U.S. patent application number 11/677202 was filed with the patent office on 2007-08-23 for testing for leaks in a two-phase liquid cooling system.
Invention is credited to Paul A. Knight, John R. Mason.
Application Number | 20070193285 11/677202 |
Document ID | / |
Family ID | 38426774 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070193285 |
Kind Code |
A1 |
Knight; Paul A. ; et
al. |
August 23, 2007 |
Testing for Leaks in a Two-Phase Liquid Cooling System
Abstract
A two-phase liquid cooling system includes an active venting
system for regulating an amount of non-condensable gas within the
cooling system. Various venting structures may be used to remove
gases from the cooling system, some of which are designed to remove
the non-condensable gases and avoid removing the vapor-phase
coolant. A control system activates the venting system to achieve a
desired pressure, which may be based on measured process conditions
within the cooling system. A venting and refilling system may serve
multiple cooling systems in a parallel arrangement. A return path
of the cooling system can be tested for coolant leaks by increasing
the pressure in the return path and placing a coolant detector near
the path.
Inventors: |
Knight; Paul A.; (Spokane,
WA) ; Mason; John R.; (Coeur d'Alene, ID) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER, 801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
38426774 |
Appl. No.: |
11/677202 |
Filed: |
February 21, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60775496 |
Feb 21, 2006 |
|
|
|
Current U.S.
Class: |
62/126 ; 62/149;
62/475 |
Current CPC
Class: |
G01M 3/228 20130101;
F25B 43/04 20130101; F25B 49/005 20130101; F25B 2500/222
20130101 |
Class at
Publication: |
62/126 ; 62/149;
62/475 |
International
Class: |
F25B 49/00 20060101
F25B049/00; F25B 45/00 20060101 F25B045/00; F25B 43/04 20060101
F25B043/04 |
Claims
1. A method for detecting leaks in a two-phase liquid cooling
system, the method comprising: operating a two-phase cooling system
in a normal mode, wherein a coolant is directed via a supply path
to one or more cooling modules and collected via a return path in a
thermal management unit for cooling and redeployment to the cooling
modules, the return path having a pressure below atmospheric
pressure; changing to a diagnostic mode, wherein the pressure of
the return path is raised to a pressure at or above atmospheric
pressure; in the diagnostic mode, using a coolant sensor to locate
any leaks in the return path; and returning the cooling system to
the normal mode, wherein the pressure of the return path is below
atmospheric pressure.
2. The method of claim 1, wherein the pressure of the return path
is increased by increasing the temperature of the coolant from the
thermal management unit to the supply path.
3. The method of claim 2, wherein the temperature of the coolant
from the thermal management unit to the supply path is increased by
restricting fluid supplied to a heat exchanger in the thermal
management unit that is configured to cool the coolant therein.
4. A method for detecting leaks in a two-phase liquid cooling
system, the method comprising: circulating a coolant through a
two-phase cooling system, wherein at least a portion of the cooling
system has a low-pressure path through which coolant is directed at
a pressure below atmospheric pressure; a step for temporarily
increasing the pressure of the low-pressure path of the cooling
system above atmospheric pressure; and testing for leaks in the
low-pressure path by checking for coolant leaving the low-pressure
path.
5. The method of claim 4, further comprising: returning the
low-pressure path to a pressure below atmospheric pressure.
6. A system for detecting leaks in a two-phase liquid cooling
system, the system comprising: a two-phase cooling system
configured to direct a coolant via a supply path to one or more
cooling modules and collect the coolant via a return path in a
thermal management unit configured to cool and redeploy the coolant
to the cooling modules, the return path having a pressure below
atmospheric pressure during normal operation of the cooling system;
a control unit coupled to the cooling system, the control unit
configured to increase the pressure of the return path to a
pressure at or above atmospheric pressure for a diagnostic mode,
the control unit further configured to return the cooling system to
normal operation; and a sensor configured to detect any coolant
leaking from the return path of the cooling system during the
diagnostic mode.
7. The system of claim 6, wherein the control unit is configured to
increase the pressure of the return path by increasing the
temperature of the coolant from the thermal management unit to the
supply path.
8. The system of claim 7, wherein the thermal management unit
comprises a heat exchanger configured to cool coolant received from
the return path, and wherein the control unit is configured to
increase the temperature of the coolant from the thermal management
unit to the supply path by restricting fluid supplied to a heat
exchanger to decrease the cooling of the coolant by the heat
exchanger.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/775,496, filed Feb. 21, 2006, which is
incorporated by reference in its entirety. This application is also
related to U.S. application Ser. No. 11/384,195, filed Mar. 17,
2006, and to U.S. application Ser. No. 11/466,076, filed Aug. 21,
2006, each of which is incorporated by reference in its
entirety.
BACKGROUND
[0002] This invention relates generally to two-phase liquid cooling
systems, such as those configured to cool rack-mounted electronics,
and more particularly to detecting coolant leaks in two-phase
liquid cooling systems.
[0003] Liquid cooling is well known in the art of cooling
electronics. As air cooling heat sinks continue to be pushed to new
performance levels, so has their cost, complexity, and weight.
Because computer power consumptions will continue to increase,
liquid cooling systems will provide significant advantages to
computer manufacturers and electronic system providers.
[0004] Liquid cooling technologies use a cooling fluid for removing
heat from an electronic component. Liquids can hold more heat and
transfer heat at a rate many times that of air. Single-phase liquid
cooling systems place a liquid in thermal contact with the
component to be cooled. With these systems, the cooling fluid
absorbs heat as sensible energy. Other liquid cooling systems, such
as spray cooling, are two-phase processes. In the two-phase cooling
systems, heat is absorbed by the cooling fluid primarily through
latent energy gains. Two-phase cooling, commonly referred to as
evaporative cooling, allows for more efficient, more compact, and
higher performing liquid cooling systems than systems based on
single-phase cooling.
[0005] An example two-phase cooling method is spray cooling. Spray
cooling uses a pump to supply fluid to one or more nozzles, which
transform the coolant supply into droplets. These droplets impinge
the surface of the component to be cooled and can create a thin
coolant film. Energy is transferred from the surface of the
component to the thin-film of coolant. Because the fluid is
dispensed at or near its saturation point, the absorbed heat causes
the thin-film to turn to vapor. This vapor is then removed from the
component, condensed (often by means of a heat exchanger or
condenser), and returned to the pump.
