How to Design a Liquid Cooled System

Dr. Pablo Hidalgo

• 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

• Heat transfer processes: − Heat transport, which strongly depends on the mass flow rate and specific heat of the fluid.

− 풒풄풐풏풗 = 풎ሶ 풄풑 푻풐 − 푻∞ − Heat convection, which is primarily governed by the heat transfer coefficient h.

− 풒" = 풉 푻풘 − 푻풎 • 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. 2 − 25 < hair < 250 W/m K 2 − 100 < hliquid < 20,000 W/m K

• 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

• 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.

Pump Cold Plate Heat Exchanger

Fan Reservoir Tubing

• Energy equation for steady pipe flow of an incompressible fluid.

3 ሶ 3 푝1 휌푉1 푝2 휌푉2 푄ሶ -푊ሶ푠 + ׬ + 푔푧1 + 푢1 휌푉1푑퐴1 + ׬ 푑퐴1=׬ + 푔푧2 + 푢2 휌푉2푑퐴2 + ׬ 푑퐴2 퐴1 휌 퐴2 2 퐴1 휌 퐴2 2

2 2 푝1 푉 푝2 푉 푄ሶ −푊ሶ + + 푔푧 + 푢 + 훼 1 푚ሶ = + 푔푧 + 푢 + 훼 2 푚ሶ 푠 휌 1 1 1 2 휌 2 2 1 2

1 푉 3 Laminar Flow, α = 2 훼 = න 푑퐴 퐴 푉ത 퐴 Turbulent Flow α ≈ 1.05

• Simplified energy equation 푝 푉2 푝 푉2 1 + 훼 1 + 푧 + ℎ = 2 + 훼 2 + 푧 + ℎ + ℎ 훾 1 2푔 1 푝 훾 2 2푔 2 푡 퐿

• Laminar flow 푝 푉2 푝 푉2 32휇퐿푉 1 + 훼 1 + 푧 + ℎ = 2 + 훼 2 + 푧 + ℎ + ℎ ℎ = ℎ = 훾 1 2푔 1 푝 훾 2 2푔 2 푡 퐿 퐿 푓 훾퐷2 Moody Diagram • Turbulent flow 푝 푝 1 + 푧 = 2 + 푧 + ℎ 훾 1 훾 2 퐿 퐿 푉2 ℎ = ℎ = 푓 퐿 푓 퐷 2푔

• The head loss produced by the flow through bends, inlets, valves, etc… is expressed by the equation: 푉2 ℎ = 퐾 퐿 2푔 • Some of those K values are shown on the adjacent table. • Energy equation is rewritten as:

푝 푉2 푝 푉2 1 + 1 + 푧 = 2 + 2 + 푧 + ෍ ℎ 훾 2푔 1 훾 2푔 2 퐿

• Where the sum of hL includes frictional losses, and losses due to fittings, contra- tions, valves, etc… that are present in the flow loop.

" • Heat source follows the Newton’s law of cooling 푞푠 = ℎ(푇푠 − 푇푚)

where Tm depends on constant heat flux or constant temperature boundary conditions and h is the LOCAL heat transfer coefficient (HTC). • Energy balance equation:

푞푐표푛푣 = 푚ሶ 푐푝 푇푚,표 − 푇푚,𝑖 • If constant surface temperature boundary condition, heat rate equation: 푞푐표푛푣 = 푈ഥ퐴푠∆푇푙푚 where 푈ഥ is the average HTC and ∆ 푇 푙푚 is the log mean temperature difference. • Heat transfer coefficient can be estimated using the Nusselt number. ℎ퐷 푁푢 = 푘 • 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:

Source Fundamentals of Heat an Mass Transfer, Incropera and DeWitt

• 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.

푞 = 푈퐴퐹∆푇푙푚 ∆푇2 − ∆푇1 푇ℎ,𝑖 − 푇푐,𝑖 − 푇ℎ,표 − 푇푐,표 ∆푇푙푚= = 푙푛 ∆푇2Τ∆푇1 푙푛 푇ℎ,𝑖 − 푇푐,𝑖 ൗ 푇ℎ,표 − 푇푐,표 1 푈 = 1Τℎ𝑖 + 1Τℎ표

• 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.

