U.S. patent application number 11/395696 was filed with the patent office on 2007-10-04 for evaporatively cooled thermosiphon.
Invention is credited to Mohinder Singh Bhatti, Ilya Reyzin.
Application Number | 20070227703 11/395696 |
Document ID | / |
Family ID | 38557130 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070227703 |
Kind Code |
A1 |
Bhatti; Mohinder Singh ; et
al. |
October 4, 2007 |
Evaporatively cooled thermosiphon
Abstract
A thermosiphon cooling assembly cools an electronic device with
a first refrigerant disposed in the lower boiling chamber of a
housing for liquid-to-vapor transformation and a second refrigerant
disposed in an upper evaporating chamber of a housing for
liquid-to-vapor transformation. The partition separating the lower
boiling chamber of the housing from the upper evaporating chamber
of the housing creates a series of vapor chambers within the lower
boiling portion for condensing vapor boiled off the first
refrigerant. The upper evaporating chamber contains a series of
refrigerant pockets interleaved vertically with the vapor chambers
to increase the surface area for heat transfer between the
refrigerant vapor and the second refrigerant for absorbing heat by
the second refrigerant for liquid-to-vapor transformation.
Inventors: |
Bhatti; Mohinder Singh;
(Amherst, NY) ; Reyzin; Ilya; (Williamsville,
NY) |
Correspondence
Address: |
DELPHI TECHNOLOGIES, INC.
M/C 480-410-202
PO BOX 5052
TROY
MI
48007
US
|
Family ID: |
38557130 |
Appl. No.: |
11/395696 |
Filed: |
March 31, 2006 |
Current U.S.
Class: |
165/104.26 ;
165/104.33; 257/E23.088 |
Current CPC
Class: |
F28D 15/02 20130101;
F28F 2215/00 20130101; H01L 2924/0002 20130101; H01L 23/427
20130101; F28D 15/0266 20130101; F28F 3/048 20130101; H01L
2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/104.26 ;
165/104.33 |
International
Class: |
F28D 15/04 20060101
F28D015/04 |
Claims
1. A thermosiphon cooling assembly for cooling an electronic
device, comprising; a housing having a partition defining a lower
boiling chamber for receiving heat from the electronic device and
an upper evaporating chamber, a first refrigerant disposed below
said partition in said lower boiling chamber of said housing for
liquid-to-vapor transformation, a second refrigerant disposed above
said partition in said upper evaporating chamber of said housing
for liquid-to-vapor transformation, said partition defining a
plurality of vapor chambers extending upwardly above said lower
boiling chamber for condensing vapor boiled off said first
refrigerant, and said upper evaporating chamber including a
plurality of refrigerant pockets interleaved with said vapor
chamber for holding said second refrigerant and an open space above
said refrigerant pockets for condensing vapor boiled off said
second refrigerant.
2. An assembly as set forth in claim 1 wherein said first
refrigerant has a higher boiling temperature than the boiling
temperature of said second refrigerant.
3. An assembly as set forth in claim 2 wherein said first
refrigerant has a first heat capacity (mh.sub.fg).sub.1 and said
second refrigerant has a second heat capacity (mh.sub.fg).sub.2 and
(mh.sub.fg).sub.1=(mh.sub.fg).sub.2 wherein m is the refrigerant
mass and h.sub.fg is the latent heat of evaporation of the
refrigerant and is defined as h.sub.fg=.alpha.(1-T/T.sub.c).sup.3/8
wherein T is the absolute temperature of interest and T.sub.c is
the critical temperature of the refrigerant and .alpha. is a
constant and is defined as .alpha.=.beta.RT.sub.c/J wherein R is
the gas constant of the refrigerant and T.sub.c is the critical
temperature of the refrigerant and J is the mechanical-to-thermal
energy conversion factor and is J=778.163 ft*lb.sub.f/Btu .beta. is
a dimensionless constant and is defined as
.beta.=ln(P.sub.n/P.sub.c)/(1-T.sub.c/T.sub.n) wherein P.sub.n is
the atmospheric pressure and P.sub.c is the critical pressure for
the refrigerant and T.sub.c is the critical temperature of the
refrigerant and T.sub.n is the normal boiling temperature of the
refrigerant.
