Transcript Vapor Compression Cooling in Electronics

Vapor Compression
Cooling (VCC) in
Electronics
Outline
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Benefits
Disadvantages
History
Basic operation
Engineering Design
 Cold plate
 Refrigerants
 Capillary tube
 Condensation
Product Reliability
Current applications:
 Supercomputer and Mainframe Cooling
Current research:
 Microscale VCRS
Benefits
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Allows cooling to below ambient temperatures,
increasing performance, reliability, and allowing
operation in higher temperatures.
High COP (around 2 to 3).
Ability to remove very large heat loads.
Widely available; compressor and fan only
moving parts, stable, and reliable. (moran, 2001).
Low mass flow rate of refrigerant needed.
Ability to transport heat away from its source.
COP up to 3 times greater than thermoelectric
coolers or more.
(peeples, 2001)
Benefits
Sub
ambient temperature operation allows
CMOS (complementary-symmetry/metaloxide semiconductor) transistors to switch on
and off faster (peeples, 2001).
CMOS circuits are a major class of
integrated circuit and include
microprocessors, microcontrollers, and other
digital logic circuits (Wikipedia, http://en.wikipedia.org/wiki/CMOS) .
Benefits
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The following physical parameters favor low temperature
operation: carrier mobility, and junction leakage.
Although at low temperatures there is an increase in the
failure rates due to hot carriers, overall failure rates
decrease at lower temps due to the overall increased
characteristics mentioned previously (Moran,2001).
Definitions:
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Carrier mobility- velocity of charge carriers in a solid material with an electric field
applied to it (Wikipedia, http://en.wikipedia.org/wiki/Electron_mobility).
 Hot carriers- high energy carriers (Moran,2001).
 Junction leakage-undesirable conductive paths in certain components, for
instance, in capacitors. Also, a pathway through which electric discharge may
slowly take place (IEEE Standard Dictionary of Electrical and Electronics Terms).
Benefits
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Much like heat pipes, vapor compression
coolers use the high heat of vaporizations of the
working liquid to remove large amounts of heat.
Therefore, low mass flow rates required.
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Advantage over a chilled liquid loop that requires a much
higher mass flow rate.
 Much like heat pipes, VCCs use the high heat of vaporization
of liquids to transport large amounts of heat, thus low
amounts of liquid are required. However, in chilled liquid loop
coolers, the liquid is heated but not necessarily evaporated.
(Cengel and Boles, 2002)
Disadvantages
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The cost of employing the cooling system may
be 10-20% of the cost of the entire system.
Large space and input power.
The disadvantages of the cooling system has to
be weighed with the large advantages. A lot of
times, traditional methods of cooling like air
cooling is not feasible so a VCC is required.
However, if the cooling requirements can be
satisfied with traditional techniques, the
traditional technique are the preferred cooling
method (Moran, 2001).
History
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The vapor compression refrigerator was first
proposed in 1805 and a model was constructed
in 1834.
Vapor compression technology is well
established but only recently used in electronics
cooling.
Kryotech was one of the first companies to
employ the technology.
By the late 90’s companies like AMD, Sun
Microsystems, IBM, and SYS Technologies had
all used Vapor Compression cooling.
(Schmidt et al., 2002)
Basic Operation
(Peeples,2001)
Basic Operation
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Ideal cycle
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1-2 The working refrigerant, a saturated vapor, is carried
through the suction tube to the compressor. The
compressor compresses the saturated vapor into a
superheated vapor which is then passed to the condenser.
 2-3 The heat of the hot and high pressure vapor is released
into the environment from the condenser. The working gas
is transformed into a saturated liquid.
 3-4 The liquid is pumped through a capillary tube or a
thermal expansion valve into the evaporator, dropping
significantly in temperature. The working fluid is a saturated
mixture.
 4-1 Heat flows into the evaporator from the heat source.
The heat vaporizes all the working liquid (refrigerant), at the
end of this stage the vapor is saturated, and the process
repeats.
(Peeples,2001)
Basic Operation
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2 represents the actual
position of the state. 2s
represents the position of
the state if it were
irreversible.
