ASHRAE Journal - March 2023 - 24

COLUMN DATA CENTERS
TABLE 3 Summary of advantages and disadvantages of data center design using x-factor reliability.
CRITERIA
Data Center Design Options
Capital Cost
Energy Cost
Maintenance Costs
Mechanical Cooling (Chillers)
Cooling Air Distribution (CRAH or Similar)
Global Warming Impact
90.4-2019 MLC Compliance
Cooling System Reliability
X-Factor Server Reliability
Capability to Host ASHRAE H1,
A1 Server Classes
MECHANICAL COOLING (BASELINE)
Flexible Deployment Topology
Higher
Baseline
Higher (More Complex Systems)
Required
Baseline (1.0x cfm)
Baseline
Challenging
Baseline
Baseline
Capable
The chillerless system had an MLC that averaged
0.09 less than the baseline system MLC. Energy costs
examined the climate impacts on cooling tower, chiller
and air distribution energy. An average annual U.S.
retail energy cost of $0.115/kWh was assumed for all
sites. Note that the potential energy savings is highest in
Climate Zones 1A and 2A because the baseline chillers
rarely operate in economizer mode and the (Scenario 1)
MLC is thus higher.
Table 3 provides a summary of the advantages and
disadvantages of a chillerless data center when
compared to a " standard " chilled water system
with mechanical cooling. As can be seen, there are
many factors to be considered, and a design without
mechanical cooling is probably not the best choice
in all cases. However, in instances in which server
specifications and failure rates are known, several
advantages exist to the chillerless design. These
include lower capital cost, lower energy cost, lower
maintenance cost, a less complex and more reliable
cooling system and no global warming potential
(GWP) refrigerant issues.
Alignment With Project and IT Equipment Specifications
The x-factor values provided in Thermal Guidelines
are based on ITE aligned with Classes A2, A3 and A4.
If equipment corresponding to high performance
Class A1 and/or the new Class H1 is being installed,
consideration should be given to the addition of
mechanical cooling even if it is not needed for
the A2/A3/A4 ITE based on the x-factor analysis.
24
ASHRAE JOURNAL ashrae.o rg M A R CH 2023
CHILLERLESS COOLING (X-FACTOR)
Aisle Containment Required
Lower (Except Possibly With Peak Server Fan Speeds at 2.5x Baseline Values)
Lower: Average MLC Reduction ±0.09 for Stated Conditions
Lower (Less Complex Systems)
Not Required
Higher (1.2 to 2.5x Higher cfm vs. Baseline)
No GWP Refrigerant Concerns (Energy GWP Savings Also Accrue)
Readily Achievable
More Reliable (Less Complex Systems, No Chillers, No Economizer Functionality, etc.)
Range Between 0.9 to 1.6, Climate Zone and System Dependent
Not Capable (Without Separate Cooling System)
Segregation of Class A1/H1 equipment (and also Class
A2 depending on Climate Zone and cooling system
design) should still allow for significant capital and
operating cost savings by using x-factor floating
temperature control when most of the IT equipment
falls into the A3 and/or A4 classes.
Summary
The x-factor reliability data has been part of ASHRAE's
Thermal Guidelines for Data Processing Environments for 10
years. Greater implementation of floating supply air
temperature control to align with x-factor reliability
data should result in lower construction and operating
costs for new data centers with only minor impact on
ITE reliability for most climate zones. The exact impact
on capital cost, energy use and annualized x-factor
reliability depends on many variables. It is important
to look at climate zone, IT equipment specs, energy use
and capital cost impact to determine whether x-factor
is a good match for your data center. This is the first
of two columns on the reliability data on data center
design and operation. The second column will focus on
optimization and the potential use of x-factor analysis in
design modifications to existing facilities.
References
1. ASHRAE. 2021. Thermal Guidelines for Data Processing
Environments, 5th Edition. Atlanta: ASHRAE.
2. ASHRAE. 2012. Thermal Guidelines for Data Processing
Environments, 3rd Edition. Atlanta: ASHRAE.
3. Beaty, D. 2014. " X-Factor explained. " ASHRAE Journal (11).
4. ASHRAE Standard 90.4-2019, Energy Standard for Data Centers.

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ASHRAE Journal - March 2023

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