[0006] Significant efforts have been expended in the development
and optimization of spray cooling. A doctorial dissertation by
Tilton entitled "Spray Cooling" (1989), available through the
University of Kentucky library system, describes how optimization
of spray cooling system parameters, such as droplet size,
distribution, and momentum can create a thin coolant film capable
of absorbing high heat fluxes. In addition to the system parameters
described by the Tilton dissertation, U.S. Pat. No. 5,220,804
provides a method of increasing a spray cooling system's ability to
remove heat. The '804 patent describes a method of managing system
vapor that further thins the coolant film, which increases
evaporation, improves convective heat transfer, and improves liquid
and vapor reclaim.
[0007] Dielectric fluids such as FLUORINERT.RTM. (a trademark of 3M
Company) are well-suited for use in electronic cooling systems, as
they are safe for electronic components and systems. The fluids
have boiling points close to atmospheric conditions and have latent
heat of vaporization values that provide efficient two-phase
cooling.
[0008] The operating parameters, such as temperature and pressure,
are important for achieving efficient and optimal cooling in a
two-phase cooling system. Leaks in the system can throw these
parameters off balance, as leaks can result in non-condensable air
added to the system or coolant loss from the system. The result of
leaks is often to degrade the thermal performance of the system.
Some leaks may be detected using a refrigerant detector moved along
a flow path of the cooling system. If the detector is located near
the vicinity of a leak in which coolant is leaking from the system,
the detector will indicate the presence of the coolant and,
therefore, a leak in the system. With many refrigerants used in
two-phase cooling systems, however, the coolant vapor in the return
side of the system is lower than atmospheric pressure. For this
reason, any leaks in the return path generally result in air
leaking into the system rather than coolant leaking out of the
system. The refrigerant detector--or any kind of external
sensor--is not helpful in such a case.
SUMMARY OF THE INVENTION
[0009] A technique is thus provided for detecting leaks in a return
path in a two-phase liquid cooling system or any other path in the
system in which, during normal operation of the system, the path is
at a pressure below atmospheric pressure. To test for leaks in such
a path, the pressure in the path is temporarily raised, causing
coolant to be expelled through any leaks in the path. A coolant
sensor can then be used to detect any leaks. Once the testing is
completed, the path is returned to normal operation for more
efficient cooling.
[0010] In one embodiment, a two-phase cooling system directs a
coolant via a supply path to one or more cooling modules and
collects the coolant via a return path in a thermal management
unit, which cools and redeploys the coolant to the cooling modules.
During normal operation of the cooling system, the return path is
at a pressure below atmospheric pressure. A control unit is coupled
to the cooling system and, when leak testing is desired, the
control unit increases the pressure of the return path to a
pressure at or above atmospheric pressure. In this diagnostic mode,
a sensor can be used to detect any coolant leaking from the return
path of the cooling system. Once the testing is completed, the
control unit can return the cooling system to its normal
operation.
[0011] These and other features, aspects, and advantages of various
embodiments of the invention will become better understood with
regard to the following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] In the course of the detailed description to follow,
reference will be made to the attached drawings. These drawings
show different aspects of embodiments of the present invention and,
where appropriate, reference numerals illustrating like structures,
components, and/or elements in different figures are labeled
similarly. It is understood that various combinations of the
structures, components, and/or elements other than those
specifically shown are contemplated and within the scope of the
present invention:
[0013] FIG. 1 is a schematic diagram of a two-phase liquid cooling
system with active venting, in accordance with an embodiment of the
invention.
[0014] FIG. 2 is a schematic diagram of a rack-mounted spray
cooling system, in accordance with an embodiment of the
invention.
[0015] FIG. 3 is a schematic diagram of a semi-permeable membrane
separator, in accordance with an embodiment of the invention.
[0016] FIG. 4 is a schematic diagram of a condensing separator, in
accordance with an embodiment of the invention.
[0017] FIG. 5 is a schematic diagram of a centrifugal separator, in
accordance with an embodiment of the invention.
[0018] FIG. 6 is a schematic diagram of a permeable tube vacuum
mechanism, in accordance with an embodiment of the invention.
[0019] FIG. 7 is a chart showing a typical saturation curve for an
example cooling liquid.
[0020] FIG. 8 is a flow diagram of a control process for activating
the active venting system to remove non-condensable gases from the
cooling system, in accordance with an embodiment of the
invention.
[0021] FIG. 9 is a schematic of a venting and refilling system for
servicing multiple liquid cooling systems, in accordance with an
embodiment of the invention.
[0022] FIG. 10 is a chart showing the process conditions during
operation of a liquid cooling system, in accordance with an
embodiment of the invention.
[0023] FIG. 11 is a flow chart of a process for detecting leaks in
the return path of a cooling system, in accordance with an
embodiment of the invention.
[0024] FIG. 12 is a system for detecting leaks in the return path
of a cooling system, in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Two-Phase Cooling System with Active Venting
[0025] FIG. 1 illustrates one embodiment of a two-phase liquid
cooling system 100 with active venting capabilities. The liquid
cooling system 100 includes at least one cooling module 105, a pump
110, a reservoir 115, and a condenser 120. The pump 110 pressurizes
a supply of liquid coolant from the reservoir 115 and delivers the
liquid coolant to the cooling module 105. The cooling module 105
places the liquid coolant in thermal contact with a heat-producing
device (not shown), such as but not limited to computer processors,
blade servers, circuit boards, memory, video cards, power devices,
and the like. In the cooling module 105, heat from the
heat-producing device transforms at least a portion of the liquid
coolant into a vapor phase fluid. The cooling fluid is transferred
to a condenser 120, which removes heat and condenses the vapor
phase fluid back into the liquid phase and delivers it to a
reservoir 115. The liquid coolant can then be recycled in the
system by the pump 110.
[0026] Although the two-phase liquid cooling system 100 is shown
with only the main components, the system 100 may include other
well known components, such as filters, heaters, manifolds,
coolers, and other components of fluid systems. In addition, the
system 100 is described as just one example of a system in which
the active venting techniques described herein can be applied. The
system 100 may be a modular cold plate type system or a global
cooling system where the cooling fluid comes directly in contact
with the electronics to be cooled. Moreover, the cooling system 100
is not limited to any particular type of two-phase liquid cooling
system. Rather, the techniques described herein can be applied to
any type of two-phase liquid cooling system, such as, but not
limited to, spray cooling, micro-channels, mini-channels, pool
boiling, immersion cooling, or jet impingement. Examples of liquid
cooling systems and their components that can be used with
embodiments of the invention are described in the following, each
of which is incorporated by reference in its entirety: U.S. Pat.
