Vapor Cooling in Electronicsa

7/26/2019 Vapor Cooling in Electronicsa

1/53

Outline Benefits Disadvantages History Basic operation Engineering Design

Cold plate Refrigerants Capillary tube Condensation

Product Reliability Current applications:

Supercomputer and Mainframe Cooling Current researc:

Microscale !CRS


2/53

Benefits "llo#s cooling to belo# ambient temperatures$ increasing

performance$ reliability$ and allo#ing operation in igertemperatures%

Hig COP &around ' to ()%

"bility to remove very large eat loads% *idely available+ compressor and fan only moving parts$

stable$ and reliable% &moran$ ',,-)% .o# mass flo# rate of refrigerant needed% "bility to transport eat a#ay from its source%

COP up to ( times greater tan termoelectric coolers ormore%

&peeples$ ',,-)


3/53

BenefitsSub ambient temperature operation allo#sCMOS &complementary/symmetry0metal/o1ide semiconductor) transistors to s#itc on

and off faster &peeples$ ',,-)%CMOS circuits are a ma2or class ofintegrated circuit and includemicroprocessors$ microcontrollers$ and oter

digital logic circuits &*i3ipedia$ ttp:00en%#i3ipedia%org0#i3i0CMOS) %


4/53

Benefits 4e follo#ing pysical parameters favor lo# temperature

operation: carrier mobility$ andjunction leakage% "ltoug at lo# temperatures tere is an increase in te

failure rates due to ot carriers$ overall failure rates

decrease at lo#er temps due to te overall increasedcaracteristics mentioned previously &Moran$',,-)% Definitions:

Carrier mobility/ velocity of carge carriers in a solid material #it an electric field

applied to it&*i3ipedia$ ttp:00en%#i3ipedia%org0#i3i0Electron5mobility)%

Hot carriers/ ig energy carriers &Moran$',,-)% 6unction lea3age/undesirable conductive pats in certain components$ for

instance$ in capacitors% "lso$ a pat#ay troug #ic electric discarge mayslo#ly ta3e place (IEEE Standard Dictionary of Electrical and Electronics Terms).


5/53

Benefits

Muc li3e eat pipes$ vapor compressioncoolers use te ig eat of vapori7ations of te

#or3ing li8uid to remove large amounts of eat% 4erefore$ lo# mass flo# rates re8uired%"dvantage over a cilled li8uid loop tat re8uires a muc

iger mass flo# rate%

Muc li3e eat pipes$ !CCs use te ig eat of vapori7ation

of li8uids to transport large amounts of eat$ tus lo#amounts of li8uid are re8uired% Ho#ever$ in cilled li8uid loopcoolers$ te li8uid is eated but not necessarily evaporated%

&Cengel and Boles$ ',,')


6/53

Disadvantages

4e cost of employing te cooling system maybe -,/',9 of te cost of te entire system%

.arge space and input po#er% 4e disadvantages of te cooling system as tobe #eiged #it te large advantages% " lot oftimes$ traditional metods of cooling li3e aircooling is not feasible so a !CC is re8uired%

Ho#ever$ if te cooling re8uirements can besatisfied #it traditional tecni8ues$ tetraditional tecni8ue are te preferred coolingmetod &Moran$ ',,-)%


7/53

History 4e vapor compression refrigerator #as first

proposed in -,; and a model #as constructed in-(,?s companies li3e "MD$ Sun

Microsystems$ @BM$ and SAS 4ecnologies ad allused !apor Compression cooling%

&Scmidt et al%$ ',,')


8/53

Basic Operation

&Peeples$',,-)


9/53

Basic Operation Ideal cycle

-/' 4e #or3ing refrigerant$ a saturated vapor$ is carriedtroug te suction tube to te compressor% 4ecompressor compresses te saturated vapor into asupereated vapor #ic is ten passed to te condenser%

'/( 4e eat of te ot and ig pressure vapor is releasedinto te environment from te condenser% 4e #or3ing gasis transformed into a saturated li8uid%

(/< 4e li8uid is pumped troug a capillary tube or atermal e1pansion valve into te evaporator$ dropping

significantly in temperature% 4e #or3ing fluid is a saturatedmi1ture%


10/53

Basic Operation

' represents te actual

position of te state% 's

represents te position of

te state if it #ereirreversible%

@n te ideal case: -' isentropic &sconst)

'( isobaric &Pconst) (< isentalpic &const)


