Best practices for keeping your data centre cool this summer


By John Jakovcevic*
Monday, 01 November, 2010



Best practices for keeping your data centre cool this summer

Data centres need a constant environment to operate efficiently. The right room design and the air-conditioning equipment will go a long way towards achieving this. There are also a number of best practices that can be applied in any data centre that will help you to keep your equipment cool this summer.

The following examines ways of increasing efficiencies and improving airflow through raised floor design, cold and hot air separation, perforated tile issues and air and water conditions. The ultimate goal of all these suggestions is to design and build the most efficient cooling system possible.

1. Airflow leakage

Airflow leakage leads to dramatic inefficiencies due to air circulating back into computer room air conditioning (CRAC) units without taking sufficient heat from the equipment. A huge amount of fan power is wasted to circulate the air in such an unwanted way. To avoid this issue, close all unwanted openings - on the floor, below the racks, near walls - and check for cable cut-outs where only a small proportion of the opening is used for cables and the remainder is left open enabling air leaks to the room. The target is to create a specified overpressure so that the perforated tiles supply an even air supply to all areas.

2. Perforated  tiles

The number of perforated tiles must be in line with the design and with the actual total airflow. If the real airflow is lower than required, the external static pressure must be maintained to ensure equal air supply anywhere in the room. If the airflow does reduce for some reason, it will lead to a reduction in static pressure in the raised floor, which in turn leads to uneven air distribution and, eventually, lack of cooling in some areas.

Note that perforated tiles with integral adjustable dampers can be used to avoid having to replace perforated tiles with solid tiles. In this case the number of tiles remains unchanged, but all need to be adjusted. It’s also possible to operate with different adjustments to vary the amount of air in different areas.

Only ever place perforated tiles in positions where cold air is required to cool equipment. Keep at least a 2 m distance between the tiles and CRAC units to avoid inducing warm air into the raised floor.

3. Close unused units in the racks with blanking panels

Recirculating cooling air inside the rack leads to overheating of servers. Air will always take the path of least resistance and blanking plates are required to fill gaps where servers have been removed or not installed, otherwise hot exhaust air from the server will circulate through the 'gap' back to the air intake of the server.

4. Airflow philosophies

There are numerous philosophies to data centre airflow such as 'hot aisle, cold aisle', 'hot aisle containment', 'cold aisle containment', 'direct in-rack supply, room return', 'room supply, direct rack-out return' and 'aisle separation'. Despite their different approaches, the aim of all these philosophies is the separation of cold supply air from the CRAC unit and hot return air.

When set up properly, this separation leads to an increased difference between supply and return air, thus increasing the efficiency of the CRAC unit and therefore the efficiency of the data centre in general. If each square metre of circulating air takes the design amount of heat from the IT equipment, the highest level of cooling efficiency is reached.

5. Raised floor height

The raised floor height has a major influence on the efficiency of the air circulation in a CRAC unit-based cooling system. Usually the raised floor contains cabling, piping and cold air. A certain obstruction-free area is required for a proper supply of cold air to any area of the room. The required free height depends on the room size, the heat density, the number and position of installed CRAC units, and on the total amount of air that has to circulate through the raised floor. As a general rule of thumb, the higher the floor, the better.

6. Return air conditions

Traditional CRAC-based cooling systems are designed to operate at return air temperatures of approximately 22 to 24°C. The cold supply air in such systems is typically around 14 to 16°C. In the past, when used with uncontrolled airflow, mixing and bypassing, it was necessary to have low room temperatures to compensate.

Nowadays, focus on delivering a more refined airflow has reduced these issues to a minimum with the result that the cooling system can operate at much higher temperatures. The server equipment to be cooled operates easily at inlet temperatures of 20°C and above, while the supply temperature level can be increased 5 to 6°C and return air temperatures of around 30°C will become common practice.

At such conditions CRAC units operate much more efficiently and free cooling systems can run many more hours per year in free cooling mode. These higher return air temperatures can easily be achieved with chilled water CRAC units. Compressor-based CRAC units can also operate at these higher return air temperatures, but typically at reduced air volumes to ensure proper conditions in the refrigeration circuit.

7. Chilled water system water conditions

The entering and leaving water temperatures of chilled water systems (chilled water CRAC units in the data centre and chiller with or without free cooling outside) have a major influence on data centre efficiency and energy consumption.

Heat loads in data centres are nearly all due to sensible cooling load (ie, heat gains due to appliance radiation and convection). There is only a minor latent cooling load associated with fresh air ventilation. Therefore, there is no reason why the chilled water temperature leaving the chiller shouldn't be elevated from a normal 6 or 7° for air conditioning applications to 10°C or even higher. This would leave the CRAC unit providing all sensible cooling and the efficiency of the chiller will increase.

8. Standby unit operation

Fan laws dictate that air volume is directly proportional to fan speed and that fan power is a cube of the fan speed. Therefore, by running the stand-by CRAC units at reduced speed (air volume) the overall fan power is greatly reduced. By using high-efficiency EC fans (fans with brushless DC motors) it is possible to obtain automatic control of fan speed as dictated by standby operation, room cooling load or underfloor static pressure.

It is more energy efficient to run all units (including the standby CRAC units) at a lower speed than to shut off or sequence off the standby unit(s). This technique also provides a more even and predictable air distribution.

Keep technology up to date

In addition to the above suggestions, try to keep your cooling technology up to date. There have been some tremendous leaps in recent years, such as the introduction of scroll technologies in compressors and the new EC fans all of which reduce demand on power and ensure a stable environment for the hardware. With better technologies and enhanced knowledge about how air cooling works, the opportunity for efficiency gains in the data centre has never been greater.

*John Jakovcevic, Managing Director, Stulz Australia

Images courtesy of iStockphoto

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