U.S. patent application number 11/384195 was filed with the patent office on 2007-08-23 for two-phase liquid cooling system with active venting.
Invention is credited to Bryan M. Darnton, Fred Hunt, Paul Knight, John R. Mason, Randall T. Palmer, Richard Thomas, Donald E. Tilton.
Application Number | 20070193300 11/384195 |
Document ID | / |
Family ID | 38426782 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070193300 |
Kind Code |
A1 |
Tilton; Donald E. ; et
al. |
August 23, 2007 |
Two-phase liquid cooling system with active venting
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.
Inventors: |
Tilton; Donald E.; (Colton,
WA) ; Knight; Paul; (Spokane, WA) ; Hunt;
Fred; (Potlach, ID) ; Palmer; Randall T.;
(Liberty Lake, WA) ; Darnton; Bryan M.; (Hayden,
ID) ; Mason; John R.; (Coeur d'Alene, ID) ;
Thomas; Richard; (Spokane, WA) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER
801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
38426782 |
Appl. No.: |
11/384195 |
Filed: |
March 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60775496 |
Feb 21, 2006 |
|
|
|
Current U.S.
Class: |
62/475 ;
257/E23.088; 62/259.2 |
Current CPC
Class: |
F28D 15/0266 20130101;
H01L 23/427 20130101; H01L 2924/0002 20130101; H05K 7/20381
20130101; H05K 7/20327 20130101; H01L 2924/0002 20130101; F25B
43/04 20130101; F28D 15/0258 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
062/475 ;
062/259.2 |
International
Class: |
F25D 23/12 20060101
F25D023/12; F25B 43/04 20060101 F25B043/04 |
Claims
1. A two-phase liquid cooling system with active venting, the
system comprising: a closed-loop flow path for circulating a
coolant; a two-phase liquid cooling module in the flow path for
evaporative cooling of a device; a venting system for exhausting
gas outside the cooling system from a volume fluidly coupled to the
flow path; and a control system operatively coupled to the venting
system, the control system configured to activate the venting
system based on process conditions within the cooling system.
2. The system of claim 1, wherein the venting system is designed to
remove non-condensable gases and substantially no coolant from the
system.
3. The system of claim 1, wherein the venting system comprises a
vacuum pump for removing gas from the volume fluidly coupled to the
flow path.
4. The system of claim 1, wherein the venting system comprises a
valve for disallowing air from outside the cooling system to enter
the venting system.
5. The system of claim 1, wherein the volume to which the venting
system is coupled is further coupled to a return line that has a
membrane that is permeable to the coolant and not permeable to the
non-condensable gases, thereby increasing the relative
concentration of non-condensable gases in the volume for being
removed from the cooling system.
6. The system of claim 1, wherein the venting system comprises a
condenser to condense the coolant and thereby separate the coolant
from the non-condensable gases.
7. The system of claim 1, wherein the venting system comprises a
centrifugal separator to separate the coolant and the
non-condensable gases based on density.
8. The system of claim 1, wherein the venting system comprises: a
semi-permeable tubing coupled to the flow path, the semi-permeable
tubing permeable to the non-condensable gases and not permeable to
the coolant; a vacuum chamber housing the semi-permeable tubing;
and a pump coupled to the housing to remove gases therefrom.
9. The system of claim 1, wherein the venting system is coupled to
a sealed chamber for storing gases vented from the cooling
system.
10. The system of claim 9, wherein the sealed chamber includes a
condenser for condensing any coolant vapor therein and a valve for
venting gas from the chamber when the pressure therein reaches a
predetermined maximum.
11. The system of claim 1, wherein the process conditions within
the cooling system include temperature and pressure.
12. The system of claim 1, wherein the control system is configured
to activate the venting system to keep the coolant in the cooling
system at a point near the coolant's saturation curve.
13. The system of claim 1, wherein the control system is configured
to activate the venting system to keep the coolant in the cooling
system within a predetermined tolerance from the coolant's
saturation curve.
14. The system of claim 1, wherein the control system is configured
to activate the venting system to maintain a desired temperature at
a cooling module.
15. The system of claim 1, wherein the control system is configured
to perform a step for regulating the pressure within the cooling
system.
16. The system of claim 1, wherein the cooling system is a
rack-mounting system.
17. The system of claim 1, wherein the cooling module is a spray
cooling module.
18. The system of claim 1, wherein the volume from which the
venting system exhausts gas is a reservoir in a heat exchanger of
the cooling system.
