Why Is Precision Air Conditioning (PAC)?

What is precision air conditioning and why is it necessary?

Compared to yesterday’s monolithic telephone system, today’s worldwide communications network is a much more complex and constantly evolving cluster of interconnected systems. And with continually more mission-critical processes reliant on transmission accuracy and 100 percent uptime, the requirements for reliability and performance have never been greater. Precision air conditioning is one cornerstone of a well- designed support system for this network.

Communications network cooling requirements differ from typical commercial or “comfort” requirements, in the following significant ways.

1     Year-Round Operation

Commercial cooling systems may be required to operate only during summer months and during normal working hours. Communications sites, however, generate heat 24 hours per day, 365 days per year, and so the cooling equipment must be able to operate reliably when outdoor ambients are as high as 120degF or as low as -20deg. Because of this constant cooling requirement, the selection of reliable and energy efficient components are essential.

Unlike common commercial air conditioning systems, these precision air conditioning units are designed to run continually, require little maintenance, and provide precise control of temperature and humidity. Standard air conditioners quickly lose efficiency and break under continual operation.

2     Electronic Load

While the complete cooling load is determined by the building construction, solar load, room pressurization or ventilation air,electronic equipment and people, the primary load in a communications site is normally the equipment. The primary loads in commercial sites are normally people and ventilation air. In the process of cooling, most air conditioners cool the air (sensible cooling) and remove moisture (latent cooling). Since electronic equipment does not release or absorb moisture, the air conditioner selected for a communications site should primarily cool the air, not remove moisture. The air conditioner should normally be selected with a “sensible heat ratio” of no lower than 78 percent. A system more typical of a commercial application, with a sensible heat ratio of only 65 percent, could consume as much as 20 percent more electricity to cool the same site, and could reduce humidity levels in the space to unacceptable levels.

Precision air conditioning is designed from the ground up to protect equipment, not people. “Comfort” air conditioning systems are designed for the comfort of people, and simply cannot provide the kind of environment required by high- performance telecom equipment.

There are fundamental differences between standard and precision air conditioning…reasons why “people coolers” won’t protect equipment. People are far more tolerant of temperature and humidity swings, whereas equipment requires precise levels of temperature and humidity maintained around the clock. This means heavy-duty systems and precision controls; people coolers don’t have either the strength or sensitivity for the job.

3     Temperature Control

The high density heat load in a telecommunications room is beyond the capacity of ordinary air conditioning systems. Equipment is best maintained in a stable environment of 72degF /-2degF (22.2degC /-1degC). As telecom equipment generates large quantities of heat in small areas, six to ten times the heat density of normal office space, the air conditioning system must have more than just enough cooling capacity. It must have the precision to react quickly to a drastic change in heat load and prevent wide temperature fluctuations, which is something a large building system cannot do.

4     Humidity Control

Telecommunications equipment must be protected from both internal condensation and static electricity discharges. Maintaining the correct humidity level in the computer room is just as important as maintaining proper temperature. Too high a humidity could cause condensation within the telecom equipment and the potential for hardware damage. If humidity is too low, static electricity could disrupt operation or even shut down the telecom system. An ordinary building system cannot normally control the environment within these boundaries.

5     Air Volume

Telecommunications equipment requires greater air volumes than ordinary air conditioning can provide. Typical comfort systems are designed to provide between 300 and 400 CFM (cubic feet per minute) per ton of cooling. Telecom systems require between 500 and 600 CFM per ton. The high-density heat load in a relatively small space requires more changes of air than a less dense “comfort” application. While a normal office space requires only two air changes per hour, a telecom room requires up to 30 changes per hour. Without proper air volume, hot spots and temperature fluctuations could develop within the space. Also, greater air volumes provide the higher sensible heat ratios required by telecom equipment.

6     Air Filtration

Filtration of both room pressurization and recirculation air is critical in a communications site, and the air filter should have an “ASHRAE 52-76” Standard Efficiency of no lower than 25 percent. This efficiency is much higher than normally provided in a commercial system.