How do They Work?

A heat pipe consists of a working fluid, a wick structure, and a vacuum-tight containment unit (envelope). The heat input vaporizes the working fluid in liquid form at the wick surface in the evaporator section.

Vapor and its associated latent heat flow toward the colder condenser section, where it condenses, giving up the latent heat. Capillary action then moves the condensed liquid back to the evaporator through the wick structure. Essentially, this operates in the same way as to how a sponge soaks up water.

Every Heat Pipe Has Three Components

1. Heat pipe working fluid, the fluid material within the envelope which transfers heat by evaporation and condensation.
2. Heat pipe envelope, the outer “shell” material which provides a leak-tight pressure vessel to contain the working fluid.
3. Heat pipe wick, the material within the envelope used to return liquid from the condenser to the evaporator using capillary forces.

Copper/Water Heat Pipes are the most commonly used fluid/envelope pair for electronics cooling.

• Gas Generation (Most Common)
• Corrosion
• Materials Transport

Heat Pipe Fluids

Selecting A Heat Pipe Working Fluid

Evaporation and condensation of the working fluid are what give heat pipes their high effective thermal conductivity, which can be as high as 100,000 W/m K. During heat pipe operation, the working fluid is vaporized in the evaporator and then condensed in the condenser, thus transferring heat.

The first step in selecting a heat pipe working fluid and envelope/wick material is to determine the operating temperature range. For a heat pipe to operate, it must be at saturated conditions, where the heat pipe contains both liquid and vapor. The practical operating temperature range for a copper/water heat pipe is roughly 25° to 150°C.

Learn more about theoretical and practical temperature ranges for heat pipes below.

Heat Pipe Fluid Selection FAQs

Heat Pipe Envelope and Fluid Parings

Once the operating temperature is known, the heat pipe designer selects the working fluid.  The envelope and wick materials are then selected from materials known to be compatible with the working fluid.  If the materials are not compatible, possible incompatibility problems include:

• Copper envelope with water as the working fluid is common for electronics cooling
• Copper or Steel envelopes with Refrigerant R134a fluid is common for Energy Recovery
• Aluminum envelope with Ammonia as the working fluid is common for Spacecraft Thermal Control
• Superalloys: Alkali Metals (Cesium, Potassium, Sodium) for High-Temperature Heat Pipes
• As discussed in Compatible Fluids and Materials there are a large number of other compatible envelope/fluid pairs that are used at other temperature ranges, or when additional factors must be considered. For example, copper/methanol is often used for electronics cooling when the heat pipe needs to operate near or below 0°C, the point at which water freezes. Fluid choices in a given temperature range are ranked by the Merit Number.

Unique Temperature Ranges

High-temperature heat pipes (also known as alkali metal heat pipes) are typically defined as heat pipes that operate between 400 and 1100°C. 

Heat Pipe Performance & Limits

Calculating Transport Capacity

The most important heat pipe design consideration is the amount of power a heat pipe is capable of transferring. Heat pipes can transfer much higher powers for a given temperature gradient than the best metallic conductors.  The maximum power that the heat pipe can carry can be set either by the heat source and heat sink conditions or by internal heat pipe limits (can be minimized through proper design).

The temperature drops across a heat pipe are:

• Conduction through the envelope wall and wick
• Evaporation
• Vapor Space Temperature Drop
  • is usually small compared to the temperature drops required to conduct heat into and out of the heat pipe.
• Condensation
• Conduction through the envelope wick and wall

In addition, there are further temperature drops to bring the heat to the heat pipe evaporator and reject the heat from the condenser, for example, using a finned heat sink and forced convection at the condenser.

Critical design point: ensuring that the heat pipes meet your maximum system requirements. If operating conditions cause the heat pipe to exceed its power capacity, the effective conductivity of the heat pipe will be significantly reduced.