No. 6,889,515, which describes a spray cooling system; U.S. Pat.
No. 6,955,062, which describes a spray cooling system for
transverse thin-film evaporative spray cooling; and U.S. Pat. No.
5,220,804, which describes a high heat flux evaporative spray
cooling; and U.S. Pat. No. 5,880,931 which describes a spray cooled
circuit card cage.
[0027] Coupled to the cooling system 100 is an active venting
system 125 for removing gases and/or adding gases to the liquid
cooling system 100. As shown in FIG. 1, the active venting system
125 may be coupled to a volume in the system 100 where gases are
present, such as the volume above liquid coolant in the reservoir
115. In other embodiments, the venting system 125 may be coupled to
other places in the flow path of the cooling system 100, such as in
a return manifold in the path from the cooling module 105 to the
pump 110 (such as return manifold 240 shown in FIG. 2, for
example). In the embodiment shown in FIG. 1, the venting system 125
comprises an auxiliary pump 130 coupled to the volume in the
reservoir 115. The auxiliary pump 130 is further coupled to a check
valve 135, which prevents air from entering the venting system
125.
[0028] A control system 140 is coupled to the venting system 125 to
provide for selective activation of the venting of gases by the
venting system 125. Using control signals (illustrated as dotted
lines in FIG. 1), the control system 140 may control the auxiliary
pump 130, thereby causing the venting system 125 to remove and/or
add gases into or out of the cooling system 100. For other
embodiments of the venting system 125, the control system 140 is
configured to provide appropriate control signals.
[0029] In one embodiment, the non-condensable gases removed by the
active venting system 125 are released into the surrounding
environment. In some applications, however, it is undesirable to
allow the non-condensable gases to be released. To address this
need, in another embodiment, the active venting system 125 vents,
pumps, or otherwise directs the non-condensable gases removed from
the cooling system 100 into a sealed chamber 160 for storage
therein. The sealed chamber allows the cooling system 100 to be
used in very sensitive areas where the non-condensable gases cannot
be introduced.
[0030] In another embodiment, the gas storage chamber 160 houses a
condenser unit 162, which may comprise condensing fins that aid in
condensing any vapor in the chamber 160. The chamber is further
coupled to a relief valve 164. The relief valve 164 is designed to
relieve the stored or collected non-condensable gases once a
certain pressure inside the storage chamber 160 is reached. In one
embodiment, the pressure relief valve 164 comprises a spring-loaded
valve that automatically opens at a 10 psi differential between the
inside of the chamber and the atmosphere. With the chamber 160 at
room temperature, the added pressure helps to ensure that only air
escapes from the system.
[0031] The control system 140 activates the venting system 125
based on process conditions within the cooling system. In this way,
the control system 140 can achieve certain desired operating
conditions in the cooling system 1 00. Although a variety of
process conditions can be used to describe the cooling system, in
one embodiment the process conditions include the pressure and
temperature of the gases above the liquid coolant in the reservoir
115. Accordingly, a pressure transducer 145 and temperature sensor
150 (which may comprise a thermocouple, thermistor, resistance
temperature detector (RTD), thermopile, infrared sensor, or any
other suitable temperature sensor) coupled to the reservoir 115
provide readings of these process conditions. The control system
140 uses these pressure and temperature readings to determine
whether and when to activate the venting system 125. Various
embodiments of algorithms that the control system 140 can be used
to activate the venting system 125 are described in more detail
below; however, it can be appreciated that the control system 140
can receive additional types of inputs and can be programmed to
perform any number of algorithms to achieve a desired effect in the
cooling system 100. Moreover, the pressure transducer 145 and
temperature sensor 150 may be located at other parts of the system,
such as in a return manifold path 240.
[0032] In one embodiment, the pressure transducer 145 is located at
or near the top of the supply manifold 230, which is fluidly
coupled to one or more cooling modules 220 in the system. This may
be beneficial in certain embodiments because pressure drop within
the supply manifold 230 is considerable in a larger manifold, so
the greatest pressure drop may occur at that point. Locating the
pressure transducer 145 in this location, therefore, may ensure
that the correct operating pressure is maintained for proper
atomization of the coolant at the uppermost connected cooling
module 220.
[0033] Although the control system 140 is illustrated as a separate
system in FIG. 1, it can be integrated into the active venting
system 125 or any other part of the cooling system 100. Moreover,
the control system may be implemented, in whole or in part, by
hardware, software, firmware, or a combination thereof.
[0034] The active venting techniques described herein can be
implemented in various types of two-phase liquid cooling systems.
For example, FIG. 2 schematically illustrates a rack-mounted spray
cooling system in which an embodiment of the active venting
technique is employed. As shown in FIG. 2, a pump 210 directs a
coolant through a plurality of spray cooling modules 220. Each
spray cooling module 220 is located in a rack-mounted device and is
configured to cool one or more heat-producing electronic devices by
spraying the coolant liquid on the devices or on a surface
thermally coupled thereto. The resulting two-phase coolant is then
returned to a thermal management unit 250 by way of a return
manifold 240. The two-phase coolant is condensed in the return
manifold 240 and/or in the thermal management unit 250, where the
liquid coolant is stored until being recycled through the system by
the pump 210.
[0035] An active venting system 260 is coupled to the return
manifold 240, where it has access to gases in the flow path of the
cooling system. As described above, the active venting system 260
may remove gases from and/or add gases to the flow path of the
cooling system to adjust the pressure therein and thus affect the
operation of the cooling system. Rather than being coupled to the
return manifold 240, the active venting system 260 may
alternatively be fluidly coupled to a volume of gas in the thermal
management unit 250 for exchanging gases therewith. In a
rack-mounted cooling system, the active venting system 260 and the
thermal management unit 250 may also be rack-mounted devices.
[0036] One problem with the startup of a rack-mounted spray cooling
system, where the supply manifold 230 and the return manifold 240
are mounted in the rack vertically, is that air can become trapped
in the supply manifold 230 above the uppermost connection that
leads to the uppermost cooling module 220. The trapped air
undesirably increases system pressure, and because there is no
fluid flow above the uppermost connection, the non-condensable
gases must dissolve back into the coolant to be removed. It has
been shown to take several days for the non-condensable gases to be
removed fully with this configuration. After a system is shut down,
moreover, a substantial amount of non-condensable gas may collect
in the supply manifold 230, which again takes significant time to
remove.
