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Precision Cooling- Frequently Asked Questions
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The following presents a brief overview of the characteristics
of precision cooling systems. It is not intended to be an
exhaustive dissertation. Most of your questions can be answered
with the information found on this web site. However, please
feel free to call us for any information you may be seeking.
We look forward to serving you.
Liebert has published several white papers relating to environmental
systems. You may access those documents from the Precision
Cooling product pages.
Cooling System Considerations
- What
is the difference between creature comfort (people) cooling
and cooling critical electronics?
- How
are these considerations manifested in the design of precision
(electronic) cooling systems?
- What
problems would likely be encountered if a comfort-cooling
unit was used for electronic cooling?
-
What
are some of the configurations of precision cooling systems?
-
What
is a "Glycool" system?
-
How
do I determine what type of system is best for my application?
Facility Design Considerations
-
What are some of the considerations that should be made
when designing an electronic facility requiring precision
cooling?
- Electronics
are commonly installed in rooms with raised floors. What
issues need to be addressed when installing precision air
conditioning in this type of facility?
- What
is the status of R-22 phase-out?
What is a BTU?
The term BTU (British Thermal Unit) is a measurement of
a quantity of heat. Specifically it is the amount of heat
required to raise the temperature of 1 lb. of water 1
°F.
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What are some of the most common
conversion factors used in cooling and heating engineering?
°F = (°C x 9/5) + 32
°C = (°F – 32)(5/9)
1 ft³ = 1728 in³
1 U.S. gal = 231 in³ = 0.1337 ft³
1 psi = 2.309 ft of water (pressure)
1 BTU = 778.17 ft lb
1 therm = 100,000 BTU
1 kw = 738 ft lb/sec = 1.341 hp = 3412.14 BTUH = 0.284
ton (refrigeration)
1 hp = 33,000 ft lb/min = 0.746 kw = 2545.1 BTUH
1 ton (refrigeration) = 12,000 BTUH = 3.517 kw
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What is the difference between
creature comfort (people) cooling and cooling critical
electronics?
People produce both heat and moisture (humidity). Electronics
produce heat and no moisture. People have a broad temperature
and humidity tolerance range. Electronics require tight
temperature and humidity tolerances to control static
electricity and moisture condensation.
The duty cycle for cooling people is typically only a
few hours of the day during the hottest months of the
year. Electronics must usually be cooled 7 x 24 x 365
(even when outside air temperatures may be subzero). Filtration
requirements for electronics is much more stringent than
required for people. Greater airflow is used in an electronic
facility to minimize hot spots; 1 or more air changes
per minute is typical for electronics and 3 to 4 air changes
per hour for comfort cooling.
Heat densities are much higher in an electronic facility
(1 ton of cooling for every 10 – 60 ft²
of space) than for a space occupied by people (1 ton of
cooling for every 200 – 400 ft² of space).
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How are these considerations
manifested in the design of precision (electronic) cooling
systems?
Precision cooling systems tend to be highly integrated,
self-contained, modularized units for cooling one room
or a small portion of a larger facility. They are relatively
easy to install. The maximum possible amount of work content
is performed at the factory to assure the highest possible
quality of installation. Given that electronic generated
heat is dry (all sensible heat), these cooling systems
were designed to have very high sensible heat ratios (sensible
cooling capacity/total cooling capacity). The result is
a highly efficient system for this application. Since
people emit moisture, comfort-cooling systems are designed
to provide both sensible and latent cooling and are efficient
for that requirement. Precision cooling systems typically
include a humidifier whereas comfort-cooling systems do
not. Because of the much more stringent duty cycle imposed
on them and the criticality of their mission, precision
cooling systems are designed to be far more robust and
reliable. Many such units incorporate dual refrigeration
systems and might make use of dual cooling sources. To
provide tight tolerance control over temperature and humidity,
precision systems commonly use advanced state of the art
microprocessor based controls which have the ability to
interface with Network Manage Systems and/or Building
Management Systems to allow remote alarming, monitoring
and control. Typically a simple thermostat controls comfort-cooling
systems. Larger fans are used in the precision models
to obtain the desired airflow. Further, filter efficiencies
of 30 to 60% are common.
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What problems would likely be
encountered if a comfort-cooling unit was used for electronic
cooling?
