Types Of Heat Pipes

Let Us study on types of heat pipes

Tubular Heat-pipes:

These are the simplest and most popular type of Heat-pipe and are used in most applications to transfer heat energy from one point to another. They can also be used as heat spreaders to isothermalise components where a uniform temperature is desired. Although Heat-pipes are predominantly used in cooling applications, they can also be used very effectively in heating applications, this negates having multiple electrical heating elements, and simplifies cabling, design and installation.

Annular Heat-pipes

These are Heat-pipes with an axial concentric hole through the middle. These may be used in a variety of situations where high heat transfer is required together with mechanical access, examples include; as a flow channel for gas or liquid, for cabling or thermocouple access, as a push rod mechanism location sleeve.Larger diameter Heat-pipe rollers have also been produced similarly with open bore for use as rotary thermal spreaders and having internal air cooling.

Thermosyphon- gravity assisted wickless heat pipe. Gravity is used to force the condensate back into the evaporator. Therefore, condenser must be above the evaporator in a gravity field.

Leading edge- placed in the leading edge of hypersonic vehicles to cool high heat fluxes near the wing leading edge.

Rotating and revolving- condensate returned to the evaporator through centrifugal force. No capillary wicks required. Used to cool turbine components and armatures for electric motors.

Cryogenic- low temperature heat pipe. Used to cool optical instruments in space

Flat Plate- much like traditional cylindrical heat pipes but are rectangular. Used to cool and flatten temperatures of semiconductor or transistor packages assembled in arrays on the top of the heat pipe

Micro heat pipes- small heat pipes that are noncircular and use angled corners as liquid arteries. Characterized by the equation: rc /r_h³1 where rc is the capillary radius, and r_h is the hydraulic radius of the flow channel. Employed in cooling semiconductors (improve thermal control), laser diodes,photovoltaic cells, medical devices.

Variable conductance- allows variable heat fluxes into the evaporator while evaporator temperature remains constant by pushing a non- condensable gas into the condenser when heat fluxes are low and moving the gas out of the condenser when heat fluxes are high, thereby, increasing condenser surface area. They come in various forms like excess-liquid or gas-loaded form. The gas-loaded form is shown below. Used in electronics cooling

Capillary pumped loop heat pipe- for systems where the heat fluxes are very high or where the heat from the heat source needs to be moved far away. In the loop heat pipe, the vapor travels around in a loop where it condenses and returns to the evaporator. Used in electronics cooling.

Diode Heat-pipes
Diode Heat-pipes have a high thermal conduction in one direction and a low thermal condution in the opposite direction. They are typically used in aerospace applications where it is possible for heat sink temperatures to exceed the Heat-pipe evaporator temperature due to solar radiation. Usually an array of diode Heat-pipes are used together to dissipate heat to a number of alternative heat sinks so that heat sinking facilities are constantly maintained.

Variable Conductance Heat-pipes
VCHP Heat-pipes provide a means for automatic temperature control. By regulation of the Heat-pipe thermal conductivity it is possible to maintain the evaporator section at near constant temperature with around only 5 °C fluctuation. VCHP’s are specifically pre-set to operated over a specific temperature range of varying ambient condidions and variable heat input loading. Where very precise control is required they can be also integrated with electronic control achieving temperature stabilization of better than 1°C with minimal electrical power comsuption for the controlling circuit.

Flexible Tubular Heat-pipes
A limited degree of flexibility is possible with most standard range Heat-pipes, as a function of their length and diameter. Often this may be sufficient to accommodate small dimensional tolerance differences encountered during assembly and installation with other components. Truly flexible Heat-pipes incorporate an intermediate convoluted bellows section providing excellent flexibility and anti-vibration characteristics.

High Performance Heat-pipe Heat Sinks
Integral Heat-pipe heat sinks built with cooling fin assemblies provide one of the most effective means of providing efficient cooling for power electronics components. The forced air cooled assembly shown opposite achives an oustanding thermal performance. Performance: Rth = 0.09°C/W.
i.e with 100 W heat dissipation the input heat block temperature is only 9°C higher than the ambient cooling air temperature!

Baffle Heat-pipes
Baffled Heat-pipes are used in water cooled applications i.e. core cooling applications used commonly in plastic injection moulding tools. The Heat-pipes are installed into manifolds and transfer heat from the heat source to the cooling water. The baffle plates direct water along the Heat-pipe cooling length to give an increased cooling area contact.

Bent Tubular Heat-pipes
Common application requirements require Heat-pipes to be bent to fit a route in a particular installation, possibly involving complex 3-D architecture. By using special internal wick structures, CRS Engineering is able to produce Heat-pipes which can be formed to shape by customers upon installation. Where exact bending is required, or where tight bending radii are involved, it is essential that CRS Engineering carry out this process during manufacture.