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Close-Coupled Cooling Solutions for Server Racks, Computer Rooms, Server Rooms & Data Centers
Close-Coupled Cooling is an ideal solution for high-density
configurations,
eliminating hot spots while improving energy efficiency.
With high performance, denser computing, the traditional
layout of perimeter CRACs, fans, and perforated floor tiles
struggles to deliver the required volume of cool air, to maintain
a uniform server inlet temperature, and to expel exhaust air
from the room. A potential remedy for these problems is close-coupled
cooling, where the once distant air conditioner is moved
closer to the actual compute load.
What is Close-Coupled Cooling?
Close-coupled cooling is a recent entry into the data center
lexicon, and many manufacturers apply the term to describe their
latest generation cooling products. Though their solutions vary
in terms of configuration and capacity, their approach is the
same. Close-coupled cooling aims to bring heat transfer closest
to its source: the equipment rack. Moving the air conditioner
closer to the equipment rack ensures a more precise delivery
of inlet air and a more immediate capture of exhaust air.
We can divide commercially-available close-coupled solutions
into two categories: open loop vs. closed loop
Open-Loop Configuration:
Open-Loop solutions, while they bring the heat transfer closer
to the equipment rack, are not completely independent of the
room in which they're installed. The air streams will interact
to some extent with the ambient room environment.
These products will use either chilled water or refrigerant
through their cooling coils. All will require remote heat rejection
via a mechanical chiller system, condensing either chilled water
or refrigerant.
In-Row, In-Line Air Conditioners
The trend in computing is densification; from chips to processors
to servers, a small footprint, these days, can pack a significant
punch. In some instances, the infrastructure has shrunk alongside
the computers. The In-Row, In-Line air conditioner brings the
functionality of the perimeter CRAC local to a data center row
with dimensions similar to the actual enclosures. These units
and the computer load benefit by proximity: neither the cold
air nor the hot exhaust air has far to travel.
End users can deploy these products in various ways. If the
infrastructure is in place, the In-Row, In-Line products may
serve as supplemental cooling solutions in existing data centers.
They may serve as localized cooling within new rows or pods,
addressing higher density installations. They may be used with
a containment strategy to achieve even greater efficiency.
Rear Door Heat Exchangers
Replacing the rear door of an existing rack, these heat exchangers
leverage the front to back air dissipation of most IT equipment.
The servers exhaust warm air, which passes through the heat
exchanger coil and is returned to the room at a palatable temperature
(often similar to server inlet temperature). These units can
remedy hot spots in existing data centers, supplementing the
existing air conditioning or for smaller loads and rooms, provide
cooling for spaces not originally designed as data centers-
data rooms, closets, labs. Installed on racks, these units do
not take up floor space-an important point in small size installations.
Overhead Heat Exchangers
Overhead, in the "traditional" cooling sense, is an alternative
to a raised floor plenum. The AC discharges air from the ceiling
into the cold aisle; exhaust air rises into vents in the ceiling.
The close-coupled version brings the air distribution and return
much closer to the enclosures. Instead of the ceiling, these
units sit directly on or above the server enclosure, making
the cold air delivery and hot air return much more precise.
Deployed overhead, the units do not occupy any floor space.
End users can use as either supplemental cooling or localized
cooling within the data center, depending on the overall load.
Closed-Loop Configuration:
Closed-loop cooling addresses the compute load independent
of the room in which it's installed. The rack and heat exchanger
work exclusively with one another, creating a microclimate within
the enclosure. Those familiar with containment strategies can
think of close-coupled, closed loop as containment fully evolved:
both hot aisle and cold aisle in the same rack footprint.
In-Rack Cooling
The In-Row/In-Line air conditioner, mentioned earlier, is a
key part of this approach. The AC is adjoined to the server
rack and both are fully sealed. The solid doors on the enclosure
and In-Row AC contain the airflow, directing cold air to the
server inlet and exhaust air, via fans, through the cooling
coil. The close-loop design allows for very focused cooling
at the rack level. Users can, therefore, install very dense
equipment exclusive of the ambient environment. As a result,
they have the flexibility to use unconventional rooms and spaces
to house the IT equipment.
