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© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential
Since 1970 • AS 9100 • ISO 9001 • ISO 14001 Certified • ITAR Registered
QF 402 Rev E
How to Design a Liquid Cooled System
Dr. Pablo Hidalgo
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential2
Outline
• Introduction to liquid cooled systems
− Air vs liquid.
− Hydrodynamical requirements.
− Thermal requirements.
• Basic principles and equations
− Hydrodynamical
− Thermal
• Essential elements needed in the circuit.
• Liquid cooled system for computing applications
• Liquid cooled system for military applications
• Summary
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential3
Air vs. Liquid Cooling
• Heat transfer processes:
− Heat transport, which strongly depends on the mass flow rate and specific
heat of the fluid.
− 𝒒𝒄𝒐𝒏𝒗 = ሶ𝒎𝒄𝒑 𝑻𝒐 − 𝑻∞
− Heat convection, which is primarily governed by the heat transfer
coefficient h.
− 𝒒" = 𝒉 𝑻𝒘 − 𝑻𝒎
• Air cooling is limited by specific heat. To dissipate large amounts of
power, a large mass flow rate is needed.
− Higher flow speed, larger noise.
• Liquid cooling is able to achieve better heat transfer at much lower
mass flow rates.
− Lower flow speed, lower noise.
• Heat transfer coefficients for air an liquid flows are orders of magnitude
apart.
− 25 < hair < 250 W/m2 K
− 100 < hliquid < 20,000 W/m2 K
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential4
Hydrodynamical Requirements
• It is critical to calculate the total pressure drop
(ΔPtotal) in the liquid line in order to size a pump.
− ΔPtotal influenced by flow regime, sudden expansions,
contractions, bends, valves, etc…
• To size a pump, two important parameters are
needed:
− Liquid flow rate
− Total head that the pump must generate to deliver the
required flow rate.
◦ Total head = static head difference + frictional head losses
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential5
Thermal Requirements
• A liquid cooled system is generally used in cases
were large heat loads or high power densities need to
be dissipated and air would require a very large flow
rate.
• Water is one of the best heat transfer fluids due to its
specific heat at typical temperatures for electronics
cooling.
• Temperature range requirements defines the type of
liquid that can be used in each application.
− Operating Temperature < 0oC, water cannot be used.
− Glycol/water mixtures are commonly used in military
applications, but the heat transfer capabilities are
significantly lower than water.
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential6
Essential Elements in a Liquid Cooled System
Pump Cold Plate Heat Exchanger
Reservoir TubingFan
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential7
Hydrodynamic Equations
• Energy equation for steady pipe flow of an incompressible fluid.
• Simplified energy equation
ሶ𝑄- ሶ𝑊𝑠 + 𝐴1𝑝1
𝜌+ 𝑔𝑧1 + 𝑢1 𝜌𝑉1𝑑𝐴1 + 𝐴2
𝜌𝑉13
2𝑑𝐴1=
ሶ𝐴1
𝑝2
𝜌+ 𝑔𝑧2 + 𝑢2 𝜌𝑉2𝑑𝐴2 + 𝐴2
𝜌𝑉23
2𝑑𝐴2
ሶ𝑄− ሶ𝑊𝑠 +𝑝1𝜌+ 𝑔𝑧1 + 𝑢1 + 𝛼1
𝑉12
2ሶ𝑚 =
𝑝2𝜌+ 𝑔𝑧2 + 𝑢2 + 𝛼1
𝑉22
2ሶ𝑚
𝛼 =1
𝐴න𝐴
𝑉
ത𝑉
3
𝑑𝐴
𝑝1𝛾+ 𝛼1
𝑉12
2𝑔+ 𝑧1 + ℎ𝑝 =
𝑝2𝛾+ 𝛼2
𝑉22
2𝑔+ 𝑧2 + ℎ𝑡 + ℎ𝐿
Laminar Flow, α = 2
Turbulent Flow α ≈ 1.05
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential8
Hydrodynamic Equations
• Laminar flow
• Turbulent flow
𝑝1𝛾+ 𝛼1
𝑉12
2𝑔+ 𝑧1 + ℎ𝑝 =
𝑝2𝛾+ 𝛼2
𝑉22
2𝑔+ 𝑧2 + ℎ𝑡 + ℎ𝐿 ℎ𝐿 = ℎ𝑓 =
32𝜇𝐿𝑉
𝛾𝐷2
𝑝1𝛾+ 𝑧1 =
𝑝2𝛾+ 𝑧2 + ℎ𝐿
ℎ𝐿 = ℎ𝑓 = 𝑓𝐿
𝐷
𝑉2
2𝑔
Moody Diagram
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential9
Hydrodynamic Equations
• The head loss produced by the flow through bends, inlets, valves, etc…
is expressed by the equation:
• Some of those K values are shown on the
adjacent table.
