Presented by:

HARSHA.M.N

(1ks09me036 )

A heat pipe heat exchanger is a simple device which is made use of to transfer heat from one location to another, using an evaporation-condensation cycle.

Heat pipes are referred to as the "superconductors" of heat due to their fast transfer capability with low heat loss.

What is a Heat Pipe?

Working Principle

• The heat input region of the heat pipe is called evaporator, the cooling region is called condenser.

• In between the evaporator and condenser regions, there may be an adiabatic region

• Container• Working fluid• Wick or Capillary structure

1.Container The function of the container is to isolate the working fluid

from the outside environment.

Selection of the container material depends on many factors. These are as follows:

Compatibility (both with working fluid and external environment)

Strength to weight ratioThermal conductivityEase of fabrication, including welding, machineability and

ductilityPorosityWettability

Container materials

Of the many materials available for the container, three are by far the most common in use—name copper, aluminum, and stainless steel.

Copper is eminently satisfactory for heat pipes

operating between 0–200◦C in applications such as electronics cooling.

While commercially pure copper tube is suitable, the oxygen-free high conductivity type is preferable.

Like aluminum and stainless steel, the material is readily available and can be obtained in a wide variety of diameters and wall thicknesses in its tubular form.

The prime requirements are:

1.compatibility with wick and wall material

2.Good thermal stability

3.wettability of wick and wall materials

4.vapor pressure not too high or low over the operating temperature range

5.high latent heat

6.high thermal conductivity

7.low liquid and vapor viscosities

8.high surface tension

9.acceptable freezing or pour point

Examples of Working Fluid

Medium Melting  Point (°C)

Boiling Point at Atm.  Pressure(°C)

Useful Range(°C)

Helium -271 -261 -271 to -269Nitrogen -210 -196 -203 to -160Ammonia -78 -33 -60 to 100Acetone -95 57 0 to 120Methanol -98 64 10 to 130Flutec PP2 -50 76 10 to 160Ethanol -112 78 0 to 130Water 0 100 30 to 200Toluene -95 110 50 to 200Mercury -39 361 250 to 650Sodium 98 892 600 to 1200Lithium 179 1340 1000 to 1800Silver 960 2212 1800 to 2300

1. It is a porous structure made of materials like steel,alumunium, nickel or copper in various ranges of pore sizes.

2. The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator.

3. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe.

4. Wicks are fabricated using metal foams, and more particularly felts, the latter being more frequently used. By varying the pressure on the felt during assembly, various pore sizes can be produce.

5. The maximum capillary head generated by a wick increases with decrease in pore size.

6. The wick permeability increases with increasing pore size.

7. Another feature of the wick, which must be optimized, is its thickness. The heat transport capability of the heat pipe is raised by increasing the wick thickness.

8. Other necessary properties of the wick are compatibility with the working fluid and wettability.

Wick DesignTwo main types of wicks: homogeneous and composite.

1.Homogeneous- made from one type of material or machining technique. Tend to have either high capillary pressure and low permeability or the other way around. Simple to design, manufacture, and install .

2.Composite- made of a combination of several types or porosities of materials and/or configurations. Capillary pumping and axial fluid transport are handled independently . Tend to have a higher capillary limit than homogeneous wicks but cost more.

htp://www.electronics-cooling.com/Resources/EC_Articles/SEP96/sep96_02.htm

Three properties effect wick design1. High pumping pressure- a small capillary pore radius

(channels through which the liquid travels in the wick) results in a large pumping (capillary) pressure.

2. Permeability - large pore radius results in low liquid pressure drops and low flow resistance. Design choice should be made that balances large capillary pressure with low liquid pressure drop. Composite wicks tend to find a compromise between the two.

3. Thermal conductivity - a large value will result in a small temperature difference for high heat fluxes.

Fig: The actual test results of heat pipe with different wick structure athorizontal and vertical (gravity assist) orientations.

Types of Heat PipesThermosyphonLeading edge- Rotating and revolving- Cryogenic pumped loop heat pipe Flat Plate- Micro heat pipes-Variable conductance-Capillary pumped loop heat pipe-

Advantages Of Heat PipesMay reduce or eliminate the need fir reheat,Allow cost effective manner to accommodate new

ventilation standards,Requires no mechanical or electrical input,Are virtually maintenance free,Provide lower operating costs,Last a very long time,Readily adaptable to new installations and retrofiting

existing A/C units andAre environmentally safe.

Ideal Thermodynamic Cycle

Heat transport limitationHeat transport limitation

description Cause Potential solution

Viscous Viscous forces prevent vapor flow in the heat pipe

Heat pipe operating below recommended operating temperature

Increase heat pipe operating temperature or find alternative working fluid

Sonic Vapor flow reaches sonic velocity when exiting heat pipe evaporator resulting in a constant heat transport power and large temperature gradients

Power/temperature combination, too much power at low Operating temperature

This is typically only a problem at start up. The heat pipe will carry a set power and the large temperature will self correct as heat pipe warms up

Entrainment(Flooding)

High Velocity vapor flow prevents condensate from returning to evaporator

Heat pipe operating above designed power input or at too low an operating temperature

Increase vapor space diameter or operating temperature

Capillary Sum of gravitational, liquid and vapor flow pressure drops exceed the capillary pumping head of the heat pipe wick structure

Heat pipe input power exceeds the design heat transport capacity of the heat pipe

Modify heat pipe wick structure design or reduce power input

Boiling Film boiling in heat pipe evaporator typically initiates at 5-10 W/cm2 for screen wicks and 20-30 w/cm2 for powder metal wicks

High radial heat flux causes film boiling resulting in heat pipe dry out and large thermal resistances

Use a wick with a higher heat flux capacity or spread out the heat load

Heat Pipe ApplicationsElectronics cooling- small high performance components

cause high heat fluxes and high heat dissipation demands. Used to cool transistors and high density semiconductors.

Aerospace- cool satellite solar array, as well as shuttle leading edge during reentry.

Heat exchangers- power industries use heat pipe heat exchangers as air heaters on boilers.

Other applications- production tools, medicine and human body temperature control, engines and automotive industry.

ApplicationsLAPTOP HEAT PIPE SOLUTION

Heat pipes used in processor

Space craft

HEAT PIPE IN CPU

Camera

Cooler Combined Heat pipe / water cooling Jacket for hi-def CCD camera.

REFERENCES

Andrews, J; Akbarzadeh, A; Sauciue, I.: Heat Pipe Technology, Pergammon, 1997.

Dunn, P.D.; Reay, D.A.: Heat Pipes, Pergammon, 1994.

www.heatpipe.com.www.cheresources.com.www.indek.comwww.wikipedia.org

THANK YOU