[0037] To address this problem, in one embodiment, a bypass flow
path 270 is placed between the supply manifold 230 and the return
manifold 240 near the tops thereof. The flow path 270 allows a
small flow (e.g., around 1% of the full flow) of gas to pass from
the top of the supply manifold 230 to the return manifold 240. The
flow path 270 may comprises a tube, and a chemical filter 265 may
be installed in the flow path 270, since this provides an ideal
service location. The bypass flow path 270 with chemical filter 265
could replace a bypass filtration line that is often used within
the thermal management unit 250. In an alternative embodiment, the
bypass path 270 can be separate from the filter 265, although it is
typically desired to reduce the number of fluid joints in the
system.
Active Venting System Embodiments
[0038] As described above, many coolants used in two-phase fluid
cooling applications may absorb a significant amount of air or
other non-condensable gases. Because the non-condensable gases
remain in gas form throughout the cooling system, they impart a
partial pressure that adds to the pressure within the cooling
system. Although a slightly increased pressure may be useful to
avoid cavitation in the pumps, it can also have detrimental effects
on the cooling performance of the system by increasing the boiling
point of the coolant. Accordingly, it is often preferable to
control the amount of non-condensable gases that are present in the
cooling system. When removing gases from the system, therefore, it
is generally preferable to remove the non-condensable gases while
leaving the coolant vapor in the system. Various embodiments of the
active venting system designed to achieve this purpose are
described below.
[0039] FIG. 3 depicts a semi-permeable membrane separator
embodiment for facilitating removal of non-condensable gases by a
venting system. This embodiment is described in the context of the
cooling system of FIG. 2, but it could be employed in any other
type of cooling system. As illustrated, a semi-permeable membrane
310 may be located in a parallel configuration with the flow path
between the return manifold 240 and a return line 320 leading to a
thermal management unit 250, or with some other portion of the flow
path. The membrane 310 is designed to be permeable to the coolant
but not to the non-condensable gases that are expected to be in the
system.
[0040] During operation of the venting system, the side of the
membrane 310 that includes the coolant and non-condensable gas
mixture is increased in pressure (e.g., by a pump, not shown). In
this way, the coolant is allowed to pass through the membrane 310
and return to the thermal management unit 250, while the
non-condensable gas remains in the manifold 240 (or another volume
from which the venting system can extract gas). This increases the
concentration of the non-condensable gas versus the coolant vapor
in the manifold 240. If the venting system takes gases from the
manifold 240, the gas mixture taken by the venting system will thus
have a relatively higher concentration of non-condensable gas
versus coolant vapor than in the rest of the system. In another
embodiment, the membrane 310 can be configured in the reverse
manner (such as in the embodiment described below in connection
with FIG. 6).
[0041] FIG. 4 shows a condensing separator embodiment of an active
venting system 410. This embodiment of the venting system 410 is
designed to receive coolant vapor air mixture from the cooling
modules, e.g., by tapping into the return manifold 240 of a cooling
system such as that shown in FIG. 2. The venting system 410 could
tap into the flow path of the cooling system downstream of a
condenser or in a reservoir of a heat exchanger, but there would be
less need for the condensing function of this embodiment since the
coolant would be expected to be primarily in the liquid phase in
those areas of the cooling system.
[0042] In operation, the venting system 410 receives a mixture of
the coolant vapor and non-condensable gases from the return
manifold 240. A valve 425 may be provided on the gas input line 420
to control when the venting system can take in the gases. The input
gases are received in a chamber of the venting system 410, where a
condenser 430 reduces the temperature of the gases until the
coolant vapor condenses and collects as a liquid in the venting
system. When a control system determines that the venting system
should be activated to expel non-condensable gas from the system,
the control system activates an auxiliary pump (as shown in FIG. 1)
or other mechanism for removing some amount of the non-condensable
gas in the venting system 410 through an exit port 460. The control
system may cause the input valve 425 to close for a period of time
before activating the auxiliary pump, thereby giving the condenser
430 sufficient time to condense the coolant vapor to ensure that
most of the gas expelled is the non-condensable gas.
[0043] At various times, such as when the venting system has a
predetermined amount of liquid coolant collected (e.g., as measured
by a level sensor, not shown), a liquid return pump 440 is
activated. The liquid return pump 440 passes the condensed liquid
coolant from the venting system 410 back to the return manifold 240
by way of a liquid return line 450. A liquid return valve 455 may
be provided in the liquid return line 450 to prevent liquid coolant
from backing up into the venting system 410. In this way, the
coolant vapor from the cooling modules is condensed so that it can
be recycled through the system, rather than being vented from it.
The pump 440 may be optional, e.g., the coolant may be gravity
drained from the reservoir and reintroduced into the cooling system
as well.
[0044] FIG. 5 illustrates a centrifugal separator embodiment of an
active venting system 510, which separates the coolant vapor from
the non-condensable gases. As illustrated, the venting system 510
may tap into the return manifold 240 of a cooling system such as
that shown in FIG. 2; however, as with the condensing separator
embodiment 410, the venting system 510 could tap into other points
in the flow path of the cooling system. The active venting system
510 thus receives a mixture of coolant vapor and non-condensable
gases in a gas input line 530, which may be opened or closed using
an input valve 535. The received mixture of gases is provided to a
centrifugal vapor pump 520, which is designed to separate the
coolant vapor and non-condensable gas based on the difference in
their densities.
[0045] The centrifugal vapor pump 520 is activated by the control
system when it is determined that the venting system 510 should
remove gas from the cooling system. The centrifugal vapor pump 520
removes dissolved non-condensable gas from the coolant vapor by
passing the mixed gas stream through a series of rapidly spinning
disks. As the rotational motion is imparted to the gas stream, the
more dense gases (e.g., FLUORINERT.RTM., in a mixture of
FLUORINERT.RTM. and air) are forced to the perimeter, while the
less dense gases continue down the center of the device and exit
the centrifugal pump. The centrifugal vapor pump 520 can be
controlled by manipulating the rotation speed of the spinning disks
by an ordinary brushless DC controller, and by the flow rate of the
vacuum pump that pulls the mixed vapor through the device and vents
to the atmosphere. Alternatively, where the coolant vapor is less
dense than the non-condensable gases, the configuration may be
changed to allow the denser gases to be removed.