4. An assembly as set forth in claim 2 including a condensing unit
operatively attached to said upper evaporating unit.
5. An assembly as set forth in claim 4 including a capillary tube
interconnecting said upper evaporating chamber of said housing and
said condensing unit.
6. An assembly as set forth in claim 5 including a wick material
disposed on said refrigerant pockets and said wick material
disposed in said upper evaporating chamber of said housing and
extending into said capillary tube for conveying liquid from said
condensing unit to said refrigerant pockets.
7. An assembly as set forth in claim 6 wherein said capillary tube
is flexible for moving said condensing unit relative to said
housing.
8. An assembly as set forth in claim 4 wherein said condensing unit
defines a lower portion and an upper portion with said upper
portion having a plurality of spaced radiation chambers extending
through said condensing unit and a plurality of condensing fins
disposed within said radiation chambers.
9. An assembly as set forth in claim 8 including an air moving
device for moving air through said radiation chambers and over said
condensing fins.
10. An assembly as set forth in claim 2 including a plurality of
boiler fins disposed in said lower boiling chamber of said housing
for enhancing heat transfer to said first refrigerant.
11. An assembly as set forth in claim 5 including a plurality of
capillary fins disposed on said capillary tube for enhancing heat
transfer from said second refrigerant.
12. A thermosiphon cooling assembly for cooling an electronic
device, comprising; a housing having a partition defining a lower
boiling chamber for receiving heat from the electronic device and
an upper evaporating chamber, a first refrigerant disposed below
said partition in said lower boiling chamber of said housing for
liquid-to-vapor transformation, a plurality of boiler fins disposed
in said lower boiling chamber of said housing for enhancing heat
transfer to said first refrigerant, a second refrigerant disposed
above said partition in said upper evaporating chamber of said
housing for liquid-to-vapor transformation, a condensing unit
defining a lower portion and an upper portion with said upper
portion having a plurality of spaced radiation chambers extending
through said condensing unit, a plurality of condensing fins
disposed within said radiation chambers, an air moving device for
moving air through said radiation chambers and over said condensing
fins, a capillary tube interconnecting said upper evaporating
chamber of said housing and said lower portion of said condensing
unit, said capillary tube being flexible for moving said condensing
unit relative to said housing, a plurality of capillary fins
disposed on said capillary tube for enhancing heat transfer from
said second refrigerant, a wick material disposed in said upper
evaporating chamber of said housing and extending into said
capillary tube for conveying liquid from said condensing unit to
said upper evaporating chamber of said housing, said partition
defining a plurality of spaced vapor chambers extending upwardly
above said lower boiling chamber for condensing vapor boiled off
said first refrigerant, said upper evaporating chamber including a
plurality of refrigerant pockets interleaved with said vapor
chambers for holding said second refrigerant and an open space
above said refrigerant pockets for condensing vapor boiled off said
second refrigerant, said wick material disposed on said refrigerant
pockets, and said first refrigerant having a higher boiling
temperature than the boiling temperature of said second
refrigerant,
13. An assembly as set forth in claim 12 wherein said first
refrigerant has a first heat capacity (mh.sub.fg).sub.1 and said
second refrigerant has a second heat capacity (mh.sub.fg).sub.2 and
(mh.sub.fg).sub.1=(mh.sub.fg).sub.2 wherein m is the refrigerant
mass and h.sub.fg is the latent heat of evaporation of the
refrigerant and is defined as h.sub.fg=.alpha.(1-T/T.sub.c).sup.3/8
wherein T is the absolute temperature of interest and T.sub.c is
the critical temperature of the refrigerant and .alpha. is a
constant and is defined as .alpha.=.beta.RT.sub.c/J wherein R is
the gas constant of the refrigerant and T.sub.c is the critical
temperature of the refrigerant and J is the mechanical-to-thermal
energy conversion factor and is J=778.163 ft*lb.sub.f/Btu .beta.=is
a dimensionless constant and is defined as
.beta.=ln(P.sub.n/P.sub.c)(1-T.sub.c/T.sub.n) wherein P.sub.n is
the atmospheric pressure and P.sub.c is the critical pressure for
the refrigerant and T.sub.c is the critical temperature of the
refrigerant and T.sub.n is the normal boiling temperature of the
refrigerant.