Since a comfort-cooling system has a low sensible heat
ratio it would be necessary to sufficiently oversize the
unit to provide the required sensible capacity. In addition
to higher initial cost and the waste of energy, this would
likely lead to over-dehumidification of the space. Temperature
would be hard, if not impossible, to control within the
desired range and there would be no control of humidity
unless a separate, stand-alone humidifier was installed.
Filtering would probably be inadequate and the ability
to monitor and control the system remotely is doubtful.
It is unlikely that system reliability and life would
be acceptable. Smaller evaporator fans would produce fewer
air changes and hot spots in the critical space could
be expected.
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What are some of the configurations
of precision cooling systems?
A complete description for each of the systems is available
in the Precision Cooling pages by drilling down into the
Guide Specifications, Technical Manuals, Installation
Manuals and Operation and Maintenance Manuals. However,
as an overview systems are classified by size (cooling
capacity), method of heat rejection (air cooled, water
cooled, glycol cooled, "Glycool" cooled or chilled
water) and mounting location (floor, wall or ceiling).
In an air-cooled system the refrigerant is directed through
a condenser (normally outdoors) where it transfers heat
to the environment. In a water-cooled system the heat
is removed from the refrigerant in a condenser (heat exchanger
normally within the indoor unit) by water. Typically the
water carries the heat to a cooling tower (outside) where
it is rejected to the atmosphere. However, in a few applications
water passes through the condenser once and is directed
down a drain. A glycol-cooled system is similar to the
water-cooled system except that a water/glycol solution
carries the heat from the indoor condenser to a drycooler
(closed system cooling coil) outside where the heat is
rejected.
Some systems have the ability to use two different sources
for cooling (commonly air-cooled refrigeration system
for primary cooling and chilled water for backup cooling).
Many options are available for each model to meet the
specific needs of the client.
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What is a "Glycool"
system?
A "Glycool" system (sometimes called a "free
cooling" system) is a glycol-cooled system that is
modified to incorporate an additional cooling coil (commonly
referred to as an econ-o-coil) upstream of the evaporator
coil, a three-way valve and additional controls including
a comparator circuit. When the temperature of the glycol
returning from the outdoor drycooler is less than the
return air temperature in the space being cooled the three-way
valve begins modulating the glycol through the econ-o-coil,
thereby cooling the return air directly from the glycol.
As the outside air temperature drops further, with a corresponding
reduction in glycol temperature, more of the cooling load
is carried directly by the glycol and less by the DX (refrigeration)
system. The system is designed so that, when the returning
glycol temperature is 45°F (corresponding to an outside
air temperature of about 35°F), or less, the entire cooling
load is carried by the glycol and the DX system is shut
off saving electrical energy and wear and tear on the
compressors.
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How do I determine what type of
system is best for my application?
Many factors enter into this decision. Some relate to the
configuration of the facility. Others depend on such things
as the characteristics of the heat load, local code requirements,
operating costs, the type of environment the system will
be operating in and, certainly, installation costs. Following
are some examples of these considerations:
How much space is available for the cooling system? If floor
space is limited but there is ceiling space, as much as
10 tons of cooling in a single system can be installed above
a dropped ceiling. Small systems (up to 3 tons) that can
be wall mounted are available. External wall mounted systems
to 5 tons are available for structures in which there is
little or no floor space, such as telecommunications shelters.
Larger systems (up to 60 tons) will be floor mounted.
What is the characteristic of the load? Are there hot spots
(areas of high heat density) that need to be cooled? Many
of the systems allow ducting of the supply air directly
to the exact point of need. Downflow systems deliver the
supply air under a raised floor where it is distributed
to the heat source through perforated tile. Otherwise, supply
air is commonly discharged through plenum grilles.
What type of heat rejection option should be used? Generally,
an air-cooled system has the least up-front cost. However,
installation cost will be more than that for a water-cooled
or glycol-cooled system since the refrigeration lines that
are run between the indoor unit and outside condenser must
include proper slopes and traps. These constraints do not
apply to water or glycol lines. Standard outside condensers
used in air-cooled systems can operate at ambient temperatures
of -20°F. The optional Lee-Temp configuration increases
this range to -30°F. Be careful to check with local codes.
To minimize energy consumption some jurisdictions require
the use of cooling systems that incorporate air or water
economizers. Introducing large amounts of outside air prohibits
the control of humidity in the critical space, which is
unacceptable. A good solution is to use a system that includes
a "free" cooling coil ("Glycool").