[0046] In the embodiment shown in FIG. 5, the venting system 510 is
designed for a cooling system in which the non-condensable gases
are less dense than the coolant vapor. The non-condensable gases
are expelled from the venting system 510 via a line 550 and through
an exhaust port 555, which preferably does not allow air to pass
into the venting system 510. The denser coolant vapor returns to
the return manifold 240 in a coolant return line 540. The coolant
return line 540 may include a valve 545 to prevent coolant from
entering the venting system 510 through the return line 540.
[0047] FIG. 6 shows another embodiment of a venting system 610 for
removing non-condensable gases from a closed-loop cooling system.
In this embodiment, at least a portion of the return path of the
cooling system is passed through a coil or bundle of semi-permeable
tubing 620, which is permeable to non-condensable gases but not
permeable to the coolant. (Although FIG. 6 shows a short length of
tubing 620 in the housing 630, having a coil or bundle of tubing
620 with a long length relative to the diameter of the tubing 620
increases the ratio of surface area to volume, thereby facilitating
removal of non-condensable gases from the system.) In one
embodiment, the coolant is FLUORINERT.RTM. and tube 620 is
impermeable to FLUORINERT.RTM. but does exhibit marked permeability
to air. The tubing 620 is located in a sealed housing 630, which is
coupled to a vacuum pump 640 by tubing 650 that is not permeable.
When the vacuum pump 640 is activated, a vacuum is applied to the
inside of the housing 630, and thus, to the outside of the
semi-permeable tubing 620. This causes the non-condensable gas to
migrate through the tubing 620, while the coolant is left inside
the tubing 620. The non-condensable gas is expelled from the
housing 630 by the vacuum pump 640 through an exhaust line 660. The
coolant, on the other hand, continues through the tubing 620 and is
returned to the cooling system to be recycled.
[0048] In one embodiment, the tubing 620 comprises a co-extrusion
having two or more layers, although the tubing 620 need not
necessarily have more than one layer. In a multilayer embodiment,
an exterior layer of the co-extruded tubing 620 may comprise ether
or ester-based polyurethane, which is appropriate due to its high
air and low PFC permeation properties. An interior layer of the
co-extruded tubing 620 may comprise polyethylene, which has
excellent fluid compatibility properties. The tubing 620 is
preferably a semi-permeable membrane. This is in contrast to the
tubing used in other parts of embodiments of the cooling system, in
which co-extruded tubing that prevents permeation and provides good
fluid compatibility while remaining flexible is used.
[0049] On one embodiment, the tubing used for some or all flexible
connections within the system is a co-extruded tubing that
comprises: [0050] an outer layer composed of an Engage 8440 with
Ampshield 1199: Ethylene Octene Co-polymer, where Ampshield is a
52% flame retardant in a low-density polyethylene carrier (0.032''
thick); [0051] a binding layer comprised of Bynel 4157: Linear low
density polyethylene (LLDPE) (0.003'' thick); [0052] a next layer
EVALCA F101: ethyl vinyl alcohol (EVOH) (0.005'' thick); [0053] a
next binding layer of Bynel 4157: Linear low density polyethylene
(LLDPE) (0.003'' thick); and [0054] an inner layer of Engage
8440:Ethylene Octene Co-polymer (0.02'' thick). The two Bynel
layers in the above construction are binding or "tie" layers. The
innermost layer is not adversely affected by fluids common to
liquid cooling of electronics. The innermost layer also remains
highly flexible at structural thicknesses and environmental
conditions typically found for rack-mounted products. The EVOH
layer is impermeable to FC-72, PF-5060, and other fluids commonly
used in the electronics cooling industry, as well as to air or
non-condensable gases. But because the EVOH layer tends to be too
stiff if implemented in greater thicknesses, it is impractical as a
flexible tubing by itself. Its presence in the co-extrusion is to
prevent cooling fluid and/or air permeation, while its minimal
thickness does little to affect flexibility. The outermost layer
adds structural integrity without adversely affecting flexibility.
In various embodiments of the system, a commercially available
version containing a flame retardant may be chosen due to its
enhanced commercial viability in the marketplace. Other
co-extrusions that implement a different layer order may be used,
as well as fewer layers or different thicknesses of the layers,
although stiffness or permeability may be sacrificed with
variations. To increase the flexibility of the tubing temporarily
(e.g., to remove stresses in an installed system), the tubing can
be heated.
[0055] Alternatively, the venting system could be designed using a
tubing that is permeable to the coolant but not to the
non-condensable gas. In such a case, the tubing could comprise
polyvinylidine fluoride (PVDF), or KYNAR.RTM., which is permeable
to FLUORINERT.RTM. but not to air. The coolant would be collected
outside of the tubing and returned to the system, while the
non-condensable gas left in the tubing would be exhausted from the
system.
[0056] Embodiments of the co-extruded tubing described herein may
be used for all fluid connections in the system where rigid tubing
is impractical, such as to connect pumps to the supply manifold,
the supply manifold to the spray modules, the spray modules to the
return manifold, and the return manifold to the condenser. The
co-extruded tubing may also connect the active venting system to
the return manifold. The selection of materials for this and any
other tubing may depend, in part, on the type of coolant used.
Operation
[0057] Controlling the pressure inside of the cooling system may be
vitally important for many applications, as demonstrated by the
saturation curve plotted in FIG. 7. The saturation curve provides
the boiling point for a particular coolant for a range of
pressures. It is often desirable to operate above the coolant's
saturation curve, since the pumps can cavitate if the pressure is
too low for a given temperature of operation. Adding air or other
non-condensable gases is one way to move above the saturation curve
to allow the pumps to operate. But with too much air the coolant
evaporates at a relatively high temperature, which causes the
two-phase cooling modules to operate at a higher temperature.
Accordingly, in one embodiment, the cooling system includes an
amount of air or other non-condensable gas in the system to balance
these competing concerns. This is illustrated by the "ideal
operating condition" curve in FIG. 7, although what is considered
ideal operating conditions may change from application to
application, so the curve in FIG. 7 is presented for illustration
purposes only.
[0058] In one embodiment, the cooling system can regulate the
amount of non-condensable gases in the cooling system using a
control algorithm implemented by the control module described
above. FIG. 8 provides one embodiment of a control algorithm for
maintaining the cooling system at or near an ideal operating curve.