14. A method of cooling an electronic device comprising the steps
of; generating heat by an electronic device, transferring the heat
generated by the electronic device to the lower boiling chamber of
a housing, a boiling a first refrigerant in the lower boiling
chamber of the housing from liquid-to-vapor at a first temperature,
transferring heat from the vapor of the first refrigerant to a
second refrigerant in the upper evaporating chamber of the housing,
and boiling the second refrigerant in the upper evaporating chamber
of the housing from liquid-to-vapor at a second temperature lower
than the first temperature.
15. A method as set forth in claim 14 including condensing the
vapor of the second refrigerant in a condensing unit and wicking
the condensed liquid refrigerant through a capillary tube to the
upper evaporating chamber of the housing.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The subject invention relates to a thermosiphon cooling
assembly for cooling an electronic device.
[0003] 2. Description of the Prior Art
[0004] The operating speed of computers is constantly being
improved to create faster and faster computers. With this, comes
increased heat generation and a need to effectively dissipate that
heat.
[0005] Heat exchangers and heat sink assemblies have been used that
apply natural or forced convection cooling methods to dissipate
heat from electronic devices that are highly concentrated heat
sources such as microprocessors and computer chips. These heat
exchangers typically use air to directly remove heat from the
electronic devices; however air has a relatively low heat capacity.
Thus, liquid-cooled units called LCUs employing a cold plate in
conjunction with high heat capacity fluids have been used to remove
heat from these types of heat sources. Although LCUs are
satisfactory for moderate heat flux, increasing computing speeds
have required more effective heat sink assemblies.
[0006] Accordingly, thermosiphon cooling units (TCUs) have been
used for cooling electronic devices having a high heat flux. A
typical TCU absorbs heat generated by the electronic device by
vaporizing the working fluid housed on the boiler plate of the
unit. The boiling of the working fluid constitutes a phase change
from liquid-to-vapor state and as such the working fluid of the TCU
is considered to be a two-phase fluid. The vapor generated during
boiling of the working fluid is then transferred to a condenser,
where it is liquified by the process of film condensation over the
condensing surface of the TCU. The heat is rejected into a stream
of air flowing through a tube running through the condenser or
flowing over fins extending from the condenser. The condensed
liquid is returned back to the boiler plate by gravity to continue
the boiling-condensing cycle.
[0007] The ability of a TCU to dissipate heat can be further
improved by adding an additional working fluid cycle. The TCU would
absorb heat generated by the electronic device by vaporizing the
first working fluid housed on the boiler plate of the unit. The
vapor generated during boiling of the first working fluid is then
transferred to a first condenser, where it is liquified by the
process of a second evaporator chamber absorbing the heat generated
by the vapor. The second evaporator chamber would absorb the heat
generated by the vapor by vaporizing the second working fluid
housed within the second evaporator chamber. The vapor generated
during the boiling of the second working fluid is then transferred
to a second condenser, where it is liquified by the process of film
condensation. The heat is rejected into air flowing over or through
the condenser, or alternatively into another refrigerant.
[0008] Additionally, with the decreasing size of computers and the
increasing portability, new challenges have been presented
regarding the ability of cooling units to effectively dissipate
heat. The decreasing size has created a demand for smaller cooling
units that are able to be fit into tighter spaces. To meet this
demand, many TCUs have been created out of a flexible material to
allow for an increased ability to be positioned in cramped spaces.
