A Technical Note from the Experts
in Business-Critical Continuity™
Technical Note: Designing the Most Efficient
and Reliable Row-Based Cooling Solution
As the global leader in data center thermal management,
Emerson Network Power has worked extensively with its
customers to develop and deliver the broadest range of
efficient and reliable thermal management technologies.
This allows us to be technology-agnostic in order to provide
the best, most unbiased solutions to our customers. As
such, Emerson Network Power in 2003 was the first to
introduce row-based cooling, launching the Liebert® XD™
line, which included the Liebert XDV™, Liebert XDO™, and
Liebert XDH™ solutions. Since that time, we have optimized
row-based cooling to provide the world’s most efficient
and reliable solution, the Liebert CRV™. This technical note
describes important considerations when evaluating or
applying this solution.
Controls Make All the Difference
The Liebert CRV incorporates the most advanced
and flexible unit and system controls – Liebert iCOM™
thermal controls. At the cooling unit level, the Liebert iCOM
unit control provides the highest protection available,
with optimal performance. At the supervisory level, the
Liebert iCOM™-S system control offers a revolutionary way
to optimize thermal system performance across the data
center, gain quick access to actionable data and automate
system diagnostics and trending.
The Liebert CRV system with Liebert iCOM unit controls
gives customers the option to control cooling operation
based on data from the supply air sensor, rack sensors (up
to 20 data points) or the return air sensor, regardless of
whether the unit is deployed by itself, in a row with other
Liebert CRV units, or in a containment design. Additionally,
the Liebert CRV system is the ONLY row-based solution that
can decouple the fan speed from the capacity to allow the
servers to receive just what they need – more airflow, cooler
temperatures or both AUTOMATICALLY.
Liebert iCOM controls provide 5 to 7% more efficiency at
the unit level and 15 – 25% greater efficiency at the system
or multi-unit level. These controls have advanced machineto-machine (M2M) communication, so units can function
as a team, coordinating cooling and eliminating fighting
between units (some heating while some cooling). Liebert
iCOM controls provide Cascade, Standby, Rotation and
Lead/Lag routines, share rack sensor data over networked
units (up to 32 units for 640 rack sensor data points) and
2
include a supply temperature compensation mode that
provides the most efficient unit and system control on
the market.
In addition, Liebert iCOM controls provide the highest
level of unit protection, with over 380 monitoring points,
the industry’s quickest cooling restart in case of a power
outage, advanced DX freeze protection and chilled water
valve auto-tuning.
Matching Airflow to Server Needs
IT equipment fan technology is not designed to support
a specific temperature difference from the air intake
to exhaust – it is designed for airflow. IT equipment
manufacturers have been lowering flow rate requirements
to increase equipment energy efficiency. This in turn has
increased the temperature difference of the air across
the equipment.
In 2007, the typical temperature difference of air across IT
equipment was between 7°C and 17°C (12.6°F and 30.6°F).
This equated to 100-250 CFM/kW. By 2010, IT equipment
temperature differences ranged from 16°C to 27°C (28.8°F
to 48.6°F), which is equivalent to about 55-120 CFM/kW.
This range is expected to tighten to between 75 CFM/kW
and 95 CFM/kW by 2020. See Figure 1.
300
250
200
CFM / kW
Emerson Network Power Row-Based
Thermal Management
150
100
50
0
2005
2010
2015
2020
Upper Range
Lower Range
Figure 1: Server Air Flow Requirements by Year of Introduction
By overlaying the Liebert® CRV™ row-based unit air flow
per kW (Figure 2), it is clear that the Liebert CRV system
better matches IT equipment needs in either high protection
or high efficiency. Other cooling products on the market are
designed for obsolete IT equipment and are wasting energy
by over provisioning air that is simply not needed.
300
250
200
150
For chilled water applications, we offer a range of
highly efficient and reliable air-cooled chillers designed
specifically for mission critical applications, using more
robust electronics and components, as well as advanced
controls that optimize economization hours and chilled
water temperatures for reduced energy usage. The chillers
generally use 15-20 percent less energy than standard
air-cooled chillers. When coupled with the Liebert CRV, the
overall system can save up to 35% in energy consumption
over the competitive designs.
Ensuring Proper Placement and Application of
Row-based Cooling
100
50
0
2005
microchannel condensers communicate back to the rowbased cooling units via Canbus communications, providing
precise control for optimal protection and efficiency.
2010
2015
2020
Competitor A
Liebert CRV High Protection
Liebert CRV High Efficiency
Figure 2: Row based Equipment Air Flow
Turning Vanes for Optimal Air Flow
There are those who believe that turning vanes in cooling
units are not necessary, because the units can capture
100% of the hot IT exhaust air. Technically, this is true, but
it requires a near perfect implementation of containment
and air flow management, which almost never happens.
At Emerson Network Power, we design our products
with turning vanes, so they perform at the highest levels
of efficiency and reliability in practical, day-to-day
applications. Turning vanes help direct the air to where
the cooling is needed, increasing efficiency and the
protection of the equipment.
Understanding when to implement row-based solutions,
along with how to implement them, is critical for customers.
These products do not fit every data center challenge nor
satisfy every customer goal. Row-based cooling is an ideal
solution for quick deployment in smaller applications or
spaces and where spot cooling is needed for higher rack
densities. For cooling larger spaces – approaching 250kW
and greater – row-based cooling becomes more expensive
and no more efficient at a system level than perimeter
cooling solutions.
In summary, row-based cooling is best used to cool
individual rows, and generally should not be used to cool
racks or rows in other pods, as it will generally be very
difficult to ensure proper airflow, given typical data center
configurations. Be sure that data center designs do not
assume perfect or ideal conditions. Make sure your designs
will achieve the protection, capacity and efficiency you need
under typical conditions.
Factoring in Heat Rejection
A large part of your thermal management system efficiency
and protection is determined by how heat is rejected into
the environment. For DX systems, we use microchannel
condensers that are over 50% more efficient and significantly
quieter than traditional fin/tube condensers. These
3
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The description of competitor transformer saturation elimination methods is based on the latest information, literature, and performance data available. It is possible that some competitors may have overcome one or the other of the
described disadvantages to some degree, but we believe the most effective is the Emerson Network Power method, which is based on continuously tracking the transformer flux and making precise transfer timing decisions for each phase
independently and employing a voltage pulsing technique which maintains a higher average voltage to the load; this method is patented and exclusively used by Emerson Network Power.
While every precaution has been taken to ensure accuracy and 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
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