In the ideal case:
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12 isentropic (s=const)
23 isobaric (P=const)
34 isenthalpic (h=const)
41 isobaric ( P=const)
(Peeples,2001)
•Qout, QH denotes the heat
released to the environment.
•Qin, QL denote heat absorbed
from the refrigerated space.
Qout = QH
Win
Qin = QL
Basic Operation
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The heat absorbed by the evaporator (Qin) released from the
condenser (Qout), and actual and isentropic work input by the
compressor can be determined from the equations:
Qin  m (h1  h4 )
Qout  m (h3  h2 )
Ws  m (h2 s  h1 )
Wa  m (h2 a  h1 )
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The efficiency of the compressor is given by:
c 
ws h2 s  h1

wa h2  h1
Where h is the enthalpy at various states, the subscripts s and a
refer to isentropic and actual, and w refers to work.
(Cengel and Boles, 2002)
Basic Operation
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Actual cycle
 The
deviation of the ideal cycle to the actual cylce is
due to irreversibilities; mainly due to fluid frictioncausing pressure drops and heat transfer to the
system or the environment.
 In the ideal cycle the state of the refrigerant is
precisely know, for example, at stage 1 the refrigerant
is a saturated vapor. However, in actuality the state of
the refrigerant may not be precisely known and is
usually a superheated vapor.
(Cengel and Boles, 2002)
Basic Operation
Note that the pressure drops between stages 8-1, 2-3,4-5 etc. cause alterations
to the TS diagram. The process 1-2’ represents a compression path that may
be more desirable than the 1- 2 pathway because the specific volume
of the refrigerant is lower and thus the work input is lower.
(Cengel and Boles, 2002)
Basic Operation
(Cengel and Boles, 2002)
Basic Operation
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The Coefficient of performance of the VCR is the ratio of heat into the
evaporator to work put into the compressor. Generally, the COP is
around 2 to 3.
Q e
COP 
Wc
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The COP can be determined by finding the ratio of the enthalpies
between the throttling valve and evaporator and between the
evaporator and the compressor.
h1  h4
COP 
h2  h1
(Cengel and Boles, 2002)
Basic Operation
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The most efficient refrigeration cycle is that of a Carnot
refrigerator.
The coefficient of performance of the Carnot refrigerator
gives the maximum performance between two hot and
and cold temperatures.
This value can be compared to the actual COP to
determine how close to ideal the refrigerator is operating.
Note that the Carnot cycle is a reversible cycle.
Reversible cycles are cycles that do not generate any
entropy due to friction, heat transfer, etc. For a reversible
cycle:
 QH

 QL

T

 H
 revers T L
(Cengel and Boles, 2002)
Basic Operation
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The coefficient of performance for a Carnot cycle is:
COPCarnot 
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1
1
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QH / Q L 1 TH /T L1
Note: the COP increases as the ratio of TH/TL decreases.
Therefore, we want TH and TL to be as close as possible.
Usually, TL is specified and TH can be altered. The
reason for TH/ TL affecting COP is that for a given TL the
greater the TH the greater the amount of work input,
decreasing COP.
(Cengel and Boles, 2002)
Basic Operation
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The area enclosed in the TS diagram
generally describes the net heat transfer
and the work of the system. For
refrigeration, work input lowers the COP
for a given heat rejection; therefore,
altering the states of the cycle to get a
smaller enclosed area will increase
performance.
(Cengel and Boles, 2002)
Engineering Goals
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The design of a cost-effective and efficient
VCC involves the following considerations:
 Cold
surfaces can not be allowed to collect
condensate from the air.
 The most suitable refrigerant for the given
application, as well as the tubing to supply and
remove the refrigerant, must be chosen.
 Compressor and condenser design.
 The cold plate must efficiently lift heat from the
device to be cooled.
(Peeples,2001)
Cold Plate/Evaporator
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The cold plate (evaporator) is a heat exchange
device which transfers heat from the heat source
to the working fluid.
