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Any air being introduced into the computer room controlled zone, for ventilation or
positive pressurization, should first pass through high efficiency filtration. Ideally,
filter air from sources outside the building using High Efficiency Particulate Air
(HEPA) filtration rated at 99.97% efficiency (DOP Efficiency MILSTD-282) or greater.
Protect the expensive high efficiency filters by multiple layers of pre-filters that are
changed on a more frequent basis. Low-grade pre-filters, 20% ASHRAE atmospheric
dust-spot efficiency, are 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 Percent
25-30
60-65
80-85
90
DOP 95
Low efficiency filters are almost totally ineffective at removing sub-micron particulates
from the air. It is also important that the filters used are properly sized for the air
handlers. Gaps around the filter panels can allow air to bypass the filter as it passes
through the air conditioner. Fill any gaps or openings using appropriate materials,
such as stainless steel panels or custom filter assemblies.
Positive Pressurization and Ventilation
A designed introduction of air from outside the computer room system will be
necessary to accommodate positive pressurization and ventilation requirements.
Design the data center to achieve positive pressurization in relation to more loosely
controlled surrounding areas. Positive pressurization of the more sensitive areas is an
effective means of controlling contaminant infiltration through any minor breaches in
the room perimeter. Positive pressure systems are designed to apply outward air
forces to doorways and other access points within the data processing center to
minimize contaminant infiltration of the computer room. Introduce only a minimal
amount of air into the controlled environment. In data centers with multiple rooms,
highly pressurize the most sensitive areas. It is, however, extremely important that the
air being used to positively pressurize the room does not adversely affect the
environmental conditions in the room. It is essential that any air introduction from
outside the computer room is adequately filtered and conditioned to ensure that it is
within acceptable parameters. These parameters can be looser than the goal conditions
for the room since minimal air is introduced. Base a precise determination of
acceptable limits on the amount of air being introduced and the potential impact on
the environment of the data center.
Because a closed-loop, re-circulating air conditioning system is used in most data
centers, it will be necessary to introduce a minimal amount of air to meet the
ventilation requirements of the room occupants. Data center areas normally have a
very low human population density; thus the air required for ventilation will be
minimal. Usually, the air needed to achieve positive pressurization exceeds that
needed to accommodate the room occupants. Normally, outside air quantities of less
than 5% make-up air is sufficient (ASHRAE Handbook: Applications, Chapter 17). A
Fractional Efficiencies Percent
3.0 micron
1.0 micron
80
20
93
50
99
90
>99
92
--
>99
Positive Pressurization and Ventilation
0.3 micron
<5
20
50
60
95
Controlling Contaminants F-7
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