U.S. patent application number 09/735415 was filed with the patent office on 2001-06-07 for spray cooling system.
Invention is credited to Bash, Cullen E., Patel, Chandrakant D..
Application Number | 20010002541 09/735415 |
Document ID | / |
Family ID | 23561655 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002541 |
Kind Code |
A1 |
Patel, Chandrakant D. ; et
al. |
June 7, 2001 |
Spray cooling system
Abstract
A spray cooling system for semiconductor devices. An ink-jet
type spray device sprays droplets of a cooling fluid onto the
semiconductor devices. The devices vaporize the liquid, which gets
passed through a roll bond panel, or other heat exchanger, and is
pumped into a spring loaded reservoir that feeds the spray
device.
Inventors: |
Patel, Chandrakant D.;
(Fremont, CA) ; Bash, Cullen E.; (San Francisco,
CA) |
Correspondence
Address: |
HEWLETT-PACKARD COMPANY
Intellectual Property Administration
P.O. Box 272400
Fort Collins
CO
80527-2400
US
|
Family ID: |
23561655 |
Appl. No.: |
09/735415 |
Filed: |
December 11, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09735415 |
Dec 11, 2000 |
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09395092 |
Sep 13, 1999 |
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6205799 |
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Current U.S.
Class: |
62/259.2 ;
257/E23.1; 62/171; 62/310 |
Current CPC
Class: |
F25B 2339/021 20130101;
H05K 7/20345 20130101; F25B 39/04 20130101; F25B 39/02 20130101;
H01L 2924/0002 20130101; H01L 23/4735 20130101; H01L 2924/0002
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
62/259.2 ;
62/310; 62/171 |
International
Class: |
F25D 023/12 |
Claims
We claim:
1. A cooling assembly for cooling a heat source with a cooling
fluid, comprising: an incremental sprayer configured to eject an
incremental amount of the cooling fluid on the heat source in
response to a control signal; and a controller configured to send
the control signal to the incremental sprayer.
2. The cooling assembly of claim 1, wherein the incremental sprayer
comprises: a body defining a chamber configured to hold a volume of
the cooling fluid, and defining an orifice in communication with
the chamber; and a heating element in thermal communication with
the chamber, the heating element being configured to vaporize a
portion of the cooling fluid held within the chamber; wherein the
orifice is configured to direct cooling fluid from the chamber to
the heat source upon the heating element vaporizing a portion of
the cooling fluid held within the chamber.
3. The cooling assembly of claim 2, wherein: the body includes a
thin-film substrate and a backing; the chambers are cavities that
are formed adjoining one side of the thin-film substrate by the
backing; the orifice is a passage through the thin-film substrate;
and the heating element is a thin-film resister.
4. The cooling assembly of claim 1, wherein the incremental amount
of the cooling fluid is a single droplet of cooling fluid.
5. The cooling assembly of claim 1, wherein the incremental sprayer
comprises a piezoelectric nozzle.
6. The cooling assembly of claim 1, and further comprising a heat
exchanger configured to cool and condense cooling fluid that was
vaporized by the heat source after being ejected by the incremental
sprayer.
7. The cooling assembly of claim 6, wherein the heat exchanger is a
roll bond panel.
8. The cooling assembly of claim 1, and further comprising a
reservoir configured to provide liquid cooling fluid to the
incremental sprayer.
9. The cooling assembly of claim 1, and further comprising: a heat
exchanger configured to cool and condense cooling fluid that was
vaporized by the heat source after being ejected by the incremental
sprayer; a reservoir configured to provide liquid cooling fluid to
the incremental sprayer; and a pump configured to pump liquid
cooling fluid from the heat exchanger to the reservoir.
10. A cooled circuit board, comprising: a circuit board; a
semiconductor device mounted on the circuit board; and the cooling
assembly of claim 1 configured to cool the semiconductor
device.