Heat Pipe Life Tests

Heat pipes are designed to operate without maintenance for long periods of time, e.g., for more than twenty years in Spacecraft Thermal Control Systems.  One requirement is that the envelope, wick, and working fluid must be compatible.  Note that compatibility requirements for heat pipes are more stringent than for many other applications.  For example, aluminum and water are not compatible, since an aluminum/water heat pipe will generate large amounts of non-condensable gas in hours or days, stopping the heat pipe from operating.

Life tests to determine compatibility have been conducted since heat pipes were rediscovered in the 1960s at the Los Alamos National Labs.  In the past, life tests have been conducted on hundreds of different working fluid/envelope/wick combinations, leading to the formation of a long list of compatible envelope/wick/material systems, including copper/water for electronics cooling, and aluminum/ammonia for spacecraft thermal management. Life tests conducted at ACT include Process Control Life Tests, High-Temperature Water Life Tests, and Intermediate Temperature Fluids.

One area of active research on compatibility is the temperature range from 150 to 400°C, see High-Temperature Water Life Tests and Intermediate Temperature Fluids Life Tests section below for more details.  ACT has been examining different envelope/fluid pairs in this temperature range for the past decade and has demonstrated several new compatible fluid pairs.

LESSONS LEARNED FROM A SERIES OF HALIDE LIFE TESTS – Read the case study

Heat Pipe Limits

There are five primary heat pipe transport limitations that must be considered during design: viscous, sonic, capillary, entrainment/flooding and boiling; see table below. These limits are a function of many variables including operating temperature, wick selection, and fluid properties. The most common limit for terrestrial applications is the capillary limit. ACT developed a heat pipe calculator for terrestrial copper-water heat pipes to help customers design accordingly.

Heat Pipe and Thermosyphon Limits Table

Heat Pipe Limit	Description	Cause
Viscous (Vapor Pressure)	Viscous forces prevent vapor flow within the heat pipe.	Heat pipe operating near triple point with a very low vapor pressure – need to use a different working fluid.
Sonic	Vapor flow reaches sonic velocity when leaving the evaporator, choking the flow.	Too much power at lower operating temperature. Typically this is seen at start-up and will self-correct.
Heat Pipe Entrainment	The power input is too high. Wick structure not designed appropriately for power and orientation.	Not enough vapor space for the given power requirement.  Occurs at low temperatures.
Thermosyphon Flooding	The power input is too high. Wick structure is not designed appropriately for power and orientation.	Not enough vapor space for the given power requirement.  Occurs at low temperatures.
Capillary	The capillary action of the wick structure cannot overcome gravitational, liquid, and vapor flow pressure drops.	High-velocity vapor flow prevents liquid return in a gravity-aided thermosyphon.
Boiling	Boiling occurs in the wick which prevents liquid return	High radial heat flux into the heat pipe evaporator.

VISCOUS (VAPOR PRESSURE)

Sonic

Note: as the vapor temperature in the heat pipe is lowered, the vapor pressure drops.  To carry a given amount of heat, the vapor velocity must increase, which in turn increases the pressure drop from the evaporator to the condenser.  At these low vapor pressures, compressible flow effects become important.

Design rule of thumb, always operate with powers less than one-half of the sonic limit.

The sonic limit is reached when the Mach number reaches 1 at the beginning of the heat pipe adiabatic section.   There are a number of ways to reach the sonic limit in compressible flow, such as decreasing the flow area, adding heat, and frictional effects.

Sonic Limit Calculation

The heat pipe sonic limit is approached by mass addition, where mass is added to the flow rate from the start to the end of the heat pipe evaporator:

1. where:
   m_{Dot_Sonic}      Mass flow at the entrance to the adiabatic section, when the flow is choked, kg/s
   ρ_V                    Vapor density, kg/m³
   V_{Sound_V}       Speed of sound in the vapor, m/s
   A_Vapor            Heat pipe vapor space area, measured perpendicular to the flow, m²
   γ                      Ratio of specific heats, c_p/c_v

2. The sonic limit is then:
   1. where:
      q_Sonic       Sonic limit, W
      λfg            Latent heat, liquid to vapor, J/kg

Sonic Limit Example

The chart to the right compares heat pipe power versus temperature for identical heat pipes using either cesium or potassium as the working fluid.  The sonic and capillary limits were calculated, and then curves using the lower of the two limits were plotted in the figure.