11/53

Basic Operation 4e eat absorbed by te evaporator &in) released from te

condenser &out)$ and actual and isentropic #or3 input by te

compressor can be determined from te e8uations:

4e efficiency of te compressor is given by:

*ere is te entalpy at various states$ te subscripts s and a

refer to isentropic and actual$ and # refers to #or3%

12

12

hh

hh

w

w s

a

s

c

==

)(

)(

)(

)(

12

12

23

41

hhmW

hhmW

hhmQ

hhmQ

aa

ss

out

in

=

=

=

=



12/53

Basic Operation

Actual cycle 4e deviation of te ideal cycle to te actual cylce is

due to irreversibilities+ mainly due to fluid friction/

causing pressure drops and eat transfer to tesystem or te environment%

@n te ideal cycle te state of te refrigerant isprecisely 3no#$ for e1ample$ at stage - te refrigerantis a saturated vapor% Ho#ever$ in actuality te state ofte refrigerant may not be precisely 3no#n and isusually a supereated vapor%



13/53

Basic Operationote tat te pressure drops bet#een stages /-$ '/($


14/53

Basic Operation



15/53

Basic Operation

4e Coefficient of performance of te !CR is te ratio of eat into te

evaporator to #or3 put into te compressor% Fenerally$ te COP is

around ' to (%

4e COP can be determined by finding te ratio of te entalpies

bet#een te trottling valve and evaporator and bet#een te

evaporator and te compressor%

c

e

W

QCOP

=

12

41

hh

hhCOP

=



16/53

Basic Operation 4e most efficient refrigeration cycle is tat of a Carnot

refrigerator%

4e coefficient of performance of te Carnot refrigerator

gives te ma1imum performance bet#een t#o ot and and

cold temperatures% 4is value can be compared to te actual COP to

determine o# close to ideal te refrigerator is operating%

ote tat te Carnot cycle is a reversible cycle% Reversible

cycles are cycles tat do not generate any entropy due tofriction$ eat transfer$ etc% Gor a reversible cycle:

L

H

reversL

H

T

T

Q

Q=



17/53

Basic Operation

4e coefficient of performance for a Carnot cycle is:

ote: te COP increases as te ratio of 4H04.decreases%

4erefore$ #e #ant 4Hand 4.to be as close as possible%

sually$ 4.is specified and 4Hcan be altered% 4e

reason for 4H0 4. affecting COP is tat for a given 4.tegreater te 4Hte greater te amount of #or3 input$

decreasing COP%

1/

1

1/

1

=

=

LHLH

CarnotTTQQ

COP



18/53

Basic Operation

4e area enclosed in te 4S diagramgenerally describes te net eat transfer

and te #or3 of te system% Gorrefrigeration$ #or3 input lo#ers te COPfor a given eat re2ection+ terefore$altering te states of te cycle to get a

smaller enclosed area #ill increaseperformance%



19/53

Engineering Foals

4e design of a cost/effective and efficient !CCinvolves te follo#ing considerations:

Cold surfaces can not be allo#ed to collectcondensate from te air%4e most suitable refrigerant for te given

application$ as #ell as te tubing to supply andremove te refrigerant$ must be cosen%

Compressor and condenser design%4e cold plate must efficiently lift eat from te device

to be cooled%

&Peeples$',,-)


20/53

Cold Plate0Evaporator

4e cold plate &evaporator) is a eat e1cangedevice #ic transfers eat from te eat sourceto te #or3ing fluid%

Cold plate design must assure efficient eattransfer from te device being cooled to terefrigerant inside te cold plate%

4e cold plate must be fabricated from tin andtermally conductive material to minimi7e termalresistance #ile maintaining structural stability%

4e mating surface bet#een te cold plate andte eated body must be flat and smoot tominimi7e contact resistance%

&Peeples$',,-)


21/53

Refrigerant Gluids

4e refrigerant?s pysical properties determineits evaporating temperature at a specifiedoperating temperature$ as #ell as$ its capacity totransport eat%

4e #ell 3no#n refrigerants R/-(


22/53

Refrigerant Gluids

*en ma3ing a decision as to #ic refrigerant to use andunder #at pressures to operate te !CR$ consideration ofte refrigerant?s temperature of vapori7ation and condensationsould be considered% @f te temperature of te refrigerant

doesn?t reac te vapori7ation point &at te operatingpressure) te !CR #ill not #or3 properly% 4o ensure a propereat transfer rate a ; to -, degree Celsius temperaturedifference sould be maintained bet#een te evaporator andte condenser #it te refrigerant%

