A Product Brief from the Experts
in Business-Critical Continuity™
Liebert® DSE™ with EconoPhase™:
Highest Efficiency DX Cooling with pumped refrigerant economizer
Introducing the Liebert® DSE™ from
Emerson Network Power
In today’s data center, minimizing annual energy usage
while maintaining reliability is increasingly important. While
chilled-water loops operating at elevated temperatures
supported by water-side economizers have helped to
decrease energy usage, new innovations are changing how
the data center industry thinks about direct expansion (DX)
cooling infrastructures with respect to efficiency, scalability
and reliability.
One such next generation DX system, the Liebert DSE
from Emerson Network Power, provides the following
key benefits:
Industry-leading energy efficiency among
DX systems
Hassle-free economization with Liebert EconoPhase™
This product brief will provide a thorough overview of
the Liebert DSE system and compare its unique design to
comparable DX and chilled-water cooling solutions you
may be considering for your data center. It also provides a
detailed explanation of the Liebert EconoPhase pumped
refrigerant economizer solution and share specific examples
for how cooling efficiency can be maximized intelligently in
accordance with your data center’s ambient climate.
The Liebert DSE direct expansion cooling system comprises
four primary components: the Liebert DSE indoor computer
room air conditioning (CRAC) unit, the Liebert MC™
micro-channel outdoor condenser, the Liebert EconoPhase
pumped refrigerant economizer, and Liebert iCOM™
thermal system controls. The components operate in
harmony to provide the industry’s most energy efficient
operation for a room-based cooling system.
It is important to note that the Liebert DSE is a
complete, integrated system— not a collection
of disparate components.
Precise control and enhanced efficiency through
intelligent controls
Uncompromising reliability and 24/7 support
Liebert iCOM
Thermal System Controls
Liebert EconoPhase: First ever
pumped refrigerant economizer
Liebert MC: Intelligent, high
efficiency condensers
Liebert DSE Indoor Unit: Next
generation data center cooling system
The Liebert DSE direct expansion cooling system comprises four primary components: the Liebert DSE indoor computer room air
conditioning (CRAC) unit, the Liebert MC micro-channel outdoor condenser, the Liebert EconoPhase pumped refrigerant economizer
and Liebert iCOM control with thermal system manager.
Indoor CRAC Unit Designed with
State-of-the-art Technologies
The Liebert® DSE™ indoor unit (DA080, DA085, DA125
or DA150) is equipped with several high-technology
components that are designed and controlled to work
together to provide a highly efficient and reliable DX system.
Key technologies serve the following functions to enhance
energy efficiency and performance:
Digital scroll compressors match cooling to
IT load
Staged evaporator coil reduces compressor power and
optimizes economizer performance
Electronic expansion valve enables part-load and low
ambient efficiency
Electronically commutated (EC) plug fans match
airflow to server needs
Digital scroll compressors (a) and EC plug fans (b) optimize
the energy efficiency of the Liebert DSE by matching cooling
and airflow to the demands of the environment.
To maximize efficiency and control at full and partial
loads, the Liebert DSE is equipped with variable capacity
digital scroll compressor technology and direct-drive,
variable-speed EC fans, which are modulated to respond
to and match changes in room load. Varying the capacity
of the compressors enables them to run at a more efficient
operating point than at full capacity. A staged evaporator
coil inside of the unit enables each refrigerant circuit to
operate over large coil surface area, increasing system
efficiency. Alternative coil lacing designs that circulate
refrigerant through only one coil slab can result in air passing
through dormant coils, resulting in lower circuit efficiency and
uneven air distribution across the coils.
The CRAC’s variable-speed, direct-drive EC fans enable
air volume to be precisely adjusted to match the needs
of the server load. The Liebert DSE’s indoor unit also is
equipped with electronic expansion valves, which enable
compressors to modulate to a lower head pressure than
traditional mechanical expansion valves (TXVs), resulting in
lower operating costs during mild temperatures.
Highly efficient Micro-Channel
Condenser Design
The Liebert® MC™ condenser integrated into the Liebert
DSE system enables the system to achieve higher efficiency,
reduced sound levels, reduced refrigerant charge and an
overall decreased footprint. As shown in Figure 1, microchannel condensers such as those found in the Liebert
DSE system are smaller, lighter (up to 40 percent) and
can require significantly less refrigerant charge (nearly 73
percent) than a traditional fin/tube design. This results in a
greener design, using less refrigerant and providing a more
favorable LEED calculation for refrigerant use.