The portability of computers has created the need for TCUs that are
able to function without the assistance of gravity. Wicking
materials have been developed which allow a TCU to transport a
liquid regardless of the effect of gravity. A typical TCU utilizing
a wicking material contains an evaporator, a condenser, and
capillary tubes lined with a wicking material interconnecting the
evaporator and condenser. The capillary tubes lined with the
wicking material allow refrigerant vapor to flow from the
evaporator to the condenser while the wicking material returns the
condensed refrigerant in the condenser to the evaporator without
the assistance of gravity.
[0009] An example of a cooling system for electronic devices is
disclosed in U.S. Pat. No. 5,647,429 to Oktay. The Oktay patent
discloses an assembly for cooling an electronic device having a
flexible housing including an evaporator and a condenser joined by
a hollow adiabatic section containing a wicking material. The
assembly is additionally adapted so that two or more consecutive
assemblies can be joined together by connecting the condenser of
one housing to the evaporator of a subsequent housing. The Oktay
patent provides for multiple TCUs to be joined together in series
to effectively cool a single electronic device.
[0010] Although the prior art dissipates heat from electronic
devices, as computing speeds increase, there is a continuing need
for alternative cooling devices having more efficient heat transfer
capabilities.
SUMMARY OF THE INVENTION AND ADVANTAGES
[0011] In accordance with the subject invention, heat generated by
an electronic device is transferred to a housing having a lower
boiling chamber and an upper evaporating chamber. A first
refrigerant is disposed within the lower boiling chamber of the
housing for liquid-to-vapor transformation, and a second
refrigerant is disposed in the upper evaporating chamber of the
housing for liquid-to-vapor transformation. The assembly employs a
partition to define the lower boiling chamber of the housing, and
the partition forms a series of vapor chambers in the lower boiling
chamber. The heat generated by the electrical device is transferred
to the lower boiling chamber of the housing and is absorbed by the
first refrigerant causing the first refrigerant to boil. The vapor
boiled off the first refrigerant as a result of the heat absorbed
from the electronic device gathers in the vapor chambers and heat
is transferred from the vapor contained within the vapor chambers
to a plurality of refrigerant pockets contained within the upper
boiling portion of the housing that are interleaved with the vapor
chambers. The second refrigerant that is disposed within the upper
evaporating chamber of the housing gathers in these refrigerant
pockets and absorbs the heat transferred from the vapor chambers.
The vapor boiled off the second refrigerant as a result of the heat
condenses in an open space above the refrigerant pockets and
returns to the refrigerant pools occupying the pockets.
[0012] The present invention utilizes the series of vapor chambers
in the lower boiling portion of the housing interleaved with the
series of refrigerant pockets in the upper evaporating portion of
the housing to increase the surface area between the lower boiling
portion of the housing and the upper evaporating portion of the
housing to enhance heat transfer between the lower and upper
portions of the housing. The series of vapor chambers and
refrigerant pockets increases the rate of heat transfer providing
for smaller TCUs that more effectively dissipate heat from an
electronic device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other advantages of the present invention will be readily
appreciated, as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings wherein:
[0014] FIG. 1 is a perspective view of the thermosiphon cooling
assembly showing the housing and condensing unit cut away;
[0015] FIG. 2 is a cross sectional view of the assembly shown in
FIG. 1; and
[0016] FIG. 3 is a cross sectional view of the assembly shown in
FIG. 1 with the condensing unit placed below the housing.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Referring to the Figures, wherein like numerals indicate
corresponding parts throughout the several views, a thermosiphon
cooling assembly 20 is shown generally for cooling an electronic
device 22.
[0018] A thermosiphon cooling assembly 20 comprises a housing 24
generally indicated having a partition 26 defining a lower boiling
chamber 28 for receiving heat from the electronic device 22 and an
upper evaporating chamber 30. The assembly 20 is used to cool the
electronic device 22 engaging or secured to the exterior of the
lower boiling chamber 28 of the housing 24. The housing 24 is
generally rectangular with the lower boiling chamber 28 being
hermetically sealed to contain a first refrigerant 32 for
liquid-to-vapor transformation.
