CRAC-systems against in-row air conditioners
By creating a solution for cooling IT load of high-density in the data center, you need to take into account the uniform distribution of air through the room and the height of the rack, as well as the potential of growth of energy consumption per rack. Air-conditioning systems based on the perimeter of air conditioners has a significant disadvantage — limited opportunities for increasing IT loads, determined by fixed height of the raised floor and the capacity of floor blocks. In addition, be aware of the fact that the racks with high power consumption typically operate with air temperature extremes at the inlet and outlet of the rack. This, in its turn, imposes restrictions on the use of air conditioners, working on DX scheme, in which inlet air temperature usually does not exceed + 35° C.
The main advantage of CRAC systems is low cost (esp. in-row air conditioning). Typical conditions of their usage are large machinery rooms with a relatively low power density per rack (up to 4-6 kWh). In such a situation, you can use the traditional scheme with the flow of cooled air under the raised floor and with air inlet directly from the premises of the server room. And in this case, the number of racks can’t be limited. CRAC-system usage for racks with greater density (up to 7-8 kWh) is also acceptable, but in this case it is necessary to organize containerization of cold aisle and fix raised floor tiles with a higher percentage of perforation.
In terms of energy efficiency and utilization coefficient of the square of the data center, the most successful variant is the use of perimeter air conditioners with fans, handed down under the raised floor (currently these conditioners are present almost at all vendors in the product line), combined with high raised floor (1000-1200 mm), active tiles to deliver air directly to the IT equipment and containerization of cold aisle. This decision will take a minimum area and provide maximum energy efficiency. However, from the viewpoint of further increasing of the power of IT equipment, this approach rests on the height of the raised floor, which in most cases is impossible to increase with the growth of the IT load (also very often basements with low ceilings are converted to data centers, and it is physically impossible to arrange a raised floor height more than 800 mm there).
The main constraints for the application of the CRAC-systems in data centers are just the power consumption per rack and the height of the raised floor. Therefore, when installing the racks in the machine hall with the capacity of more than 6 kWh using of in-row air conditioning is optimal. Then, with the growth of power consumption of racks or increase of their number, you can simply add the required amount of air conditioning without significant changes in data center planning.
Sum it all up, we note that the selection of a solution in the design of air conditioning systems of the data center should be based on the current site capacity and the prospects for further growth of the IT load. In one situation CRAC-system wins, in the other — in-row air conditioning. There is no universal solution for all occasions.