Since 1970© 2016 • AS Aavid 9100Thermacore, • ISO 9001 Inc. • AllISO Rights 14001 Reserved. Certified • ITAR Registered Aavid Thermacore Proprietary & Confidential QF 402 Rev E Computer Desktop Liquid Cooling System

Heat Exchanger

Liquid Pump

Connective Tubing

Fluid Not Shown

Cold Plates

• 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.

Heat Exchanger – Sub 1U

• 4 x 95W AMD CPU’s • 90 CFM per Module 4 PMCP Cold • 0.20 °C/W (c-a) Plates • 2 Pumps PCB Powered Low • Sub 1U PCB Spacing Perm Tubing

2 Liquid Pumps

Examples of Cold Plate Technologies Vertical Fin Cold Plate (VFCP) Powder Metal Cold Plate (PMCP)

• Utilizes closely spaced vertical fins • Uses high surface area density to to dissipate heat dissipate heat • Moderate heat transfer coefficients • High effective heat transfer possible coefficients possible • Many flow geometries possible

Thermacore Technology Inlet Outlet Inlet

Metal Powder Particles

Heat Enters from Bottom

Advantages Well-Bonded Porous • High Surface Area Cool Single Phase Metal Matrix Warm Single Phase Coolant In Coolant Out • High Heat Transfer Coefficient • High Heat Flux (> 300 W/cm2) • Low Thermal Resistance • Low Profile Packaging Heat Source • Low Mass (< 75 grams) e.g.: computer chip, particle beam, EM radiation, laser diode array

Liquid cooled heat sinks make use of high surface area and effective heat transfer available in a well-bonded porous metal matrix.

a Performance Comparison - VFCP vs. PMCP

0.12 Lower Thermal Resistance = Better Performance 0.10

0.08 VFCP Heat Input: 12mm x 12mm

0.06 Benefit PMCP Heat Input: 7mm x 7mm 0.04 >40%

0.02

Resistance (deg-C/W/cm^2) Resistance Courtesy: Dr. Kevin Wert 0.00 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Coolant Flow Rate (GPM)

• 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. Desktop Heat Exchanger − 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 1U Server Heat Exchanger

1U Server Heat Exchanger Desktop Heat Exchanger

HEX Spec. – Desktop Chassis HEX Spec. – 1U Server Chassis

Material Aluminum Material Aluminum

Length Application Specific: 150mm shown Length Application Specific: 130mm shown Typical: 100 -150mm Typical: 100 – 275 mm

Height Application Specific: 150mm shown Height Application Specific: 40 mm shown Typical: 100 -150mm Typical: 30 - 50mm

Depth 25mm Depth 25mm

Mass Application Specific: 350 grams Mass Application Specific: 85 grams Typical: 250 -350mm Typical: 85 -150mm

Compact Form Factor Pump

Pump Spec. Pump Spec. – 1U Server Chassis Flowrate ~ 0.25gpm @ 3.6 psi head Flowrate ~ 0.125 gpm @ 3psi head Acoustic Power Target ≤ 3.3 BA Acoustic Power Target ≤ 3.3 BA Dimensions 62 mm W x 38mm H x 32 mm W x 32mm H x 89mm L 87mm L w/ barbs Dimensions

Mass 200 grams Mass 135 grams

Power 12W Power 10W

Voltage/Amps 12Vdc / 1A continuous Voltage/Amps 12Vdc / 0.6A continuous

• 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.

HX with chilled Pumps Reservoir water

Heater

3-way temp. controlled valve

Filter Purge Line Strainer Antenna

• 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

Internal Heat External Heat Exchanger Exchanger

Cold Plates

Heat Pipe Assy’s

Rugged, Liquid Cooling • Application: System (rLCS) − Ruggedized Electronics Cooling • Thermal Load/Power: 1 kW − Designed / Tested to MIL Specs. • Cooling System includes: − Heat pipes Redundant Pumps − Liquid-cooled cold plates − Internal brazed aluminum liquid-to-air heat exchanger − Dip brazed aluminum cold plates • Sealed air-tight chassis − An external brazed aluminum liquid-to-air heat • Upgradeable Electronics exchanger

• 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

• 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.

• 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…