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Paper Series – Continuos Cooling

Accredited Tier Designer
Technical Paper Series:
Continuous Cooling
October 2017
Continuous Cooling | October 2017
This technical paper clarifies the requirements for Continuous Cooling in the context of Uptime
Institute’s Tier Standard: Topology. Tier IV is the only Tier that requires Continuous Cooling.
However, Uptime Institute recommends Continuous Cooling at densities beyond 4 kilowatts (kW) per
rack, regardless of the Tier level.
The Role of Continuous Cooling
As the power densities in data centers continue to increase, the need for Continuous Cooling becomes more profound. The
risk of a loss of cooling during an uninterruptible power supply (UPS) ride-through event and the associated impact can be
catastrophic to a business. IT equipment may fail or be damaged.
Depending on the cooling or UPS technology deployed in a facility, the requirements for Continuous Cooling can widely
differ. This paper clarifies the definition of Continuous Cooling and details deployment considerations for various cooling
Continuous Cooling is defined as the ability to provide a stable thermal environment for the critical IT equipment without
any interruption. Continuous Cooling requires a consistent server inlet temperature for the amount of time it takes for the
mechanical system to restart after any interruption of power to the cooling systems (including the time for a transfer to
engine generators, if applicable). It also requires adequate maintenance of the as-designed Cold Aisle temperature.
Maintaining a stable thermal environment helps mitigate the chances of sudden increases to the UPS output load.
Increases in load may occur due to increased server fan power consumption to help make up for a temporary loss of
cooling. If the load is not well managed, the increase could overload the UPS, impacting the operation of the entire facility.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), in concert with major IT
equipment manufacturers, established Thermal Guidelines for Data Processing Environments that include recommended
computer equipment inlet air temperatures necessary to enable reliable operation of servers, storage equipment, and
network devices. As of 2015, the ASHRAE Guideline (accepted worldwide) recommends that the device inlet be maintained
between dry bulb temperatures of 18-27°C (66-81°F), with dew point temperatures of -9°C to 15°C (-15-59°F) and relative
humidity below 60% to meet the manufacturer’s established criteria. It should be noted that the requirements of Continuous
Cooling are independent of the ASHRAE Guideline for actual server inlet temperature. Compliance with the ASHRAE
Guideline for server inlet temperature is an owner decision based on an organization’s individual needs.
However, Uptime Institute Continuous Cooling requirements are tied to the ASHRAE Guideline for allowable rate of change
as it relates to the definition of a stable thermal environment. ASHRAE Guidelines state the maximum allowable change in
inlet temperature for IT equipment. For a data center with tape storage (specifically) the maximum allowable temperature
change for typical IT equipment is limited to 5°C in an hour. All other IT equipment is limited to maximum allowable
temperatures of up to 20°C in an hour. For a data center with any kind of devices, ASHRAE limits this rate of change to 5°C
within any 15-minute period. It is important to note that this is not a rate of change, but rather a discrete temperature
increase/decrease limitation.
Continuous Cooling | October 2017
These parameters have many impacts in application. As an example:
Consider a data center with Cold Aisle containment where chilled water computer
room air handling (CRAH) units operate on supply air temperature control
methodologies and with a normal set point of 20°C. If the control deadband is set
at 1°C, this means that during normal operations the unit would provide an IT
equipment inlet temperature of 19-21°C, assuming no air mixing between the
CRAH unit and the IT equipment.
If a fault or loss of utility power occurs while the unit is supplying 19°C air to the
critical environment, and the fault causes the CRAH unit to increase its
temperature to 24°C at its peak, then the inlet temperature cannot, within any
15-minute period, deviate between 19-24°C. This means that if the unit recovers
within a 15-minute period, the unit cannot overcool to the point where the supply
air temperature drops below 19°C.
Maintaining this temperature range necessitates close attention to the control
algorithms used to direct the cooling of the CRAH units. Using the same example, if
a fault were to instead cause a peak supply air temperature of 22°C before
recovery, the recovery could cool down to 17°C in order to stay within the 5°C
deadband in any 15-minute period.
A Continuous Cooling solution
must be capable of providing
the stable thermal
environment for the entire
amount of time it takes for the
mechanical cooling system to
Consideration must also be given to the time required to restore mechanical
cooling. While ASHRAE uses 15-minute periods to define the maximum allowed
temperature changes, the Tier Standard: Topology additionally requires that a
Continuous Cooling solution is capable of providing the stable thermal
environment for the entire amount of time it takes for the mechanical cooling
system to restart following any interruption of the cooling production or the loss of
utility power. The mechanical system restart time is measured from the moment of
the loss of utility up until the engine generators (or other on-site power production
systems) start and close into the critical load, when the mechanical system has
power restored, is in operation, and is providing rated cooling in a steady state
operating condition. For example, a chiller is not officially operational until it is
restarted and running with normal supply and return water conditions and flow
Although manufacturers are reducing equipment restart times, the interval between
the loss of power and a resumption of the system’s ability to produce stable
cooling needs to be incorporated as a data point in determining the ride-through
time. For example, if it takes 10 minutes to resume stable mechanical cooling after
a loss of power, then the thermal energy storage (TES) must be able to provide 10
minutes of chilled water storage.
