Alternative Air Isolation Approach

Recognizing the shortcomings of using traditional cooling approaches on today’s computer and data center equipment, an alternative has emerged. Rather than focus only on the supply of cool driven by the return air temperature in the room, a more effective approach is to concentrate on isolating the heated return air from the cooler supply air to begin with. By adopting this fundamental approach, the design now focuses on options to create enclosures around the computer equipment that effectively capture the heat from the computer equipment and return it to the CRAC without mixing it with the cooler supply air. In the process two other room components become important contributing factors. The first is the plenum space above a suspended ceiling. As an open area for air flow, this can be an effective way to capture and deliver return air back to the CRAC unit just as it is commonly done in other building design applications. Alternatively, the plenum space could be used for ductwork to perform the same function where that is warranted by engineering design. The second room component to consider is a raised floor that has been commonly used in computer rooms and other energy efficient buildings in general. The space between the raised floor and the actual structural floor creates a second plenum space below the equipment for the supply of cool air to flow through. 

Hot Air Isolation is an approach that separates the hot air from the supply air, thus eliminating hot spots and any concern about return air temperature. Images courtesy of Chatsworth Products, Inc.

 

Adopting this fundamental approach to maintain the environmental control in computer rooms has a number of distinct advantages:

• Elimination of hot spots: In the traditional, air-mixed approach, hot spots are common and often drive the decision of where to set the temperature control to suit this “worst case” in the room. Thus, the system operates much less efficiently since it is over compensating for the non hot-spot areas. Isolating the heated return air from the cooler supply air means that distribution is designed to meet the demand of different equipment and the hot spots can be effectively eliminated. Hence cooling temperature set points can be increased meaning that less cooling is needed and controlled accordingly.
• Allows higher heat and power densities: In designs that use cabinets to stack the computer servers or related equipment, the heat and power limits inside of the cabinet become an important consideration. Determining the density of equipment that can be put into the cabinet based on power required and heat produced becomes an issue of testing and rating. By using methods that connect directly to the cabinets to isolate and channel heat from the equipment back to the CRAC, those ratings can be improved dramatically, meaning more equipment can be installed and cooled in fewer square feet. Independent testing has shown that heat and power densities of up to 4 times higher are possible. In a traditional system, 6 kW of power and associated heat would be typical, while using air isolation techniques, this can be increased to over 30kW—potentially a higher rating on the cabinet than could ever be accommodated inside it.
• Full utilization of supply air: Part of the cooling equipment assembly includes a Computer Room Air Handler (CRAH) that employs the fans to move the air within the room. In a standard, open room, traditional approach with hot and cold aisles, the CRAHs are required to produce significantly more chilled air than is directly required by the computer equipment. The design requirement for this surplus is to be sure the cold aisles are adequately filled with a volume of cold air to minimize the effect of the hot aisle re-circulating over the top of the server cabinets or wrapping around the ends of cabinet rows. This practice typically results in some amount of bypass air flow that returns to the air handlers without having conducted any effective heat transfer in any of the computer equipment. In addition, there is frequently inadvertent bypass where cold air is short-circuited back to an adjacent air handler without picking up any heat load. Surveys and audits have found this over-production to range over 2.5 times the actual airflow demand of the data center cooling load.  With containment, all the cool air produced can only return to the cooling units after having passed through the computer equipment and thereby conducting heat transfer and eliminating the need for any over-production. Because fan energy does not have a linear relationship to fan flow, the elimination of this wasted production is can be significant, up to a calculated 64% in some cases.
• More effective heat transfer: Another effect of isolating the return air is to achieve a much higher return air temperature, which is not necessarily a bad thing. In actuality, higher return air temperatures actually bump up the performance ratings of chilled water cooling units. Note that direct expansion (DX) units have a more or less fixed temperature difference that they work within so higher return temperatures will actually drive up the supply temperatures accordingly. However, with chilled water CRACs cooling performance efficiency actually increases with higher return air temperatures. In addition, as the temperature rise increases, the amount of air required for the same amount of cooling decreases. Anecdotal cases have demonstrated data centers that were experiencing significant over-heating problems evolve to actually placing half of their cooling unit capacity in reserve merely by effectively managing these variables.
• Chiller efficiency improvement: As previously illustrated, traditional, open return air data centers will typically operate cooling units with a return air set point around 72 °F, resulting in an under-floor supply temperature around 54 °F – 55 °F. These low supply temperatures were shown to be necessary so that, after re-circulation and mixing, computer equipment would not see temperatures exceeding the ASHRAE recommended maximum threshold of 80.6 °F. Since isolated air systems eliminate the effect of re-circulated return air on the supply air, it allows the supply air to be set in the mid-to-upper 70s, while still assuring that computer equipment is seeing appropriate temperature air. Most data center chiller plants are sending water to the CRAH cooling coils around 42 °F – 45 °F, in order to produce that 54 °F – 55 °F supply air; however, with the supply set at 75 °F or higher, that water temperature coming from the chiller plant can now be set around 65 °F or 20 degrees higher! Depending on the age and style of the chiller, operating costs are reduced by 1.5 – 4% per degree increase in the water temperature. Since the chiller constitutes anywhere from 65% to 90% of the total data center cooling cost, these potential savings of 30% to 80% exceed all other potential efficiency improvement opportunities other than turning off the chiller completely for economization. 
• Economizer improvements: Economizer functions on cooling systems use outdoor air for cooling on days where the outdoor temperature and humidity conditions are favorable for that to happen. That is true in CRAC units as well, so finding ways to optimize this “free” cooling makes considerable sense for this application. As we have noted, isolated air systems operate quite well at higher thermostat settings in comparison to traditional systems. This means that the number of days and hours when the outdoor air is suitable for using the economizer cycle can be increased dramatically. Depending on the climate and equipment used at a facility, the difference can be as dramatic as a jump from using an economizer cycle for 5% of the operating hours of a traditional system to over 90% of the operating hours of an isolated air system.
• Allows for design options: Open return air data centers have inherent design restrictions due to the need to deliver an adequate amount of cool supply air to each point of use, (i.e, rack or cabinet) and by the need to locate CRACs and CRAHs to minimize dragging warm return air over or through cold aisles. By contrast, isolated air systems remove both of those design constraints because the complete removal of waste warm air eliminates the criticality of the CRAC/CRAH physical location. They also allow supply air to be delivered merely to maintain room pressure rather than to push an adequate volume to a particular point of use. Therefore, a raised floor becomes an option but is no longer a necessity with the associated perforated floor tile located directly at a point of need. In addition, if rooftop air-side economizers are being used, the cool air can be ducted directly down into the data center without having to add fan energy to overcome duct loss to rout that supply air around the data center and under the floor. Similarly, in some cases, the cool air can be delivered through a wall again without having to apply the additional fan energy required to maintain adequate static pressure under the floor. In short, isolated air systems allow the data center design to be more flexible and respond to a higher level of business and operational issues.

Clearly, then, the energy saving impact can be quite significant by changing from a traditional open air approach to a design strategy that isolates the warm return air from the cooler supply air. How much of an impact will depend on the particular conditions of a given facility of course, but it will also depend on the selection of the most appropriate specific strategy employed by the design team to achieve that separation.