Containment refers to physical barriers used in a hot aisle/cold aisle layout that further eliminate the mixing of cold ("supply") air and hot exhaust air. Containment barriers include plastic curtains and Plexiglas sheets that prevent hot exhaust air from flowing over the tops of server racks, mixing with cold supply air, and thereby reducing its cooling capacity.
With containment in place, CRAC units can reduce their fan speeds, CRAH units can use higher temperature chilled water, and economizers can be utilized more often for “free cooling.” Despite these energy-saving benefits, a 2014 Uptime Institute survey indicated that only 30% of operators have at least ¾ of their data center using some form of containment. Less than half of all survey respondents had at least 50% of their data center benefiting from containment.1
Hot Aisle / Cold Aisle Layout: Good, But You Can Do Better
The hot aisle/cold aisle layout helps prevent cold “supply” air from mixing with hot exhaust air, resulting in cooling savings from 10 to 35 percent. While hot aisle/cold aisle layout is a sound start on the road to an energy-efficient data center, hot exhaust air can still flow over the top of servers, leading to higher server inlet temperatures at the upper portion of server racks. (See Figure 1.) Likewise, air from the cold aisle can “short circuit” by flowing around the sides or over the tops of racks to mix with hot return air. Both of these scenarios result in energy waste because fans, CRACs and CRAHs must work harder to make up for higher temperatures in the cold aisle.
A variety of different containment barriers can prevent the mixing of hot exhaust air with cold supply air:
- Flexible strip curtains. Similar to plastic refrigeration sheets you might see at a supermarket, flexible strip curtains are an inexpensive option. They are easily installed with an aluminum track system that allows curtains to attach to drop ceilings and to the tops of racks (as shown in Figure 2). Google has successfully deployed these devices in its data centers (see inset).2
- Rigid Containment. This configuration effectively turns either the cold aisle or the hot aisle into its own room by sealing the aisle with doors, sidewalls, and roofs (as shown in Figures 3 and 4). These are often used for racks with very high densities (and therefore high heat loads.)
- Rigid Containment with Chimneys and an Overhead Return Plenum. Two chimneys of vertical ductwork direct hot exhaust air from the hot aisle up into the overhead return plenum, which is created by a drop ceiling. (See in Figure 5.) Hot air in the plenum returns to the CRAC intakes where it is cooled.
Saving and Costs
- Cold aisle containment can be implemented easily and quickly, and can reduce cooling-related energy costs by as much as 30 percent.2 It typically and carries a payback time measured in a few weeks to a few months, depending on whether utility rebates are available to help offset project costs.
- PG&E's experience with containment retrofits indicated that typical paybacks can be less than two years.3
- Google’s Green Data Centers: Network POP Case Study (PDF, 4 MB) examines a small data center’s experience with vented tile optimization, temperature and humidity adjustments, cold aisle containment, and CRAC air return extensions. Taken together, these measures showed a return on investment (ROI) in less than one year.
- An analysis of hot aisle versus cold aisle containment revealed hot aisle containment can save 40% more than cold aisle containment4 for two main reasons. One is that the higher return air temperatures produced by hot aisle containment allow cooling units to work more efficiently. The second is that cold aisle containment can result in the uncontained area (where workers spend most their time) becoming too hot. Uncomfortable working conditions force staff to increase the use of mechanical cooling.
- Other potential costs for consideration include:
- HVAC system adjustments/tuning after containment barriers are installed. For example, CRAC unit operation can no longer be governed by return air temperatures -- they must use server inlet temperatures instead
- Electrical costs for reconfiguring power distribution to the racks, and
- Associated labor, overtime, and vendor costs, if applicable.
Tips and Considerations
- Depending on their location inside the data center, adjustments to fire detection and/or fire suppression systems may be necessary. Staff at Kaiser Permanente’s ENERGY STAR certified data center deployed special above-aisle cold aisle containment tiles that shrink and drop to the floor at high temperatures – avoiding the need for a fire suppression system. See their case study (PDF, 79 KB) and public service announcement (PDF, 2 MB).
- When the cold aisle is contained, the rest of the data center becomes warmer. This may result in uncomfortable working conditions for staff and unsafe operating temperatures for some equipment.
- Though many CRAC units use return air temperature to indicate room temperature, this will not work with containment because the return air is concentrated and therefore much hotter. (Return air refers to warm air entering the CRAC for cooling.) Instead, CRAC unit operation should be governed by server inlet temperatures.
- A paper by American Power Conversion (APC) (PDF, 657 KB) argued for hot aisle containment over cold aisle containment for three reasons:
- Air returning to the air conditioning unit is hotter, resulting in better cooling unit efficiency.
- Increased economizer mode hours.
- Server inlets have access to a large air mass (the rest of the data center room) that can help prevent overheating in case of a cooling failure.
2 Source: 2010 Green.net Conference
5 Focused Cooling Using Cold Aisle Containment, https://www.vertivco.com/globalassets/shared/focused-cooling-using-cold-aisle-contaiment.pdf (PDF, 582 KB)
6 Conversation with Mark Bramfitt, former Principal Program Manager, High Technology Energy Efficiency Team, PG&E, July 14, 2010.
7 Source: http://www.facilitiesnet.com/whitepapers/pdfs/APC_011112.pdf (PDF, 684 KB)