How to Design a Liquid Cooled System - semi-therm.org · © 2016 Aavid Thermacore, Inc. All Rights Reserved. Aavid Thermacore Proprietary & Confidential Since 1970 • AS 9100 •

© 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


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


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


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.


Essential Elements in a Liquid Cooled System

Pump Cold Plate Heat Exchanger

Reservoir TubingFan


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



• 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



• 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


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


Thermal Equations (Heat Source)

Source Fundamentals of Heat an Mass Transfer, Incropera and DeWitt


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



QF 402 Rev E

Liquid Cooled System for Computing

Applications


Computer Desktop Liquid Cooling System

Fluid Not Shown

Cold Plates

Heat Exchanger

Liquid Pump

Connective Tubing


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.


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


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


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.


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


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


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


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



QF 402 Rev E

Liquid Cooled System for Military

Applications


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.


LCS Flow Diagram

Pumps

Filter

Purge Line Strainer

Heater

3-way temp. controlled valve

HX with chilled waterReservoir

Antenna


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



• 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)



• 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


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.


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…

Documents

How to Design a Liquid Cooled System - semi-therm.org · © 2016 Aavid Thermacore, Inc. All Rights Reserved. Aavid Thermacore Proprietary & Confidential Since 1970 • AS 9100 •