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The growing use of electronic components means modern office equipment and systems are much more sensitive to fluctuations in temperature and relative humidity than those of decades past. They require special air conditioning considerations.  Here, EMERSON/Liebert Corp. takes a look at the difference between precision ac and comfort cooling.

Precise control over both temperature and relative humidity within the office is mandatory to safe-guard sensitive electronic equipment from damage.  The high cost of equipment downtime and/or replacement makes it imperative that the air conditioning system chosen for a critical area be appropriate for the types and amounts of equipment it will hold.

While people can generally tolerate moderate fluctuations in temperature, electronic equipment can’t.  If the temperature of a critical area is too high or too low, data integrity and operating reliability can be compromised.  Moreover, electronic equipment generates heat, which compounds the difficulty of controlling the temperature precisely.  To prevent overheating that could damage delicate circuits or void the manufacturer’s warranty, computer and other electronic equipment makers specify tight temperature tolerances, often in the range of 72-75° F, (±) 2° F.

Controlling the relative humidity of critical areas is just as important as regulating their temperature. Environmental standards for many types of equipment call for 45-50% relative humidity with a maximum swing of (±) 5% per hour.  Too much moisture in the air allows moisture to condense on electronic circuitry.  This condensation combined with trace contaminants in the air produces corrosion that can cause malfunctions and equipment failures.

In fact, corrosion of contacts and components on circuit boards due to excessive humidity accounts for about 30-40% of all equipment failures.  Excess humidity can also cause paper to swell in size, creating serious problems in the operations of any paper-handling devices such as computer printers, plotters, fax machines, word processors, copiers, etc.  Jams and form mis-feeds are common occurrences in these situations.

Excessively low humidity is just as much of a problem as high humidity.  In an environment with very dry air, a touch from a finger can produce a tiny jolt of static electricity that can cause equipment malfunctions, fry components, and scramble or erase data.  Magnetic media such as disks or tapes can suffer oxide shed, increasing the possibility of altered or lost data.  Static charges on paper can result in jamming or mis-feeds as separate sheets cling together.

Once they become aware of the importance of an air conditioning system to protect their equipment investment, many network managers might assume they need only supplement the capacity of their existing comfort air conditioning system to accommodate the increased heat load produced by electronic equipment.  However, there are some important differences between cooling electronics and cooling people.  For example, people add humidity to a room, but electronics don’t.  That’s why it’s important to consider a system’s latent cooling capacity (its ability to remove humidity), as well as its sensible cooling capacity (its ability to remove heat).

The comfort air conditioners found in many office environments are designed with sensible cooling ratios of roughly 0.60 to 0.70.  That means that 60-70% of the work a comfort system does is lowering the temperature of the air, while the remaining 30-40% of the work is removing moisture.

By contrast, today’s high efficiency precision air conditioning systems have a ratio of sensible-to-total cooling capacity of 0.85 to 0.95.  In other words, 85-95% of the work done by a precision air conditioning system is devoted to cooling the air, while 5-15% is removing humidity.  Even though precision systems tend to be slightly more expensive initially than comfort cooling systems, the difference in sensible-to-total cooling capacities often makes a precision air conditioning system a better choice over the life of the system for a number of reasons:

First, it takes much more comfort capacity to do the same job as a precision air conditioning system.  Remember this rule of thumb:  It takes three tons of total comfort capacity to do the same sensible cooling job as two tons of precision total capacity.

Second, a comfort system will pull the relative humidity down below the acceptable range for a significant part of the year.  That may make it necessary to install an add-on re-humidification system to put moisture back into the air to prevent static and paper jamming problems.  In contrast, precision air conditioning systems are designed with multiple steps of cooling, which allow the system to match the room load conditions, providing closer control of temperature and humidity and significant energy savings.  Many have built-in re-humidification capabilities to maintain the level recommended by the manufacturer of the electronic equipment.

Third, load density is an important consideration when choosing among various air conditioning options.  Since many types of electronic equipment generate heat, the rooms that house them need a lot more cooling capacity per square foot of floor space than rooms that house only people.

The rule of thumb here is:  It takes a ton of comfort air conditioning capacity for every 250-300 sq. ft. of office space and about a ton of precision air conditioning capacity for every 50-100 sq. ft. of equipment room.  As more and more electronic equipment is added to an office, this load density becomes more apparent.