4e ne1t slide so#s te P/4 curves of R/


23/53

Refrigerant Gluid Saturation

Pressure and 4emperature

&Peeples$',,-)


24/53

Capillary 4ube

4e capillary tube as te function of transporting te#or3ing li8uid from te condenser to te evaporator%

4e small diameter and long lengt of te tube produces

a large pressure drop% Refrigerants cosen ave a large 6oule/4omson

coefficient$ #ic tells us o# muc te temperaturedrops as te pressure drops at constant entalpy%

Since pressure drops create performance losses carefuldesign must be ta3en so tat te tube lengts anddiameters are minimal%

&Heydari$ ',,')


25/53

Condensation

Condensation$ muc li3e in 4EC$ is a

problem because te surfaces of te !CR

may be lo#er tan te de# point% Since #ater is a7ardous to electronic

assemblies$ condensation must be

minimi7ed by insulating surfaces from airin spot/cooling applications% &Peeples$',,-)

More on sealants later on in te slides%


26/53

Product Reliability

Electro/mecanical

systems generally ave

product life cycles called

Ibattub curvesJ% 4e curve as tree

distinct regions @nfant mortality/ rate of

failure decreases #it time ormal use/rate of failure

relatively constant

*ear out/ rate of failure

increases #it time% &Peeples$',,-)


27/53

@mproving Performance

@n some applications te eat re2ection demands&efficiency or amount) are iger tan #at canbe andled by a vapor compression cyclerunning on a regular cycle% @n tese cases$modifications of te cycle must occur%

.i3e previously mentioned$ modifying te 4H04.to get tem as as lo# as possible #ill increaseperformance but larger modifications may needto be made%



28/53

@mproving Performance

"n e1ample is a cascade cycle$ #ic performs terefrigeration process in t#o cycles tat are inseries% 4is is useful in situations &industrial)$ #ere

tere is a large temperature difference bet#een teot and cold side for one cycle to be practical%

@f te fluid used in te cascade system is te same$te eat e1canger can be replaced by a mi1ing

camber$ 3no#n as a flas camber #ic asbetter eat transfer caracteristics%



29/53

Cascade Refrigeration



30/53

Refrigeration System #0 Glas

Camber



31/53

Supercomputer and Mainframe

Cooling 4e e1tremely ig cooling demands of

mainframes and supercomputers are ideal

applications for !CRs because otercooling systems can not provide te

necessary cooling capacity% "n e1ample of

is @BM?s F< mainframe &so#n on te ne1tslide)

&Scmidt et al%$',,')


32/53


Cooling



33/53


Cooling 4e bul3 po#er assembly at te top

provides ';, volts dc to te mainframe%

Belo# te bul3 po#er is te centralelectronic comple1 #ere te MCM

&multicip module) is located% 4e MCM

ousing te -' processors%


34/53


Cooling Belo# te MCM are blo#ers tat provide air cooling for

all of te components in te processors e1cept for teprocessor module$ #ic is cooled by refrigeration%

Belo# te blo#ers are t#o modular refrigeration units&MRs/te !CR) #ic provide cooling via teevaporator mounted on te processor module%

@n te bottom of te mainframe are te input0 output&@0O)connections and t#o blo#ers% 4e blo#ers coolte @0O connections$ as #ell as$ provide te cooling forte condenser of te MRs%



35/53

Multicip Module &MCM)

4e mainframe?s processing unit$ MCM$ is

constructed as follo#s%

ote te evaporator above te cips%



36/53

Modular Refrigeration nit

&MR) 4e MR &!CR)

ouses all terefrigeration

components e1cept teevaporator% 4e MRcontains te: Condenser

4ermostatic e1pansionvalve

DC rotary compressor



37/53

Condensation Protection

4o avoid moisture condensation onte MCM ard#are$ all te coolingard#are including te evaporator

copper cold plate is contained inan airtigt metal enclosure #itone open face%

'K, grams of silica gel desiccant is

3ept tere to absorb any moisturelea3ing into te enclosure%



38/53

Condensation Protection

4e figure on te previous slide also so#s a flat boardtat replaced te planar board in order to test teeffectiveness of various sealants in 3eeping moisture outof te evaporator cavity%

4e results so# tat te

Butyl L- rubber sealant

displayed te best

sealant caracteristics$

allo#ing te least amount

of umidity to enter%



39/53

Microscale !CRs

4o utili7e !CRs in laptops$ personal computers$ oroter cooling applications of small si7e$ te !CRsi7e must be reduced to fit #itin a small enclosure%

