12
h. The minimum humidity level for Classes A1, A2, A3, and A4 is the higher (more moisture) of the 12°C dew point and the 8%RH.
These intersect at approximately 25°C. Below this intersection (~25°C) the dew point (–12°C) represents the minimum moisture level,
while above it, RH (8%) is the minimum.
i. Based on research funded by ASHRAE and performed at low RH, the following are the minimum requirements:
1) Data centers that have non-ESD floors and where people are allowed to wear non-ESD shoes may want to consider increasing
humidity given that the risk of generating 8kV increases slightly from 0.27% at 25% RH to 0.43% at 8% RH (see Appendix D of
reference [1] for more details).
2) All mobile furnishing/equipment is to be made of conductive or static dissipative materials and bonded to ground.
3) During maintenance on any hardware, a properly functioning and grounded wrist strap must be used by any personnel who contacts
ITE.
j. To accommodate rounding when converting between SI and I-P units, the maximum elevation is considered to have a variation of
±0.1%. The impact on ITE thermal performance within this variation range is negligible and enables the use of rounded values of 3050m
(10,000ft).
k. See Appendix L of reference [1] for graphs that illustrate how the maximum and minimum dew-point limits restrict the stated relative
humidity range for each of the classes for both product operations and product power off.
l. For the upper moisture limit, the limit is the minimum absolute moisture of the DP and RH stated. For the lower moisture limit, the
limit is the maximum absolute moisture of the DP and RH stated.
m. Operation above 3050m requires consultation with IT supplier for each specific piece of equipment.
Applying the ASHRAE thermal guideline classes listed in Table 1 looks straightforward but, in
practice, an older data center may have equipment that was designed to different versions of those
guidelines. For example, the recommended envelope from the 2004 ASHRAE guidelines was 20
– 25°C (68-77°F) whereas newer equipment is typically designed to Class A2 (35°C, 95°F) or even
A3 (40°C, 104°F) and A4 (45°C, 113°F). The life of a typical data center is 15 to 20 years so some
of the data center equipment may be original to the time the data center was built and some will
have been recently refreshed. One of the challenges with implementing new energy saving cooling
technologies, such as economization, is identifying an environmental control window that is
compatible with all of the different thermal specifications, types of equipment, and equipment
vintages in the data center.
2.2 Increasing Use of Economization in IT Equipment Spaces
Economization is the use of outdoor air to cool the data center [7].The premise of economization
is simple – why run an air conditioner or chiller if there is already an unlimited supply of cold air
outside? Many world-wide locations can economize for as much as 50% of the hours in a year
within the ASHRAE recommended range of 18-27°C (64-81°F). Larger percentages of
economization (energy savings) require a wider allowable temperature and humidity range.
There are several forms of economization: air-side economization [7], water-side economization
[7], and refrigerant economization. Air-side economization typically accomplishes cooling by
bringing filtered outside air directly into the data center without any air conditioning or humidity
control. In winter months when the outside air is cold, a comfortable working temperature inside
the data center is maintained by mixing the incoming air with hot exhaust air from the IT
equipment. It should be noted there are forms of indirect air-side economization that use a thermal
wheel, heat pipes, or a plate air–to-air heat exchanger [8]. These indirect air-side economization
methods bring only a very small amount of outside air into the IT space of the data center. The
disadvantage of a wheel or plate is the heat exchange step required makes it less efficient in full
economizer cooling mode compared to air-side economization. The disadvantage of direct air-
side economization is, depending on the local air quality, it can bring pollutants into the data center
[9] which can require additional filtration to remove. Another disadvantage of direct air-side
economization is, in some climates, the humidity of the outdoor air is outside the recommended
(or allowable) limits for the IT equipment. This can preclude use of air-side economization, even