11. The cooled circuit board of claim 10, and further comprising: a
heat exchanger configured to cool and condense cooling fluid that
was vaporized by the semiconductor device after being ejected by
the incremental sprayer; a reservoir configured to provide liquid
cooling fluid to the incremental sprayer; and a pump configured to
pump liquid cooling fluid from the heat exchanger to the
reservoir.
12. A cooled circuit board assembly, comprising: a plurality of the
cooled circuit boards of claim 10; a heat exchanger configured to
cool and condense cooling fluid that was vaporized by the
semiconductor device of each of the plurality of cooled circuit
boards after being ejected by the incremental sprayer; a reservoir
configured to provide liquid cooling fluid to the incremental
sprayer of each of the plurality of cooled circuit boards; and a
pump configured to pump liquid cooling fluid from the heat
exchanger to the reservoir.
13. The cooled circuit board assembly of claim 12, wherein each of
the plurality of cooled circuit boards includes a roll bond panel
configured to cool and condense cooling fluid that was vaporized by
the semiconductor device after being ejected by the incremental
sprayer.
14. A method of cooling a heat source, comprising: ejecting an
incremental amount of a cooling fluid onto the heat source using an
incremental sprayer; and repeating the step of ejecting at time
increments.
15. The method of claim 14, wherein the step of spraying comprises:
providing cooling fluid to a chamber defined in the incremental
sprayer; energizing a heating element in thermal communication with
the chamber to vaporize a portion of the cooling fluid within the
chamber; wherein the incremental sprayer defines an orifice in
communication with the chamber, the orifice being configured to
direct cooling fluid from the chamber to the heat source upon the
heating element's vaporizing of a portion of the cooling fluid in
the chamber.
16. The method of claim 15, wherein: in the step of providing, the
chamber is a cavity formed in a backing, the cavity adjoining a
thin-film substrate; the orifice is a passage through the thin-film
substrate; and in the step of heating, the heating element is a
thin-film resister.
17. The method of claim 14, and further comprising: sensing the
temperature of the heat source; sensing the pooling of the sprayed
cooling fluid; adjusting the mean flow rate of cooling fluid
ejected by the incremental sprayer based on the sensed temperature
of the heat source and pooling of the sprayed cooling fluid.
18. The method of claim 17, wherein the mean flow rate is adjusted
by adjusting the frequency with which the step of ejecting is
repeated.
19. The method of claim 14, wherein the step of repeating is
conducted with a frequency that varies with a sensed indication of
pooling of the sprayed cooling fluid.
20. The method of claim 14, and further comprising: sensing the
temperature of the heat source; sensing whether pooling is
occurring; and varying the frequency with which the step of
repeating is conducted based upon the sensed temperature and the
sensed result of whether pooling is occurring; wherein if the
sensed temperature is above a previously determined maximum
temperature and the sensed result of whether pooling is occurring
indicates that pooling is occurring, the frequency with which the
step of repeating is conducted is varied downward in the step of
varying; and wherein if the sensed temperature is above a
previously determined maximum temperature and the sensed result of
whether pooling is occurring indicates that pooling is not
occurring, the frequency with which the step of repeating is
conducted is varied upward in the step of varying.
21. A method of cooling a heat source, comprising: spraying a
cooling fluid onto the heat source; sensing whether pooling of the
sprayed cooling fluid is occurring; and varying the flow rate of
the spraying based upon the sensed indication of whether pooling is
occurring.
22. The method of claim 21, and further comprising sensing the
temperature of the heat source, wherein, in the step of varying,
the flow rate is also varied based upon the sensed temperature.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to cooling systems for
heat-generating devices and, more particularly, to a spray cooling
system and a method of using the spray cooling system to cool a
heat source.
[0002] With the advent of semiconductor devices having increasingly
large component densities, the removal of heat generated by the
devices has become an increasingly challenging technical issue.