On the left side of the graph, the maximum heat pipe power is set by the sonic limit (the roughly parabolic part of the curve), while on the right side of the graph, the maximum power is set by the capillary limit (the roughly flat part of the curve).

At lower temperatures, more power can be carried with cesium, since it has a higher vapor density (and higher sonic limit) at any given temperature.  Once the temperature is increased above roughly 500°C, the potassium heat pipe carries more power (for this particular design).  This is the reason that cesium is normally used at temperatures below 600°C, and is replaced by potassium and then sodium at higher temperatures.

Heat Pipe Entrainment/Thermosyphon Flooding

Like the viscous and sonic limits, the entrainment or flooding limit is related to the vapor velocity and is more significant at lower temperatures.  The reason is that the vapor pressure and vapor density decrease as the temperature is lowered, so the vapor velocity must increase to carry the same power.

Design rule of thumb, always operate with powers less than 0.75 of the entrainment or flooding limit.

Heat Pipe Entrainment

In a heat pipe, the vapor flows from the evaporator to the condenser, with countercurrent liquid flow from the condenser to the evaporator.  The entrainment limit is reached when the vapor velocity in the heat pipe is high enough to shear liquid from the wick.

The entrainment limit is given by:

1. where:
   q_Entrain    Entrainment limit, W
   A_Vapor      Heat pipe vapor space area, measured perpendicular to the flow, m²
   λ_fg             Latent heat, liquid to vapor, J/kg
   σ                Surface tension, N/m
   ρ_V             Vapor density, kg/m³
   r_c              Wick pore radius, m

Thermosyphon Flooding Limit

In a thermosyphon, the vapor flows from the evaporator at the bottom of the thermosyphon to the condenser at the top of the thermosyphon.  At the same time, the condensed liquid drains back to the evaporator along the wall (driven by gravity).  The flooding limit is reached when the vapor velocity in the thermosyphon is high enough that shear stresses prevent liquid from returning to the evaporator.

1. Note: loop thermosyphons are sometimes used to eliminate the flooding limit since the vapor and liquid flows are separated from each other.

One flooding correlation that is often used was developed by A. Faghri and published in the book, Heat Pipe Science and Technology, CRC Press, pp. 387-397, 1995.

The first step is to define the Bond number, a dimensional number that measures the importance of surface tension forces versus body forces:

Capillary

During heat pipe operation the working fluid evaporates in the evaporator and condenses in the condenser, transferring the latent heat from one end of the heat pipe to the other.  The liquid condensate is passively returned to the evaporator by capillary forces in the wick.

Design rule of thumb: the capillary limit states that the capillary force generated by the wick must be larger than the sum of the pressure drops in the wick.

Capillary Limit Calculation

The maximum power that the heat pipe can carry and still return the condensate by capillary forces is the capillary limit:

1. where:
   ΔP_c      Capillary force generated in the wick, Pa
   ΔP_g      Pressure drop due to gravitation and acceleration, Pa
   ΔP_L      Liquid pressure drop in the wick, Pa
   ΔP_V      Vapor pressure drop in the heat pipe, Pa
   
   1. ΔP_C, Capillary Force
   1. The capillary pumping capability depends on surface tension and two radii of curvature of the liquid/vapor interface, measured perpendicular to each other:

1. ΔP_G, Gravitational Pressure Drop The gravitational pressure drop is:

P_L AND ΔP_V, Liquid and Vapor Pressure Drops

Effective Length

Capillary Limit Example

Boiling

When a low heat flux is applied to a heat pipe evaporator, the heat is conducted through the wick, and liquid vaporizes on the inner surface of the wick, into the vapor chamber.  As the heat flux increases, the temperature difference across the wick increases linearly.  The boiling limit or heat flux limit takes place when the transverse heat flux into the evaporator is enough to create nucleate boiling in the wick of the evaporator section. This generates vapor bubbles, which can become trapped in the wick, blocking the liquid from coming back, which can result in evaporator wick dry-out.