In this control process, the pressure transducer 145 and
temperature sensor 150 measure 810 the pressure and temperature in
a location in the cooling system (such as a volume over a reservoir
or a point in the return path). Based on this measured pressure and
temperature, the control system calculates the ideal operating
pressure of the system for the measured temperature. If 820 the
difference between this ideal pressure and the saturation curve at
the measured temperature is above a predetermined maximum
differential (e.g., 3 psi), the control system activates 830 the
venting system to reduce the pressure in the cooling system.
Otherwise, the control system turns or keeps 850 off the venting
system, after which the pressure and temperature are measured 810
again in a subsequent interval.
[0059] In one embodiment, the control system activates 830 the
venting system according to a predetermined profile, which
specifies an amount of time on and off for the venting system. The
on period of the profile allows the system to exhaust a
non-condensable gas for a period of time, while the off period
allows the venting system to separate the coolant vapor from the
non-condensable gas. The off period also allows the system as a
whole to come into equilibrium, while other entrained
non-condensable gases are moved to the venting system so they can
be extracted. Although the particular profile used may depend on
the system parameters, in one embodiment the profile is 10 seconds
of venting followed by 3 minutes off. The venting system runs
(e.g., according to the profile) until the control system
determines 840 that the system pressure is within a predetermined
differential (e.g., 2.5 psi) of the saturation curve at the system
temperature. Once this condition is met, the control system turns
850 the venting system off, and the control cycle repeats.
[0060] In one embodiment, the control system may check the pressure
difference between the system and the saturation curve so that it
can maintain the system above a minimum differential (e.g., a 1.8
psi). This checking may occur, for example, continually during the
running of a profile for the venting system. If the cooling system
does come within the predetermined minimum differential of the
saturation curve, the control system automatically shuts the
venting system off. This helps to prevent the pumps from cavitating
due to too low of a pressure in the cooling system.
[0061] During startup of the cooling system there may be different
venting needs than during normal operation. For example, there is
typically more need for venting since there is more air that has
seeped into the cooling system. Moreover, the system can tolerate
faster venting because the system is stagnant at startup;
therefore, the vapor and air are more separated from one another.
Once fluid is pumped through the system, the air and vapor tend to
mix and extraction has to be done more slowly. Accordingly, a
startup profile may be run until the cooling system reaches a
desired point from the saturation curve, where the startup profile
has more aggressive venting than the regular profile. In one
embodiment, the startup profile runs the venting system for 55
seconds on and 5 seconds off, for up to 5 minutes or until the
cooling system reaches 5 psi above the saturation curve. As with
the regular venting profile, various other startup profiles may be
defined based on other system parameters and needs.
[0062] Rather than trying to maintain the cooling system at an
ideal operating curve, the control system can also be used to
maintain the cooling system at a given temperature. This may be
useful, for example, as a tool for the testing or burn-in of
semiconductors. Because non-condensable gases within the working
fluid of the system affect the component temperatures, adding the
gases to the system or allowing the gases to remain in the system
can raise the temperature of the components being cooled by the
system. The control system may therefore receive additional inputs,
such as the temperature of a particular component attached to the
cooling system. By adjusting the gases within the cooling system,
the control system can maintain these inputs at desired values.
Venting and Refilling of Multiple Cooling Systems
[0063] In other embodiments, the automated venting of two-phase
liquid cooling systems can be applied by a central system coupled
to a plurality of cooling systems in a parallel arrangement. The
cooling systems may be rack-mounted cooling systems, where the
overall system extends the automated venting techniques described
herein to the multi-rack level. In addition to automated venting of
multiple cooling systems, embodiments of the invention can also
provide for the automated refill of multiple cooling systems from a
central reservoir.
[0064] FIG. 9 illustrates one embodiment of a centralized system
for providing venting and refilling for multiple two-phase liquid
cooling systems 910. Each cooling system 910 may comprise a
plurality of two-phase liquid cooling modules, such as those
described above. The plurality of cooling modules may further be
configured within electronic devices arranged in a rack-mounted
system, as found in computer server environments, or they may be
any other type of two-phase liquid cooling systems for which
venting and/or refilling are desired. The components of the
centralized venting and refilling system are coupled to the cooling
systems 910 in a parallel arrangement. This arrangement allows the
various liquid cooling systems 910 to be vented and/or refilled
concurrently, and it also allows a subset of the cooling systems
910 to be serviced at any one time by closing the corresponding
fluid path, as described below.
[0065] In the embodiment shown, the centralized venting and
refilling system comprises a compressor 930, a separation column
940, a reservoir 950, an exhaust valve 970, and a fill valve 980.
The compressor 930 of the centralized system is coupled to an
exhaust path from each of the liquid cooling systems 910. The
exhaust path for a particular cooling system 910 may be coupled
from a simple vent from a condenser in the cooling system 910, or
it may be coupled to any other vent or port designed to exhaust
gases from the cooling system 910. A vent valve 920 in each exhaust
path separates the compressor 930 from the corresponding cooling
system 910. Each vent valve 920 can be opened and closed to control
when gases are allowed to vent from the corresponding cooling
system 910.
[0066] The centralized venting and refilling system may also
include return paths that couple the system to an input port of
each liquid cooling system 910. These return paths allow selective
refilling of each of the cooling systems 910 with the coolant
liquid from the reservoir 950. A refill valve 960 couples the input
port of each cooling system 910 to the reservoir 950, thereby
allowing control of when and how much each cooling system 910 is
refilled. In one embodiment, the reservoir 950 is held at a higher
pressure than each cooling system 910, so the coolant liquid in the
reservoir 950 naturally flows into each cooling system 910 when the
corresponding refill valves 960 are opened. Alternatively, a pump
or other means may be used to cause the coolant liquid to flow from
the reservoir 950 into the cooling systems 910 when desired. In one
embodiment, the vent valves 920 and/or refill valves 960 may be
automatically controllable valves, such as solenoid valves, which
facilitate control of the valves' state from the centralized,
automated venting and refill system.
