Avoiding Contamination

   

All surfaces within the controlled zone of the data center should be kept clean. This should be done by:

  • Keeping contaminants out. Keeping contaminants from entering the data center should be done by minimizinging traffic through the room, adequate air filtering, avoidance of improper chemical use, and positive pressurization of the room. Also, a properly constructed data center uses only non-shedding and non-gassing materials. If the data center is a retrofit of an existing structure, it might be necessary to change out or seal some existing construction materials.

  • Regularly scheduled cleanings. Cleanings should be performed by trained professionals on a regular basis. These cleanings should be done with the same concern and regularity as data backups .

Unfortunately, the data center cannot be a hermetically sealed environment. It must have several breaches for both humans and atmosphere. These are points of potential exposure to contaminants and must be clearly addressed in the design of the center.

Exposure Points

Breaches in the controlled zone of the data center must be controlled and monitored . All doors must fit snugly in their frames and be sealed with gaskets and sweeps . Automatic doors should be carefully controlled to avoid accidental triggering, especially by people without proper security clearance. A remote door trigger might be necessary so that personnel pushing carts can easily open the doors. In highly sensitive areas, a design with double sets of doors and a buffer in between will limit direct exposure to outside contamination.

To maintain environmental control, the data center is an isolated cocoon. Windows leading to uncontrolled parts of the building should be avoided. Also, seal all penetrations between the data center and adjacent areas. Do not share subfloor or ceiling plenums with any other part of the building. The subfloor void is of particular concern and is covered in the following subsection.

Figure 12-3. Unfilled Void Between Data Center Room and Subfloor Plenum

graphics/12fig03.jpg

Damaged or poorly protected building materials are often sources of contamination. Unprotected concrete, masonry block, plaster, or gypsum wall-board will deteriorate over time, shedding fine particulate into the airstream. Corrosion on parts of the air conditioning system past the filters can also release particulate.

Subfloor Void

The subfloor void in a downward-flow air conditioning system functions as the supply air plenum. This area is pressurized by forced conditioned air, which is then introduced to the data center room through perforated tiles. Since all air moving into the room must travel through the subfloor void, it is critical that this area be kept at a high level of cleanliness. Contaminant sources can include degrading building materials, operator activity, or infiltration from areas outside the controlled zone.

Clutter in the subfloor plenum should be avoided. Tangled cables or stored materials can form "air dams" that allow particulate matter to settle and accumulate. When these items are moved, the particulate is stirred up and reintroduced to the supply airstream. Store supplies in outside storage areas, and keep all subfloor cabling organized in wire basket cable trays.

All surfaces of the subfloor area, particularly the concrete deck and the perimeter walls, should be properly sealed, ideally before the raised floor is installed. Unsealed concrete, masonry, and similar materials degrade over time. Sealants and hardeners used in normal construction are not meant for the surfaces of a supply air plenum. Only appropriate materials and methodologies should be used in the encapsulation process. Here are some guidelines:

  • Spray applications should never be used in an online data center. The spraying process forces sealant particulate into the supply airstream. Spray applications could be appropriate if used in the early stages of construction.

  • Use a pigmented encapsulant. The pigmentation makes the encapsulant visible, ensuring thorough coverage and helping to indicate areas damaged over time.

  • The encapsulant must have a high flexibility and low porosity to effectively cover the irregular surface textures and to minimize moisture migration and water damage.

  • The encapsulant must not out-gas harmful contaminants, particularly in an online data center. Some encapsulants are highly ammoniated or contain other chemicals harmful to hardware. The out-gassing might not cause immediate failure but could contribute to corrosion of contacts, heads, or other components . If out-gassing is short lived and the area is well ventilated, this might not be a problem in a new construction data center.

Positive Pressurization and Ventilation

Positive pressurization of the data center applies outward air forces to doorways and other access points within the room, keeping outside air, insects , and particulate matter from entering. In a closed-loop, recirculating air conditioning system, very little outside air needs to be introduced, however, some outside air is required to maintain positive pressurization and ventilation. This air must also be filtered and conditioned. Ventilation is important to the health of the occasional operators and visitors in the data center, but the air required for positive pressurization will likely exceed what is needed for occupants . The introduction of outside air should be kept to the minimum necessary to achieve the positive pressurization and ventilation requirements of the room.

