ASHRAE Journal - November 2014 - 84
At first glance, jumping to the highest temperature row
would indicate an average relative failure rate of 1.76. In
other words, there is a 76% higher probability of failure
if operating with a constant inlet temperature of 113°F
(45°C) compared to operating with a constant inlet temperature of 68°F (20°C). However, this is not the same as
saying that there is a 76% chance of server failure since
the value is only relative to what would normally occur
regarding server failures.
The reality is that operating at a constant elevated temperature actually can have a minimal impact on server
failure. When you consider that the typical annual
server failure rate is around 2% to 4%, Table 2 shows that
even using the 4% rate, a constant elevated inlet temperature of 104°F (40°C) only raises the annual server
failure rate by an additional 3%.
Variable Inlet Temperature Impact
Data center cooling was once referred to as precision
cooling. This is an indication of design conditions that
typically have very tight tolerances. However, x-factor
values in Table 1 show that for inlet temperatures lower
than the baseline, the reliability actually improves.
Therefore, a variable inlet temperature can potentially
84
ASHRAE JOURNAL
ashrae.org
N OVEM BER 2014
52.4
23.2
16.7
7.7
-
X-Factor (R2)
0.865
1.130
1.335
1.482
-
Failure Rate
(R3 = R1 × R2)
0.453
0.262
0.223
0.114
1.052
take advantage of these lower temperature reliability
"credits" to offset the higher temperature impact.
A variable inlet temperature is linked to the use of free
cooling and the elimination of mechanical refrigerationbased cooling. Therefore, it is linked to the variation in
external temperature for a given location. An annualized time at temperature histogram for a given location can help show the exterior temperature profile
distribution.
Figure 1 shows a typical time at temperature histogram for Atlanta using weather data from the ASHRAE
Weather Data Viewer software, which provides statistical
averages based on the most recent 25 years of weather
data. This histogram assumes air mixing occurs to
maintain a minimum air temperature of 59°F (15°C) and
shows that roughly 52% of all hours fall below the baseline x-factor temperature of 68°F (20°C).
The time at temperature histogram can be used to
construct a time-weighted average x-factor by simply
proportionally weighting the number of hours per year
©ASHRAE
table reformatted by DLB Associates
Atlanta %
Hours (R1)
Analyzing the X-Factor Data
©ASHRAE
Average Hours Per Year
Average Hours Per Year
data for all of the internal
8,760 (1 yr)
8,760 (1 yr)
components (processors,
99.8% 100% 100%
8,000
98.1%
8,000
memory, hard drives,
7,000 88.1%
7,000
etc.) within a piece of IT
6,000
6,000
equipment, and since the
5,000
5,000 52.4%
configurations can vary,
4,000
4,000
a range of x-factors are
3,000
3,000
provided with upper and
23.2%
2,000
2,000
lower bound values in
16.7%
1,000
addition to the average.
1,000
7.7%
Therefore, the baseline
0
0
59≤ T ≤68 68< T ≤77 77< T ≤86 86< T ≤95
RECOMMENDED A1
A2
A3
A4
x-factor of 1.00 is actuDry-Bulb Temperature (°F)
Class
ally the average failure
rate value for an inlet
FIGURE 1 Annualized time at temperature histogram for Atlanta.
ambient temperature of
68°F (20°C) for a typical
TABLE 3 Time-at-Temperature Weighted Failure Rate Calculation for ITE in
configuration of a volume server; the corresponding
Atlanta. (Weather data from ASHRAE Weather Data Viewer.)
upper and lower bound of 1.14 and 0.88, respectively,
ANNUAL
would cover alternate configurations. Table 1, (Page
FAILURE
LOCATION 59 ≤ T ≤68°F 68
ASHRAE Journal - November 2014
Table of Contents for the Digital Edition of ASHRAE Journal - November 2014
Contents
ASHRAE Journal - November 2014 - Cover1
ASHRAE Journal - November 2014 - Cover2
ASHRAE Journal - November 2014 - 1
ASHRAE Journal - November 2014 - 2
ASHRAE Journal - November 2014 - Contents
ASHRAE Journal - November 2014 - 4
ASHRAE Journal - November 2014 - 5
ASHRAE Journal - November 2014 - 6
ASHRAE Journal - November 2014 - 7
ASHRAE Journal - November 2014 - 8
ASHRAE Journal - November 2014 - 9
ASHRAE Journal - November 2014 - 10
ASHRAE Journal - November 2014 - 11
ASHRAE Journal - November 2014 - 12
ASHRAE Journal - November 2014 - 13
ASHRAE Journal - November 2014 - 14
ASHRAE Journal - November 2014 - 15
ASHRAE Journal - November 2014 - 16
ASHRAE Journal - November 2014 - 17
ASHRAE Journal - November 2014 - 18
ASHRAE Journal - November 2014 - 19
ASHRAE Journal - November 2014 - 20
ASHRAE Journal - November 2014 - 21
ASHRAE Journal - November 2014 - 22
ASHRAE Journal - November 2014 - 23
ASHRAE Journal - November 2014 - 24
ASHRAE Journal - November 2014 - 25
ASHRAE Journal - November 2014 - 26
ASHRAE Journal - November 2014 - 27
ASHRAE Journal - November 2014 - 28
ASHRAE Journal - November 2014 - 29
ASHRAE Journal - November 2014 - 30
ASHRAE Journal - November 2014 - 31
ASHRAE Journal - November 2014 - 32
ASHRAE Journal - November 2014 - 33
ASHRAE Journal - November 2014 - 34
ASHRAE Journal - November 2014 - 35
ASHRAE Journal - November 2014 - 36
ASHRAE Journal - November 2014 - 37
ASHRAE Journal - November 2014 - 38
ASHRAE Journal - November 2014 - 39
ASHRAE Journal - November 2014 - 40
ASHRAE Journal - November 2014 - 41
ASHRAE Journal - November 2014 - 42
ASHRAE Journal - November 2014 - 43
ASHRAE Journal - November 2014 - 44
ASHRAE Journal - November 2014 - 45
ASHRAE Journal - November 2014 - 46
ASHRAE Journal - November 2014 - 47
ASHRAE Journal - November 2014 - 48
ASHRAE Journal - November 2014 - 49
ASHRAE Journal - November 2014 - 50
ASHRAE Journal - November 2014 - 51
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ASHRAE Journal - November 2014 - 53
ASHRAE Journal - November 2014 - 54
ASHRAE Journal - November 2014 - 55
ASHRAE Journal - November 2014 - 56
ASHRAE Journal - November 2014 - 57
ASHRAE Journal - November 2014 - 58
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ASHRAE Journal - November 2014 - 96
ASHRAE Journal - November 2014 - Cover3
ASHRAE Journal - November 2014 - Cover4
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