[0067] FIG. 10 illustrates the temperature and pressure conditions
in the venting and refilling systems during operation of the system
in accordance with one embodiment. As with certain embodiments
described above, the operation of this system is described using
the coolant liquid PF-5060; however, it can be appreciated that
embodiments of the invention can be practiced with a variety of
other coolant fluids, also as described above. In the example
shown, the desired operating point of an individual cooling system
910 (e.g., one rack in a multi-rack system) is identified on the PF
5060 saturation curve as point 1. It is often desirable to operate
the cooling system 910 at a sub-atmospheric pressure to provide
lower possible CPU temperatures. Operating at sub-atmospheric
pressures also tends to cause any leakage that occurs in the system
to be air ingress only, thereby avoiding coolant vapor escape into
the atmosphere.
[0068] During normal operation of the cooling systems 910, air will
typically leak into the system. This may occur due to any servicing
operation (e.g., when the servers containing the individual cooling
modules are attached to the cooling system's manifold), during
servicing of the thermal management units of each cooling system
910, or from permeation or minor leaks through seals and joints in
the gas lines. As air leaks into each cooling system 910, the
pressure climbs towards point 2 on the PF-5060 saturation curve. As
the saturation pressure in the cooling module increases with the
overall system pressure, the temperature of the cooling module--and
hence, of a CPU or other device being cooled--likewise
increases.
[0069] When the automated venting system determines that the
pressure or temperature has reached a predetermined maximum (e.g.,
at point 2 on the chart of FIG. 10), the vent valves 920 for the
cooling systems 910 are opened. The system may open all of the vent
valves 920 at this time, or it may open only a subset of the vent
valves 920 corresponding to the cooling systems 910 that are
determined to need venting. This latter embodiment may be useful
when the cooling systems 910 are monitored individually, and the
system may determine that some, but not all, of the cooling systems
910 need venting to reduce the pressures therein.
[0070] Once the desired vent valves 920 are opened, the compressor
930 is turned on while the exhaust valve 970 is kept shut. This
causes air--which may accumulate in the top of a vent manifold or
condenser of the cooling system--along with a small amount of
coolant vapor to be pumped into the separation column 940 from the
cooling systems 910. The compressor 930 may continue to pump until
the separation column 940 exceeds a predetermined amount, such as
20 psi. The vent valves 920 are then closed and the compressor 930
is turned off, trapping an amount of air and coolant vapor at the
increased pressure.
[0071] The gases and fluid in the separation column 940 are then
allowed to cool, e.g., to room temperature (or about 20.degree.
C.). At this point, the system is at point 3 on the chart of FIG.
10, where the saturation pressure of the PF-5060 coolant vapor is
around 4 psi, and the remaining pressure in the column 940 is due
to air and any other gasses in the system. The coolant vapor in the
separation column 940 will tend to stratify, as PF-5060 vapor is
more than ten times denser than the air in the column 940. The
exhaust valve 970 near the top of the separation column 940 is then
opened, which causes gases to vent from the column 940 due to the
increased pressure in the column 940. Because the air is mostly at
the top of the column 940, the majority of the gases being vented
will tend to be air, thus limiting the amount of coolant vapor lost
from the system. Alternatively, any one or any combination of the
separation methods described herein may be used in lieu of or in
combination with the separation column 940.
[0072] Over time, and from venting multiple cooling systems 910,
coolant liquid will tend to accumulate in the system. In one
embodiment, the separation column is coupled to the reservoir,
which receives condensed coolant liquid from the column 940. This
coolant liquid can be used to refill any individual cooling system
910 that is running low by simply opening the refill valve 960 for
the corresponding cooling system 910. An increased pressure in the
reservoir 950, or an optional pump, may be used to cause the
coolant liquid to flow from the reservoir 950 into the desired
cooling system 910. As needed, coolant fluid can be added to the
system centrally by adding coolant fluid to the reservoir 950 via
the fill valve 980.
[0073] As described herein, the entire system may be automated to
vent individual or multiple cooling systems 910 based on the
pressure in the corresponding cooling systems 910. The system may
also be automated to fill the cooling systems 910 as needed based
on the cooling liquid level in each cooling system 910. Embodiments
of the system may be applied to rack-mounted systems, in which each
rack containing multiple servers or other computer systems is
treated as a single cooling system. The racks are then vented and
refilled using the central system, thus allowing for scaling of the
cooling systems without a proportional scaling of the associated
maintenance. The centralized system also allows for automation,
further reducing the maintenance of a multi-rack or otherwise
plural cooling systems configuration. This system may thus greatly
improve the ease of servicing in large data centers.
Testing for Leaks in the Cooling System
[0074] FIG. 11 illustrates a technique for testing for leaks in a
two-phase liquid cooling system. FIG. 12 is a schematic diagram of
a cooling system that can be used to test for leaks in accordance
with the process shown in FIG. 11, although the leak detection
technique described herein can be used with other types of cooling
systems. In the cooling system shown, a coolant is cycled from a
thermal management unit 1250 via a pump 1240 and supply manifold
1210. The coolant is then provided to one or more cooling modules
1230, in which heat energy from cool components being cooled by the
system cause at least some of the coolant to evaporate. The coolant
is then collected in a return manifold and received by the thermal
management unit 1250 to be recycled through the system. To cool
and/or condense the coolant before reusing it in the cooling
modules 1230, the thermal management unit 1250 includes a heat
exchanger 1260, which cools the coolant by passing a cooler liquid
(such as water) through a path in the heat exchanger that is
thermally coupled to the coolant path.
[0075] As illustrated in FIG. 11, the cooling system has a normal
operation mode and a diagnostic mode. In the normal mode, the
cooling system is operated 1110 such that a portion of the path
through which the coolant is directed is under negative pressure
(i.e., lower than atmospheric pressure). Typically, this
low-pressure portion is the return path of the cooling system,
where the coolant is returning to the thermal management unit 1250
after being evaporated within the cooling modules 1230. In this
normal operation mode, any leaks in this path will result in air
entering the system, due to the negative pressure of the return
path.
[0076] When an operator wants to test the system for leaks, the
operator puts the system in a diagnostic mode. In the diagnostic
mode, the pressure in the return path is brought 1120 to or above
atmospheric pressure. In one embodiment, the pressure in the return
path is increased by increasing the temperature of the coolant
provided by the thermal management unit 1250. Preferably, the
temperature of the coolant from the thermal management unit 1250 is
increased so that the cooling modules 1230 still operate to cool
components (albeit less efficiently). With the pressure in the
return path at or exceeding atmospheric pressure, leaks in the
return path can be detected 1130 by placing a coolant sensor along
the return path. For example, a leak can be inferred upon an
indication by the sensor of the presence of coolant outside the
return path. The operator can then more closely examine the portion
of the return path near the detected leak so that the path can be
repaired or replaced. Once any leak problems are resolved, the
operator may return the cooling system to normal operation
1110.