Because the Liebert MC condenser is equipped with a coil
that is only about one 1 inch thick, the condenser is able
to achieve lower pressure drop on the air side, resulting in
reduced fan power. Micro-channel condenser coils allow
for the same heat transfer within a smaller footprint,
providing efficiency gains as much as 52 percent compared
to traditional fin/tube condensers. Even higher efficiency
levels can be realized during low load or lower ambient
temperatures, when the fan speed can be reduced, utilizing
the condensers’ variable speed fans controlled by Liebert
iCOM™ controls.
The Liebert® MC™ condenser offers other benefits beyond
the intrinsic efficiency gains.The micro-channel coil design
eliminates the need for copper/aluminum junctions,
improving the condenser’s resistance to corrosion and
extending the life of key internal components. And the
condenser’s optimized fan blade design significantly
reduces fan noise at full load - lower operating speeds can
result in even lower sound levels.
Hassle-free Economization with
Liebert EconoPhase™ Pumped
Refrigerant Economizer
In addition to the aforementioned components, the
integrated Liebert EconoPhase pumped refrigerant
economizer – the industry’s first – is a key component to
maximizing the Liebert DSE™ system’s efficiency during
lower outdoor ambient conditions. The Liebert EconoPhase
economizer is unique from other systems particularly
because it is designed to operate without consuming
water, eliminating the need for expensive water treatment
systems and removing the risk of water leaks by not
introducing water into the data center environment. The
Liebert EconoPhase also eliminates the risk of outside
air contamination and does not require maintenance
associated with air-economizers such as damper and
louver maintenance.
Changeover between compressor mode and refrigerant
economizer mode is almost instantaneous, whereas
traditional economizer systems typically operate in
economizer mode only when the economizer season is
stable—not changing back-and-forth during the night, for
example. The Liebert EconoPhase solution is equipped
with variable speed pumps controlled to run at the most
efficient levels according to heat loads and ambient
outdoor temperatures, providing maximum economization
with minimum pump power consumption.
The Liebert EconoPhase is also unique in that it is designed
with reliability in mind. The Liebert iCOM™ control
monitors pump operation to ensure that the pumps do
not cavitate - improving system reliability and service life.
If free-cooling mode is temporarily disrupted, reliability
is maintained by switching to DX operation without
interruption. If a Liebert EconoPhase pump needs to be
serviced, it can be isolated from the system and replaced
without loss of mechanical cooling.
Liebert MC
-4.3 dBA
Footprint (ft2)
Weight (lb)
Efficiency (COP)
Sound Power (dBA)
Refrigerant Charge (lb)
Figure 1: Breakdown of typical benefits offered by the Liebert MC’s micro-channel condenser design, compared to a traditional
fin/tube design found in most DX cooling systems.
by automatically transferring between mechanical cooling
and economizer operation modes. Built-in diagnostics
ensure that system reliability is maintained, and information
about system operation is reported.
Comparing Economizer Designs and
Efficiency Implications
Liebert® iCOM™ Controls: The Industry’s
Most Advanced Control System
The Liebert DSE™ is managed by the latest Liebert
iCOM intelligent controls, which enable the system to
continuously adapt to the data center IT load and outdoor
conditions to take full advantage of economization while
providing consistent cooling to IT equipment. The controls
monitor all cooling system components as well as inlet rack
temperatures throughout the data center.
Monitoring inlet temperatures provides two primary
benefits: first, it enables the Liebert DSE system to adjust
cooling capacity to the exact amount of cooling needed.
Second, the inlet rack temperature may be reported back
to a building management system and recorded to ensure
service level agreements are met.
The Liebert iCOM controls scale the operation of the CRAC
to variably modulate the air volume and compressor
capacity to precisely match the data center’s IT load,
enabling the CRAC unit to deliver the right amount of air at
the right temperature. Liebert iCOM also monitors the load
and outdoor conditions to control the outdoor Liebert MC™
micro-channel condenser, and during cooler temperatures,
automatically activates the Liebert EconoPhase pumped
refrigerant economizer to provide economization when
conditions are optimal to do so.
The system’s integrated Liebert iCOM controls also monitor
several parameters within each sub-system and coordinate
operation to provide efficient and reliable cooling. The
Liebert iCOM control system provides hassle-free operation
Traditional direct expansion systems packaged with waterside economizers utilize a direct expansion cooling coil
overlaid by a closed loop of water/glycol refrigerant solution.
When ambient temperatures are low enough, the system’s
pump moves the water/glycol solution from an outdoor fluid
cooler (or drycooler) to the system’s indoor economizer coil.
