Hot aisle
In data center operations Hot Aisle refers to the arrangement of 19" rack computer cabinets within the data center so that hot and cold air are separated into Hot and Cold aisles thereby improving cooling efficiency.
Airflow
Data Centers typically have a raised floor or plenum floor arrangement where cold air is delivered under pressure, causing it to escape from every opening. Typically a certain number of the tiles in the floor are perforated or integrate an opening vent. Hot air rises to the top of the computer room, where it is captured by Computer Room Air Conditioners (CRAC) and chilled to be pumped back under the floor. An efficient data center ensures that as much of the cold air is drawn across hot computer parts as possible and that hot and cold air are not allowed to mix.
Front to Back Cooling
Most computer and communications equipment draws cold air from the front and exhausts hot air at the back. By mounting equipment in a cabinet and ensuring that all cabinets are oriented in rows or aisles, front to front and back to back, hot air and cold air mixing can be reduced. It is important to have open floor tiles in the cold aisle and closed tiles in the hot aisle. Cabinets should be fully populated with blanking plates to minimize airflow short circuits occurring. Cable entry holes in the floor should be stopped up to reduce airflow leakage.
Improving Efficiency
Reducing the ability of hot and cold air to mix improves the cooling efficiency of computer room air conditioners, reducing cooling costs and improving the heat load that can be removed from the data center. Using containment systems such as curtains, or proprietary container systems 1 can make very significant improvements to the thermal performance of the CRAC units.
Alternatives to the Hot Aisle
Vertical Cooling
At least one vendor has a patented technology that uses bottom to top cooling (Vertical Cooling) 2 using specialized racks or containers. This turns the hot aisle, cold aisle concept through 90 degrees and the result is a more natural cold layer at the bottom of the rack with a hot layer above. Cold air, hot air mixing is reduced.
Liquid Cooling
A number of vendors offer solutions that deliver cold water cooling either through coils fitted to the rear doors of computer cabinets 3 or alternatively through cold plates 4 attached directly to the CPU chips. The latter has the advantage of reducing the case temperature so much that the CPUs can be clocked at a very high rate, much higher than that which can be supported with air cooling.
References
- O'Donnell, S (2008). "Using Curtains to manage data center airflow", 'The Hot Aisle'', http://www.thehotaisle.com/2008/04/18/being-smart/
- Sun Microsystems. "Rack-mounting the Systems", "Site Planning Guide for Entry-Level Servers", http://docs.sun.com/source/816-1613-14/Chapter3.html