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Selecting the Optimal Data Center Cooling Solution
By Peter Sacco, President & CEO, PTS Data Center Solutions,
Inc.
Date: August 30, 2011
A better way to think about data center cooling is
to forget the notion of adding ‘cold’ to a room. Rather, think about
air conditioning as removing heat from the room. To that end, there
are two components that make up any effective cooling solution. They
are first, the Heat Removal section, often referred to as the CRAC,
CRAH, or the evaporator, and the second, the Heat Rejection section
which is named according to the cooling approach executed and
referred to as the condenser, dry-cooler, fluid-cooler, cooling
tower, etc.
As such, the first step in selecting an appropriate
data center cooling solution always starts with establishing
suitable design criteria. Further, selecting the optimal
cooling solution involves a deep understanding and comparison
between the performance characteristics, capital expense (CAPEX),
and operational expense (OPEX) of each potential configuration.
Establish Suitable Design Criteria
Key design criteria definition establishes the
critical load, site availability requirements, and IT equipment
deployment density for a specific computer room improvement,
expansion, or new construction project. These three (3) high-level
decisions serve as variables in a cost model matrix to identify
their affect on overall project costs across multiple scenarios,
thus allowing a narrowing of the scope of the project to one that is
affordable, yet meets the business’ needs. Following this the
narrowed scope serves as a basis of engineering and design, as well
as the subsequent construction.
Key Design Criteria for Load
The load design criteria is defined as two
(2) separate parameters:
1.
The Day-One Load, and
2.
The Maximum Permissible Load after all phases, upgrades,
expansions, and improvements.
Understanding the day-one load is important is
because air conditioners cannot operate effective with too little
load as much they cannot handle too much load. Conversely,
understanding the maximum load is vital for overall space and power
budgeting.
Key Design Criteria for Availability
Availability is a measure of the probability that a
data center’s site and supporting infrastructure will stay active
and useable, but more importantly an estimate on the amount of
downtime that may be realized in making different design decisions.
Design characteristics such as reliability, redundancy,
non-disruptive maintainability, and fault tolerance are all
availability improvement drivers as well as impacting on cost. The
first organization that made a meaningful contribution has been The
Uptime Institute with their paper, ‘Tier Classifications Define Site
Infrastructure Performance’. However, most experienced data center
design firms have their own rating guidelines to estimate
availability. Typically, these ratings have been developed using a
much broader sample of data center designs, across a wider
definition of infrastructure configurations, and provide more
detailed CAPEX and OPEX impacts.
Key Design Criteria for Density
The equipment deployment density is a statement of
the watts-per-square foot (W/SF), on a per-rack, cabinet, or
enclosure basis, that the IT load will be deployed. While this
approach is divergent from its predecessor, describing the W/SF of
the computer room itself, it is a far better measure since today’s
load density diversity can stretch from 0 W/SF for a cable
infrastructure rack to 25+ kW/SF for an ultra high-density blade
cabinet – all in the same computer room. Therefore, this more
granular statement of load geometry allows the designer to better
adapt cooling systems approach to the needs of the load with respect
to supply and return air control.
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Computer Room Air Conditioning Approaches
Room-Based vs. Row-Based vs. Rack-Based Heat Removal
Regardless of the cooling system’s heat rejection
method, the air side configuration is 50% of the cooling equation.
The guiding principal for the air side of the system is heat
removal. This is the process of using the ‘cold’ coil to absorb heat
from the room (as created by the server) by passing the room air
across the coil via a fan or blower.
 Regardless of the approach, the main focus to effective heat removal is to
remove it as close to the source it is generated as is practical.
This means that Rack-Based Cooling will win, as compared to the
other approaches, every time. This is because in a rack-based
cooling scenario the heat is absorbed within the rack, and sometime
even right at the chip set! However, the cost for achieving these
efficiencies is often high because each load source has to have its
own heat removal equipment.
As such, the next best options are Row-Based cooling
solutions. Row-Based cooling equipment intersperses the air
conditioners in between rows of cabinets that are the heat load.
Therefore, the heat removal, as well as the cold air supply, is
closely coupled to their intended targets. The benefit to this
approach is that one air conditioner unit can serve multiple load
cabinets in a given area or zone. In addition, this allows
adaptation to varying deployment densities.
Finally, the traditional standby approach has been
perimeter located down-flow air conditioners pumping cold air into a
raised floor plenum. In this approach, the heat is removed via the
top of the air conditioner. This is traditionally been done via free
air, however in later years this approach has been augmented by
utilizing the space above the suspended ceiling in the room as a
return air plenum.
In all cases, the ‘secret’ is to reduce air mixing.
All techniques and approaches work toward achieving that goal from
the various air conditioner configurations, to the bevy of air
containment systems available, even the adaptation of the hot aisle
/ cold aisle strategy itself.