While this type of system can provide economization and
reduce compressor operation, its efficiency is hampered by
the energy required for the pump to move a highly viscous
water/glycol solution. In addition, the glycol solution itself—
especially a propylene glycol solution—significantly reduces
heat transfer of the economizer coil, resulting in very low
ambient temperatures required to provide full cooling.
The Liebert EconoPhase requires much less power to pump
refrigerant than alternative systems that pump glycol.
The Liebert EconoPhase maintains full cooling capacity by
taking advantage of the two-phase properties of refrigerant.
Alternative glycol-based economizers can experience
capacity reduction as much as 40 percent compared to
full-DX operation resulting in a need to oversize the
economizer to reach system rated capacity. Water-side
economizers require extra fan power year-round, due to
the additional heat exchange that overlays the DX coil. The
Liebert DSE system requires no additional economizer coils,
keeping air-side pressure drop lower, resulting in lower
indoor fan energy consumption1.
Another option is to use chilled water air handlers
(CRAHs) in conjunction with a chiller plant and an air-side
economizer. In this configuration, the chiller plant produces
cold water during warmer times of the year, and outdoor
air is brought into the data center when conditions are
right. However, some end-users do not wish to change
According to ASHRAE 90.1-2010, data centers are now required to meet mandatory requirements for minimum SCOP (full-load,
design day) to achieve a satisfactory efficiency rating per ASHRAE rating Standard 127. The ASHRAE 90.1 requirements also mandate
the utilization of economizers in many regions of the United States. This minimum efficiency target for direct expansion cooling systems
ranges from 1.8 to 2.6, depending on the cooling capacity, condensing type, and airflow configuration.
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.
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
configuration 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 efficiency 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
filtration. In addition, some end users are not comfortable
introducing outside air into the data center due to
contamination concerns.
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 endusers 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 efficiency 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 percent1.
Other economizer systems involve chilled water air
handlers (CRAHs) connected to a water-cooled chiller plant,
with cooling towers providing water-side economization
Figure 2: Summer
and High Ambient
DX-Only Operation
Check Valve
For a load of 88
kW, the cooling
system consumes
only 24.1 kW of
power, resulting in a
sensible coefficient
of performance
(SCOP) of 3.65, or a
mechanical PUE
of 1.28.
MC Condenser
Solenoid Valve
Refrigerant Pump
Check Valve
Expansion Valves
circuit 2
circuit 1
Optimizing Cooling System Operating Modes
to Ambient Conditions
The operation of the Liebert DSE and corresponding Liebert
EconoPhase™ system is adaptable based on the ambient
seasonal temperatures and the overall IT load of the
data center. The following examples model a data center
operating at 88 kW of load on the system (70 percent load
when using a 125kW, DA125 model).
Summer and High Ambient Temperatures: DX-Only Operation
The example in Figure 2 represents a data center operating
at 70 percent load, experiencing ambient outdoor
temperatures of 95°F. During warm summer months, the
Liebert® EconoPhase™ refrigerant pump system is idled,
routing refrigerant instead through the evaporator, dual
compressors and outdoor condenser. To maximize efficiency
in this scenario, the variable-speed evaporator fans, variable
speed condenser fans, and the digital scroll compressors
automatically adjust to match the load and optimize
energy usage.
However, even in this highly efficient system (which achieves
SCOP of 3.65, compared to the industry standard 1.9
at 75°F return air), more than 70 percent of the cooling
system’s total energy draw is utilized by the compressors,
emphasizing the overarching importance of compressor
efficiency or reduced compressor operation– particularly in
warm climates.
During cooler parts of the year or at night, the
refrigerant economizer may have the ability to
provide partial free cooling, offsetting some of
the compressor power.
Nighttime and Cooler Ambient Temperatures:
Mixed-Mode Operation
During cooler times, such as mild seasons and at night, the
refrigerant economizer has the ability to provide partial free
cooling, offsetting some of the compressor power usage.
Assuming an 85°F return air temperature to the CRAC unit,
when the outdoor ambient temperature drops to 65°F (as
shown in Figure 3), Liebert EconoPhase can begin to reduce
energy even further by offering partial economization.
Because the Liebert DSE™ is equipped with a staged
evaporator coil, the system’s refrigerant Circuit One is
seeing the highest return air temperatures (85°F in this
case), meaning the return air is warmer inside than the
outdoor ambient air temperature. In this case, the outer
cooling coil acts as the primary economizer circuit by using
one Liebert EconoPhase pump to provide cool refrigerant to
the indoor heat exchanger, essentially pre-cooling the air.
The remainder of cooling required is accomplished by the
compressor on the second circuit.
