Heat Pipes & Vapor Chambers – Useful Guidelines for Heat Sink Implementation

Webinar
Overview:
Heat pipes, and increasingly vapor chambers, are common devices used to improve heat sink thermal performance by over 30% when compared to solid metal alternatives. This webinar will cover two-phase device similarities, differences, misconceptions, best uses, sizing and performance modeling through the presentation of numerous examples.

Who Should Attend: Engineers interested in learning about how to best incorporate heat pipes and/or vapor chambers into their next heat sink design.

Speaker: George Meyer

Speaker: George Meyer

This course is being taught by George Meyer, a thermal industry veteran with over three decades of experience in electronics thermal management. He holds over 70 patents in heat sink and heat pipe technologies and currently serves as the CEO of Celsia Inc.

Below are the audience questions asked during the live presentation.
Click on the question to view the answer.

CFD Questions

How do you model a heat pipe or a vapor chamber in CFD?

Assign an effective thermal conductivity to the heat pipe or vapor chamber into the model. As these figures are highly application specific, we suggest contacting us with rough design guidelines and we’ll calculate the appropriate figures for model input.

When simulating heat pipes, do you use an equivalent conductivity? If yes, how much is it?

Effective thermal conductivity of two-phase devices typically ranges between 1,000 W/mK to fifty times that. If you’re unsure but need to model a potential solution, use 5,000 W/mK or contact Celsia and we’ll run the numbers based on a rough design to get you a more accurate figure.

The fact that thermal conductivity of heat pipes increases with the length of the pipe seems counterintuitive. Can you explain?

Yes, heat pipe delta-Ts do not increase much with added length. This added “L” with little or no increase in delta-T results in higher effective conductivity numbers.

Application Questions

Data Center / Server – Can I route heat pipes from various heat sources to a central condenser?

Yes, this is done routinely in applications such as laptop PCs.

How do I determine the area and length of the evaporator and condenser sections?

The evaporator is fixed by the size of the heat input area and similarly the condensation takes place where the heat sink is located. Typical heat pipe and vapor chamber designs do not have specific evaporator and condenser areas.

  1. For heat pipe applications, you must ensure that the pipe(s) fully cover the heat source to avoid hot spots.
  2. For vapor chamber applications, where spreading the heat is important, a minimum ratio of 20:1 vapor chamber area to heat source area is recommended.
  3. For quick estimates on heat sink size requirements which sets the condenser area you can do a quick estimate as shown below.
  4. Heat Sink Volumetric Calculation

In a thermal cycling application, what is the transient response of 2-phase devices?

The evaporation, vapor flow and condensation take place immediately as heat is applied. The time constant is due to the thermal capacitance of the system being heated or cooled.

2-Phase Technology, Manufacturing and Attachment Questions

What are the most common working fluids used in heat pipes and vapor chambers?

I’m just going to take a section from Wikipedia as it explains it nicely.

“The most commonly used envelope (and wick)/fluid pairs include:
Copper envelope/Water working fluid for electronics cooling is by far the most common type of heat pipe.
Copper or Steel envelope/Refrigerant R134a working fluid for energy recovery in HVAC systems
Aluminum envelope/Ammonia working fluid for Spacecraft thermal control.
Superalloy envelope/Alkali Metal (Cesium, Potassium, Sodium) working fluid for high temperature heat pipes, most commonly used for calibrating primary temperature measurement devices.”

What are the most important parameters regarding the design of the wick structure?
  1. Power densities, operating orientations are the two driving factors for determining the best wick structure.
  2. This was touched on in a couple of the presentation slides, one of which I’ll show here. In addition to the cost and performance increases as you move from grooved wick to mesh and then to a sintered wick, each type can be optimized to meet specific application requirements. For instance, changes to the wick thickness and/or porosity of a wick has a dramatic effect on performance characteristics.
  3. 2-Phase Similarity: Wick Structures
Is there a minimum delta T between evaporator and condenser ends for the heat pipe to work?

No, a specific delta-t is not required.

How do you solder/seal the different layers of the vapor chamber? Is it a mechanical cold soldering process by pressure or an electrical soldering process?

They can be sealed using a number of processes, welding or brazing are the two most common.

What are the main theories and studies about the inner workings and performance of vapor chambers?

Same as heat pipes, for reference get a copy of Dunn & Reay, Heat pipes published by Pergamon Press.

What are the definitions of the wick limit and vapor limit for heat pipes and vapor chambers respectively?
  1. Wick Limit – also known as the capillary limit, it measures the effectiveness with which the working fluid inside the heat pipe can move from the condenser section back to the evaporator area. As mentioned earlier, changes to the wick structure and dramatically change the wick limit of a given diameter heat pipe.
  2. Vapor Limit – is related to the amount of vapor that can move down the interior open space in the device, referred to as the sonic limit.
What environmental stresses can heat pipes survive?
  1. Temperature extremes: Operating and non-operating temperatures especially through freeze thaw cycles and high elevated temperatures.
  2. Exposure to corrosive and humid conditions are often addressed by using a protective coating such as nickel plating.
  3. G-forces and shock and vibe
Where is Celsia’s manufacturing?

Celsia is headquartered in the San Francisco Bay area of California with a wholly owned production facility in Taiwan.

What is the best solder material and process to attach heat pipes or vapor chambers to an aluminum metal sheet?

Nickel plate the aluminum and then use any solder that is appropriate for the application.

Does soldering damage heat pipes and vapor chambers?

No, provided the solder application is below 180 deg C. For vapor chambers, special fixtures must be designed to contain the positive internal pressures.

Do vapor chambers allow bends in the XY plane?
  1. Two piece vapor chambers can be stamped into almost any configuration along the xy plane.
  2. Narrow one piece VCs can be bent in the XY with the same bend radius guidelines as heat pipes. Larger one piece VCs cannot be bent in the XY direction
What is the advantage of nickel plating on copper fins?

Three advantages: for soldering, for corrosion protection and for cosmetics.

For cost reduction purposes, what are the parameters do I need to consider to change from a vapor chamber to a heat pipe solution?

Two key points, potential hot spots in the device being cooled and the added conduction losses in a heat pipe system.

What is a good rule of thumb for de-rating heat pipe Qmax for flattening, bending, orientation, etc?
  1. Flattening: The closest one can get to a rule of thumb is ‘larger diameter heat pipes can be flattened more – in terms of flattened thickness to original diameter – before the Qmax is affected. Let me explain. For terrestrial applications, the two limits which affect the Qmax of a given heat pipe are wick limit (also known as capillary limit) and vapor limit (which is a combination of blah and blah). Unless the wick is unusually thick, the round heat pipe will have excess vapor capacity, meaning that that the wick limit will determine Qmax. For smaller diameter heat pipes like 3mm, there’s only a small excess so flattening it to even 2mm will bring the vapor limit below the wick limit and reduce Qmax. However, in an 8mm heat pipe there’s a massive amount of excess vapor space, so you’d have to flatten it to around 3mm before Qmax would be affected.
  2. Bending: reduce the rated Qmax by 2.5% for each 45 degree bend. I have a blog post which explains this in further detail here.
  3. Orientation: See the chart below. It’s important to remember that heat pipes can be specifically designed to work against gravity. For instance, the performance of a standard 8mm heat pipe will drop significantly depending on orientation. However, by tweaking the internal structure, we can raise the performance when working against gravity, but it will be at the expense of Qmax when working with gravity.
  4. Orientation Affects Qmax