Containment vs. Non-Containment
In keeping with the ‘air mixing is bad’ theme, it
was inevitable that containment solutions evolved. Containment is
the execution of containing the hot air and/or cold air to improve
the effectiveness and efficiency of whatever cooling solution is
deployed. In fact, the fight has been downright contentious over
which is better: cold aisle containment (CAC) or hot aisle
containment (HAC).
I contend that the problem isn’t one of containment,
rather one of air flow balancing. Picture a fish tank being filled
by a hose, but with a spigot that can let water out. If you fill the
tank faster than you let water out, then the tank overflows. This is
analogous to the hot aisle. If the IT load pumps heat into the hot
aisle faster than the CRAC/CRAH can remove the heat then the excess
heat billow out and mixes with the cold air targeted for the IT
equipment. Conversely, if you let water out faster than the hose can
fill, the opposite is true. In this instance, the hot aisle will
draw in cold air from the surrounding to make up for the lack heat
available to be rejected. In either case, no degree of non-perfect
sealed containment will prevent air mixing. Hence, the problem is
air flow, not containment.
Direct Expansion (DX)-Based vs. Chilled Water-Based Heat
Rejection
As stated earlier, the other half of the cooling
approach is heat rejection. This is the process of rejecting the
heat from the room where it was located and transferring that energy
to the ambient environment preferably outside of the building.
The two main mediums for transferring heat from one
location to another are via refrigerant or water. While water is a
far better for transferring heat as it can hold far more heat per
its volume than refrigerant can, refrigerant is lighter than water
and therefore makes for a less complicated albeit limited method.
Further, the characteristic that makes
refrigerant-based air conditioners attractive is that it can exist
in both a liquid and gaseous state at much lower temperatures than
water which can be controlled by pressure, or by the act of
compressing the liquid into a gas which in turn adds heat. This
compressed gas can be moved to the outdoor condenser section where a
fan blowing ambient air across the coil can reduce the temperature
of the refrigerant. Thus, when it makes its way back to the indoor
air conditioner the pressure can be released by allowing the
refrigerant to expand and thereby get cold. This condition is called
the refrigeration process and is the basis of all direct expansion
(DX)-based air conditioner solutions.
To further complicate matters, the DX process can be
accomplished in a few different ways including, air cooled, glycol
cooled (which uses a water / glycol mixture), and water cooled
(which utilizes a cooling tower). Common to all of them is that a
compressor is utilized in the air conditioning system to compress
refrigerant. Often the indoor portion of this system is referred to
as a Computer Room Air Conditioner (CRAC) unit.
A chilled water cooling approach utilizes a Computer
Room Air Handling (CRAH) unit. This means, that the CRAH does not
rely on a compressor within to create the refrigeration process.
Rather, a chiller creates cold water via a number approaches outside
the scope of this paper. In turn, the chilled water is pumped to the
CRAH and a blower passes the room air across the cold water coil
which absorbs the heat from the room and carries it to the heat
rejection equipment.
Each of these approaches carries with it a unique
CAPEX / OPEX condition. Choosing the appropriate cooling solution
for any data center project therefore requires an analysis by a
design professional familiar with comparing the different
approaches. However, in general DX air cooled solutions can
accommodate the majority of computer room cooling application given
its relative low cost, ease of installation and operation, its
scalability due to the 1:1 nature of its indoor to outdoor
components.
Energy Usage Improvements Using Economization and/or Evaporative
Cooling Techniques
Ask any data center manager for a list of priorities
for effective operations and on the top of the list will be
availability. In fact, availability trumps energy efficiency every
time. That said there are methods that can be employed that do not
overly affect availability while at the same time able to provide
energy usage efficiency. One such method is the use of economizers
in concert with the heat rejection approach. There are two distinct
ways to employ economization air-side and water-side. Both leverage
leveraging the ambient condition to offset using mechanical-based
cooling.
Air-side economizers accomplish this by actually
bringing in outside air into the facility while at the same time
exhausting heat out of the facility in balance with the amount
coming in. Typically, the outside air must be tempered, meaning
filtered of particulates and adjusted for humidity content prior to
introducing it into the critical environment. The danger is that no
amount of tempering is failsafe in preventing air infiltration when
the air outside becomes toxic or worse.
Water-side economizers provide less efficiency
gains, but are far less risky to data center operations. This
approach is typically utilized with chilled water operation. The
ambient condition is used to reject the heat from the ‘warmed’
water, thereby making ‘cold’ water in lieu of having to use a
mechanical process to do so. The result is energy saving from not
having to run the chillers all the time.
A third technique to produce cooling with very low
energy cost is evaporative cooling. An evaporative cooler is a
device that cools air through the evaporation of water. Evaporative
cooling differs from typical air conditioning systems which use
vapor-compression or absorption refrigeration cycles. Evaporative
cooling works by employing water's large enthalpy of vaporization,
or the amount of energy it takes to convert a substance into a gas.