 Another big difference between comfort and precision systems is the volume of air that must be moved.  Typically, a comfort system will move air through its coils at about 350-400 cubic feet per minute (CFM) per ton of cooling capacity.  However, a precision system can move air at nearly twice that rate – 500-600 CFM – producing a high sensible cooling ratio, which is needed to deal with the dense heat load in areas with lots of electronic equipment.

A precision system’s ability to move larger volumes of air also contributes to better dust filtration.  Dust in the heads of tapes and disk readers can physically damage storage media.  Dust also accumulates quickly on charged electronic components.  Get enough of it, and cooling capacity diminishes, causing the affected components to operate at temperatures in excess of design specifications.  That could lead to shorter service life and premature failure.  Comfort systems typically use disposable filters, similar to those used in residential forced-air furnaces.  These types of filters are typically only about 10% efficient.  Precision air conditioning systems, on the other hand, have filters that are significantly better at extracting dust from the air (from 20-85%, depending on the specific room/equipment requirements).

The number of hours the air conditioning system must operate each day is another important factor in choosing between comfort and precision systems.  A growing number of networks have computer systems operating almost constantly.  In these situations, the air conditioning system must operate 24 hours a day, 365 days a year, for a total of 8,760 operating hours per year.  Precision systems are designed with high-durability components that can handle constant operation.  Most comfort systems, however, are designed with the assumption they will be operating about eight hours a day, five days a week, and only during the summer cooling season for an average of about 1,200 hours a year.  This “cooling season only” assumption built into comfort systems raises an important issue – cold weather operation.  Comfort systems with outside heat exchangers typically can’t operate when outside temperatures drop below 32°F, due to lack of low ambient controls and freeze protection.  Precision systems, by contrast, can operate at temperatures down to –30° F.

While the initial purchase prices of comfort and precision systems typically aren’t compatible, precision air conditioning offers a big economic advantage in terms of operating costs.  Consider this very basic example, based upon the following assumptions:

1. Each ton of total cooling requires 1.0 horsepower (or 0.746 kw).
2. The motor on the system’s compressors and fans is 90% efficient.
3. Electricity costs $0.06 per kilowatt hour.
4. A precision air conditioning system with a 0.90 sensible heat ratio (SHR).
5. A comfort cooling system with a 0.60 SHR.

The first step is to calculate the cost per ton for a year:

Assume the cost of electricity is BD$0.46 per kwh (Energy + Fuel charge):

0.746 kw/ton x 8,760 hrs/yr x $0.46/kwh  = $3,340.10/ton/year

                     0.9 efficiency

The cost per sensible ton of cooling for the precision system would then be:

                          $3,340.10       = $3,711.22

                     0.90 SHR

If using a comfort system, the cost per sensible ton of cooling would be:

                          $3,340.10       = $5,566.83

                     0.60 SHR

That means it costs an extra $1,855.61 per ton of sensible load to run a comfort system for a year.  To take this comparison process a step further, consider the cost of re-humidification.  To calculate the latent cooling that occurs per ton of sensible cooling for a precision system:

12,000 Btu/ton        12,000 Btu/ton  = 1,333 latent Btu/ton

0.90 SHR

For a comfort system, that figures out to:

12,000 Btu/ton        12,000 Btu/ton  = 8,000 latent Btu/ton

0.60 SHR

That’s a difference of 6,667 Btu.  In other words, the comfort system expends 6,667 Btu of energy per ton of sensible cooling in removing humidity from the air.  This humidity must be replaced to maintain the specified moisture content of 45-50% (±) 5%.  The added cost of re-humidification is:

6,667 Btu/ton x 8,760 hrs/yr x $0.46/kwh  = $7,871.47/yr/ton

                     3,413 Btu/hr/kw

Adding the cooling and rehumidification costs, it costs an extra $9,727.08/ton of sensible cooling to run a comfort-based system at $0.46/kwh.  As this example illustrates, the higher operating costs associated with comfort systems soon offset whatever purchase price advantage they offer initially.

IAQ Solutions Inc. are authorized agents for Liebert Precision Conditioning Environmental systems through the Caribbean Authorised Distributors: Precision Power & Air (Caribbean) Limited.

EMERSON/Liebert Corporation is a World Leader in computer support systems providing the following:

1. Environmental Control (Precision Air Conditioning for Electric Controls,

            Communication Shelters and Laboratories)

Power Protection

1. Site Monitoring/Control
2. Service Parts and Training

EMERSON/Liebert Corporation design, manufacture and market complete systems for improvement of computer uptime and performance.  The result is improved business operations, increased productivity and higher return on the computer investment.