[0019] The first refrigerant 32 is disposed below the partition 26
in the lower boiling chamber 28 of the housing 24 for
liquid-to-vapor transformation. The first refrigerant 32 is
confined to the lower boiling chamber 28 of the housing 24 and
absorbs heat generated by the electronic device 22 and transferred
into the lower boiling chamber 28 of the housing 24. The first
refrigerant 32 is boiled and evaporated by the heat transferred
from the electronic device 22 and the resultant vapor is later
condensed and returned to the refrigerant pool. The first
refrigerant 32 is preferably essentially dielectric in character to
prevent any electrocution hazard in the event of leakage of the
fluid. A refrigerant like R-134a is suitable for this purpose.
[0020] A plurality of boiler fins 34 extend from the bottom of the
lower boiling chamber 28 of the housing 24 for increasing heat
transfer from the electronic device 22 to the interior of the lower
boiling chamber 28 of the housing 24. The boiler fins 34 extend
upwardly toward the upper evaporating chamber 30 of the housing 24.
The boiler fins 34 are disposed in the lower boiling chamber 28 of
the housing 24 for transferring heat from the electronic device 22
disposed on the exterior of the lower boiling chamber 28 of the
housing 24 by boiling the first refrigerant 32 in the lower boiling
chamber 28 of the housing 24.
[0021] A second refrigerant 36 is disposed above the partition 26
in the upper evaporating chamber 30 of the housing 24 for
liquid-to-vapor transformation. The second refrigerant 36 absorbs
heat transferred to the upper evaporating chamber 30 by the first
refrigerant 32 disposed within the lower boiling chamber 28. The
second refrigerant 36 has a thermal conductivity larger or greater
than that of air for increasing the heat transfer between
refrigerant vapor and the second refrigerant 36. The second
refrigerant 36 is also preferably essentially dielectric in
character to prevent any electrocution hazard in the event of
leakage of the fluid. A refrigerant like R-134a is suitable for
this purpose.
[0022] The assembly 20 has a condensing unit 38 generally indicated
having a lower portion 40 and an upper portion 42 with the upper
portion 42 having a plurality of spaced radiation chambers 44
extending through the condensing unit 38. The lower portion 40 of
the housing 24 is generally rectangular and the upper portion 42
generally consists of a plurality of rectangular radiation chambers
44 extending perpendicular to the lower portion 40 of the
condensing unit 38 at spaced intervals. The radiation chambers 44
house refrigerant vapor awaiting condensation while the lower
portion 40 provides a reservoir for the cooled liquid refrigerant.
The condensing unit 38 is preferably oriented as illustrated in
FIG. 2 so that the radiation chambers 44 are positioned above the
lower portion 40.
[0023] A plurality of condensing fins 46 extend from the exterior
of the radiation chambers 44 of the condensing unit 38 for
increasing the heat transfer from refrigerant vapor contained
within the radiation chambers 44 to the exterior environment. The
condensing fins 46 as illustrated in FIG. 1 preferably extend from
one radiation chamber 44 to an adjacent radiation chamber 44 and
back to the original radiation chamber 44 forming a channel for air
to pass through. The condensing fins 46 are disposed within the
radiation chambers 44 for transferring heat contained within the
condensing unit 38 to the exterior environment of the assembly
20.
[0024] As illustrated in FIG. 1, an air moving device 48 for moving
air through the radiation chambers 44 and over the condensing fins
46, here shown as a single axial fan, can be positioned within the
proximity of the condensing unit 38. The air moving device 48
increases the flow of air over the condensing fins 46 to increase
the rate at which heat contained within the condensing unit 38 is
transferred to the surrounding environment. The air moving device
48 can either be of the push or pull type.