The temperature of dry air can be dropped significantly through the
phase transition of liquid water to water vapor, which requires much
less energy than refrigeration. In extremely dry climates, it also
has the added benefit of conditioning the air with more moisture to
the benefit of the electronics. However, unlike refrigeration, it
requires a water source, and must continually consume water to
operate.
There are a number of approaches to evaporative
cooling however usually one is applicable to data center
applications. Traditional evaporative coolers use only a fraction of
the energy of vapor-compression or absorption air conditioning
systems. Unfortunately, except in very dry climates they increase
humidity to high levels. Two-stage, or indirect-direct evaporative
coolers do not produce humidity levels as high as that produced by
traditional single-stage evaporative coolers. In the first stage of
a two-stage cooler, warm air is pre-cooled indirectly without adding
humidity (by passing inside a heat exchanger that is cooled by
evaporation on the outside). In the direct stage, the pre-cooled air
passes through a water-soaked pad and picks up humidity as it cools.
Since the air supply is pre-cooled in the first stage, less humidity
is needed in the direct stage to reach the desired cooling
temperatures. The result is cooler air with a relative humidity
between 50 and 70 percent, depending on the climate, compared to a
traditional system that produces about 70–80 percent relative
humidity air. Since ASHRAE’s TC 9.9 standard widened the allowable
operating humidity limits indirect-direct evaporative cooling has
become more commonplace in drier climates.
In general air conditioning will account for
approximately 60% of the facility construction budget and
potentially more energy consumption than the load itself.
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The Role of Consultants in the Process
Sufficient Planning Prior to Detailed Engineering and/or Construction
Given the variety of approaches to architect a data
center cooling solution, each with its pluses and minuses, it is
imperative to develop an overall game plan prior to detailed
engineering design or construction. Qualified Data Center design
consultants:
-
leverage
proven processes to ascertain client near- and long-term
requirements for data center power, space, and cooling requirements,
-
utilized
design tools such as measurement solutions, monitoring tools, and
Computational Fluid Dynamics (CFD) design software to accurately
depict existing and new load requirements; and,
-
call upon
experiential designs with similar requirements and scenarios to help clients design a cooling approach which will support client IT loads requirements.
CFD Tools to Analyze Steady-State and Failure Conditions in
Comparing Designs
Consultants utilize powerful 3-D CFD software for the design, operational
analysis, and maintenance for data center and computer rooms of all
types and sizes. The intent is to provide better designs and
management of mission critical facilities throughout their lifetime.
The goal is to minimize the risk of cooling problems as well as
increase the overall manageability and performance of the site.
CFD modeling gives engineers the luxury to consider
several design options in the minimum amount of time. As a result,
the final design is not based on a tentative approach, but is a
result of a professional design process considering several options
and selecting the optimum solution. This can save on capital and
operational costs as well as save time by avoiding mistakes and
during commissioning.
A typical CFD modeling process includes:
-
Evaluation of
the Architectural Shell and Room Type (I.e. raised floor, ducted
supply)
-
Maps
Obstructions
-
Raised Floor
& Suspended Ceiling Layouts
-
Computer Room
Air Conditioner (CRAC) Methodology
-
Development
of CFD Model(s)
-
Model
Analysis & Update
-
Site
Engineering & Construction Documents
Analysis of CAPEX/OPEX Ramifications of Cooling Design Decisions
Using experiential-based modeling, experienced Data
Center Design Consultants provide clients with an analysis of
CAPEX/OPEX ramifications related to a new data center cooling
design. By looking at data centers with similar size, IT
infrastructure, and potential cooling designs, consultants add
valuable experience in terms of the real costs associated
with building a data center cooling system.
In addition, consultants can build operating cost
models to assist in final decision making regarding design
approaches. These models are then validated against real data center
case scenarios to confirm accuracy.
Summary
To select the optimal cooling system for a data
center there is no panacea. Each approach to data center cooling
design (I.e. Hot Aisle Containment vs. Cold Aisle Containment or
Room vs. Row vs. Rack cooling approaches) has its merits.
Ultimately, key design criteria for a specific facility and IT
environment as well as pressures related to CAPEX versus OPEX
budgets drive the proper approach for a given company.
By selecting and leveraging experienced data center
design consultants, a company can avoid costly mistakes and, worse,
potential and expensive design changes down the road. By leveraging
proven design approaches, CFD design tools, and significant prior
experiences, data center cooling design consultants should be
considered as an integral part of the data center design team long
before orders are placed, floors are installed, and cooling systems
provisioned.
To learn more about PTS recommended Air Conditioning Equipment
& Systems, contact us or visit:
To learn more about PTS consulting services to support Air
Conditioning Equipment & Systems deployments and support, contact us or visit:
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