Close-Coupled Cooling Solution
Close-Coupled Cooling Efficiencies
Modular and Scalable Infrastructure
Data center professionals must understand their current requirements
for space, power, and cooling and predict how those needs will
grow over time; otherwise, the data center can quickly become
obsolete. A past approach addressed these concerns with excess-bigger
spaces, more racks and air conditioners, and large chiller plants.
This left ample headroom, it was thought, for redundancy and
growth.
For today's data center, immersed in discussions of high
density and even higher energy costs, this approach is problematic.
As Sun Microsystems states in its Energy Efficient Datacenter
paper, "Building datacenters equipped with the maximum power
and cooling capacity from day one is the wrong approach. Running
the infrastructure at full capacity in anticipation of future
compute loads only increases operating costs and greenhouse
gas emissions, and lowers datacenter efficiency"
Close-coupled cooling embodies two of the industry's favorite
buzzwords: modularity and scalability. Instead of building larger
spaces and installing more air conditioners, professionals can
"right-size" from the very beginning. Perhaps a 40kW installation,
which is ordinarily spread over 10 racks, is now spread over
5 racks, when using a rear door heat exchanger. Perhaps a planned
5000 sq ft facility becomes a 2000 sq ft facility, with fewer
CRACs supplemented with overhead cooling. Due to the modularity
of these products, end users can add pieces in a "building-block"
fashion, increasing capacity as business needs dictate.
The close-coupled design offers consistency in cooling performance,
independent of the raised floors, floor tiles, and fans associated
with traditional cooling. These products scale to successfully
support the full gamut of rack loads, from minimal to high
density. A user with a closed-loop, close-coupled design
knows that a predictable capacity is available as his operation
grows.
Fan Energy
In the traditional layout, fans must move the air from the perimeter
of the room, under the raised floor, and through a perforated
floor tile into the server intake. This process, of course,
requires energy, which varies from facility to facility. Often
impediments (large cable bundles, conduits) exist under the
raised floor, which require additional fan energy to move the
required volume of cold air.
The In-Row air conditioner reduces fan energy by proximity.
The AC unit is embedded within the row of racks, ensuring the
air does not have far to go. In addition, the AC delivers air
directly to the row; there are no underfloor impediments to
overcome.
In addition, certain close-coupled products operate with
variable speed fans, where fan velocity aligns with the
installed compute load within the rack. For fans, this point
is not insignificant; speed is directly proportional to energy
consumption. A SearchDataCenter.com article puts the savings
into perspective: "[I]f you can reduce fan speed by 10%, fan
power consumption decreases by 27%. If you reduce fan speed
by 20%, fan power drops by 49%" (Fontecchio, 2009).
Furthermore, certain rear door heat exchangers function without
fans. These systems use the servers' internal fans to propel
hot air through the heat exchanger where it's cooled and released
into the room.
Higher Chilled Water Temperatures
As documented on this site, chilled water supply temperatures
typically range from 42 to 45 degrees F. The cold water is needed
to produce cold air, which offsets the mixing that occurs on
the data center floor. As cold inlet air and warm exhaust air
interact, the hope is the resulting inlet temperature falls
somewhere in the ASHRAE recommended range of 64.6-80 degrees
F.
Some close-coupled designs can accept warmer inlet water
temperatures. Due to the proximity of the heat transfer and
the design of the cooling coil, a warmer water temperature can
provide a desired server inlet temperature within ASHRAE's guidelines.
This point is significant for three reasons:
- Chillers, depending on the source, are estimated to
represent 33% to 40% of a facility's energy consumption,
due in large part to the mechanical refrigeration process
- A higher inlet water temperature maximizes the number
of hours in which "free cooling" is possible through the
use of water side economizers if the environment permits.
- Chiller efficiencies in kW/Ton increase at a higher
supply water temperature
However, the process is not as simple as raising the temperature.
It may not be practical for facilities which share a chiller
plant with offices or other spaces.
Installation Considerations
The close-coupled products will require connections to a
chiller system; new supply and return lines will have to be
run to the heat exchangers. Thus, there are numerous design
and installation considerations.
Infrastructure
- Will the supply and return pipes be run under the floor
or overhead? What are the challenges and risks with either
option?
- Does the existing chiller plant have sufficient qualities-
pressure differential, water quality, flow rate-to ensure
the products function as advertised?
