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• Automatic doors should be avoided in areas where they can be accidentally
triggered. An alternate means of control would be to remotely locate a door
trigger so that personnel pushing carts can open the doors easily. In highly
sensitive areas, or where the data center is exposed to undesirable conditions, it
may be advisable to design and install personnel traps. Double sets of doors with
a buffer between can help limit direct exposure to outside conditions.
• Seal all penetrations between the data center and adjacent areas.
• Avoid sharing a computer room ceiling or subfloor plenum with loosely
controlled adjacent areas.
Filtration
Filtration is an effective means of addressing airborne particulate in a controlled
environment. It is important that all air handlers serving the data center are
adequately filtered to ensure appropriate conditions are maintained within the room.
In-room process cooling is the recommended method of controlling the room
environment. The in-room process coolers re-circulate room air. Air from the
hardware areas is passed through the units where it is filtered and cooled, and then
introduced into the subfloor plenum. The plenum is pressurized, and the conditioned
air is forced into the room, through perforated tiles, which then travels back to the air
conditioner for reconditioning. The airflow patterns and design associated with a
typical computer room air handler have a much higher rate of air change than typical
comfort cooling air conditioners so air is filtered much more often than in an office
environment. Proper filtration can capture a great deal of particulates. The filters
installed in the in-room, re-circulating air conditioners should have a minimum
efficiency of 40% (Atmospheric Dust-Spot Efficiency, ASHRAE Standard 52.1). Low-
grade pre-filters should be installed to help prolong the life of the more expensive
primary filters.
Any air being introduced into the computer room controlled zone, for ventilation or
positive pressurization, should first pass through high efficiency filtration. Ideally, air
from sources outside the building should be filtered using High Efficiency Particulate
Air (HEPA) filtration rated at 99.97% efficiency (DOP Efficiency MILSTD-282) or
greater. The expensive high efficiency filters should be protected by multiple layers of
pre-filters that are changed on a more frequent basis. Low-grade pre-filters, 20%
ASHRAE atmospheric dust-spot efficiency, should be the primary line of defense. The
next filter bank should consist of pleated or bag type filters with efficiencies between
60% and 80% ASHRAE atmospheric dust-spot efficiency.
ASHRAE 52-76
Dust spot efficiency %
25-30
60-65
80-85
90
DOP 95
December 2012
Fractional Efficiencies %
3.0 micron
1.0 micron
80
20
93
50
99
90
>99
92
--
>99
Filtration
0.3 micron
<5
20
50
60
95
Controlling Contaminants 95
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