• Energy equation is rewritten as:
• Where the sum of hL includes frictional
losses, and losses due to fittings, contra-
tions, valves, etc… that are present in the
flow loop.
𝑝1𝛾+𝑉12
2𝑔+ 𝑧1 =
𝑝2𝛾+𝑉22
2𝑔+ 𝑧2 +ℎ𝐿
ℎ𝐿 = 𝐾𝑉2
2𝑔
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential10
Thermal Equations (Heat Source)
• Heat source follows the Newton’s law of cooling
where Tm depends on constant heat flux or constant temperature
boundary conditions and h is the LOCAL heat transfer coefficient (HTC).
• Energy balance equation:
• If constant surface temperature boundary condition, heat rate equation:
where is the average HTC and is the log mean
temperature difference.
• Heat transfer coefficient can be estimated using the Nusselt number.
• Multiple correlations exists for laminar flow, turbulent flow, fully
developed flow, developing flow, heat source boundary conditions, etc…
that can be summarized in the following table:
𝑞𝑠" = ℎ(𝑇𝑠 − 𝑇𝑚)
𝑞𝑐𝑜𝑛𝑣 = ሶ𝑚𝑐𝑝 𝑇𝑚,𝑜 − 𝑇𝑚,𝑖
𝑁𝑢 =ℎ𝐷
𝑘
𝑞𝑐𝑜𝑛𝑣 = ഥ𝑈𝐴𝑠∆𝑇𝑙𝑚 ഥ𝑈 ∆𝑇𝑙𝑚
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential11
Thermal Equations (Heat Source)
Source Fundamentals of Heat an Mass Transfer, Incropera and DeWitt
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential12
Thermal Equations (Heat Exchanger)
• Counterflow heat exchangers are the most efficient ones to be used.
− Cross-flow heat exchangers are typical in these applications but the thermal
characteristics are very similar to that of counterflow but a correction factor
must be applied.
• Overall energy balance is used to estimate maximum heat transfer rate
given certain input parameters (i.e. mass flow rate, fluid temperature, etc…)
• Heat exchanger calculations are based on the log mean temperature
difference.
• hi and ho can be calculated using the Nusselt number correlations shown
earlier.
• Another way to size a heat exchanger would be to use the effectiveness-
NTU method.
∆𝑇𝑙𝑚=∆𝑇2 − ∆𝑇1𝑙𝑛 Τ∆𝑇2 ∆𝑇1
=𝑇ℎ,𝑖 − 𝑇𝑐,𝑖 − 𝑇ℎ,𝑜 − 𝑇𝑐,𝑜
𝑙𝑛 ൗ𝑇ℎ,𝑖 − 𝑇𝑐,𝑖 𝑇ℎ,𝑜 − 𝑇𝑐,𝑜
𝑞 = 𝑈𝐴𝐹∆𝑇𝑙𝑚
𝑈 =1
Τ1 ℎ𝑖 + Τ1 ℎ𝑜
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential
Since 1970 • AS 9100 • ISO 9001 • ISO 14001 Certified • ITAR Registered
QF 402 Rev E
Liquid Cooled System for Computing
Applications
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential14
Computer Desktop Liquid Cooling System
Fluid Not Shown
Cold Plates
Heat Exchanger
Liquid Pump
Connective Tubing
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential15
Key Reliability Issues
• Pump Reliability
• All Electro-mechanical devices such as pumps have finite life which
leads to reliability issues.