19. The system of claim 1, wherein the volume from which the
venting system exhausts gas is a return manifold in the flow
path.
20. A cooling system comprising: a closed-loop fluid path having an
internal volume, the internal volume for holding: a cooling fluid
having a vapor portion occupying a partial amount of the internal
volume and a liquid portion occupying a partial amount of the
internal volume, and a non-condensable gas occupying a partial
amount of the internal volume; one or more two-phase liquid cooling
modules in the fluid path; and a means for regulating an amount of
the non-condensable gas within the internal volume.
21. The system of claim 20, wherein the means for regulating is
configured to remove substantially no coolant from the system.
22. The system of claim 20, wherein the means for regulating is
coupled to a sealed chamber for storing gases vented from the
cooling system.
23. The system of claim 22, wherein the sealed chamber includes a
condenser for condensing any coolant vapor therein and a valve for
venting gas from the chamber when the pressure therein reaches a
predetermined maximum.
24. The system of claim 20, wherein the means for regulating is
configured to control an amount of the non-condensable gas within
the internal volume based on at least one of a temperature and a
pressure at a location in the internal volume.
25. The system of claim 20, wherein the means for regulating is
configured to keep the cooling fluid at a point near the cooling
fluid's saturation curve.
26. The system of claim 20, wherein the means for regulating is
configured to keep the cooling fluid within a predetermined
tolerance from the cooling fluid's saturation curve.
27. The system of claim 20, wherein the means for regulating is
configured to maintain a desired temperature at the cooling
modules.
28. The system of claim 20, wherein the cooling system is a
rack-mounting system.
29. The system of claim 20, wherein the cooling module is a spray
cooling module.
30. A closed-loop liquid cooling system having an internal volume,
the system comprising: a cooling fluid having a vapor portion
occupying a partial amount of the internal volume and a liquid
portion occupying a partial amount of the internal volume; a
non-condensable gas occupying a partial amount of the internal
volume; and a means for regulating the amount of the
non-condensable gas within the internal volume.
31. The system of claim 30, wherein the means for regulating is
configured to remove substantially no coolant from the system.
32. The system of claim 30, wherein the means for regulating is
configured to control an amount of the non-condensable gas within
the internal volume based on at least one of a temperature and a
pressure at a location in the internal volume.
33. The system of claim 30, wherein the means for regulating is
configured to keep the cooling fluid at a point near the cooling
fluid's saturation curve.
34. The system of claim 30, wherein the means for regulating is
configured to keep the cooling fluid within a predetermined
tolerance from the cooling fluid's saturation curve.
35. The system of claim 30, wherein the cooling system is a
rack-mounting system.
36. The system of claim 30, wherein the cooling system further
comprises one or more spray cooling modules.
37. A method for venting a non-condensable gas from a two-phase
liquid cooling system, the method comprising: passing a coolant
through a closed-loop flow path, the flow path including one or
more cooling modules; cooling a heat producing device using the
cooling modules, the cooling resulting in the formation of coolant
vapor; collecting the coolant vapor and a non-condensable gas in a
volume fluidly coupled to the flow path; and venting an amount of
the non-condensable gas from the volume to outside the cooling
system, the venting based on measured process conditions within the
cooling system.
38. The method of claim 37, wherein the venting removes
non-condensable gases and substantially no coolant from the
system.
39. The method of claim 37, wherein the volume from which the
non-condensable gas is vented is coupled to a return line that has
a membrane that is permeable to the coolant and not permeable to
the non-condensable gas, thereby increasing the relative
concentration of non-condensable gases in the volume for being
removed from the cooling system.
40. The method of claim 37, further comprising: condensing coolant
in the volume to separate the coolant from the non-condensable gas
being vented.
41. The method of claim 37, further comprising: separating the
coolant from the non-condensable gas being vented using a
centrifugal separator.
42. The method of claim 37, wherein the venting system comprises:
directing a flow of the coolant and non-condensable gas through a
semi-permeable tubing in a vacuum chamber, the semi-permeable
tubing permeable to the non-condensable gases and not permeable to
the coolant; reducing the pressure in the vacuum chamber to collect
non-condensable gas from the tubing to the vacuum chamber; and
pumping the non-condensable gas from the vacuum chamber.
43. The method of claim 37, wherein the vented non-condensable gas
is directed to a sealed chamber.