4e ne1t section discusses progression in tis area% Since miniature !CRs ave ig eat loads totransfer a#ay$ te condenser and evaporator mustbe designed suc tat tey transfer enoug eat tosatisfy te eat removal demands%

4e most difficult part in designing a miniature !CRis te compressor%


40/53

Microscale Evaporators

Cirac et al% &',,;) suggest using an evaporator #it

microcannels troug te center as an option to

miniaturi7e te evaporator% 4e microcannels transfer

large amounts of eat reducing te evaporator si7eneeded to transfer te eat load from te eat source%


41/53

Microscale Condenser

Ciriac et al% also describe a condenser #it

microcannels troug te center% " eat sin3

surrounds te outside of te condenser #ile a fan

blo#s air over te eat sin3%


42/53

Refrigerant Gluids

Heydari &',,') performed asimulation #it a miniature !CR#ic included a miniaturecompressor to cool a computersystem%

4e figure so#s ammonia aste igest COP relative to teoter refrigerants% Ho#ever$#en te refrigerant?s cost$

environmental impact &o7onedepletion and global #armingpotential)$ and safety issues#ere considered$ Heydariconcluded te optimalrefrigerant to use is R-(


43/53

Condenser 4emperature and

Performance Heydari found tat #it

te computer cip

2unction temperatureand eat absorbed by

te evaporator fi1ed$

decreasing te

condenser temperaturedecreases te COP%


44/53

Evaporator 4emperature and

Performance .astly$ Heydari found tat

for a fi1ed 2unction

temperature and te

amount of eat absorbedby te evaporator fi1ed$

increasing te evaporator

temperature increases

te COP+ but te amountof eat condensed

decreases%


45/53

Refrigerant Gluids

*en it comes to te performance of refrigerants inminiature !CRs$ Pelan et% "l% &',,


46/53

Refrigerant Gluids

Pelan et% "l% &',,


47/53

Refrigerant Gluids

Gor eac condition$ ammonia as te igestCOP%

4e iger COP values are due to ammonia?sgreater latent eat of vapori7ation% Ho#ever$ due to ammonia?s greater adverse

environmental and pysiological effects$ R/-(


48/53

Microscale !CRs tili7ing !CRs for electronics cooling applications as

been limited by teir large si7e due to te use oftraditional components li3e pistons$ lin3ages andpressure vessels%

niversity of @llinois as a D"RP" grant to developminiature !CR?s for use #it cooled military uniforms foruse in desert #arfare% 4is 4ecnology could also beused for electronics applications% Ho#ever$ te miniaturecompressor as been difficult to acieve%

Researc is ongoing on a Stirling cycle MEMS coolerbeing developed in "S" Flenn%

&Moran$',,-)


49/53

Microscale !CRs

4e Stirling Cycle is muc li3e te vapor

compression cycle and is so#n belo#%

&Moran$',,-)


50/53

Microscale !CRs

sing diapragms instead of pistons$ te MEMS Stirlingcooler is fabricated #it semiconductor processingtecni8ues to provide a device #it planar geometry%

4e result is a flat cold surface for e1tracting eat and anopposing flat ot surface for termal dissipation% " typicaldevice #ould be composed of numerous suc cellsarranged in parallel and0or series #it all layers 2oined atte peripery of te device to ermetically seal te#or3ing gas%

&Moran$',,-)


51/53

Microscale !CRs

&Moran$',,-)


52/53

Microscale !CRs

4e e1pansion and compressiondiapragms are te only moving parts%

E1pansion of te #or3ing gas directlybeneat te e1pansion diapragm in eaccycle creates a cold top end for e1tractingeat$ #ile compression at te oterbottom end creates a ot region fordissipating eat%

&Moran$ ',,-)


53/53

References Cengel and Boles$ ',,'$ Aunus "%$ Boles$ Micael "% &',,')% Thermodynamics: an Engineering Approach.

e# Aor3: A: McFra#/Hill% Ciriac$ Glorea+ NCiriac$ !ictor &',,;)% "n alternative Metod for te Cooling of Po#er Microelectronics

sing Classical Refrigeration%ASME/Pacific im Technical !onference and E"hi#ition on Integration andPac$aging of MEMS% &EMS% and Electronic Systems: Ad'ances in Electronic Pac$aging% pp

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Vapor Cooling in Electronicsa