[0025] A capillary tube 50 interconnects the upper evaporating
chamber 30 of the housing 24 and the lower portion 40 of the
condensing unit 38 to transfer vapor from the upper evaporating
chamber 30 of the housing 24 to the lower portion 40 of the
condensing unit 38 and to transfer condensed vapor from the lower
portion 40 of the condensing unit 38 to the upper evaporating
chamber 30 of the housing 24. The upper evaporating chamber 30 of
the housing 24 and the condensing unit 38 are hermetically sealed
about the capillary tube 50 to contain the second refrigerant 36
for liquid-to-vapor transformation. The capillary tube 50 is
preferably operatively attached to the top surface of the upper
evaporating chamber 30 opposite the second refrigerant 36 and
preferably operatively attached to the bottom surface of the lower
portion 40 of the condensing unit 38 opposite the radiation
chambers 44 as illustrated in FIG. 2.
[0026] The capillary tube 50 is flexible to move the condensing
unit 38 relative to the housing 24 for increasing the ability to
position the assembly 20 in tight spaces. The condensing unit 38
may be either positioned above the housing 24 as illustrated in
FIG. 2 or the condensing unit 38 may be positioned below the
housing 24 as illustrated in FIG. 3.
[0027] A plurality of capillary fins 52 extend from the exterior of
the capillary tube 50 for increasing heat transfer. The capillary
fins 52 preferably extend perpendicularly from the capillary tube
50. The capillary fins 52 are disposed on the capillary tube 50 for
transferring heat from refrigerant vapor contained within the
capillary tube 50 to the exterior environment of the assembly
20.
[0028] A wick material 54 is disposed in the upper evaporating
chamber 30 of the housing 24 and extending into the capillary tube
50 for conveying liquid from the condensing unit 38 to the upper
evaporating chamber 30 of the housing 24. Refrigerant vapor travels
through the capillary tube 50 to the condensing unit 38 to be
cooled whereupon the wick material 54 is able to return the liquid
refrigerant back to the upper evaporating chamber 30 of the housing
24 substantially unaided by gravity. A wick material 54 formed by a
cladless metal coating process, which is capable of yielding about
fifty percent porosity required to wick the refrigerant liquid,
could be used.
[0029] As illustrated by FIG. 2, the partition 26 separating the
lower boiling chamber 28 of the housing 24 from the upper boiling
chamber of the housing 24 defines a plurality of spaced vapor
chambers 56 extending upwardly above the lower boiling chamber 28
for condensing vapor boiled off the first refrigerant 32. The vapor
chambers 56 are rectangular and extend from the lower boiling
chamber 28 towards the upper evaporating chamber 30. The vapor
chambers 56 provide a receptacle for refrigerant vapor boiled off
the first refrigerant 32.
[0030] The refrigerant vapor is able to occupy the vapor chambers
56 until it is condensed to liquid form whereupon gravity will
return the condensed refrigerant to the reservoir for the first
refrigerant 32. The vapor chambers 56 preferably extend
perpendicularly above the lower boiling chamber 28.
[0031] The upper evaporating chamber 30 of the housing 24 includes
a plurality of refrigerant pockets 58 interleaved with the vapor
chambers 56 for holding the second refrigerant 36 and an open space
60 above the refrigerant pockets 58 for condensing vapor boiled off
the second refrigerant 36. FIG. 2 illustrates the refrigerant
pockets 58 containing the second refrigerant 36. The refrigerant
pockets 58 are preferably rectangular in shape and extend from the
upper evaporating chamber 30 towards the lower boiling chamber 28
and are adjacent to two separate vapor chambers 56. The refrigerant
pockets 58 also preferably extend downwardly from the upper
evaporating chamber 30 creating a depth that is equal to the height
the vapor chambers 56 extend upwardly from the lower boiling
chamber 28. As shown in FIG. 2, the vapor chambers 56 and
refrigerant pockets 58 are located adjacent to one and other where
the partition 26 is a common side to two adjacent chambers 28, 30,
44, 56. The refrigerant pockets 58 increase the surface area
between the upper evaporating chamber 30 and the lower boiling
chamber 28 for increasing the heat transfer from the evaporated
refrigerant collected within the vapor chambers 56 and the second
refrigerant 36 contained within the refrigerant pockets 58.
