ASHRAE Journal - May 2024 - 17

COLUMN DATA CENTERS
TABLE 1 Comparison between the max allowed MLC values in ASHRAE Standard
90.4-2016 and IECC 2021 for various climate zones.
ANNUALIZED MLC VALUES
CLIMATE ZONE
0A
0B
1A
2A
3A
4A
5A
6A
1B
2B
3B
4B
5B
6B
3C
4C
5C
7
8
STANDARD 90.4-2016
0.37
0.40
0.36
0.35
0.33
0.33
0.33
0.32
0.38
0.36
0.35
0.35
0.33
0.34
0.32
0.32
0.32
0.32
0.32
IECC 2021
0.19
0.20
0.18
0.19
0.18
0.17
0.17
0.17
0.16
0.18
0.18
0.18
0.16
0.17
0.16
0.16
0.16
0.16
0.16
centers use signifi cant amounts of water, a scarce
resource in many parts of the country. And a chillerless
data center using direct outdoor air is typically confi ned
to cool climates.
This column looks at IECC 2021 compliance in
Phoenix, Ariz., in ASHRAE Climate Zone
2B. Table 2 provides basic information
on this climate zone. As a baseline,
we will look at an air-cooled chiller
(ACC) without any economizer, then
an air-cooled chiller with an integrated
closed-loop water economizer. Finally,
we will look at scenarios that make
use of a direct airside economizer.
The annualized server failure rate, as
characterized by x-factor, is also tracked
to make sure code-compliant energy
solutions do not result in an increase in
server failure rates.
Scenario Assumptions
In this column, operational scenarios
CLIMATE
ZONE
2B
for a sample ACC are examined, each designed to assess
compliance with ASHRAE Standard 90.4-2016 and the
IECC 2021 code while operating in a high-temperature
climate like Phoenix. These scenarios (Table 3) include
variations in chilled water supply (CHWS) temperatures
and different types of economizers, including direct free
cooling with outdoor air. More detailed descriptions of
the cases are below.
Scenario 1 (Baseline):ACC, no economization
(chiller year-round, CHWS temperature of 68°F
[20°C]). This scenario serves as a baseline, representing
typical operating conditions for ACCs without
the implementation of economization strategies.
By maintaining a constant CHWS of 68°F (20°C)
(corresponding to a 76°F [24°C] cold aisle or server inlet
temperature) throughout the year, the chiller operates
solely based on mechanical cooling.
Scenario 2-A: ACC with integrated economizer
(chiller on year-round, but at reduced or no load as a
function of outdoor air dry-bulb temperature, CHWS
of 68°F [20°C]). This scenario introduces integrated
economization, whereby the return water from the
computer room air handler (CRAH) unit is passed
though a coil to reduce its return temperature prior to
reaching the evaporator. The water coil is only active
when the outdoor dry-bulb temperature is below the
return temperature of the water. It is placed in series
with the chiller's refrigerant condenser coil, so the
condenser fan requires more energy to force airfl ow
through both coils. To maximize economization hours,
TABLE 2 Details of Phoenix, Ariz., climate conditions.
DETAILS
DESCRIPTION
Hot Dry
REPRESENTATIVE
CITY
Phoenix
TABLE 3 Details of scenarios evaluated.
SCENARIO
1 (Baseline)
2-A
2-B
3-A
3-B
WEATHER LOCATION
Phoenix Sky Harbor
International Airport
ANNUAL COOLING DESIGN
CONDITIONS, DB (0.4%)
110.3°F
MCWB
69.5°F
SCENARIO DESCRIPTION
Air-Cooled Chiller, No Econ
Air-Cooled Chiller w/Integrated Economizer
Air-Cooled Chiller w/Integrated Economizer
Air-Cooled Chiller, No Econ & Airside Free Cooling
(Min Server Inlet Maintained at 64.4°F)
Air-Cooled Chiller, No Econ & Airside Free Cooling
(Min Server Inlet Maintained at 59°F)
CHILLER OPERATING
DESCRIPTION
Year-Round
Year-Round
Year-Round
If OA>80.6°F
If OA>80.6°F
DETAILS
CHW SUPPLY
68°F
68°F
68°F if OA >65°F
60°F if OA <65°F
68°F
68°F
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