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Cooling Made Easy

Precision versus Comfort Cooling

Is your cooling solution built for your Data Centre or server room needs? Or only adapted to them? Comfort cooling systems consist of central air conditioners in offices and other commercial buildings. They are primarily designed to balance temperature and humidity in order to keep people comfortable. Precision cooling systems are designed to strictly regulate atmospheric conditions within a very narrow range to provide a stable environment for sensitive electronics.

Although comfort cooling solutions are initially less expensive to implement, the overall total cost of ownership can be higher than with precision air conditioning both in terms of operational expense and costs associated with increased downtime.

People can handle fluctuations in temperature or humidity, but IT equipment cannot. Today’s higher-density Data Centre or server room deployments have far more demanding requirements than did yesterday’s mainframes. The following list delineates how the cooling requirements for an IT environment are quite different from those for an office environment.
1. Heat load profile

Almost the entire cooling load in an IT environment (such as a Data Centre or server room) is dry (sensible) heat thrown off by the equipment. Consequently, the air conditioner should primarily cool the air, now lower humidity. The required “sensible heat ratio” for a Data Centre or server room air conditioner is very high, 095-0.99. Comfort air conditioners typically have a sensible heat ratio of only 0.65-0.70, which means that they remove too much moisture from the air. Continuous re-humidification would be needed to maintain the required atmospheric conditions for IT equipment.

2. Heat load density

The heat density of an IT environment may be five times higher and far less uniform that that of a typical office. Precision air conditioning manages the increased load by maintaining a higher sensible heat ratio and operating at a much higher airflow rate which means more air is moved through the space to ensure a more even distribution of air and reduce the chances of localised hot spots in a Data Centre or server room.

3. Temperature and humidity control

Precision air conditioners have sophisticated, fast-acting microprocessor-based controls that react quickly to maintain strictly regulated set-point target temperature (­+1oF/+.56oC and humidity (+4% levels).

Comfort air conditioners simply are not designed to maintain such precise control over the temperature or relative humidity that can be created in such environments as Data Centres or server rooms or to react with the necessary speed to rapidly changing conditions.

4. Year-round operation

Precision cooling systems are built to run nonstop, 24/7, 365 days a year 8,760 hours annually. They are designed to incorporate redundancy and respond to the full range of outside ambient temperatures.

Comfort cooling systems are not designed to run continuously or to operate in winter conditions. They are meant for summertime or use during normal business hours (9 to 5, five days a week), up to an expected maximum of 1,200 hours per year. Comfort air conditioners quickly lose efficiency and break down under the strain of nonstop operation.

5. Air quality

Comfort air conditioners typically employ inefficient residential-style flat air filters, which are inadequate to remove a sufficient percentage of air-borne particles.

Precision air conditioners on the other hand, use deep-pleated, higher-efficiency filter banks that sufficiently minimise dust and other particles that can damage sensitive IT equipment that may be found in Data Centres or server rooms.

7. Efficiency

The separation of cold delivery and hot air return on most comfort air conditioners is very small. For wall-mounted units the cool air is delivered via the oscillating blades and the warm air returns via the static grill which is normally just above the cool air blades. In ceiling cassette units the perimeter oscillating blades again deliver the cool air and the central grill captures the warm air return.

Perimeter room precision air conditioners are either configured as down-flow or up-flow. There is a significant separation of the cold air supply and the hot air return. Indeed in a down-flow, pressurised under floor cool air delivery system the cold air can be directed to the air intakes of the IT equipment by the use of strategically placed open grille tiles. The air conditioners are then located to capture the hot return air from the rear of the IT racks. In an up-flow design, cold air is delivered at high level and is directed over racks to the IT equipment air intakes. The air conditioners are again located to capture the hot air return. Up-flow are used when there is no raised floor or it is very shallow.

This philosophy is employed as all air conditioners work more effectively and more efficiently when the temperature difference between the cold air delivery and the hot air return is at maximum levels. For example – for exactly the same equipment and with exactly the same running costs an air conditioner rated at 10kW of sensible heat load will deliver 10kW of cooling at a temperature difference of 6°C and only 7.5kW of cooling if the temperature difference is reduced to 4°C. If the load is effectively producing 10kW of heat load the easiest way to make it work is reduce the cold air temperature set-point. This significantly increase running costs and could lead to a secondary process of continuous dehumidification and humidification. Again, this is very expensive and back to the running a kettle on and off all day – and all night.

Why does the difference vary is different installations?

It is all down to the ability of the cold and hot air to mix and create warm air. The causes of air mixing are numerous and include a direct bypass from the cold air supply to the hot air return (especially prevalent in comfort units due to the close proximity of both the cold and hot air vents), distance of the heat generating equipment from the air conditioner, a mixed layout that does not have distinct hot and cold aisles and very little physical separation of the cold and hot air.

The higher the density of heat being produced in a rack the greater the problem and isolated hot-spots occur. Indeed in medium and high density installations it is necessary to place the air conditioners next to the IT equipment and or containing at least one section of the air in either a hot or cold aisle containment system.
A simple way to check on the efficiency of your air conditioning is to buy a combined temperature/humidity probe (costs around £100-£150 for an accurate device and worth getting the humidity option as well) and measure the temperature at the air conditioner cold air outlet, measure again at the IT equipment inlet (mid rack at the front), again at the IT equipment outlet (mid rack at the back) and finally at the air conditioner hot air return inlet. The closer the cold air outlet and the hot air inlet temperature are together the less efficient the operation. Don’t be surprised to find minimal differences with some installations. Measuring at the IT equipment inlet against the air conditioner supply shows how effectively the cold air is reaching your equipment and the IT equipment outlet against the air conditioner return shows how effectively the hot air is being returned to the air conditioner.

Where installations incorporate comfort cooling (typically offices converted to server rooms), installing more comfort air conditioners to combat hot spots or air mixing problems is mostly a wasted investment. It just costs even more to run and is not addressing the real problem.

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