If the building has an existing cooling tower with enough
capacity to allow the addition of the precision cooling
system on the loop, then a water-cooled unit may be a good
alternative. Similarly, if the building has an existing
chiller with the capacity to support the precision cooling
load, a chilled water unit would be an excellent choice.
A glycol-cooled system provides ease of installation (no
special routing considerations for the glycol piping other
than not placing it over electronic equipment) and good
performance in cold climates. If a glycol cooled system
is chosen, upgrading to a "Glycool" configuration
should be considered. It is not uncommon for the added capital
cost to be recovered, by energy savings alone, in 6 to 18
months, depending on climatic conditions.
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What are some of the considerations
that should be made when designing an electronic facility
requiring precision cooling?
Location of the space within the building is an important
consideration. Locating it within the core of the building
provides isolation from seasonal environmental load influences.
The space should not be adjacent to any mechanical room
or unconditioned area to prevent thermal impact on the space.
When calculating the cooling load it is important to consider
all of the load factors, not just the electronic equipment
heat rejection. These factors include heat from the adjacent
areas, including from above and below; heat load from windows
if on an outside wall (considering direction of exposure);
heat from people regularly in the room and, importantly,
heat from lighting (usually in the range of 3 watts/ft²).
To maintain the desired humidity in the controlled space
and avoid costly humidifier run times and dehumidification
cycles it is imperative to minimize (if not eliminate) the
incursion of outside air. One of the key factors in doing
this is to seal the room with a vapor barrier. For example,
plastic sheets placed between sheetrock in the walls in
new construction provides an excellent barrier. A rubber-
or plastic-based paint can be used on concrete walls and
floors. Doors should not be undercut or have grilles. A
proper vapor barrier can reduce moisture migration by as
much as 80%.
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Electronics are commonly installed
in rooms with raised floors. What issues need to be addressed
when installing precision air conditioning in this type
of facility?
Raised floors provide a great and flexible alternative for
routing cables and piping as well as distributing cooling
air. For this type of application a downflow cooling system
delivers the cold air to the space under the floor where
it is directed to the desired location either through vents
or, more commonly, perforated tiles. The space under the
floor is, in essence, a supply air plenum. Raised floor
heights are commonly in the 12 to 18 inch range. However,
they may be as low as 6 inches or as high as 24 inches.
Cooling systems are heavy. Therefore, floorstands fabricated
to the height of the raised floor are normally, but not
always, used to provide structural support. Obviously, the
strength of floor must be evaluated when making this decision.
Using a floorstand also allows the cooling unit to be installed,
piped, wired and inspected prior to the installation of
the raised floor to allow easier access. A floorstand also
provides vibration isolation while eliminating the need
for cutting special floor panel openings under the unit.
Floorstands can be manufactured to meet local seismic requirements.
It is important when installing the system that the floorstand
be bolted to the subfloor and the cooling unit bolted to
the floor stand. Otherwise there would be no restraint in
a seismic event. If the height of the raised floor is less
than 12 inches a turning vane should be ordered with the
floorstand and installed to assure proper air distribution.
For underfloor air distribution, the units (if more than
one) should not be placed too close together or in a long,
narrow space or the effectiveness of the air distribution
will be reduced. Air supply grilles or perforated panels
should be selected to minimize circuit pressure loss. Air
volume dampers on grilles are usually detrimental to airflow.
Care should be taken when laying out the piping, wiring,
etc. under the floor to avoid blocking the free flow of
cooling air. Wherever possible all piping should be run
parallel to the airflow.
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What is the status of R-22 phase-out?
R-22 has been the refrigerant of choice used by most cooling
system manufacturers for decades. Because it is mildly toxic
to the atmosphere it was included in the provisions of the
Clean Air Act Amendments of 1990. This Act stipulated phase-out
dates for various refrigerants, including HCFC-22 (a Class
II substance). Essentially it says that no new products
will be built containing R-22 after January 1, 2010 and
no R-22 will be produced after January 1, 2020. Systems
operating with R-22 will be able to continue using that
refrigerant after the 2020 date. However, with the cessation
of R-22 production replacement refrigerant will become more
difficult to obtain. Equipment manufacturers will undoubtedly
develop products using new, acceptable refrigerants prior
to the cutoff date in 2010. In fact, Liebert is beginning
to sell products using R-407C refrigerant (although those
same products can be ordered with R-22 until the phase-out
date). R-407C was designed to have operating characteristics
similar to R-22.
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