Cold plate design must assure efficient heat
transfer from the device being cooled to the
refrigerant inside the cold plate.
The cold plate must be fabricated from thin and
thermally conductive material to minimize thermal
resistance while maintaining structural stability.
The mating surface between the cold plate and
the heated body must be flat and smooth to
minimize contact resistance.
(Peeples,2001)
Refrigerant Fluids
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The refrigerant’s physical properties determine
its evaporating temperature at a specified
operating temperature, as well as, its capacity to
transport heat.
The well known refrigerants R-134a and R-404a
(and others) perform well for cooling high power
electronics due to their high heat transport
characteristics, low possibility of environmental
harm, corrosion, and risk of explosion.
(Peeples,2001)
Refrigerant Fluids
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When making a decision as to which refrigerant to use and
under what pressures to operate the VCR, consideration of
the refrigerant’s temperature of vaporization and
condensation should be considered. If the temperature of the
refrigerant doesn’t reach the vaporization point (at the
operating pressure) the VCR will not work properly. To ensure
a proper heat transfer rate a 5 to 10 degree Celsius
temperature difference should be maintained between the
evaporator and the condenser with the refrigerant.
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The next slide shows the P-T curves of R-404a and R-134a
describing the refrigerant’s lowest practical operating limit.
(Peeples,2001)
Refrigerant Fluid Saturation
Pressure and Temperature
(Peeples,2001)
Capillary Tube
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The capillary tube has the function of transporting the
working liquid from the condenser to the evaporator.
The small diameter and long length of the tube produces
a large pressure drop.
Refrigerants chosen have a large Joule-Thomson
coefficient, which tells us how much the temperature
drops as the pressure drops at constant enthalpy.
Since pressure drops create performance losses careful
design must be taken so that the tube lengths and
diameters are minimal.
(Heydari, 2002)
Condensation
Condensation, much like in TEC, is a
problem because the surfaces of the VCR
may be lower than the dew point.
 Since water is hazardous to electronic
assemblies, condensation must be
minimized by insulating surfaces from air
in spot-cooling applications. (Peeples,2001)
 More on sealants later on in the slides.
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Product Reliability
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Electro-mechanical
systems generally have
product life cycles called
“bathtub curves”.
The curve has three
distinct regions
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Infant mortality- rate of
failure decreases with time
 Normal use-rate of failure
relatively constant
 Wear out- rate of failure
increases with time.
(Peeples,2001)
Improving Performance
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In some applications the heat rejection demands
(efficiency or amount) are higher than what can
be handled by a vapor compression cycle
running on a regular cycle. In these cases,
modifications of the cycle must occur.
Like previously mentioned, modifying the TH/TL
to get them as as low as possible will increase
performance but larger modifications may need
to be made.
(Cengel and Boles, 2002)
Improving Performance
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An example is a cascade cycle, which performs
the refrigeration process in two cycles that are in
series. This is useful in situations (industrial),
where there is a large temperature difference
between the hot and cold side for one cycle to
be practical.
If the fluid used in the cascade system is the
same, the heat exchanger can be replaced by a
mixing chamber, known as a flash chamber
which has better heat transfer characteristics.
(Cengel and Boles, 2002)
Cascade Refrigeration
(Cengel and Boles, 2002)
Refrigeration System w/ Flash
Chamber
(Cengel and Boles, 2002)
Supercomputer and Mainframe
Cooling
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The extremely high cooling demands of
mainframes and supercomputers are ideal
applications for VCRs because other
cooling systems can not provide the
necessary cooling capacity. An example of
is IBM’s G4 mainframe (shown on the next
slide)
(Schmidt et al.,2002)
Supercomputer and Mainframe
Cooling
(Schmidt et al.,2002)
Supercomputer and Mainframe
Cooling
The bulk power assembly at the top
provides 250 volts dc to the mainframe.
 Below the bulk power is the central
electronic complex where the MCM
(multichip module) is located. The MCM
housing the 12 processors.
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Supercomputer and Mainframe
Cooling
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Below the MCM are blowers that provide air cooling
for all of the components in the processors except
for the processor module, which is cooled by
refrigeration.
