ASHRAE Journal - April 2013 - 54

Data Centers

Standard 127-2012

Testing CRAC & CRAH
By Donald L. Beaty, P.E., Fellow ASHRAE

O

ver the past 25 years, but particularly in the past 10, there has
been a significant evolution in environmental conditions for data

centers. One of the ways this change can be observed is by analyzing
the evolution of one of ASHRAE’s standards, Standard 127, Method
of Testing for Rating Computer and Data Processing Room Unitary Air
Conditioners, to see how design conditions have changed between the
initial version of Standard 127 (in 1988), and the version that was
recently released, Standard 127-2012.

The types of units covered by this
standard usually are: “computer room
air conditioner” (CRAC), which uses
dedicated compressors and refrigerant
cooling coils rather than chilled-water
coils; or “computer room air handler”
(CRAH), which uses chilled-water coils
for cooling rather than dedicated compressors. Ratings for both types of units
are covered in this standard.
Some baby steps between 1988 and 2007
have turned into giant strides between the
2007 and 2012 revisions of this standard,
and will be highlighted in this column.

Early History of Standard 127
The title of Standard 127, Method of
Testing for Rating Computer and Data
Processing Room Unitary Air Conditioners, acknowledges that most data centers
are different from spaces that must be
comfort cooled, and as a result, a unique
class of equipment has evolved to condition these spaces.
For many years, there was no standard
rating system for this equipment, which
led to difficulty in comparing ratings
between manufacturers, or even in know54

ASHRAE Journal

ing if the rating of a single manufacturer
applied to one’s own data center. This
vacuum led to the creation of Standard
127, whose purpose is to “establish a
uniform set of requirements for rating
computer and data processing room unitary air conditioners (CDPR).”
While Standard 127 uses “CDPR” as
its primary definition for rated equipment, this acronym was never widely
adopted by the HVAC industry. Instead,
CRAH and CRAC have become the
popular nomenclature.
While it is difficult to list all of the
differences between a data center and a
comfort cooling space that resulted in the
evolution of a unique product line, among
the most important are:
a. Significantly higher internal heat
density (resulting in much higher duty
cycles on a 24/7 basis);
b. Lower external static pressure
(there is usually an underfloor plenum
air distribution system with minimal
pressure drop, and often no return ductwork at all);
c. Higher sensible heat ratios on the
coils (lower latent cooling capability);
ashrae.org

d. A legacy requirement for close
temperature and relative humidity conditions; and
e. Acknowledgement that computers are not people, and thus may have
significantly different environmental
requirements.
In looking at today’s data centers, (a)
through (c) are still true, with equipment
density still on the increase,1 while (d) and
(e) are rapidly changing, mostly in an effort to reduce energy costs. The relaxation
of tight temperature control, along with
alignment with ASHRAE’s book Thermal
Guidelines for Data Processing Environments, have driven most of the changes to
Standard 127, especially the latest revision.

Changes in 2007
The design conditions for data centers
in the 1999 ASHRAE Handbook—HVAC
Applications, Chapter 16, Data Processing and Electronic Office Areas, listed
the recommended temperature at 70°F to
74°F (21.1°C to 23.3°C), and the recommended humidity range at 45% to 55%
RH. When ASHRAE Standard 127 was
reissued in 2001, it aligned reasonably
well with the Handbook—HVAC Applications, and the rating point for “air entering
and surrounding the indoor portion of the
unit” was set at 71.6°F (22°C) dry bulb
and 60.8°F (16°C) wet bulb, corresponding to 54% relative humidity.
Probably not without coincidence,
this design point also falls within the
range of ASHRAE Standard 55, Thermal
Environmental Conditions for Human
Occupancy, allowing for both adequate
cooling of the data center spaces and for
human comfort, since legacy data centers
had more human occupancy than today’s
data centers.
The 2007 version of Standard 127 still
listed a single return air design point, but
April 2013