Furthermore, typical processor boards can, in some instances,
include multiple CPU modules, application-specific integrated
circuits (ICs), and static random access memory (SRAM), as well as
a dc-dc converter. Heat sinks can be used to increase the
heat-dissipating surface area of such devices. However, heat sinks,
and their interfaces to the cooled devices, can provide
interference in the heat flow, and can lead to uneven cooling.
[0003] Known cooling methods for semiconductors include
free-flowing and forced-air convection, free-flowing and
forced-liquid convection, pool boiling (i.e., boiling a liquid
cooling fluid off of a submerged device), and spray cooling (i.e.,
boiling a liquid cooling fluid off of a device being sprayed with
the liquid). Because liquids typically have a high latent heat of
vaporization, these latter two methods provide for a high
heat-transfer efficiency, absorbing a large quantity of heat at a
constant temperature. Typically, the cooling fluid used has a
relatively low boiling point (the temperature to maintain) and is
inert to the heat source. For semiconductor devices, FED. CIR.-72,
i.e., Fluorinert.RTM., sold by 3M Corporation, is one of a number
of known suitable cooling liquids.
[0004] The use of these boiling/vaporizing methods is limited to a
maximum power density, the critical heat flux (CHF). At higher
densities, the vaporized cooling fluid forms a vapor barrier
insulating the device from the liquid cooling fluid, thus allowing
the wall temperature of the device to increase greatly. This
phenomenon is referred to as pooling. When a coolant is properly
sprayed, it can disperse such a vapor layer, and its CHF can be
well over an order of magnitude higher than the CHF of a pool
boiling system. This high CHF is reliant on having a uniform spray.
Thus, spray cooling presently provides the most efficient cooling
for a heat-generating device, such as a semiconductor device.
[0005] Typically, current sprayer designs employ either pressurized
liquid spraying or pressurized gas atomizing. A number of factors
affect the performance of spray cooling, thus affecting the heat
transfer coefficient h and/or the CHF. It is commnonly understood
that surface roughness and wettability of the sprayed component are
two of these factors, and the orientation of the surface being
sprayed can be a third. In particular, it is believed that h is
higher for rough surfaces when using a pressurized liquid spray,
and for smooth surfaces when using gas atomizing. Surfaces with
decreased wettability appear to have a marginal increase in h.
[0006] Critical to consistent, controlled cooling is the controlled
application of the liquid cooling fluid in a desired distribution,
flow rate, and velocity. For example, at a low mass flow rate, CHF
and h increase with the mass flow rate. However, at a critical mass
flow rate, the advantages of increased mass flow are diminished due
to pooling and/or due to a transition to single phase heat
transfer. Thus, a spray cooling system is preferably operated
uniformly at a mass flow rate defined at a point before the
critical mass flow rate is reached. All of these factors make
critical the design of the sprayer, i.e., the design of the nozzle
and its related spray devices.
[0007] Also important to the cooling system design is its operating
temperature. In particular, it is desirable to configure the system
to operate at a high h, which will occur with a design temperature
above the boiling temperature and below a temperature that will dry
out the sprayed coolant. The amount of heat to be dissipated must
be less than the CHF.
[0008] For pressure-assisted spraying, consistent, controlled
spraying requires one or more high pressure pumps that provide a
precise pressure to pump the liquid through a nozzle, even at
varying flow rates. Both the distribution and the flow rate of the
sprayed liquid can change with variations in the driving pressure
and/or small variations in the nozzle construction. Thus, the
cooling system is a sensitive and potentially expensive device that
can be a challenge to control.
[0009] For gas atomizing, consistent, controlled spraying requires
a pressurized gas that is delivered to a sprayhead design in a
precise manner. Because the gas must be pressurized separately from
the cooling fluid, such systems are not typically closed systems.
The gas must be bled out for the condenser to run efficiently.
Furthermore, both the distribution and the flow rate of the cooling
fluid can change with variations in the gas pressure. Thus, the
cooling system is a sensitive and potentially expensive device that
can be a challenge to control.
