ASHRAE Journal - March 2023 - 21

COLUMN DATA CENTERS
TABLE 1 Relative ITE failure rate x-factor as a function of constant ITE air inlet
temperature.1
DRY-BULB
TEMP, °F
59.0
63.5
68.0
72.5
77.0
81.5
86.0
90.5
95.0
99.5
104.0
108.5
113.0
LOWER BOUND
0.72
0.80
0.88
0.96
1.04
1.12
1.19
1.27
1.35
1.43
1.51
1.59
1.67
FAILURE RATE X-FACTOR
AVERAGE BOUND
0.72
0.87
1.00
1.13
1.24
1.34
1.42
1.48
1.55
1.61
1.66
1.71
1.76
UPPER BOUND
0.72
0.95
1.14
1.3 I
1.43
1.54
1.63
1.69
1.74
1.78
1.81
1.83
1.84
Relative hardware failure rate x-factor for volume servers is shown as a function of
continuous operation.
condition. In this instance, the cooling system can
be designed for only a portion of the full IT load and
associated reduction in capital costs and energy
consumption.
Impact on Data Center Equipment Sizing and Capital Costs
The x-factor was originally developed to exploit the
concept of reducing or even eliminating mechanical
cooling for data centers. The primary capital cost
savings associated with implementing variable supply
temperature control based on x-factor is the reduced
cost of the cooling system. For this column, we examine
the trade-offs for a chilled water system, though similar
analyses could be performed for direct or indirect
outside air cooling or other systems.
Chilled water systems using cooling towers have
an aggregate approach temperature (temperature
difference between cold aisle supply temperature and
ambient wet-bulb temperature) in the range of 15°F
to 25°F (8.3°C to 13.9°C). The aggregated approach
temperature is typically lowest near the design
condition due to the higher effi ciency of cooling towers
at high wet-bulb temperatures.
The capital cost of a system with chillers vs. a system
without chillers depends on two main parameters:
* Cost savings associated with eliminating the
chillers (mechanical cooling); and
* Cost penalty associated with increased airfl ow
requirements in a warm data center due to server fan
speed increases.
The overall capital cost savings associated with
the elimination of the chillers can be normalized at
$1,300/ton ($370/kW) , but a signifi cant range in CapEx
is possible depending on the equipment type, physical
location, system topology and labor availability. For a
1,000 kW (1 MW) data center with two 350 ton ( 1231 kW)
chillers and N + 1 redundancy, the capacity and CapEx
of installed chillers would be 350 tons × 2 chillers =
700 tons × $1,300/ton = $910,000.
The reason airfl ow requirements need to increase in
a data center without chillers is that the data hall will
be operating near the top of the allowable range for a
signifi cant number of hours per year. Figure 1 shows
that the airfl ow requirement for servers can increase
signifi cantly with higher inlet temperatures, to as much
as 250% of the baseline value corresponding to 65°F to
68°F (18°C to 20°C). This higher airfl ow will result in the
need for more CRAH units or other forms of terminal
air distribution. If we assume that the airfl ow rate
increase requirement is 180%, 1.8 times as many CRAH
units would be needed. The CapEx of additional airfl ow
is assumed to be about $4.50/cfm ($9.54/L·s) . For the
example provided in this column, an increase in airfl ow
of 104,820 cfm (49 470 L/s) is needed, corresponding to a
cost of $471,690. Adding these two values together yields
FIGURE 1 Server flow rate increase vs. server inlet temperature.1
2.8
2.6
2.0
2.4
1.8
2.2
1.6
1.2
1.4
1.0
50
59
68
77
86
Ambient Temperature, °F
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Server Airflow Rate Increase
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