- Does the capacity of the product vary based on the chilled
water temperature? Can the facility use elevated temperatures
to improve energy efficiency?
- Will there be an isolated data center loop, fed by a
coolant distribution unit (CDU)? If so, what's the interaction
between building chilled water and the CDU? Does the particular
product require a CDU?
- How is redundancy accomplished?
Compatibility
Certain close-coupled solutions are compatible with existing
server rack products. The rear door heat exchangers are intended
to retrofit onto 3rd party racks. The overhead heat exchangers,
assuming the architecture is in place, can be deployed above
any rack. Existing facilities may find these features friendly,
as there seems less need to re-rack servers and other equipment
into different rack.
The In-Row air conditioners, in a close-loop design, often
require a proprietary rack enclosure to ensure the entire operation
is properly sealed. Even in an open-loop design, the products
install at the row-level, which may require new rows or disrupting
existing rows. Furthermore, these products do occupy floor space-something
that smaller facilities must consider.
If the facility is willing to re-rack existing equipment
or is deploying new equipment, they can greatly reduce the total
required floor space, due to the increased capacities.
The brown field/ green field discussion is very pertinent for
close-coupled cooling. Existing facilities must discern the
complexity and practicality of deploying the technology within
their data centers. Green field facilities have time to plan
and design the entire cooling scheme from soup to nuts.
Conclusion
The considerations for close-coupled designs are many. Product
selection depends largely on the individual installation and
requires input from a number of groups, from IT and facilities
staff, consulting engineers, and vendor representatives. Yet,
the end result of this concerted effort can be considerable.
Consider the following examples:
- An EYP presentation compares conventional cooling architecture
with 45 degree chilled water vs. a close-loop, close-coupled
cooling strategy with 55 degree chilled water. The presentation
projects annual energy savings of over $ 1 million with
the closed-coupled design.
- For facilities where it's impractical to raise the chilled
water temperature, a manufacturer study with a 45 degree
chilled water supply using an In-Row product reports a $60,000
annual energy savings over perimeter CRAH technology (Bean
& Dunlap, 2008),
- The oft-cited EPA's Report to Congress christens close-coupled
liquid cooling as a "State of the Art" technology-a key
element in curbing cooling plant energy consumption.
- Close-coupled cooling can accommodate a "Zone" approach
to data center expansion, addressing the higher performance
loads. The approach allows for, according to Gartner, "adequate
power distribution and cooling…without having to create
a power envelope for the entire data centre that supports
high density" This zone approach reduces first-costs and
the continuing electricity costs. (Cappuccio, 2008)
The final point is key. The benefits of energy efficiency
are not exclusive to the environment. Close-coupled cooling
products, along with other best practices, offer financial incentives.
From potential utility rebates to lower utility bills, the business
case is there. The ecological results are simply an added bonus.
Bibliography
Bean, J., & Dunlap, K. (2008). Energy Efficient Data Centers:
A Close-coupled Row Solution. ASHRAE Journal , 34-40.
Cappuccio, D. (2008). Creating Energy- Efficient Low Cost, High
Performance Data Centers. Gartner Data Center Conference,
(p. 18). Las Vegas.
EPA. (2007, August 2). EPA Report to Congress on Server and
Data Center Energy Efficiency. Retrieved January 5, 2009,
from Energy Star:
http://www.energystar.gov/ia/partners/prod_development/downloads/EPA_Report_Exec_Summary_Final.pdf
EYP Mission Critical Facilities. (2006, July 26). Energy
Intensive Buildings Trends and Solutions: Data Centers.
Retrieved February 2, 2009, from Critical Facilities Roundtable:
http://www.cfroundtable.org/energy/072106/myatt.pdf
Fontecchio, M. (2009, January 21). Data Center Air Conditioning
Fans Blow Savings Your Way. Retrieved January 22, 2009,
from Search Data Center:
http://searchdatacenter.techtarget.com/news/article/0,289142,sid80_gci1345584,00.html
Sun Microsystems. (2008). Energy Efficient Data Centers:
The Role of Modularity in Data Center Design. Sun Microsystems.
About 42U
Since 1995, 42U has been a leader in providing
data center efficiency
solutions for data center and facilities managers. Our vendor and technology
agnostic approach leverages our best-practice expertise in monitoring, airflow
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This work is licensed under a
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Unported License.
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