[0032] The wick material 54 is disposed on the refrigerant pockets
58 for enhancing heat transfer from the vapor chambers 56 to the
refrigerant pockets 58. The wick material 54 lining the refrigerant
pockets 58 enhances heat transfer by promoting nucleate boiling
within the upper evaporating chamber 30. A wick material 54 formed
by a cladless metal coating process, which is capable of yielding
about fifty percent porosity required to wick the refrigerant
liquid, could be used.
[0033] The present invention utilizes the first refrigerant 32
disposed in the lower boiling chamber 28 of the housing 24 and the
second refrigerant 36 disposed in the upper evaporating chamber 30
of the housing 24 to remove heat by ebullition. The heat transfer
rate of the second refrigerant 36 for liquid-to-vapor
transformation is inherently higher than that of air or a
single-phase fluid enhancing the cooling capacity of the TCU. Thus
to ensure the second refrigerant 36 will undergo liquid-to-vapor
transformation, the second refrigerant 36 preferably has a boiling
temperature less than the boiling temperature of the first
refrigerant 32. Assuming the assembly 20 initially starts with a
negligible amount of energy, a certain amount of energy will be
transferred to the assembly 20 as heat from the electronic device
22 causing the first refrigerant 32 to boil, and a specific amount
of energy determined by the heat of vaporization of the first
refrigerant 32 will be used up to completely vaporize a unit of the
first refrigerant 32. As a result, an amount of energy less than
that required to boil the first refrigerant 32 will be transferred
from the refrigerant vapor to the second refrigerant 36. Therefore,
having a second refrigerant 36 with a lower boiling temperature
than the first refrigerant 32 will ensure ebullition of the second
refrigerant 36 and increase the effectiveness of the assembly
20.
[0034] The present invention additionally utilizes a criterion that
the heat capacity of the first refrigerant 32 must be equal to the
heat capacity of the second refrigerant 36 in order to ensure a
proper charge balance. The efficiency of any cooling system suffers
if there is too much or too little refrigerant charge. In a single
coolant system, if there is too little charge the evaporator
capacity is reduced because less of its surface is wetted and the
average evaporator temperature differential increases across the
evaporator. If there is too much charge, refrigerant may back up in
the condenser reducing the effective surface area of the condenser
and increasing the average temperature differential across the
condenser. Therefore, to ensure efficient heat transfer in the
present assembly 20, the charge between the first refrigerant 32
and the second refrigerant 36 must be balanced so as to not affect
the charge balance of the assembly 20. If the charge between the
first refrigerant 32 and the second refrigerant 36 is too high, the
evaporator capacity of the lower boiling portion of the housing 24
will be reduced. If the charge between the first refrigerant 32 and
the second refrigerant 36 is too low, the evaporator capacity of
the upper evaporating chamber 30 of the housing 24 will be reduced.
Therefore, to ensure efficient heat transfer, the charges of the
two refrigerants 32, 36 must be balanced by equating the heat
capacities of the two refrigerants 32, 36. The first refrigerant 32
has a heat capacity equal to (mh.sub.fg).sub.1 and the second
refrigerant 36 has a second heat capacity equal to
(mh.sub.fg).sub.2 and the charge balance of the two refrigerants
32, 36 is based on the criterion that the heat capacity of the two
refrigerants 32, 36 must be equal. Thus
(mh.sub.fg).sub.1=(mh.sub.fg).sub.2 (1)
[0035] where the subscripts 1 and 2 refer to the two refrigerants
32, 36 and
[0036] m is the refrigerant mass, lb.sub.m
[0037] h.sub.fg is the latent heat of evaporation of the
refrigerant, Btu/lb.sub.m
[0038] The latent heat of evaporation of the refrigerant is given
by the relation h.sub.fg.alpha.(1-T/T.sub.c).sup.3/8 (2)
[0039] where
[0040] .alpha. is a constant defined below, Btu/lb.sub.m
[0041] T is the absolute temperature of interest, .degree.R
[0042] T.sub.c is the critical temperature of the refrigerant,
.degree.R .alpha.=.beta.RT.sub.c/J (3)
[0043] where
[0044] .beta. is a dimensionless constant defined below
[0045] R is the gas constant of the refrigerant,
ft*lb.sub.f/lb.sub.m*.degree.R
[0046] T.sub.c is the critical temperature of the refrigerant,
.degree.R
[0047] J is mechanical-to-thermal energy conversion factor=778.163
ft*lb.sub.f/Btu .beta.=ln(P.sub.n/P.sub.c)/(1-T.sub.c/T.sub.n)
(4)
[0048] where
[0049] P.sub.n is the atmospheric pressure, lbf/ft.sup.2
[0050] P.sub.c is the critical pressure of the refrigerant,
lbf/ft.sup.2
[0051] T.sub.c is the critical temperature of the refrigerant,
.degree.R
[0052] T.sub.n is the normal boiling temperature of the
refrigerant, .degree.R
[0053] Based on the foregoing simple relations the charge balance
for the two refrigerants 32, 36 can be determined in a
straightforward fashion as follows:
[0054] 1. Determine the dimensionless constant .beta. for the first
refrigerant 32 from Eq. (4) knowing the refrigerant specific
constant values.