Below the blowers are two modular refrigeration
units (MRUs-the VCR) which provide cooling via the
evaporator mounted on the processor module.
In the bottom of the mainframe are the input/ output
(I/O)connections and two blowers. The blowers cool
the I/O connections, as well as, provide the cooling
for the condenser of the MRUs.
(Schmidt et al.,2002)
Multichip Module (MCM)
The mainframe’s processing unit, MCM, is
constructed as follows.
 Note the evaporator above the chips.
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(Schmidt et al.,2002)
Modular Refrigeration Unit
(MRU)
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The MRU (VCR)
houses all the
refrigeration
components except the
evaporator. The MRU
contains the:
 Condenser
 Thermostatic
expansion
valve
 DC rotary compressor
(Schmidt et al.,2002)
Condensation Protection
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To avoid moisture condensation on
the MCM hardware, all the cooling
hardware including the evaporator
copper cold plate is contained in
an airtight metal enclosure with
one open face.
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260 grams of silica gel desiccant is
kept there to absorb any moisture
leaking into the enclosure.
(Schmidt et al.,2002)
Condensation Protection
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The figure on the previous slide also shows a flat board
that replaced the planar board in order to test the
effectiveness of various sealants in keeping moisture out
of the evaporator cavity.
The results show that the
Butyl #1 rubber sealant
displayed the best
sealant characteristics,
allowing the least amount
of humidity to enter.
(Schmidt et al.,2002)
Microscale VCRs
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To utilize VCRs in laptops, personal computers,
or other cooling applications of small size, the
VCR size must be reduced to fit within a small
enclosure. The next section discusses
progression in this area.
Since miniature VCRs have high heat loads to
transfer away, the condenser and evaporator
must be designed such that they transfer
enough heat to satisfy the heat removal
demands.
The most difficult part in designing a miniature
VCR is the compressor.
Microscale Evaporators
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Chirac et al. (2005) suggest using an evaporator with
microchannels through the center as an option to
miniaturize the evaporator. The microchannels transfer
large amounts of heat reducing the evaporator size
needed to transfer the heat load from the heat source.
Microscale Condenser
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Chiriac et al. also describe a condenser with
microchannels through the center. A heat sink
surrounds the outside of the condenser while a fan
blows air over the heat sink.
Refrigerant Fluids
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Heydari (2002) performed a
simulation with a miniature VCR
which included a miniature
compressor to cool a computer
system.
The figure shows ammonia has
the highest COP relative to the
other refrigerants. However,
when the refrigerant’s cost,
environmental impact (ozone
depletion and global warming
potential), and safety issues
were considered, Heydari
concluded the optimal
refrigerant to use is R134a.
Refrigerant COP
Ammonia
Refrigerant
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R718
R134a
R-22
R-407c
R507a
0
1
2
COP
3
4
Condenser Temperature and
Performance
Heydari found that with
the computer chip
junction temperature
and heat absorbed by
the evaporator fixed,
decreasing the
condenser temperature
decreases the COP.
Effect of Variation of Tcond on
COP (Junction Temp Fixed)
6
5
COP
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4
3
2
1
0
45
50
55
60
Condenser Temp (C)
65
Evaporator Temperature and
Performance
Effect of Variation of Tevap on Qcond
195
190
Qcond(Watts)
Lastly, Heydari found that
for a fixed junction
temperature and the
amount of heat absorbed
by the evaporator fixed,
increasing the evaporator
temperature increases
the COP; but the amount
of heat condensed
decreases.
185
180
175
170
165
0
5
10
15
20
25
Tevap (deg C)
Effect of Varation of Tevap on COP
4
3.5
COP
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3
2.5
2
1.5
1
0.5
0
0
5
10
15
Tevap (deg C)
20
25
Refrigerant Fluids
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When it comes to the performance of refrigerants in
miniature VCRs, Phelan et. Al. (2004) performed
experimental analysis comparing three refrigerants:
ammonia, R-134a, and R-22 to determine which of the
three produce the highest COP for a miniature VCR
under various conditions.
