between mechanical cooling and economizer cooling several
times during the year. Instead, they may elect to change
to economizer mode only when the system will stay in this
conguration for a long period of time (only when the dew
point of the outside air is within a certain window: 40°F
to 59°F, for example). This can result in missing many partial
economizer hours during nighttime or mild “shoulder”
months. As such, the efciency of this particular option
is largely dependent of the outdoor ambient climate,
and can vary widely depending on geography and
environmental conditions.
In addition, air economizer energy savings can be negatively
impacted due to the need for powered exhaust fans and
the resultant added fan energy to overcome outside air
ltration. In addition, some end users are not comfortable
introducing outside air into the data center due to
contamination concerns.
Other economizer systems involve chilled water air
handlers (CRAHs) connected to a water-cooled chiller plant,
with cooling towers providing water-side economization
during cooler outdoor temperatures. In this case, water is
consumed year-round, even during economizer operation,
adding additional operating costs. Regular water treatment
is required for the open cooling tower system. Many end-
users do not operate these systems during ‘shoulder seasons’
when the system could changeover during nighttime and
daytime. Instead, they prefer to changeover to economizer
operation only when the system will stay in that operating
mode for an extended period of time. This results in a
reduction in economizer use, compared to the Liebert DSE
system that automatically changes between economizer
mode and DX mode in a matter of a few seconds.
The Liebert DSE system’s combination of DX mode operation
during warmer temperatures and refrigerant economizer
mode during cooler temperatures results in a mechanical
power usage effectiveness (PUE) between 1.05 and 1.3.
However, in some climates, economization is not always
necessary to meet ASHRAE 90.1 efciency standards, nor
is a fast payback always attainable to justify investment in
economizer systems. As such, it is important to note that the
Liebert DSE, even when used solely in the standard direct
expansion operating mode, is still capable of exceeding
minimum efficiency standards by up to 53 percent
1
.
Figure 2: Summer
and High Ambient
Temperatures:
DX-Only Operation
For a load of 88
kW, the cooling
system consumes
only 24.1 kW of
power, resulting in a
sensible coefcient
of performance
(SCOP) of 3.65, or a
mechanical PUE
of 1.28.
Refrigerant Pump
Solenoid Valve
circuit 1
circuit 2
Compressor
Check Valve
Electronic
Expansion Valves
Check Valve
Check
Valve
8.5kW
8.5kW
3.9kW
3.2kW
Liebert
MC Condenser
The Liebert
®
DSE™ system combines DX mode
operation during warmer temperatures and
refrigerant economizer mode during cooler
temperatures, resulting in a mechanical power
usage effectiveness (PUE) between 1.05 and 1.3.
6
(24 pages)
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