Although Tier IV is the only Tier level that requires Continuous Cooling, data centers
with higher than average IT load densities should consider Continuous Cooling to
mitigate large temperature increases due to a loss of utility power or component
failure. As a point of reference, Uptime Institute conducted a demonstration on a
6-kW/rack average computer room. Intake air temperatures in the computer room
exceeded the top value in this range within 60 seconds after a loss of cooling or
even after just loss of air movement. Additionally, this demonstration showed that
a 1-minute loss of cooling required 20 minutes to recover.
Continuous Cooling | October 2017
Continuous Cooling provides
the bridge to enable the
thermal environment to
remain stable until the
mechanical or other cooling
system resumes.
Consider the scenario of a public utility failure during which the UPS continues to
power the IT devices, but mechanical plant operation is interrupted. Depending on
the technology of the cooling deployed, this interruption may continue for several
minutes. During this time, any elevation in temperatures in the computer room may
damage IT equipment. Continuous Cooling provides the bridge to enable the
thermal environment to remain stable until the mechanical or other cooling system
resumes. A properly designed Continuous Cooling solution will prevent any
increase in the average server inlet air temperature.
Application of Continuous Cooling
Tier IV is the only Tier that requires Continuous Cooling. The following are a few
examples of ways to achieve Continuous Cooling. The Tier Standards are not
prescriptive but provide outcome- and performance-based standards, thus other
methods may exist to accomplish the goal of Continuous Cooling beyond those
described below.
Example 1:
Continuous Cooling for a chilled water system is generally accomplished with TES
capability (also known as chilled water storage). Secondary pumps and CRAHs are
required to be on Fault Tolerant UPS power sources as well. The power source can
be the IT UPS or a separate, Concurrently Maintainable and Fault Tolerant UPS
system that is dedicated to the mechanical systems. If the cooling system is in a
primary-direct configuration, then the primary pumps are required to be on a UPS.
Additionally, consideration must be given to the thermal storage tanks and how
they are connected to the chilled water distribution. Depending on the tanks’
connection to the chilled water distribution system, supply and return (hence
warmer) chilled water might be mixed, thereby potentially decreasing the amount
of time that a chilled water storage tank can support a stable thermal environment
for the IT equipment.
Example 2:
Continuous Cooling for direct expansion (DX) systems requires both the computer
room air conditioners (CRACs) and the external condensers to be on a
Concurrently Maintainable and Fault Tolerant UPS system. Additionally, DX
systems may require some additional engineering analysis to meet Continuous
Cooling requirements. Compressor cycling during the normal operation of CRACs
can cause the compressors to pause operation for several minutes for
compressor protection. In higher than average density environments, these gaps
may cause large temperature swings during both normal operations and during
faults and loss of utility power, making the stable thermal environment more
difficult to achieve.
Continuous Cooling | October 2017
Example 3:
Continuous Cooling for evaporative systems such as direct evaporative or indirect
evaporative heat exchangers, requires the water pumps (secondary or system
circulating) and the distribution fans to be on Fault Tolerant UPS power sources.
Example 4:
Continuous Cooling for 100% outside air systems that can provide cooling
throughout the year require the fans (or the system that delivers the air to the
computer room) to be on Fault Tolerant UPS power sources.
When rotary UPS systems are deployed as the IT UPS, and the cooling systems are
on a no-break bus, additional Continuous Cooling measures may not be needed
because the mechanical system would not experience an interruption during a
transfer from utility to engine-generator (or other on-site) power. If there is a chilled
water system deployed in tandem with a rotary UPS with no batteries, the site must
demonstrate that the average server inlet temperature will not increase beyond the
limits stated previously if there is no TES installed. Each specific case should be
reviewed to ensure the requirement for a stable thermal environment will be met
during a loss of power event.
Continuous Cooling is a
justifiable safeguard for any
facility with average densities
above 4 kW in light of the
potential damage to facilities
and IT assets.
By providing thermal stability to the IT environment during any interruption in the
cooling system, such as the transition from utility outage to engine-generator
power, Continuous Cooling ensures that a utility event does not result in costly heat
damage to IT hardware or critical equipment, nor increase the UPS output load to
the point of overload. Continuous Cooling is a requirement only for Tier IV
Certification, but is a justifiable safeguard for any facility with average densities
above 4 kW in light of the potential damage to facilities and IT assets.
Continuous Cooling | October 2017
ATD Technical Paper Series: Continuous Cooling, Version B. All updates
specific to this version are effective October 2017.
Related Publications
Tier Standard: Topology
Accredited Tier Designer Technical Paper Series
Further information can be found at www.uptimeinstitute.com.
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