• Fluid Permeation Loss. Fluids tend to permeate through polymer
materials and joints. If too much fluid is lost due to permeation, the LCS
could eventually stop working.
• Fluid Leakage
• Environmental Impact: Environmental concerns with cooling fluid
leakage and disposal are issues.
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential16
Sub 1U LCS for High End Server Compute Module
Heat Exchanger –
Sub 1U
4 PMCP Cold
Plates
2 Liquid Pumps
Low
Perm
Tubing
• 4 x 95W AMD CPU’s
• 90 CFM per Module
• 0.20 °C/W (c-a)
• 2 Pumps PCB
Powered
• Sub 1U PCB Spacing
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential17
Cold Plate Technology
Vertical Fin Cold Plate (VFCP)
• Utilizes closely spaced vertical fins
to dissipate heat
• Moderate heat transfer coefficients
possible
Powder Metal Cold Plate (PMCP)
• Uses high surface area density to
dissipate heat
• High effective heat transfer
coefficients possible
• Many flow geometries possible
OutletInlet Inlet
Heat Entersfrom Bottom
Metal PowderParticles
Thermacore
Technology
Examples of Cold Plate Technologies
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential18
Cold Plate – Porous Metal
Advantages
• High Surface Area
• High Heat Transfer Coefficient
• High Heat Flux (> 300 W/cm2)
• Low Thermal Resistance
• Low Profile Packaging
• Low Mass (< 75 grams)
a
Cool Single PhaseCoolant In
Warm Single PhaseCoolant Out
Heat Sourcee.g.: computer chip,
particle beam, EM radiation,laser diode array
Well-Bonded PorousMetal Matrix
Liquid cooled heat sinks make
use of high surface area and
effective heat transfer
available in a well-bonded
porous metal matrix.
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential19
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Coolant Flow Rate (GPM)
Resis
tan
ce (
deg
-C/W
/cm
^2)
Lower Thermal Resistance =
Better Performance
Benefit
>40%
VFCP Heat Input: 12mm x 12mm
PMCP Heat Input: 7mm x 7mm
Courtesy: Dr. Kevin Wert
Performance Comparison - VFCP vs. PMCP
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential20
Liquid-to-Air Heat Exchanger – Flat Tube with Rolled Fins
• All Aluminum Liquid-to-Air Heat Exchanger
• Maximizes Heat Transfer Efficiency & Volume
− Flat, low profile tubes that provide more
surface area.
− Metallurgical bond between components.
• Highly Reliable and Durable
− One-piece integral structure.
− Components are joined together by an
aluminum brazing process .
− Leak-tight .
• Custom designed for the specific application
− Desktop Chassis
− 1U Server Chassis
− Vertical position blade server
Desktop Heat Exchanger
1U Server Heat Exchanger
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential21
Example Heat Exchanger Specs
HEX Spec. – Desktop Chassis
Material Aluminum
Length Application Specific: 150mm shown
Typical: 100 -150mm
Height Application Specific: 150mm shown
Typical: 100 -150mm
Depth 25mm
Mass Application Specific: 350 grams
Typical: 250 -350mm
HEX Spec. – 1U Server Chassis
Material Aluminum
Length Application Specific: 130mm shown
Typical: 100 – 275 mm
Height Application Specific: 40 mm shown
Typical: 30 - 50mm
Depth 25mm
Mass Application Specific: 85 grams
Typical: 85 -150mm
Desktop Heat Exchanger1U Server Heat Exchanger
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential22
Liquid Pump Specs
Pump Spec.
