Often times the need to have back-up AC systems to guard against system failures, service, and maintenance outages is better and more easily served with the presence of the computer room cooling units - locally present, right within the vicinity of the critical heat load. This not only gives the IT management great peace of mind, but, depending upon the degree of redundancy (N+1), keeps the space at least partially cooled during such events to avoid thermal runaway conditions from developing.

Comfort air conditioners typically employ inefficient residential-style flat air filters, which are inadequate to remove a sufficient percentage of airborne particles. Precision air conditioners, on the other hand, use deep-pleated, higher-efficiency filter banks that sufficiently minimize dust and other particles that can damage sensitive IT equipment.

Precision air conditioners have sophisticated, fast-acting microprocessor-based controls that react quickly to maintain strictly regulated set-point temperature tolerance of ±1°F (±.56°C) and humidity control of ± 4%. Comfort air conditioners simply are not designed to maintain such precise control over the temperature or relative humidity or to react with the necessary speed to rapidly changing conditions.

Precision cooling systems are built to run nonstop, 24 hours a day, 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 use during normal business hours (9 to 5, 5 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.

The IT environment is continually losing moisture that is added to the space though natural ventilation and air conditioning. If the humidity is too low in an IT environment electrostatic discharge (ESD) can be experienced; similar to shocking someone after shuffling along a carpeted floor in stocking feet. An ESD event can shut down electronic equipment and possibly damage it. ASHRAE recommends the relative humidity be within the 40% to 55% range to protect IT equipment. Humidification would be needed to maintain the required atmospheric conditions.

Cooling IT environments require primarily sensible cooling, meaning there is little to no moisture removal requirements. The higher the CRAC unit’s SHR, the more efficient the IT environment since the unit is designed for high sensible cooling applications. CRAC units achieve a higher SHR through special cooling coil design and operating at a higher airflow rate than a typical air conditioner. In addition to the benefits of the higher SHR, the unit is also moving more air throughout the space, thus helping to ensure better air distribution – reducing the chances of localized hot spots.

Almost the entire cooling load in an IT environment is sensible heat (dry heat) thrown off by the equipment. The heat density of an IT environment may be five times higher or more, and far less uniform, creating potential hot spots that are not in a typical office.