[0010] Accordingly, there has existed a need for an accurate,
reliable and cost-efficient spray cooling system. The present
invention satisfies these and other needs, and provides further
related advantages.
SUMMARY OF THE INVENTION
[0011] The present invention provides a spray cooling system for
cooling a heat source, embodiments of which can exhibit improved
accuracy, reliability and/or cost efficiency. Embodiments of the
invention typically feature an incremental sprayer configured to
eject an incremental amount of the cooling fluid on the heat
source. The cooling fluid is sprayed in response to a control
signal, which is sent to the sprayer by a controller.
[0012] Advantageously, these features provide for accurate delivery
of cooling fluid at precise and controllable rates. The technology
for this type of incremental sprayer is well developed in the
ink-jet printer arts, and it is relatively inexpensive to
manufacture. Furthermore, the design can be modular, offering
quickly and easily replaceable units.
[0013] The invention further features the use of thermal ink-jet
technology in designing the sprayer. In particular, the embodiment
of the invention may have a body defining a chamber configured to
hold a volume of the cooling fluid, and defining an orifice in
communication with the chamber. A heating element is in thermal
communication with the chamber, and is configured to vaporize a
portion of the cooling fluid held within the chamber. The orifice
is configured to direct cooling fluid from the chamber to the heat
source upon the heating element vaporizing a portion of the cooling
fluid held within the chamber.
[0014] This technology generally provides for efficient delivery of
the cooling fluid to the heat source. Some known inert cooling
fluids have viscosities and boiling points similar to that of
ink-jet ink, and the ink-jet sprayers are typically adaptable to
use with the cooling fluids. Furthermore, unlike typical ink-jet
ink, cooling fluid does not contain particulate matter that can
clog the system. Thus, the system is both reliable and cost
efficient to design.
[0015] The invention further features the ejection of incremental
amounts of a cooling fluid on the heat source, using an incremental
sprayer, spaced over a number of time increments. Either the
incremental time or the amount ejected can be varied to adjust the
flow rate to an optimal level. The system can be controlled by
monitoring, either directly or indirectly, the temperature of the
heat source and the amount of pooling or dry-out that is occurring,
if any. This can provide for optimized cooling of a heat
source.
[0016] Other features and advantages of the invention will become
apparent from the following detailed description of the preferred
embodiments, taken in conjunction with the accompanying drawings,
which illustrate, byway of example, the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic representation of a cooling system
embodying features of the present invention.
[0018] FIG. 2 is a cross-sectional view of a sprayer for the
cooling system represented in FIG. 1.
[0019] FIG. 3 is a cut-away perspective view of a first embodiment
of the cooling system represented in FIG. 1.
[0020] FIG. 4 is a cross-sectional view of the embodiment depicted
in FIG. 3.
[0021] FIG. 5 is a cut-away perspective view of a second embodiment
of the cooling system represented in FIG. 1.
[0022] FIG. 6 is a cross-sectional view of a third embodiment of
the cooling system represented in FIG. 1.
[0023] FIG. 7 is a control system block diagram for controlling the
operation of the embodiment depicted in FIG. 6.
[0024] FIG. 8 is a control system block diagram for controlling the
operation of the embodiment depicted in FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] An embodiment of a cooling assembly 10 for cooling a
heat-generating semiconductor device 12, according to the present
invention, is schematically shown in FIG. 1. The assembly includes
a one or more incremental sprayers 14 for spraying an incremental
amount of a liquid cooling fluid 16, preferably from a reservoir
18, onto the semiconductor device to evaporatively cool the
semiconductor device. The assembly also includes a heat exchanger
20 to extract the heat from the vaporized cooling fluid, and
thereby liquify or condense it. The assembly further includes a
pump 22 to pump the liquified cooling fluid back into the reservoir
that feeds the sprayers.