The factors tested were the evaporator and condenser
temperatures, as well as, the efficiency of the
compressor which was predicted to decrease as the size
of the compressor decreased. The results are shown on
the next slide.
Refrigerant Fluids
Phelan et. Al. (2004)
Refrigerant Fluids
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For each condition, ammonia has the highest
COP.
The higher COP values are due to ammonia’s
greater latent heat of vaporization.
However, due to ammonia’s greater adverse
environmental and physiological effects, R-134a
is more predominantly used.
Ammonia is typically used only with a secondary
loop due to its toxicity.
(Phelan et. al 2004)
Microscale VCRs
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Utilizing VCRs for electronics cooling
applications has been limited by their large size
due to the use of traditional components like
pistons, linkages and pressure vessels.
University of Illinois has a DARPA grant to
develop miniature VCR’s for use with cooled
military uniforms for use in desert warfare. This
Technology could also be used for electronics
applications. However, the miniature compressor
has been difficult to achieve.
Research is ongoing on a Stirling cycle MEMS
cooler being developed in NASA Glenn. (Moran,2001)
Microscale VCRs
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The Stirling Cycle is much like the vapor
compression cycle and is shown below.
(Moran,2001)
Microscale VCRs
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Using diaphragms instead of pistons, the MEMS Stirling
cooler is fabricated with semiconductor processing
techniques to provide a device with planar geometry.
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The result is a flat cold surface for extracting heat and an
opposing flat hot surface for thermal dissipation. A typical
device would be composed of numerous such cells
arranged in parallel and/or series with all layers joined at
the periphery of the device to hermetically seal the
working gas.
(Moran,2001)
Microscale VCRs
(Moran,2001)
Microscale VCRs
The expansion and compression
diaphragms are the only moving parts.
 Expansion of the working gas directly
beneath the expansion diaphragm in each
cycle creates a cold top end for extracting
heat, while compression at the other
bottom end creates a hot region for
dissipating heat.
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(Moran, 2001)
References
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Cengel and Boles, 2002, Yunus A., Boles, Michael A. (2002). Thermodynamics: an Engineering
Approach. New York: NY: McGraw-Hill.
Chiriac, Florea; &Chiriac, Victor (2005). An alternative Method for the Cooling of Power Microelectronics
Using Classical Refrigeration. ASME/Pacific Rim Technical Conference and Exhibition on Integration and
Packaging of MEMS, NEMS, and Electronic Systems: Advances in Electronic Packaging. pp 425-430.
Heydari, Ali. (2002). Miniature Vapor Compression Refrigeration System for Active Cooling of High
Performance Computers. 8th Intersociety Conference on Thermal and Thermommechanical phenomena
in Electronic Systems. pp. 371-378.
IEEE Standard Dictionary of Electrical and Electronics Terms. (1973). New york: Wiley-Interscience.
Moran, Mathew E. (2001). Micro-Scale Avionics Thermal Management. 34th International Symposium on
Microelectronics sponsored by the International Microelectronics and Packaging Society.
Peeples, John. W.(2001). Mechanically Assisted Cooling for High Performance Applications. Advances in
Electronics Packaging; Procedings of the Joint ASME/JSME Conference on Electronics Packaging. Pp.
899-904.
Phelan, Patrick; Chiriac, Florea; &Chiriac, Victor (2004). Designing a Mesoscale Vapor-Compression
Refrigerator for Cooling High-Power Microelectronics. Intersociety Conference on Thermal Phenomena,
1, pp. 218-223.
Schmidt, Roger R., and Notohardjono, Budy D. (2002). High-End Server Low-Temperature Cooling. IBM
Journal of Research and Development, 46 ( 6), pp. 739-750.
Wikipedia the Free Encyclopedia (August 2006). CMOS. Retrieved August 2006.
http://en.wikipedia.org/wiki/CMOS .
Wikipedia the Free Encyclopedia (August 2006). Electron mobility. Retrieved August 2006.
http://en.wikipedia.org/wiki/Electron_mobility