Normally, outside air quantities of about 5 percent new (make-up) air should be sufficient (ASHREA Handbook: Applications, Chapter 17). A volume of 15 CFM (Cubic Feet per Minute) outside air per occupant or workstation should be enough for the ventilation needs of the room (Uniform Building Code, Chapter 12).

In data centers with multiple rooms, the most sensitive areas should be the most highly pressurized.

Filtration

Warm air from the data center hardware returns to the HVAC units where it is cooled and reintroduced to the room to continue the cooling cycle. The air change rate in a data center is much greater than a typical office environment and proper filtration is essential to arresting airborne particulate. Without high efficiency filtration, particulate matter will be drawn into computers with the probability of clogging airflow, gumming up components, causing shorts, blocking the function of moving parts, and causing components to overheat.

The following figure shows the filters placed in the top of an HVAC unit.

Figure 12-4. HVAC Filters

graphics/12fig04.jpg

The filters installed in recirculating air conditioners should have a minimum efficiency of 40 percent Atmospheric Dust-Spot Efficiency (ASHRAE Standard 52.1). Air from outside the building should be filtered with High Efficiency Particulate Air (HEPA) filters rated at 99.97 percent efficiency (DOP Efficiency MIL-STD-282) or greater. To prolong their life, the expensive high-efficiency filters should be protected by multiple layers of lower grade prefilters that are changed more frequently. The first line of defense should be low-grade 20 percent ASHRAE Atmospheric Dust-Spot Efficiency filters. The next level of filtration should consist of pleated or bag type filters with efficiencies between 60 and 80 percent. All of these filters should fit properly in the air handlers. Gaps around the filter panels decrease the filter efficiency. These gaps should be filled with appropriate materials such as stainless steel panels or custom filter assemblies.

Refer to the following table for a comparison of filter efficiencies. As the table demonstrates , low efficiency filters are almost totally ineffective at removing submicronic particulate from the air.

Table 12-2. Typical Efficiencies of Various Filters
ASHRAE 52-76 % of Dust-Spot Efficiency % of Fractional Efficiencies
3.0 micron 1.0 micron 0.3 micron
25 to 30% 80% 20% <5%
60 to 65% 93% 50% 20%
80 to 85% 99% 90% 50%
95% >99% 92% 60%
DOP 95% -- >99% 95%

Copyright 1995, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., www.ashrae.org. Reprinted by permission from ASHRAE Journal-June 1995.

Taking Out the Trash

Trash should never be allowed to collect in any part of the data center, even in designated trash receptacles. If there are trash receptacles, they should be removed from the center and emptied often. Loose papers, rags, and chemical containers all pose fire hazards.

Regularly Scheduled Cleanings

Hardware performance and longevity are important reasons to perform regularly scheduled cleanings of the data center. Contaminants, even in small quantities, infiltrate the room and settle on room surfaces and within machinery. Excessive exposure to contaminants will result in increased component failure and interruption of services. Even a well-designed and constructed data center will require regularly scheduled maintenance and cleanings. Data centers with design flaws or rough retrofits will require more extensive effort to maintain appropriate levels of cleanliness.

Another, possibly less obvious reason for maintaining a clean data center has to do with psychology. Operators working in a clean and organized data center will be more inclined to respect the room and keep it clean and organized, thus maintaining its efficiency. Visitors to the data center will show similar respect and interpret the overall appearance of the room as a commitment to quality and excellence.

When designing the data center, keep regularly scheduled decontaminations in mind. A well-designed data center is easy to maintain.

   


Enterprise Data Center Design and Methodology
Enterprise Data Center Design and Methodology
ISBN: 0130473936
EAN: 2147483647
Year: 2002
Pages: 142
Authors: Rob Snevely

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