[0055] 2. Determine the dimensionless constant .beta. for the
second refrigerant 36 from Eq. (4) knowing the refrigerant specific
constant values.
[0056] 3. Determine the dimensional constant .alpha. for the first
refrigerant 32 from Eq. (3) knowing the refrigerant specific
constant values.
[0057] 4. Determine the dimensional constant .alpha. for the second
refrigerant 36 from Eq. (3) knowing the refrigerant specific
constant values.
[0058] 5. Determine the latent heat of evaporation h.sub.fg of the
first refrigerant 32 in the lower boiling chamber 28 corresponding
to T=T.sub.b where T.sub.b is the boiler plate temperature.
[0059] 6. Determine the latent heat of evaporation h.sub.fg of the
second refrigerant 36 in the upper evaporating chamber 30
corresponding to T=T.sub.60 where T.sub..alpha. is the cooling
medium temperature.
[0060] 7. Knowing h.sub.fg values of the two refrigerants 32, 36
from steps 5 and 6 determine the mass ratio of the two refrigerants
32, 36 from Eq. (1).
[0061] The operation of the assembly 20 is incorporated into a
liquid cooling system as illustrated in FIG. 1. The electronic
device 22 generates an amount of heat to be dissipated and the heat
is transferred from the electronic device 22 to the lower boiling
chamber 28 of the housing 24. The heat is then conducted from the
lower boiling chamber 28 to the boiler fins 34 and thence to the
first refrigerant 32. Vapor boiled off the first refrigerant 32
generates an amount of heat to be dissipated and the heat is
transferred from the vapor to the upper evaporating chamber 30 of
the housing 24. The heat is then conducted from the upper
evaporating chamber 30 to the second refrigerant 36. Vapor boiled
off the second refrigerant 36 generates an amount of heat to be
dissipated and the heat is transferred from the vapor through a
capillary tube 50 to a condensing unit 38. The heat is then
conducted from the condensing unit 38 to condensing fins 46 to air
forced through the condensing fins 46 by an air moving device
48.
[0062] The invention therefore provides a method of cooling an
electronic device 22 by transferring heat generated by an
electronic device 22 to a lower boiling chamber 28 of a housing 24,
boiling a first refrigerant 32 in the lower boiling portion of the
housing 24 from liquid-to-vapor at a first temperature, and
transferring heat from the vapor of the first refrigerant 32 to a
second refrigerant 36 in an upper evaporating chamber 30 of the
housing 24. The method is distinguished by boiling the second
refrigerant 36 in the upper evaporating chamber 30 of the housing
24 from liquid-to-vapor at a second temperature lower than the
first temperature. The method is further distinguished by
condensing the vapor of the second refrigerant 36 in a condensing
unit 38 and wicking the condensed liquid refrigerant through a
capillary tube 50 to the upper evaporating chamber 30 of the
housing 24.
[0063] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. The
invention may be practiced otherwise than as specifically described
within the scope of the appended claims.
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