[0026] While incremental sprayers 14 that can be used as part of
the present invention can be based on other types of ink-jet
droplet expelling technology, such as piezoelectric technology
(i.e., piezoelectric nozzles), they are preferably based on thermal
ink-jet technology. Examples of this technology are discussed in
numerous U.S. Patents, including Pat. Nos. 5,924,198, 4,500,895,
and 4,683,481, which are incorporated herein by reference. Other
thermal ink-jet technologies can likewise be appropriate for use
with this invention. A highly preferable cooling fluid for use with
a thermal incremental sprayer is 3M Fluorinert.RTM., which is
easily adaptable to existing thermal ink-jet technology because it
has a viscosity and boiling point similar to that of the inks
typically used in ink-jet printers.
[0027] With reference to FIG. 2, which depicts two simplified,
exemplary incremental sprayers 14, each sprayer includes structure
defining a chamber 30 for receiving a predetermined portion of the
cooling fluid and a heater 32 for vaporizing a portion of the
cooling fluid, to create the pressure to eject an incremental
amount of the cooling fluid through an orifice 34 that directs the
ejected cooling fluid toward the semiconductor device 12 (FIG. 1).
The orifices are formed in a flexible polymer tape 36, e.g., tape
commercially available as Kapton.TM. tape, from 3M Corporation.
[0028] Affixed to a back surface 38 of the tape 36 is a silicon
substrate 40 containing the heaters 32, in the form of individually
energizable thin-film resistors. Each heater is preferably located
on a side ofthe chamber 30 across from the chamber's orifice 34.
Cooling fluid is preferably drawn and loaded into the chamber by
capillary action, as is typical for an ink-jet type device. A
computerized controller (not shown) energizes the heater,
vaporizing the portion of the cooling fluid adjacent to the heater.
The vaporized cooling fluid expands, expelling most of the
non-vaporized cooling fluid out of the orifice, typically in the
form of a single droplet.
[0029] Depending on the configuration of the sprayer, the
incremental amount of the fluid sprayed from the sprayer could be
in the form of a single droplet, or in the form of multiple
droplets. Multiple droplets could be produced by multiple orifices
related to a single heater, or by sprayers having larger chamber
volumes and appropriately shaped orifice nozzles to cause the
incremental amount of fluid to break into droplets. After the
chamber has been fired by the heater, capillary action again loads
the chamber for a subsequent firing.
[0030] The liquid spray from the incremental sprayers 14 can be
highly controllable. For example, by increasing or decreasing the
frequency that the sprayers are energized, the flow rate can be
accurately adjusted. Furthermore, because the sprayers can be
configured to deliver very small quantifies of cooling fluid, and
because a large number of sprayers can be fit into a small area,
the heat distribution over that area can be accurately controlled
by energizing some of the sprayers at a rate greater than that of
other sprayers.
[0031] With reference again to FIG. 1, to aid the reservoir 18 in
providing the cooling fluid to the incremental sprayers 14, the
reservoir can be configured with a spring assist mechanism 42.
Alternatively, the reservoir can be positioned such that the
cooling fluid receives a gravity assist in flowing to the sprayers
14. In addition to serving as a source of liquid cooling fluid, the
reservoir also serves to separate any gas leaving the condenser. A
filter (not shown) can be used, either in the reservoir or in some
other portion of the system, to remove board level contaminants
that are present in the system.
[0032] The pump 22 serves to replenish the reservoir 18, and can be
a low-cost apparatus that does not provide either high pressure or
consistent and controlled flow. Preferably the pump should be self
priming to remove trapped gas.
[0033] The precise order of the components can be varied. For
example, the pump 22 could be placed prior to the heat exchanger
20, so long as it can pump both vapors and fluids. Likewise,
depending on the type of sprayer, the reservoir could be
eliminated, and the pump could be used to directly feed the
sprayers 14. The entire assembly 10, including the circuit board,
is preferably a field-replaceable unit.
