless of kW loadings of the cabinets. The data center is providing the overall quantity of air required based on the design load. As long as you replace the air taken from the room with cool air of the same quantity, the performance will be unchanged cabinet to cabinet regardless of kW loading. A common fallacy is that the cooling tile in front of the server cabinet is somehow magically used only by that cabinet.

In reality the room is a container of air (hopefully cool air) that servers draw from to cool servers. In the process they raise the temperature of the cool air. Your challenge is to get that heated air directly back to the cooling units without letting it mingle with other cool air in the data center thereby depleting your ability to effectively cool other servers. A traditional data center design does a very poor job of this. Passive cooling (chimney) cabinets do a very good job of this by collecting the heated air and sending it to the return ceiling plenum. The cooling units are also connected to this return plenum. This creates a closed circuit for the air to travel and insures all the cool air you produce provides cooling to servers and is not depleted by mixing with heated air from other equipment in the room. Of course you will want to use blanking panels in empty rack spaces to stop short circuiting at the cabinet level. You will find a very good improvement in cooling efficiency (lower cost of operation).

TeraFrame Cabinets with Duct

You can use traditional cabinets intermixed with passive cooling (chimney) cabinets. I would suggest that you use a hot aisle/cold aisle configuration and deploy all the usual best practices you probably already use. Place the non-chimney cabinets more in the middle of the row with a return grill in back of these cabinets only. It is also a good idea to maintain a under floor static pressure of 0.1 to 0.15 inches of water column. This will assist in creating a non-physical air curtain to help separate hot and cold air at the non-chimney locations.

Our support engineers can assist you in laying out your data center for best performance and flexibility. Please contact us for an available support engineer (800) 834-4969. Bill Watts, Sr. Data Center Architect

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