Boiling Limit Calculation

Experimental Boiling Limits

1. Rules of thumb for the boiling limit in some typical heat pipe wicks:
   1. Sintered Wicks with Water: ~ 75 W/cm²
   2. Screen Wicks with Water: ~ 75 Wcm²
   3. Grooved, Aluminum Wicks with Ammonia: ~ 15 W/cm²
   4. In special cases, wicks can be designed with much higher boiling limits.  The photo at the right shows a specially designed copper/water vapor chamber wick, which can remove 750 W/cm² over a 1 cm² area, shown in the center of the figure.

Heat Pipe Performance Limit

Water Heat Pipe Parameters and Limitations

There are three general limitations for passive two-phase devices, which include heat pipes, HiK™ plates, and vapor chambers:

• Temperature
• Vertical Height
• Acceleration

Operating Temperature

All heat pipes have a temperature range over which they have the best performance.   Maximum power versus operating temperature for typical water heat pipes is shown in the Copper Water Heat Pipe Calculation output above.  The maximum power is high from roughly 60 to 200°C, falling off gradually at lower temperatures (the power also falls off at higher temperatures, but this is typically not a concern for electronics cooling).

The drop-off in power as the temperature is reduced is set by the fluid properties of the working fluid.  As the temperature (and associated saturation pressure) of the water is reduced, the water vapor density is also reduced.  To carry a given amount of power, the vapor velocity in the heat pipe must increase, which in turn increases the pressure drop in the heat pipe while reducing the power that can be carried.

Adverse Verticle Height

Heat pipes return liquid from the condenser to the evaporator through a wick, which allows them to operate in any orientation. During operation, capillary forces in the wick must overcome the sum of liquid and vapor pressure drops as well as the adverse gravity head and acceleration. As the adverse elevation is increased (evaporator located above condenser), more of the wick pumping capability is used to counteract the adverse gravity head, and the maximum heat pipe power is reduced. This can be seen when comparing the Copper Water Heat Pipe Calculation output from above, sides (a) and (b), which show power with a level heat pipe, and a heat pipe with an adverse 4-inch (10 cm) elevation, respectively. It can be seen that the maximum power is reduced significantly. Other adverse elevations can be examined through calculations with ACT’s heat pipe calculator.

In general, water heat pipes can operate with the evaporator elevated a maximum of 9-10 inches (23-25 cm) above the condenser. This sets the maximum elevation for spot-cooling heat pipes and vapor chambers. This elevation is doubled for HiK™ plates to 18-20 inches (46-50 cm), when the HiK™ plates are cooled on both the top and the bottom, and a double set of heat pipes is embedded. Note that if the electronics and sink can be arranged so that the electronics are lower than the heat sink, then the heat pipe behaves as a thermosyphon. In this orientation, gravity returns the liquid to the evaporator rather than capillary forces, and the heat pipe length is essentially unlimited.

Acceleration

During operation, capillary forces in the wick must overcome the sum of liquid and vapor pressure drops as well as the adverse gravity head and acceleration. Water heat pipes will stop operating under high adverse acceleration when the wick can no longer return condensate to the evaporator, and the heat pipe de-primes, or dries out. The wick will quickly re-prime after the acceleration stops. Heat is stored by a rise in temperature during acceleration. Most accelerations are relatively short, and this temperature rise is acceptable.

Different Types of Heat Pipes