[0034] With reference now to FIGS. 3 and 4, multiple cooling
systems within one computer (or other device) can be configured to
share components. The computer can contain a plurality of circuit
boards 50 carrying heat-generating components 52 such as CPUs, each
circuit board typically being mounted on a backplane 54.
Incremental sprayers 56 are located adjacent to the components, and
are configured to spray the components with a cooling fluid. The
components and sprayers are enclosed in a compartment 58 that
prevents vaporized cooling fluid from escaping the system. A roll
bond panel 60 serves as a first heat exchanger, condensing some or
all of the vapor. The roll bond panel is formed as a wall of the
sealed compartment. Suitable roll bond panels can be obtained from
Showa Aluminum Corporation, of Tokyo, Japan, or from Algoods, of
Toronto, Canada. A suitable low-boiling point working fluid, e.g.,
3M Fluorinert.RTM., is carried within fluid channels in the roll
bond panel. Alternatively, working fluids such as hydrofluoroether
or alcohol could be used.
[0035] A second heat exchanger 62, which can also be a roll bond
panel, is located externally from the circuit board compartments
58, and provides for the additional condensing of vaporized cooling
fluid. The second heat exchanger receives the cooling fluid, which
can be both liquid and vaporized, from the compartments of each
circuit board 50. After the cooling fluid has been further cooled
by the second heat exchanger, a commonly shared pump 64 delivers
the cooling fluid to a commonly shared reservoir 66, which in turn
returns the cooling fluid to the sprayers 56 of each circuit
board.
[0036] With reference now to FIG. 5, the entire cooling system can
be incorporated into a single circuit board assembly 70. The
circuit board assembly will typically include heat-generating
components 72 such as CPUs, mounted on a circuit board 74.
Incremental sprayers 76 are located adjacent to the components, and
are configured to spray the components with a cooling fluid. The
components and sprayers are enclosed in a sealed compartment 78
that prevents vaporized cooling fluid from escaping. One or more
roll bond panels 80 preferably are incorporated into one or more
compartment walls, and are configured to condense vapor and release
it into a collection reservoir 82 in the bottom of the circuit
board assembly. The pool also receives non-vaporized coolant that
drips from the components. A pump 84 pumps the cooling fluid up
into a main feed reservoir 86, preferably being above the sprayers,
which provides the cooling fluid to the sprayers. As an alternative
to the reservoir's being located above the sprayers, which causes a
gravity feed effect, the reservoir could incorporate some type of
pressure mechanism, such as a spring.
[0037] Generally speaking, for embodiments of the invention to
function at optimal efficiency, the sprayers' mass flow rate ({dot
over (m)}.sub.s) should be adjusted to avoid having the
semiconductor device become either dry or immersed. This rate is
controlled by having a controller adjust the rate that the thermal
jets are fired. The optimum mass flow rate can change as the heat
flux of the semiconductor device changes. Thus, for a controller to
correctly control the mass flow, parameters of the semiconductor
device and/or cooling system need to be sensed.
[0038] To determine whether the mass flow rate is at an optimal
level, sensors can be used to track one or more of the system
parameters. The types of parameters that are available vary with
the type of system employed. For example, if the heat exchanger is
external to the chamber where the spraying occurs, then the liquid
and the vapor can be removed from the chamber through separate
passages (with the assistance of a resistive mesh to inhibit entry
of vapor into the liquid passage), and the mass flow of the vapor
({dot over (m)}.sub.v) and/or mass flow of the liquid ({dot over
(m)}.sub.l) are available to be measured. However, these are not
available if the heat exchanger is within the spray chamber, such
as in the embodiment of FIG. 5. Instead, the vapor pressure within
the spray chamber (P.sub.v) and the semiconductor device's junction
temperature can be sensed.
