# Heat Pipe Limits

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.

Figure 1. Temperature drops across a heat pipe. The vapor space temperature drop is usually small compared to the temperature drops required to conduct heat into and out of the heat pipe.

In a properly designed heat pipe, the maximum power is set by the source and sink conditions.  As shown in Figure 1, the temperatures drops across a heat pipe are:

Heat Pipe Assembly used to move heat away from the hot side of TEC to external heat sink.

• Conduction through the envelope wall and wick
• Evaporation
• Vapor Space Temperature Drop
• 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.

However, if operating conditions cause the heat pipe to exceed its power capacity, the effective conductivity of the heat pipe will significantly reduce. Therefore, assuring heat pipes meet your maximum system requirements is a critical aspect of design.

As shown in Table 1, there are five primary heat pipe transport limitations that must be considered during design: viscous, sonic, capillary, entrainment/flooding and boiling; see Table 1. 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 to help customers design accordingly.

Table 1.  Heat Pipe and Thermosyphon Performance Limits.

 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 High velocity vapor flow strips liquid from the wick. Not enough vapor space for the given power requirement.  Occurs at low temperatures. Thermosyphon Flooding High velocity vapor flow prevents liquid return in a gravity aided thermosyphon. 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. Power input too high. Wick structure not designed appropriately for power and orientation. Boiling Boiling occurs in the wick which prevents liquid return High radial heat flux into the heat pipe evaporator.

To calculate the heat pipe performance limit, the different heat pipe limits are plotted as a function of temperature; see Figure 2 (Top).  Note that the viscous limit is not shown, since it is not relevant in the normal operating temperature range.   The lowest limit at each temperature is then the heat pipe performance limit curve; see Figure 2 (Bottom).

Figure 2. Heat pipe performance limits. (Top) A plot of the individual limits, showing that the entrainment and capillary limits are controlling over certain temperature ranges. (Bottom) The heat pipe performance limit curve, calculated by taking the lowest limit at each temperature.

The viscous, sonic, and entrainment/flooding limits are all related to the vapor velocity, and are 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.