The Data Center Hot/Cold Aisle Containment Debate

Optimizing air flow in your data center has several key benefits, including reductions in energy cost and carbon footprint. Most server equipment manufactured today is designed to draw in air through the front and exhaust it out the rear. This allows equipment racks to be arranged to create hot aisles and cold aisles. This 'hot/cold aisle containment' approach positions racks so that rows of racks face each other, with the front of each opposing row of racks drawing cold air from the same aisle (the cold aisle). Hot/cold aisle containment systems often use makeshift design solutions like vinyl plastic sheeting used in meat lockers to prevent the mixing of hot and cold air and there is some debate at the moment about whether it is better to contain the hot or cold air.Hot Aisle Containment

In hot aisle containment (HAC), racks are positioned in rows back to back. The hot aisle in between racks will be covered on the top and at the end of the rows and ducted back to the computer room air conditioning (CRAC) unit. The result is a full separation between supply and return air. A cold supply of air will be delivered into the room, as shown in Figure 1. The room itself will be at a low temperature level.

Cold Aisle Containment

In cold aisle containment (CAC), racks are positioned in rows front to front. The cold aisle in between racks will be covered on the top and at the end of the rows. The result of this is a full separation between supply and return air. Cold air will be supplied through a raised floor into the contained cold aisle; hot return air is exhausted from the racks into room and back to the CRAC unit, as shown in Figure 2. The room itself will be at a high temperature level.

In a recently posted white paper, Emerson Network Power looked at both hot and cold aisle containment and came to the conclusion that cold aisle containment is a better solution. The paper asserts that cold aisle containment is a more focused cooling approach since with CAC you have cold air only where you want it. "The HAC approach can result in high efficiency of the cooling unit, but because the cold air distribution to the servers is open and exposed to disturbances from the room, it has a higher risk of not providing the server with its required input temperature... For HAC to work correctly and avoid mixing hot and cold air, the hot return air must be ducted all the way from the HAC to the air inlet of the CRAC." This means that hot air containment may be difficult to install as a retrofit, especially without interrupting data center operations, because of the additional required ducting.

John Bean, director of innovation in racks and cooling, with APC by Schneider Electric takes a contra position in support of hot aisle containment. "Both hot aisle and cold aisle offer significant improvement in data center cooling efficiency and effectiveness. There are isolated cases that cold aisle may be simpler to implement. However, in most cases, hot aisle containment may be just as easily implemented and offers a superior level of performance."

Bean says that hot aisle containment effectively captures the heated IT exhaust air very close to its point of origin, cooling it immediately. "This approach eliminates risk of hot air wandering off into sections of the data center that could be adversely impacted." He points out that it is difficult to achieve an airtight seal of raised floors used in many CAC solutions, and some portion of the cold air will be mixed into hot return air reducing the overall effectiveness of the cooling solution. Another issue is the fact that in cold aisle containment the server room will be kept at a high temperature, as opposed to just hot aisles with hot aisle containment, and will adversely affect the comfort of the workers in the space.

William Tschudi, a project manager in the Energy Technologies Division at Lawrence Berkeley National Laboratory, who has studied the effectiveness of both hot-aisle and cold-aisle containment, agrees that both approaches offer significant improvement in data center cooling efficiency and effectiveness, but disagrees that one or the other way is necessarily best. "It's a design option. From a thermodynamic point of view, it makes sense to try to get as great a temperature difference as you can because the HVAC (heating, ventilating, and air conditioning) equipment works more efficiently with a high temperature difference." Whether or not you decide containing hot or cold air is a more manageable solution, the Emerson white paper advises contacting the local fire authority to discuss any aisle containment plans to make sure that codes are followed. If you put plastic sheets over or around your server racks, you want to make sure that your overhead sprinkler system can still put out any potential fires.