[0039] FIG. 6 depicts a cooling system 90 having a heat exchanger
92 external to a spray chamber 94. The spray chamber contains
incremental sprayers 96 that spray a cooling fluid onto
semiconductor devices 98 on a circuit board 100. Depending on the
temperature of the semiconductor devices, some of the cooling fluid
may vaporize, and some may run off to form a pool 102. Vapor exits
the spray chamber through a vapor passage 104, while liquid exits
via a liquid passage 106. A mesh 108 is used to prevent vapor from
entering the liquid passage, while gravity prevents the liquid from
entering the vapor passage. The vapor and liquid are combined and
inserted into the heat exchanger 92, which removes heat and
liquefies the vapor. A pump 110 draws the cooled cooling fluid up
into a reservoir 112, where it is again provided to the
sprayers.
[0040] A number of potentially useful system parameters can be
sensed in this system, including: The temperature of the
semiconductor devices (T.sub.j) (i.e., the junction temperature),
which can often be sensed from within the device; The ambient
temperature (T.sub.a) and pressure (P.sub.a), as well as the vapor
pressure (P.sub.v), in the spray chamber 94, which can be sensed
using temperature and pressure sensors 114 within the spray
compartment; The mass flow of the vapor ({dot over (m)}.sub.v) and
the mass flow of the liquid ({dot over (m)}.sub.l), which can be
sensed using appropriate sensors 116, 118 in respective vapor and
liquid passages 120, 122; The temperature (T.sub.sc of the
sub-cooled liquid coming out of the heat pump 92, which can be
sensed by a temperature sensor; And the temperature (T.sub.s) of
the liquid being received by the sprayer.
[0041] With reference to FIG. 7, a method of adjusting the
sprayers' mass flow rate ({dot over (m)}.sub.s) for the device
depicted in FIG. 6 begins with the steps of by starting the cooling
system 120 and setting 122 the sprayers' initial mass flow rate at
an initial value ({dot over (m)}.sub.s,init). This value typically
would be based on prior experience with this system, or with
systems of its type, but could also be based on calculated heat
generations rates and cooling rates. A limited amount of time (t)
is preferably allowed to pass 124 so that the system can begin
functioning, and then the sensing logic begins to take action,
i.e., the cooling system begins sensing and monitoring parameters
and adjusting the sensors' mass flow rate.
[0042] In particular, the temperature of the semiconductor devices
(T.sub.j) is sensed 126, and the resulting sensor value is compared
128 to a selected maximum value T.sub.max. If the resulting sensor
value is below the selected maximum value then no action is taken,
and the monitoring of parameters is repeated. If, however, the
semiconductor device has reached the selected maximum value, then
sensors are used to determine if pooling is occurring. Preferably,
to detect pooling, the mass flow of the vapor ({dot over
(m)}.sub.v) is sensed 130 and compared 132 to a selected minimum
value ({dot over (m)}.sub.v,min) to verify that it is above that
value {dot over (m)}.sub.v,min. The selected minimum value ({dot
over (m)}.sub.v,min) typically would be based on prior experience
with this system, or with systems of its type, but could also be
based on calculated heat generations rates and cooling rates.
[0043] If the mass flow of the vapor ({dot over (m)}.sub.v) is
above the selected minimum value, then pooling is not a problem,
and the sprayers' mass flow rate should be increased 134 to reduce
the temperature (T.sub.j). However, if the mass flow rate of the
vapor is not above the minimum value, then pooling is occurring and
the sprayers' mass flow rate is decreased 136 to increase the
cooling system's effectiveness. After the sprayers' mass flow rate
is incrementally adjusted, either up or down, the monitoring is
continued by again sensing the temperature of the semiconductor
devices (T.sub.j). It should be noted that the orientation of the
sprayed surface (with respect to gravity) might have an effect on
the accurate sensing of pooling, and that experimentation can be
used to verify and/or adjust the selected minimum value {dot over
(m)}.sub.v,min accordingly.
