Heat Pipe

Celsia offers heat pipe manufacturing in all shapes, sizes, and performance characteristics for use in its custom heat sink designs. Their high thermal conductivity, ease with which they can be bent and shaped, and exceedingly long life make heat pipes an ideal upgrade to solid metal heat sinks or, in some cases, an alternative to pumped liquid cooling systems.

Heat Pipe Technology

In order to understand heat pipe technology, it’s first important to understand the function of each of its component parts.

1. Enclosure/Envelope – the heat pipe enclosure is a vacuum-sealed vessel (tube) that houses the wick structure and a working fluid. Once the wick material and liquid are added, the device is vacuum-sealed.
2. Heat Pipe Wick Material – affixed to the inner walls of the heat pipe, the wick absorbs (for most wicks) and helps distribute liquid from the cooler end of the heat pipe to the hotter end.
3. Working Fluid – heat pipe working fluid turns to vapor that carries heat from one end to the other. Because it’s in a vacuum, water turn to vapor at much lower temperatures.
4. Heat Pipe Vapor Space – the amount of internal space not taken up by water and wick that can be used to transport vapor. Not really a ‘part’ but a consequence of other ‘part’ choices.

The most popular heat pipe technology combination as a copper enclosure using a copper sintered wick with water as the working fluid. This combination of metal enclosure, wick type and fluid, when used for cooling electronics, allows for the widest range of operating temperature, the highest heat pipe power carrying capacity, and the ability to handle the highest power densities when compared to other combinations. The chart below shows the operating temperature ranges for different types of fluid.

Working Fluid

When it comes to other working fluids, the most common combination is methanol (fluid) with a copper enclosure and copper sintered wick. However, this type of heat pipe runs a distant second to copper/water as they are mainly used in colder terrestrial environments where the heat pipe needs to operate at temperatures below freezing. Other combinations such as aluminum enclosure with ammonia (liquid) are reserved primarily for space/satellite applications

While most heat pipe manufacturers only offer off-the-shelf products, Celsia optimizes enclosure wall thickness, wick properties and fluid-loading for each application. During production, every heat pipe is burned-in and helium tested for leakage and thermal performance before being integrated into the heat sink. Further, every completed heat sink is tested and verified to meet or exceed quoted thermal and dimensional requirements.

How Do Heat Pipes Work

Heat Pipe Working Principles

For heat pipes to work, heat is applied to one area of the heat pipe turning the liquid to vapor. This phase change happens at temperatures well below the open-air vapor point of the liquid because the entire vessel is vacuum-sealed.  However, sintered copper water heat pipes, that will freeze at 0 ^oC won’t enter their operational ‘sweet spot’ until around 25 ^oC.

Next, vapor moves to an area of lower pressure (away from the heat source) where it cools and returns to liquid form. Finally, the cooled liquid is absorbed into the heat pipe wick material and gets redistributed (by capillary action) to the evaporator area. The capillary action of a heat pipe wick is analogous to how a paper towel will become soaked if only one corner is placed in water. The process continuously repeats so long as heat is applied.

Heat pipes are offered in a variety of diameters ranging from 2mm to 12mm. Generally speaking, the larger the diameter, the more heat (measured in watts of power) it can handle. It depends on the heat pipe wick material, the angle at which the heat pipe must operate, the degree to which the heat pipe is bent and/or flattened, and the altitude at which the device operates.

Heat Pipe Wick Material

There are three commonly used heat pipe wick materials, each with unique cost and performance characteristics. Ordered from highest to lowest by cost and ability to work against gravity they are: sintered copper wick, screen mesh wick, and grooved wick.

Heat Pipe Wick Material

Sintered Copper 

Because they can handle high power density and work well against gravity, sintered copper heat pipes are the most widely used for most electronics cooling applications. The sintered material starts as a powder, sometimes containing different size particles, that gets baked onto the walls of a copper pipe.

Screen / Mesh

Screen mesh with perpendicular metal strands, like a screen door, or braided copper threads, running in the same direction, are another type of heat pipe wick material. They are generally used to allow heat pipes to be made as thin as possible, although the trade-off is lower power density and ability to work against gravity.