Figure 3: Nighttime
and Cooler Ambient
Liebert EconoPhase
Check Valve
MC Condenser
Solenoid Valve
Refrigerant Pump
Check Valve
Expansion Valves
circuit 2
circuit 1
The refrigerant
bypasses the CRAC’s
first compressor,
allowing it to idle.
Pump One of the
Liebert EconoPhase
is activated, using
only 0.6 kW—a net
savings of 6.8kW.
The resulting system
efficiency is a lower
cooling PUE of 1.17,
or a system SCOP
of 5.83.
Figure 4:
Operation: Pumped
Economization with
Compressor Bypass
Liebert EconoPhase
Check Valve
MC Condenser
Refrigerant Pump
PUE as low as 1.05
Full economizer mode
Check Valve
Expansion Valves
circuit 2
circuit 1
Cold-Weather Operation: Pumped Refrigerant Economization
with Compressor Bypass
Finally, as shown in Figure 4, when outdoor temperatures are
at their lowest (particularly in winter months), the Liebert DSE
can leverage the Liebert EconoPhase system to operate at
full economization. In this scenario (45°F ambient), all of
the Liebert DSE system’s compressors are idled and
bypassed, replaced entirely by the Liebert EconoPhase
system’s refrigerant pumps. Condenser fans also operate
at lower power because colder outdoor air requires less
condenser airflow.
This translates to SCOP of 10.7, more than 5.5 times the
ASHRAE minimum efficiency requirement at 75°F return
air conditions. When operating at full economizatation at
temperatures below 45°F or at lower load, the Liebert DSE
is able to achieve a cooling PUE as low as 1.05 and SCOP
above 20– nearly ten times the required efficiency targets
in ASHRAE 90.1. These efficiency levels also can be realized
at higher ambient temperatures by increasing the supply air
temperature to the CRAC unit.
In essence, a total
of 14.8 kW of
compressor power
has been replaced
by 1.2 kW of pump
power. The result
is a cooling PUE of
only 1.09, or 9 kW
of power for the
cooling system for
every 100 kW of
IT load.
better understand and quantify the progression of efficiency
gains as outdoor temperatures change, consider the example
illustrated in Figure 5.
As previously discussed, the Liebert DSE is capable of
operating above SCOP of 5.0 during DX operation when
operating with 85°F return air to the cooling unit and 80
percent load (assuming outdoor ambient is less than design
day temperature). As the ambient temperature decreases,
the first stage of economization is automatically activated
by the Liebert DSE iCOM™ control system, bringing the
SCOP to about 12. Once the ambient temperature reaches
approximately 40°F, full economization is realized and
efficiency continues to increase as ambient temperatures
decrease – with SCOP peaking at about 22 kW/kW, or a
cooling PUE of about 1.05.
Efficiency Profile Based on Outdoor
Ambient Temperatures
As this example illustrates, the Liebert DSE transition from
mechanical (compressor) cooling to economizer mode
is seamless and automatic, capable of moving in and out
of economization several times a day, if needed. At these
conditions, significant economizer hours can be realized. The
system operates as an integrated economizer, meaning it
is capable of operating in partial economizer and partial DX
mode, if needed.
As these examples have demonstrated, the Liebert DSE is
designed to adapt its operation to the ambient environment
in order to maximize the efficiency and cooling effectiveness
of its DX CRAC unit and pumped-refrigerant economizer. To
It is important to note that while this particular example
illustrates an 85°F return air application, this scenario scales
for other return air conditions. To further illustrate this point,
consider Figure 6. When operating at 70 percent load with
80% Load (100kW), 85°F return air Mixed mode and Liebert EconoPhase Efficiency
Mixed Mode
Full Economization
DX Cooling
Outdoor Temperature F°
Figure 5: Liebert® EconoPhase™ efficiency progression according to outdoor ambient temperature (operating at 80 percent load).
DSE System Load (%)Model DA125
75°F Return
85°F Return
95°F Return
At 70% load and 85°F, full
economization is available
at 50°F outdoor
105°F Return
Outdoor Ambient °F
Figure 6: Liebert EconoPhase efficiency progression according to outdoor ambient temperature and system operating load.
return air temperatures of 85°F, 100 percent economization
is achieved at an outdoor ambient temperature of 50°F.
Increasing the return air to 105°F at 70 percent operating
load results in 100 percent economization at just over
70°F outdoor ambient temperature, enabling even more
economizer hours.
The key takeaway is that, in order to maximize
economization, three factors must be optimized:
return air temperature, operating load and outdoor
ambient temperatures.
