ASHRAE Journal - February 2024 - 9

COLUMN DATA CENTERS
fan speed, which is typically defi ned as the server
airfl ow at 68°F (20°C) inlet temperature (Figure 1).
In a new data center, server specifi cations inclusive
of airfl ow rates are typically known or can be specifi ed,
and CRAH capacities can be designed for the " worst
case " airfl ow. For an existing data center, however,
CRAH units may be undersized for operation at high
temperatures, particularly on an airfl ow basis. But use
of redundant HVAC equipment, knowledge of IT loads
and improvements in containment can help alleviate
this constraint.
Strategies to implement x-factor within existing data
centers with CRAH airfl ow limitations could include:
* Making use of redundant and/or unused CRAH
airfl ow. For instance, " N " CRAH units may be designed
with suffi cient airfl ow capacity to match server airfl ow
with a 20°F (11°C) ∆T. If N = 10 units, and two additional
units are installed (i.e., N + 2 redundancy), by operating
all 12 units the CRAH airfl ow capacity exists to match
a server ∆T of 16.7°F (9.3°C) at full IT load (servers
operating at higher inlet temperatures have lower ∆T).
* Take advantage of the fact that data centers are
rarely fi lled to capacity with IT equipment. This will
allow for more CRAH airfl ow per cfm of server airfl ow.
* Add or improve hot/cold aisle containment.
Sealing of openings between supply and return air to
the servers, and typically maintaining a slight negative
pressure in the hot vs. the cold aisle, optimizes both data
center airfl ow and temperature management.
* While the addition of containment is one of the
most complementary improvements that can be made
to align with x-factor control, fi re protection must be
considered (refer to NFPA 75/76/13). This is an issue that
must be coordinated with the latest NFPA standards and
applicable codes.
* Empirical data can be taken with an existing facility
to better determine the potential constraint of CRAH
airfl ow on x-factor implementation.
* Energizing an existing chiller system for a limited
number of the hottest days of the year can signifi cantly
decrease the maximum server airfl ow by limiting server
inlet temperatures. The maximum temperature can be
tailored to the existing CRAH airfl ow limitations, server
and/or SLA temperature limits.
Limits of Existing Server and SLA Maximum Temperatures
The concept of x-factor was created by ASHRAE
FIGURE 1 Typical range of server airflow as a function of inlet air temperature.2
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
10
15
20
25
30
Ambient Temperature, °C
TC 9.9 to align with the design of " chillerless data
centers " with Class A3 and/or Class A4 servers. The
concept of " chillerless " simply refers to the elimination
of the vapor-compression cycle for cooling a data
center. A Class A3 server has a maximum allowable
inlet temperature of 104°F (40°C), which should be
satisfi ed with a chillerless data center with water-cooled
heat rejection in almost all ASHRAE climate zones.
Similarly, a Class A4 data center, which has a maximum
allowable inlet temperature of 113°F (45°C), should be
satisfi ed with a chillerless data center with direct airside
economizer in almost all ASHRAE climate zones.
An existing data center, however, may have Class A1 or
A2 servers, with maximum allowable inlet temperatures
of 89.6°F (32°C) and 95°F (35°C), respectively. In this
case, or in the case of a non-negotiable SLA upper
temperature limit, an existing facility could operate
as a chillerless facility for most of the year but switch
to chiller operation as needed to maintain a specifi ed
maximum server inlet temperature. The example
calculation presented in this column provides examples
of such an operating strategy.
Increased monitoring of server inlet temperatures will
be a critical part of the transition and ongoing operation
at higher temperatures. Many industry solutions are
now available for achieving this via wireless and wired
monitoring at a granular level, as desired.
Practical Example
To illustrate the use of x-factor optimization in an
existing facility, TMY3 weather data from the city of
F E B R U A RY 2 0 2 4 ashrae.org ASHRAE JOURNAL
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Server Airfl ow Rate Increase
Server Airfl ow Rate Increase
http://www.ashrae.org
ASHRAE Journal - February 2024

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