ASHRAE Journal - January 2015 - 62

COLUMN DATA CENTERS

Geographic Considerations

FIGURE 2 Number of hours per year of chiller operation required for air-side econo-

Economizers are most commonly used in cooler climate zones, such as Zones 5 and higher per older versions of the energy code. More recent changes to the
energy code push for economizers in all climate zones
except Zone 1A.
Hypothetically, the expanded range of the Thermal
Guidelines permits economizer mode in any climate
zone, but only if the data center is operating in the
allowable ranges or with higher class ratings of the
installed hardware.
In terms of potential energy savings, the airside
economizers with direct exchange of outdoor air will
often offer the greatest potential energy savings from a
thermodynamic standpoint, assuming there is not an
indoor humidity requirement or contamination concern. This can be seen in the chart of annual mechanical
refrigeration (chiller) hours in several global locations
in ASHRAE's Thermal Guidelines publication (Figure 2).
The relatively low number of chiller hours indicates the
operation of airside economization for the majority of
hours throughout the year.

mizer for selected U.S. cities.

Class A2
Class A3
Class A4

8
7
6
5
4
3
2

Phoenix

Dallas

Denver

Houston

Washington, D.C.

Atlanta

Helena, Mont.

Boston

Chicago

Miami

Madison, Wis.

Los Angeles

Seattle

1
San Francisco

Chiller Hours Per Year (In Thousands)

on the failure rates of the IT equipment as discussed in
November's column ("X-Factor Explained").

Airborne and Gaseous Contamination Considerations
With direct exchange airside economizers (the most
common application), airborne particulate and gaseous
contamination need to be considered.
Recent changes to the manufacturing of electronics brought about by RoHS (discussed in December's

Thermal Guidelines

ASHRAE JOURNAL

ashrae.org

JAN UARY 2015

SOURCE: ASHRAE. MODIFIED BY DLB

Dew-Point Temperature °F

We
t-

Tem

rat

ure

°F

Equally important as the geographical environmental
conditions, the internal environmental conditions of a data
center play a significant role in
FIGURE 3 ASHRAE thermal guidelines expanded envelopes for recommended and allowable entering air conditions.
the decision for economizers. If
higher inlet temperatures and
Relative Humidity
90% 80% 70% 60% 50%
40%
30%
wider humidity conditions are
80
80
deemed acceptable, the opportunities for savings can greatly
75
75
increase. At the extreme, chillA3
A4
ers can be eliminated from
70
70
select designs.
A2
20%
65
The third edition
65
of ASHRAE's Thermal
60
A1
60
Guidelines for Data Processing
55
Environments introduced
55
50
wider allowable entering air
10% 50
45
Recommended
45
envelopes (Figure 3).
40
40
35
Energy efficient operation,
35
30
through expanded allowable
20
environmental envelopes,
10
0
was previously tempered by
35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
unknown impact that higher
Dry-Bulb Temperature °F
temperatures would have

ASHRAE Journal - January 2015

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