44. The method of claim 38, further comprising: condensing coolant
vapor in the sealed chamber; and venting gas from the sealed
chamber through a valve when the pressure in the sealed chamber
reaches a predetermined maximum.
45. The method of claim 37, wherein the process conditions within
the cooling system include temperature and pressure.
46. The method of claim 37, wherein the venting is controlled to
keep the coolant in the cooling system at a point near the
coolant's saturation curve.
47. The method of claim 37, wherein the venting is controlled to
keep the coolant in the cooling system within a predetermined
tolerance from the coolant's saturation curve.
48. The method of claim 37, wherein the venting is controlled to
maintain a desired temperature at a cooling module.
49. The method of claim 37, wherein the cooling modules are spray
cooling modules.
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.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates generally to two-phase liquid cooling
systems, and more particularly to two-phase liquid cooling systems
that have an active venting system for regulating the pressure
within the system by removing gases such as non-condensable gases
from the cooling system.
[0004] 2. Background of the Invention
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] A significant challenge in the use of some two-phase cooling
systems is presented by non-condensable gases. Dielectric fluids
like FLUORINERT.RTM. can contain significant amounts of air and
other non-condensable gases in solution. When the dielectric fluid
is placed into a system at atmospheric conditions, the fluid may
thus contain a significant amount of air dissolved in the fluid.
During use within a thermal management system, according to Henry's
Law, as the fluid approaches its saturation temperature the amount
of air in solution decreases. The air that was previously in
solution occupies a volume within the system. According to the
ideal gas law, the partial pressure of the air will raise the
boiling point of the cooling fluid above the natural saturation
curve. This, in turn, reduces the cooling efficiency of the system
because it raises the boiling point of the fluid to a level that
renders less than ideal cooling performance. But some amount of air
is useful for some cooling systems, for example, to avoid pump
cavitation. The actual amount of air in the system can vary as air
seeps into the system during operation, so it can be difficult to
maintain the amount of air within the system at an optimal
level.
[0011] For the foregoing reasons, there is a need for a two-phase
liquid cooling solution that can maintain an ideal amount of air or
other non-condensable gas within the system. With changing
conditions inside a cooling system, there is a need for a method of
regulating the non-condensable gases in the cooling system. Such a
cooling system would result in significant improvements in both the
performance and reliability of the two-phase liquid cooling
process.
SUMMARY OF THE INVENTION
[0012] To avoid at least some of the problems encountered with
existing two-phase liquid cooling systems, as described above, a
cooling system with active venting is provided. An active venting
system actively regulates the pressure within the cooling system,
for example, by regulating the amount of non-condensable gases in
the cooling system. With appropriate control of the active venting
system, the performance and reliability of the system can be
increased and maintained over long and continuous periods of
operation.
[0013] Embodiments of the invention include liquid cooling systems
and methods that can provide thermal management for one or more
electronic components. In one embodiment, a cooling system includes
a cooling liquid, or coolant, that is circulated through a closed
loop by one or more pumps. The cooling fluid enters one or more
cooling modules as a liquid or saturated liquid, and changes phase
in the cooling module by means of latent energy gains. The
resulting liquid and vapor mixture is then removed from the cooling
module and condensed so that it can be returned to the pump and
circulated back through the system. An active venting system is
coupled to a volume in the cooling system to regulate the pressure
in the cooling system. A control system is coupled to the active
venting system to activate the venting based on any of a number of
criteria, such as process conditions within the cooling system.
[0014] The active cooling system can exhaust gases out of the
system using various mechanisms. In one embodiment, a vent is
located between the cooling module and the pump, and an auxiliary
pump is coupled to the vent to pump a desired amount of gas out of
the system. The control system is coupled to the auxiliary pump and
vent to provide the ability to regulate the amount of gas removed
from the system. The active venting system may also be capable of
adding gases into the cooling system (e.g., by pumping air into the
system) when a pressure increase is desired. In this way, the
cooling system can regulate the pressure in the cooling system to
achieve a desired overall cooling efficiency. Adding air to the
cooling system may also help avoid cavitation in the pumps.