The Liebert DSE can deliver a payback in less
than three years compared to traditional aircooled DX systems, but without the hassles
and risks of other types of economizer systems.
about 1.05, thanks to its ability to dynamically optimize
operations according to return air and outdoor ambient
temperatures. A chilled water system with air economizer is
also shown for comparison (blue line). The cooling PUE for
these systems start at about 1.28, and plateau at about 1.2.
However, in theory, if air economizers are working perfectly
and humidity control is not assumed, the minimum cooling
PUE of this system can be as low as 1.13.
Comparing Efficiency and Total Cost
of Ownership
As illustrated by the previous examples, the efficiency
of economizer-based cooling systems can vary widely
according to the outdoor ambient temperature. This fact
is what makes the Liebert® DSE™ system’s flexible and
intelligent mixed-mode operating capabilities an ideal
choice for maximizing cooling efficiency.
As shown in Figure 7, a traditional DX system (red line)
starts at about 1.5 cooling PUE on design day, and then
levels out to about 1.35 as the ambient temperature drops.
By comparison, the Liebert DSE (green line) system starts
with a cooling PUE of just over 1.3, and bottoms out at
While this accurately portrays PUE at a specific outdoor
temperature, the annualized PUE depends largely on how
many hours the system operates at each temperature. As
such, colder climates will log more hours at lower ambient
temperatures, resulting in a lower annualized PUE than a
climate that does not have as many of these low ambient
hours available. However, as previously demonstrated, lower
annualized PUE is attainable for data centers in warmer
climates by deploying the Liebert DSE and raising return air
Efficiency Comparison at 85°F Return Air
Traditional DX
Liebert DSE
with EconoPhase
Chilled Water with
Air Economizer
Air Economizer Theoretical
Liebert DSE at 105°F return air, part
load and Optimized Aisle
Outdoor Ambient (°F)
Figure 7: Comparing cooling system efficiency changes based on outdoor ambient temperatures.
Regardless of the system used, an intelligent control system
that evaluates room load, room temperature (or supply air
temperature) and outdoor temperature must be employed
to determine the most optimal operation of system.
When considering total cost of ownership, the Liebert®
DSE™ with Liebert EconoPhase™ system also compares
very favorably to alternative cooling systems. In order to
compare systems, a complete picture of system costs must
be known and compared. As shown in Figure 8, the Liebert
DSE can deliver a payback in less than three years compared
to traditional air-cooled DX systems, but without the
hassles and risks of other types of economizer systems.
As this brief has demonstrated, the Liebert DSE cooling
system from Emerson Network Power represents the
industry’s most flexible, reliable and efficient DX solution.
The Liebert DSE achieves industry-leading efficiency by
offering a modular, end-to-end cooling infrastructure,
complete with a high-efficiency indoor CRAC unit, a
hassle-free pumped-refrigerant economizer, micro-channel
outdoor condensers with variable-speed fan technology
and a comprehensive data center intelligence and
management system built-in.
Collectively, these components make up a cooling
infrastructure capable of delivering the lowest possible
total cost of ownership and the highest levels of reliability
compared to competitive solutions – all backed by the
largest and most experienced sales and support team in the
data center industry.
Air-Cooled DX
Chilled Water
Chilled Water
Liebert DSE with
Air Economizer
Pumped Refrigerant
Energy Cost
Water Cost
Total Utility
4.9 years
5.6 years
2.7 years
Annual Cooling PUE
Capital Cost
15-Year PV
Figure 8: Analysis of total cost of ownership across cooling system options, taking into consideration capital expenditure and operating
expenditure for a data center with a 1,500 kW load, $0.10/kWh utility cost and 40 degree dew point limit, based on Columbus, Ohio
weather data. The chilled water systems assume an entering water temperature of 50°F, with a water temperature difference of 12°F.
Emerson Network Power
Liebert Corporation World Headquarters
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P.O. Box 29186
Columbus, Ohio 43229
United States Of America
800 877 9222 Phone (U.S. & Canada Only)
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614 841 6022 FAX
Emerson Network Power
Caribbean and Latin America
Office – United States of America
+1-954-984-3452 Phone
24 x 7 Tech Support
800 222 5877 Phone
614 841 6755 (outside U.S.)
Emerson Network Power.
The global leader in enabling
Business-Critical Continuity™.
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completeness in this literature, Liebert Corporation assumes no
responsibility, and disclaims all liability for damages resulting
from use of this information or for any errors or omissions.
© 2013 Liebert Corporation. All rights reserved throughout
the world. Specifications subject to change without notice.
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