The next generation of cooling for computer rooms

CO2
The first installation of Trox’s carbon-dioxide cooling system for computer cabinets is at Imperial College. The units bolt on to existing racks and can remove up to 24 kW of heat, returning air to the space at the same temperature as it entered the front of the cabinet.
With the cooling requirements of data centres outstripping conventional close-control air conditioning, Trox has been working on a radically new approach.Whenever computing equipment in IT rooms and data centres is replaced, the heat density will be higher. The power density of blade servers which are installed in rows and rows of racks in such centres has grown so much that a standard 19 in rack could generate at least 20 kW of heat. Unfortunately, traditional close-control air conditioning is limited to heat loads of about 8 kW per cabinet. With this approach, air is drawn through cabinets by fans in the blade servers to cool them. The hot air is delivered into the space, passed through close-control units for cooling and then returned to the space. Introducing cooling into the cabinets using chilled-water coils can handle greater heat loads, but its limit is about 15 kW per rack. In response to what is clearly a growing problem, Trox has spent two years developing a new approach to cooling with the potential to handle up to 30 kW per rack. Guy Hutchins of Trox Advanced IT Cooling Systems (AITCS) explains that this radically new approach is based on using liquid carbon dioxide to remove heat directly from racks before it enters the space. CO2OLrac, as it is called, can already be used with racks producing 24 kW of heat, and work continues to increase capacity. To put these heat loads into perspective, the test rig at Trox involved in developing this concept uses several electric fan heaters to simulate heat loads from blade servers. Each rack with a high heat output has CO2OLrac unit fixed to its rear door, as long as it has 50% free area. Its weight is supported by three castor wheels. These are conventional racks, so CO2OLrac can be added to existing computer rooms. Lots of cooling coil and five fans provide the redundancy required in mission-critical computer centres. Room air is drawn in through the front of a rack at, say, 22°C and picks up the heat from the blade servers. As this air passes through the CO2 coils, all that heat is removed and the air returns to the room at the same temperature it entered the rack. Without the cooling unit, air would be discharged into the space at 35 to 45°C. Liquid CO2 at 14°C, typically above dew point to avoid condensation, is pumped through pipework to the coolers, and some of this liquid evaporates to remove heat generated by the blade servers. The return pipework thus carries liquid and vapour — still at 14°C. A range of units to convert CO2 vapour back to liquid has been developed by Trox AITCS and Star Refrigeration. They provide the interface between the CO2 circuits in the computer room and a conventional chilled-water circuit or an air-cooled or water-cooled R134a chiller. Indeed, an existing chilled-water system of adequate capacity can be used, and Trox offers a short-term trial in such cases.
Trox’s approach to cooling computer cabinets is based on liquid carbon dioxide being pumped to the rack at 14°C and liquid and vapour returning at 14°C. The vapour is recondensed in this unit developed by Star Refrigeration, with the latent heat being rejected to chilled water.
There are many reasons why carbon dioxide was chosen as the cooling medium. HFC-type refrigerants were rejected because of oil-management issues. Water was rejected because its heat absorption is a sensible process; with flow/return of 6/12°C, 1 kg of water absorbs 25.2 kJ of heat. The evaporation of 1 kg of CO2 absorbs seven times more heat — 182 kJ. Not all the CO2 can evaporate of course, but even if a seventh does, the heat-absorption capability is the same as water at the same flow. The higher temperature of the CO2 circuit (flow and return are both 14°C compared with, say 12/6°C for a chilled-water system coupled with reduced pumping power) make possible more efficient chiller operation — perhaps as much as 30%. Using less power to cool a computer room makes more power available for IT equipment. CO2 has the additional benefit that should a leak occur, it is electrically benign. Imagine the consequences of a leak of water or refrigerant. While a leak of CO2 could be a health-and-safety hazard, the risk is minimised by CO2 detection as part of the system. The CO2 circuitry in the computer room is rather special. It comprises high-grade fully welded stainless-steel pipework. Tap-offs are provided for flexible connections to the rack-mounted units using high-quality couplings that are readily available. The flexible couplings enable cooling units to be hinged back for access to the interior of the rack without having to disconnect. Even when the unit is hinged back, sufficient air is drawn through the cooling coil to absorb 60 to 70% of the heat, so that only 30 to 40% is discharged into the aisle — increasing the local ambient temperature by 1 to 2 K, which is in turn absorbed by surrounding units — making the system intrinsically resilient. CO2OLrac units are carefully sized and shaped so that the door to which they are fixed can be fully opened. There is no need to reposition racks — as is evident from the photograph. This CO2 approach to cooling blade servers has been utilised as part of an upgrade of a computer room at Imperial College in London by consulting engineers hurleypalmerflatt. It comprises 15 CO2OLrac units, with heat rejected by a 300 kW air-cooled chiller using R134a. Without some means of handling the ever-increasing heat from blade servers, fewer cabinets could be installed in a space with a particular cooling capacity provided by conventional close-control cooling. Trox’s CO2-based approach is estimated to achieve up to 50% saving in rent and space which when combined with the energy saving provides a payback for the system of two to three years. The system was being developed for the last two years for a formal launch on 1 February. With the project at Imperial College in progress, word leaked out, and interest became widespread. Indeed, Guy Hutchins, says, ‘Everyone with blade-server technology wanted to know about this system even before we launched. This could change computer-room cooling for ever.’
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