[0015] Within the cooling system there may be one or more
non-condensable gases. Non-condensable gases may include any gases
or mixtures thereof that do not condense into liquid form under
conditions experienced during normal operation of the two-phase
liquid-cooling system. Air is a common non-condensable gas in
cooling systems, since they are typically run at pressures below
atmospheric so that air tends to seep in slowly through points in
the system that are not completely sealed or otherwise allow air
permeation into the system. The non-condensable gases cause a
partial pressure within the closed volume of the cooling system,
which alters the boiling point of the cooling fluid and thus
affects the operation of the cooling system. While removing the
gases from the system, it is often desirable to remove the
non-condensable gases while minimizing the removal of the coolant
in vapor phase. Otherwise, over time the cooling system would lose
coolant and would need to have the coolant replaced. By removing
non-condensable gases rather than vapor-phase coolant from the
cooling system, the need to replace coolant is reduced.
Accordingly, the active venting system may be configured to remove
an amount of the non-condensable gases from the system.
[0016] Various embodiments of the system include mechanisms in the
venting system for separating the coolant vapor from the
non-condensable gases to be removed. By separating the coolant
vapor from the non-condensable gases, the active venting system can
remove only the non-condensable gases and allow the coolant vapor
to recycle through the cooling system. In one embodiment, the
active cooling system includes using a semi-permeable membrane
separator coupled between the cooling module and the return line,
allowing only coolant vapor to recycle through the system. In other
embodiments, the active venting system comprises a condensing
separator, a centrifugal gas separator, or a semi-permeable
membrane separator (such as a permeable tube vacuum system) to
separate the vapor cooling fluid from the non-condensable gases to
be removed.
[0017] In one embodiment, the control system measures process
conditions such as the temperature and pressure within a volume of
the cooling system. Based on the measured temperature and pressure,
the control system determines whether the process conditions within
the system are inside a desired range. In one embodiment, the
control system determines that removal of non-condensable gases is
needed based on the saturation curve of the coolant. For example,
the control system may detect when the pressure and temperature
inside the system deviate from the saturation curve of the coolant
by a predetermined amount. When the control system determines that
venting is needed, it activates the venting system, for example,
causing the active venting system to open the vent and turn on the
auxiliary pump to remove gases in the system.
[0018] 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
[0019] 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:
[0020] FIG. 1 is a schematic diagram of a two-phase liquid cooling
system with active venting, in accordance with an embodiment of the
invention.
[0021] FIG. 2 is a schematic diagram of a rack-mounted spray
cooling system, in accordance with an embodiment of the
invention.
[0022] FIG. 3 is a schematic diagram of a semi-permeable membrane
separator, in accordance with an embodiment of the invention.
[0023] FIG. 4 is a schematic diagram of a condensing separator, in
accordance with an embodiment of the invention.
[0024] FIG. 5 is a schematic diagram of a centrifugal separator, in
accordance with an embodiment of the invention.
[0025] FIG. 6 is a schematic diagram of a permeable tube vacuum
mechanism, in accordance with an embodiment of the invention.
[0026] FIG. 7 is a chart showing a typical saturation curve for an
example cooling liquid.
[0027] 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 system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Two-Phase Cooling System with Active Venting
[0028] 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.
[0029] 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.
[0030] 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 (as 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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 100. 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 use 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 a
return manifold (see FIG. 2).
[0035] 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.
[0036] 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 supply manifold 130 to 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.
[0037] 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.
[0038] 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.
[0039] To address this problem, in one embodiment, a bypass flow
path 260 is placed between the supply manifold 230 and the return
manifold 240 near the tops thereof. The flow path 260 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 260 may comprises a tube, and a chemical filter 265 may
be installed in the flow path 260, since this provides an ideal
service location. The bypass flow path 260 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 260 can be separate from the filter 265, although it is
typically desired to reduce number of fluid joints in the
system.
Active Venting System Embodiments
[0040] 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 efficiency 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.
[0041] 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.
[0042] 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).
[0043] 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.
[0044] 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.
[0045] 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 venting 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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. 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.
[0050] In one embodiment, the tubing 620 comprises a co-extrusion
having at least two layers. 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 a coextruded tubing that prevents
permeation and provides good fluid compatibility while remaining
flexible is used. This coextruded tubing 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 coextruded 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.
[0051] On one embodiment, the tubing used for some or all flexible
connections within the system is a co-extruded tubing that
comprises: [0052] 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); [0053] a binding layer comprised of Bynel 4157: Linear low
density polyethylene (LLDPE) (0.003'' thick); [0054] a next layer
EVALCA F101: ethyl vinyl alcohol (EVOH) (0.005'' thick); [0055] a
next binding layer of Bynel 4157: Linear low density polyethylene
(LLDPE) (0.003'' thick); and [0056] 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.
[0057] 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.
Operation
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
Summary
[0064] 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.
* * * * *