Flowrate ~ 0.25gpm @
3.6 psi head
Acoustic Power Target ≤ 3.3 BA
Dimensions 62 mm W x 38mm H x
87mm L w/ barbs
Mass 200 grams
Power 12W
Voltage/Amps 12Vdc / 1A continuous
Pump Spec. – 1U Server Chassis
Flowrate ~ 0.125 gpm @ 3psi head
Acoustic Power Target ≤ 3.3 BA
Dimensions 32 mm W x 32mm H x 89mm L
Mass 135 grams
Power 10W
Voltage/Amps 12Vdc / 0.6A continuous
Compact Form Factor Pump
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential
Since 1970 • AS 9100 • ISO 9001 • ISO 14001 Certified • ITAR Registered
QF 402 Rev E
Liquid Cooled System for Military
Applications
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential24
System Requirements
• Military applications have much tighter and controlled requirements
compared to computing liquid cooled systems.
− Subject to MIL specs.
− Extreme temperature ranges (-55oC to +70oC).
− Extreme environmental conditions.
− Air-tight enclosures.
− Low accessibility for servicing.
− Shock and vibration requirements.
− Feedback controllers for optimized heat removal in any conditions.
− Multiple sensors to monitor faults in the system.
− Redundant elements are generally required.
• In airborne applications, low weight materials need to be used, (i.e.
aluminum), which have worse thermal conductivity than copper.
− Thermal path from the electronics to the heat exchanger is critical to
reduce thermal resistance.
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential25
LCS Flow Diagram
Pumps
Filter
Purge Line Strainer
Heater
3-way temp. controlled valve
HX with chilled waterReservoir
Antenna
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential26
Intelligent Thermal Management System (iTMS)
• Application:− Airborne Mapping & Imaging
− Laser Diode Cooling
• Power: 1.1kW− Thermal Technologies
− TEC’s
− Heat Pipe Cold Plate
− Al Vacuum Brazed Cold Plates
− Pumped Liquid Cooling
− Sub-ambient Cooling
− Sophisticated Control System
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential27
Intelligent Thermal Management System (iTMS)
• Application:
− Ruggedized Electronics Cooling
• Thermal Load/Power: 1 kW
− Designed / Tested to MIL Specs.
• Cooling System includes:
− Heat pipes
− Liquid-cooled cold plates
− Internal brazed aluminum liquid-to-air heat exchanger
− Dip brazed aluminum cold plates
− An external brazed aluminum liquid-to-air heat exchanger
Cold Plates
External Heat
ExchangerInternal Heat
Exchanger
Heat Pipe
Assy’s
Redundant Pumps
• Sealed air-tight chassis
• Upgradeable Electronics
Rugged, Liquid Cooling
System (rLCS)
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential28
Intelligent Thermal Management System (iTMS)
• Application− Airborne Computer Cooling
− Dissipates thermal load into ambient air at 75k feet
• Primary Components− Vacuum Brazed Aluminum HXs
− Vibration Isolation (40G operational)
− Brushless DC Pumps
− PTFE Hoses
− Custom Machined Chassis & Reservoir
− Custom Motor Control
• Key Features− Sub Ambient Cooling
− PID temperature control
− Conditioning heaters to facilitate rapid “cold start”
− Liquid level sensors
− Fault Tolerance/Safety flow switch provides visual and electrical confirmation of coolant flow
− PLC control of pumps, heaters, valves, etc.
− LED status indicators
− Data logging
− Color touchscreen display/interface panel
− Shock Mounted for Vibration Isolation
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential29
Summary
• Liquid cooling is a necessary technology applied in
cases where power densities are too high to be
managed by traditional air cooling.
− Liquid heat transport capabilities are far much greater than
air.
• Liquid cooled systems can be simple but in some
applications can have very complex architecture.
− Basic elements: pump, cold plate, heat exchanger, liquid
line.
• Total pressure head is necessary to be estimated to
properly size a pump.
− Static head, difference in elevation.
− Frictional head losses calculated using known documented
friction factors.
© 2016 Aavid Thermacore, Inc. All Rights Reserved.Aavid Thermacore Proprietary & Confidential30
Summary
• Heat balance equation and heat rate equation are
used in sizing a heat exchanger.
− Necessary to know fin area and flow rate to dissipate the
heat.
• Selection of liquid will depend on application and
materials used in the system.
• Computing liquid cooled applications don’t require
strict requirements compared to military applications.
− Redundant systems, extreme temperatures, shock and
vibrations, etc…