Groove

When heat pipes are gravity aided with the evaporator below the condenser, the condensed liquid returns without aid of capillary action as it’s simply able to flow downward. The primary benefit of grooved heat pipest is the low price as the wick structure is formed as part of the heat pipe extrusion process.

Heat Pipe Performance

More commonly known as Qmax, the maximum heat pipe carrying capacity generally increases with the diameter of the pipe. However, each wick type can be tuned to optimize specific performance parameters and this is especially true of sintered wicks.

For example, the chart below graphs Qmax for typical sintered wick heat pipes of varying diameters against the orientation in which the pipe is required to operate. The grey line represents a10mm pipe designed to maximize Qmax when flat (0-degree orientation). Like all heat pipes the Qmax increases as the evaporator is moved below the condenser. The opposite is also true and there can be as much as a 95% drop in Qmax from one orientation extreme to the next. However, the internal structure – wick thickness, wick porosity, and amount of working fluid – can be changed to optimize for specific conditions.

Heat Pipe Carrying Capacity (Qmax) by Orientation

If the engineer knows that the heat pipe will be required to operate in orientations between -50 to -90 degrees, the wick structure can be optimized to increase the capillary pumping action. As seen in the chart below, a gravity optimized 6mm heat pipe now has a higher power carrying capacity (Qmax) than its non-optimized 6mm counterpart, to meet the needs of this application. The trade-off? Its Qmax is lower than the non-gravity optimized 6mm pipe in orientations above -45 degrees.

Gravity Optimized Heat Pipe

Heat Pipe Shapes

Heat pipes can be made into virtually any shape by bending and/or flattening them; subject to certain parameters. The typical minimum bend radius is 3-times the diameter of the tube. However, bending will reduce its Qmax, the maximum power transport capacity. Smooth, gradual bends will have less of an effect than tight ones, but a good rule of thumb is for every 45-degree bend Qmax will decrease by 2.5%. Please contact Celsia engineering@celsiainc.com for detailed information regarding your application.

Flattening a heat pipe to one-third of its original diameter is generally considered the maximum, although this figure decrease with smaller pipes (2-4mm) and increases with larger ones (>10mm). ). Performance can be affected as the tube is flattened. The chart below offers some insight into how flattening affects heat pipe performance. Provided a heat pipe is properly matched to the application, its Qmax is determined by the lower of the wick limit or the vapor limit. For instance, for a round 6mm standard heat pipe the wick limit (black dotted line) is just under 60 watts. Flattening it to 4, 3.5, or 3mm has no effect on its Qmax as the vapor limit is above the wick limit. Note that flattening a round 8 or 10mm heat pipe to 3mm or 2.5mm will have a substantial effect on its Qmax.

Maximum Heat Pipe Carrying Capacity is the Lesser of the Wick & Vapor Limits

Heat Pipe Effective Thermal Conductivity

Knowing the thermal conductivity of a heat pipe is important when performing CFD modeling of two-phase devices which are integrated into a thermal solution.

Regularly published heat pipe thermal conductivities range from 5,000 to 100,000 W/m-K. That’s up to about 250 times that of solid copper and 500 times that of solid aluminum. However, engineers should confirm the conductivity numbers for their specific application. Unlike solid metal, the effective thermal conductivity of heat pipes varies tremendously.

Two-phase device performance varies with a number of factors including temperature and power densities the effective conductivity of these devices is a snapshot of operation at a given operating condition. This is typically done at the worst case condition for the device. The effective conductivity numbers are derived from a calculated delta-t factored with the power (Q), the cross-sectional area (A) and the length the heat is being moved (L). Length is the dominant factor in conductivity calculations.

The figure below illustrates the effect of length on heat pipe thermal conductivity. In this example, we used three (6mm) heat pipes to transport heat from a 75 watt power source. While thermal conductivity of 10,000 W/m-K is achieved at just 75mm heat pipe length, a 200mm length has a conductivity of just over 44,000 W/m-K.

Figure 1: Heat Pipe Effective Thermal Conductivity as Function of Length

Detailed information on heat pipe thermal conductivity can be found here.