[0044] In the alternative, other sensors can be used to determine
if pooling is occurring. For example, the vapor pressure (P.sub.v)
in the spray chamber is a more direct measure of whether pooling is
occurring.
[0045] FIG. 8 is a flowchart depicting a method of adjusting the
sprayers' mass flow rate ({dot over (m)}.sub.s) for a cooling
device having an internal heat exchanger, such as the device
depicted in FIG. 5. The method begins with the steps of starting
the cooling system 140 and setting the sprayers' initial mass flow
rate 142 at an initial value ({dot over (m)}.sub.s,init). This
value typically would be based on experimentation, and/or prior
experience with this system or systems of its type, but it could be
based on an analysis of temperature generation rates and cooling
rates. A limited amount of time (t) is preferably allowed to pass
144 prior to starting the actions of the sensing logic, so that the
system can begin functioning, and the cooling system can begin
sensing and monitoring parameters and adjusting the sensors' mass
flow rate. The time (t) is related to the time constant of the
system, i.e., the time needed for the system to reach operating
temperatures.
[0046] In particular, the temperature of the semiconductor devices
(T.sub.j) is sensed 146, and the resulting sensor value is compared
148 to a selected maximum value T.sub.max. If the resulting sensor
value is below the selected maximum value then no action is taken,
and the monitoring of parameters is repeated. If, however, the
semiconductor device has reached the selected maximum value, then
sensors are used to determine if pooling is occurring. Preferably,
to detect pooling, the vapor pressure (P.sub.v) in the spray
chamber is sensed 150 and compared 152 to a selected minimum value
(P.sub.v,min) to verify that it is above the selected minimum
value. To aid in the accurate sensing of the vapor pressure, the
system is preferably configured with an internal pressure below
atmospheric pressure. The selected minimum value (P.sub.v,min) is
not easy to calculate, and is preferably determined
empirically.
[0047] If the vapor pressure (P.sub.v) in the spray chamber is
above the selected minimum value, then pooling is not a problem,
and the sprayers' mass flow rate should be increased 154 to reduce
the temperature (T.sub.j). However, if the vapor pressure (P.sub.v)
is not above the minimum value, then pooling is occurring and the
sprayers' mass flow rate is decreased 156 to increase the cooling
system's effectiveness. After the sprayers' mass flow rate is
incrementally adjusted, either up or down, the monitoring is
continued by again sensing the temperature of the semiconductor
devices (T.sub.j).
[0048] More generally, it will be seen that any sensor reading
indicative of the semiconductor's temperature, including direct
readings or indirect readings (such as heat dissipation when the
heat generation rate is known) can be used to judge whether the
cooling is adequate. Furthermore, it will be seen that any sensor
reading indicative of pooling, such as vapor flow rate, liquid flow
rate, vapor pressure, or others, can be used to judge whether the
cooling would be improved by increasing or decreasing the spray
flowrate. Additionally, it will be appreciated that, while the
order of sensing and decision making contributes to the efficiency
of the system, it can be varied within the scope of the invention.
For example, both temperature and pooling can be sensed prior to
any comparisons. Likewise, pooling can be sensed and compared to a
reference value prior to sensing the semiconductor (or other
heat-generating device) temperature.
[0049] From the foregoing description, it will be appreciated that
the present invention provides an accurate, reliable and cost
efficient spray cooling system. The system includes a sprayer
configured to deliver cooling fluid to a heat-generating device in
limited increments. Preferably, the sprayer is thermally driven in
a fashion similar to that of an ink-jet print head.
[0050] While a particular form of the invention has been
illustrated and described, it will be apparent that various
modifications can be made without departing from the spirit and
scope of the invention. Thus, although the invention has been
described in detail with reference only to the preferred
embodiments, those having ordinary skill in the art will appreciate
that various modifications can be made without departing from the
invention. Accordingly, the invention is not intended to be
limited, and is defined with reference to the following claims.
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