|
|
| 
Precision Cooling- Frequently Asked Questions
|
| 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.
Top of page
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
Top of page
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).
Top of page
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.
Top of page
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.
Top of page
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.
Top of page
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.
Top of page
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.
Top of page
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%.
Top of page
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.
Top of page
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.
Top of page
|
|
|
|