[0077] In one embodiment, the cooling system includes a control
unit 1280, which an operator may use to put the cooling system into
the normal and diagnostic modes. In one embodiment, the control
unit 1280 is coupled to control a valve 1270 in the water supply
path of the heat exchanger 1260. To put the cooling system into
diagnostic mode for leak detection, the control unit restricts the
flow of water to the heat exchanger 1260 by controlling the valve
1270. This restriction can cause the coolant temperature throughout
the cooling system--and thus the pressure of the coolant vapor in
the return path--to rise to a level where the return path of the
cooling system is at or above atmospheric pressure. A refrigerant
detector 1290 can then be used to locate any leaks in the return
path. The detector 1290 can be coupled to the control unit 1280 for
data acquisition purposes, or it can be used independently
thereof.
Thermal Management Unit Design
[0078] In one embodiment of the system, the reservoir and heat
exchanger are preferably constructed as follows. Heated coolant,
which may comprise fluid as saturated vapor or some mixture of
liquid and vapor, enters the heat exchanger, is condensed and/or
cooled, and then falls into the reservoir. From the reservoir, the
fluid then enters a pump inlet and is pumped into the rest of the
system. For such a fluid flow to occur, the reservoir is preferably
positioned gravitationally below the exit of the heat exchanger and
is formed to create a sump from which the pump draws fluid. In one
embodiment, the reservoir is constructed to have an air/vapor space
above the sump during operation of the cooling system. This
construction facilitates a location from which non-condensable
gasses can be extracted from the system. The heat exchanger may be
of a commonly known bar and plate heat type that uses water as the
heat exchange medium.
[0079] In one embodiment, the system uses a heat exchanger that
occupies a greater area than the sump area and is slopped to
facilitate drainage into the sump area. With this construction and
relative sizing, the sump area can be made to contain only enough
fluid to keep the pumps flooded, with some reserve for adding dry
spray units and fluid losses such as permeation.
[0080] In another embodiment, a fluid return port of a pump
connected to the reservoir is disconnectably matable to a self
sealing connector mounted in a through port of the reservoir. The
return port of the pump can be inserted into the connector, thereby
biasing a seal in the connector. The pump can then be hard-mounted
to the outer surface of the reservoir using a fastening means such
as one or more threaded fasteners. The pump supply port is then
fluidly connected to a flexible line that couples it to the rack
supply manifold. Beneficially, the flexible supply line permits the
pump to be engaged to the self sealing connector without adversely
affecting alignment of the seals, where a dual o-ring seal on an
outer circular structure of the pump return may be implemented. The
flexible supply line and the rigid return connection structures may
be reversed, and other non-o-ring sealing members (such as but not
limited to face seals or gaskets) may be implemented.
[0081] In another embodiment, the thermal management unit has a
modular design and includes variable flow valves controllably
connected to a control unit that monitors temperature, pressure,
and water leaks. The control unit is capable of increasing a flow
rate of cooling water through the thermal management unit if a
temperature or pressure measured by the control unit is too high,
or decreasing the flow rate through the thermal management unit if
a measured temperature or pressure is too low. Finally, the control
unit is capable of reducing or shutting off water flow if a water
leak is detected within the modular system. In many instances, the
facility water loop heat exchanger systems are designed to have a
constant water temperature rise (e.g., 35.degree. C. inlet and
30.degree. C. outlet). By varying the water flow to the heat
exchanger, flow can be reduced to maintain optimal water
temperature.
[0082] In another embodiment, in the liquid to liquid heat
exchanger the cooling water loop is connected directly to a
facility cooling tower. The desired water temperature entering the
facility cooling tower is maintained using a variable flow valve,
controllably connected to a system controller that reads a
temperature in the exit water flow of the thermal management unit.
The valve may control flow into or out of the thermal management
unit. Typically, the water entering a facility cooling tower needs
to be maintained at a temperature of approximately 38.degree. C.
The thermal management unit exit water temperature can be
maintained by regulating the flow of cooling water circulating
through the thermal management unit. One benefit that may be
realized by this implementation is that water towers generally
provide a good heat rejection system because they operate within a
narrow range (typically and ideally, a 10.degree. F. rise). The
change in temperature is measured and controlled to the desired
change in temperature of the water. This functionality can be put
into water safety valves for when there is a water leak.
[0083] In another embodiment, during operation of the cooling
system, adequate vapor space above the fluid level may be desirable
for easily separating non-condensable gasses from the working
fluid. If too little vapor space exists, a significant amount of
liquid may be pulled out with the vented gasses. This is highly
undesirable. This condition may occur when an entire rack is
populated with servers, and therefore with cooling modules. The
condition might also occur when a cooling module is connected to
the uppermost fluid port in a vertically mounted supply and return
manifold, since the fluid levels in the manifolds rise according to
the height of the connected units. To avoid this scenario, the
height of the manifold may be increased. However, such a solution
may be impractical because the manifold would typically be required
to retrofit a standard size rack. In another embodiment, the
scenario can be avoided by positioning the non-condensable gas
extraction pump at the top of the rack (as would normally be done)
and fluidly connect it to a vapor space in the reservoir at the
bottom of the rack. This connecting fluid line then becomes a
vertical separation tube, since due to buoyancy the air or other
non-condensable gasses will naturally rise to the top of the tube.
Even in a configuration in which non-condensable gases are pulled
out of the heat exchanger, it may still be desirable to locate the
active vent pump at the top of the rack so that the space between
can be used for gravitational separation of the non-condensable
gasses and coolant vapor.
SUMMARY
[0084] The foregoing description of the embodiments of the
invention has been presented for the purpose of illustration; it is
not intended to be exhaustive or to limit the invention to the
precise forms disclosed. For example, many of the fastening,
connection, manufacturing, and other means and components that are
described in various embodiments are widely known in the relevant
field, and their exact nature or type is not necessary for a person
of ordinary skill in the art or science to understand the
invention. Persons skilled in the relevant art can appreciate that
many modifications and variations are possible in light of the
above teachings. It is therefore intended that the scope of the
invention be limited not by this detailed description, but rather
by the claims appended hereto.
* * * * *