Heat Pipes

SEMINAR ON “HEAT PIPES” PRESENTED BY: [email protected] m

WHAT IS A HEAT PIPE 

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Heat pipe is a simple device that can quickly transfer heat from one point to another. They are often referred to as the "superconductors" of heat as they possess an extra ordinary heat transfer capacity & rate with almost no heat loss. The idea of heat pipes was first suggested by R.S.Gaugler in 1942.In 1963, G.M.Grover invented its remarkable properties & serious development began.

CONSTRUCTION It consists of a sealed hollow tube using thermoconductive metal such as copper or aluminium.  On the internal side of the tube's side-walls a wick structure exerts a capillary force.  It is similar to a thermosyphon. The pipe Consists of a small amount of coolant and the rest of the pipe is filled 

DESIGN CONSIDERATIONS 

Three basic components of a heat pipe are:   

the the the

container working fluid wick or capillary structure

CONTAINER

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The function of the container is to isolate the working fluid from the outside environment.

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It has to therefore be leak-proof, maintain the pressure differential across its walls, and enable transfer of heat to take place from and into the working fluid.

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Selection of the container material depends on Compatibility (both with working fluid and external environment)

Working fluid 

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The pipe contains a small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with vapour phase of the working fluid. The materials and coolant chosen depends on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications to mercury for high temperature conditions. However, the vast majority of heat pipes uses either ammonia or water as working fluid.

Wick or Capillary Structure 

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The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. This is typically a sintered metal powder or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the coolant.

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Working Inside the container is a

liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state.When that happens,the liquid picks up the latent heat of

Contd… 

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The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe.

Contd… Heat pipes have an effective thermal conductivity many times that of copper.  The heat transfer or transport capacity of a heat pipe is Specified by its " Axial Power Rating (APR)“.  . It is the energy moving axially  along the pipe. The larger the heat pipe diameter, greater is the APR. Similarly, longer the heat pipe lesser is the APR. Heat pipes can be built in almost any size and shape. 

ADVANTAGES 

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The advantage of heat pipes is their great efficiency in transferring heat. They are actually a vastly better heat conductor than an equivalent cross-section of solid copper. Heat flows of more than 230MW/m^2 have been recorded (4 times the heat flow from the surface of the sun). Heat pipes contain no moving parts and typically require no maintenance. relatively space efficient.

LIMITATIONS 

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below a certain temperature, the working fluid will not vaporize at all, and the thermal conductivity will be reduced to that of the solid metal casing . When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensation process will cease to occur; in such conditions thermal conductivity will be reduced to that

APPLICATIONS 

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Heat pipes are used in a wide range of products like air-conditioners, refrigerators, heat exchangers, transistors, capacitors, space staction,etc. Heat pipes are also used in laptops to reduce the working temperature for better efficiency. VAPIPE is recent developed heat pipe used in a car engine to vaporise gasoline by exhaust gases .

THANK YOU….