ASHRAE Journal - April 2013

Table of Contents for the Digital Edition of ASHRAE Journal - April 2013

ASHRAE Journal - April 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Ground-Coupled Heat Pump and Energy Storage
Fan Efficiency Requirements for Standard 90.1-2013
Technology Award Case Studies:
Geothermal for 5 Ecosystems
Holistic HVAC Design
Standing Columns
Engineer’s Notebook
Data Centers
Emerging Technologies
IAQ Applications
Refrigeration Applications
Special Products
People
International Column
Energy Modeling
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2013 - ASHRAE Journal - April 2013
ASHRAE Journal - April 2013 - Cover2
ASHRAE Journal - April 2013 - 1
ASHRAE Journal - April 2013 - 2
ASHRAE Journal - April 2013 - Contents
ASHRAE Journal - April 2013 - Commentary
ASHRAE Journal - April 2013 - 5
ASHRAE Journal - April 2013 - Industry News
ASHRAE Journal - April 2013 - 7
ASHRAE Journal - April 2013 - 8
ASHRAE Journal - April 2013 - Letters
ASHRAE Journal - April 2013 - Meetings and Shows
ASHRAE Journal - April 2013 - 11
ASHRAE Journal - April 2013 - 12
ASHRAE Journal - April 2013 - 13
ASHRAE Journal - April 2013 - Ground-Coupled Heat Pump and Energy Storage
ASHRAE Journal - April 2013 - 15
ASHRAE Journal - April 2013 - 16
ASHRAE Journal - April 2013 - I1
ASHRAE Journal - April 2013 - I2
ASHRAE Journal - April 2013 - I3
ASHRAE Journal - April 2013 - I4
ASHRAE Journal - April 2013 - 17
ASHRAE Journal - April 2013 - 18
ASHRAE Journal - April 2013 - 19
ASHRAE Journal - April 2013 - 20
ASHRAE Journal - April 2013 - 21
ASHRAE Journal - April 2013 - 22
ASHRAE Journal - April 2013 - 23
ASHRAE Journal - April 2013 - Fan Efficiency Requirements for Standard 90.1-2013
ASHRAE Journal - April 2013 - 25
ASHRAE Journal - April 2013 - 26
ASHRAE Journal - April 2013 - 27
ASHRAE Journal - April 2013 - 28
ASHRAE Journal - April 2013 - 29
ASHRAE Journal - April 2013 - 30
ASHRAE Journal - April 2013 - 31
ASHRAE Journal - April 2013 - Geothermal for 5 Ecosystems
ASHRAE Journal - April 2013 - 33
ASHRAE Journal - April 2013 - 34
ASHRAE Journal - April 2013 - 35
ASHRAE Journal - April 2013 - 36
ASHRAE Journal - April 2013 - 37
ASHRAE Journal - April 2013 - 38
ASHRAE Journal - April 2013 - 39
ASHRAE Journal - April 2013 - Holistic HVAC Design
ASHRAE Journal - April 2013 - 41
ASHRAE Journal - April 2013 - 42
ASHRAE Journal - April 2013 - 43
ASHRAE Journal - April 2013 - 44
ASHRAE Journal - April 2013 - 45
ASHRAE Journal - April 2013 - 46
ASHRAE Journal - April 2013 - 47
ASHRAE Journal - April 2013 - Engineer’s Notebook
ASHRAE Journal - April 2013 - 49
ASHRAE Journal - April 2013 - 50
ASHRAE Journal - April 2013 - 51
ASHRAE Journal - April 2013 - 52
ASHRAE Journal - April 2013 - 53
ASHRAE Journal - April 2013 - Data Centers
ASHRAE Journal - April 2013 - 55
ASHRAE Journal - April 2013 - 56
ASHRAE Journal - April 2013 - 57
ASHRAE Journal - April 2013 - 58
ASHRAE Journal - April 2013 - 59
ASHRAE Journal - April 2013 - Emerging Technologies
ASHRAE Journal - April 2013 - 61
ASHRAE Journal - April 2013 - 62
ASHRAE Journal - April 2013 - 63
ASHRAE Journal - April 2013 - IAQ Applications
ASHRAE Journal - April 2013 - 65
ASHRAE Journal - April 2013 - 66
ASHRAE Journal - April 2013 - Refrigeration Applications
ASHRAE Journal - April 2013 - Special Products
ASHRAE Journal - April 2013 - People
ASHRAE Journal - April 2013 - International Column
ASHRAE Journal - April 2013 - 71
ASHRAE Journal - April 2013 - Energy Modeling
ASHRAE Journal - April 2013 - 73
ASHRAE Journal - April 2013 - Products
ASHRAE Journal - April 2013 - 75
ASHRAE Journal - April 2013 - 76
ASHRAE Journal - April 2013 - 77
ASHRAE Journal - April 2013 - 78
ASHRAE Journal - April 2013 - Classified Advertising
ASHRAE Journal - April 2013 - Advertisers Index
ASHRAE Journal - April 2013 - Cover3
ASHRAE Journal - April 2013 - Cover4
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