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1. INTRODUCTION
A 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 development of the heat pipe originally started with Angier March Perkins who worked initially with the concept of the working fluid only in one phase (he took out a patent in 1839 on the hermetic tube boiler which works on this principle). Jacob Perkins (descendant of Angier March) patented the Perkins Tube in 1936 and they became widespread for use in locomotive boilers and baking ovens. The Perkins Tube was a system in which a long and twisted tube passed over an evaporator and a condenser, which caused the water within the tube to operate in two phases. Although these early designs for heat transfer systems relied on gravity to return the liquid to the evaporator (later called a thermosyphon), the Perkins Tube was the jumping off point for the development of the modern heat pipe. The concept of the modern heat pipe, which relied on a wicking system to transport the liquid against gravity and up to the condenser, was put forward by R.S. Gaugler of the General Motors Corporation. According to his patent in 1944, Gaugler described how his heat pipe would be applied to refrigeration systems. Heat pipe research became popular after that and many industries and labs including Los Alamos, RCA, the Joint Nuclear Research Centre in Italy, began to apply heat pipe technology in their fields. By 1969, there was a vast amount of interest on the part of NASA, Hughes, the European Space Agency, and other aircraft companies in regulating the temperature of a spacecraft and how that could be done with the help of heat pipes. There has been extensive research done to date regarding specific heat transfer characteristics, in addition to the analysis of various material properties and geometries.
2. DESIGN CONSIDERATIONS
The three basic components of a heat pipe are :
1. The container.
2. The working fluid.
3. The wick or capillary structure.
2.1. CONTAINER
The function of the container is to isolate the working fluid from the outside environment. 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.
Selection of the container material depends on many factors. These are as follows:
¢ Compatibility (both with working fluid and external environment)
¢ Strength to weight ratio
¢ Thermal conductivity
¢ Ease of fabrication, including welding, machineability and ductility
¢ Porosity
¢ Wettability
Most of the above are self-explanatory. A high strength to weight ratio is more important in spacecraft applications. The material should be non-porous to prevent the diffusion of vapor. A high thermal conductivity ensures minimum temperature drop between the heat source and the wick.
2.2. WORKING FLUID
A first consideration in the identification of a suitable working fluid is the operating vapour temperature range. Within the approximate temperature band, several possible working fluids may exist, and a variety of characteristics must be
examined in order to determine the most acceptable of these fluids for the application considered.
The prime requirements are:
¢ compatibility with wick and wall materials
¢ good thermal stability
¢ wettability of wick and wall materials
¢ vapor pressure not too high or low over the operating temperature range
¢ high latent heat
¢ high thermal conductivity
¢ low liquid and vapor viscosities
¢ high surface tension
¢ acceptable freezing or pour point
The selection of the working fluid must also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe like, viscous, sonic, capillary, entrainment and nucleate boiling levels.
In heat pipe design, a high value of surface tension is desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force. In addition to high surface tension, it is necessary for the working fluid to wet the wick and the container material i.e. contact angle should be zero or very small. The vapor pressure over the operating temperature range must be sufficiently great to avoid high vapor velocities, which tend to setup large temperature gradient and cause flow instabilities.
A high latent heat of vaporization is desirable in order to transfer large amounts of heat with minimum fluid flow, and hence to maintain low pressure drops within the heat pipe. The thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid viscosities. Tabulated below are a few mediums with their useful ranges of temperature.
Table 2.1 : TABLE OF A FEW MEDIUMS WITH THEIR USEFUL RANGES OF TEMPERATURES
MEDIUM MELTING PT.
( C ) BOILING PT. AT
ATM. TEMP. ( C ) USEFUL RANGE
( C )
Helium
Nitrogen
Ammonia
Acetone
Methanol
Flutec PP2
Ethanol
Water
Toluene
Mercury
Sodium
Lithium
Silver -271
-210
-78
-95
-98
-50
-112
0
-95
-39
98
179
960 -261
-196
-33
57
64
76
78
100
110
361
892
1340
2212 -271 to -269
-203 to -160
-60 to 100
0 to 120
10 to 130
10 to 160
0 to 130
30 to 200
50 to 200
250 to 650
600 to 1200
1000 to 1800
1800 to 2300
2.3. WICK OR CAPILLARY STRUCTURE
It is a porous structure made of materials like steel, alumunium, nickel or copper in various ranges of pore sizes. They 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 produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt.
Fibrous materials, like ceramics, have also been used widely. They generally have smaller pores. The main disadvantage of ceramic fibres is that, they have little stiffness and usually require a continuos support by a metal mesh. Thus while the fibre itself may be chemically compatible with the working fluids, the supporting materials may cause problems. More recently, interest has turned to carbon fibres as a wick material. Carbon fibre filaments have many fine longitudinal grooves on their surface, have high capillary pressures and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fibre wicks seem to show a greater heat transport capability.
The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. 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. Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid.
The maximum capillary head generated by a wick increases with decrease in pore size. The wick permeability increases with increasing pore size. 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. The overall thermal resistance at the evaporator also depends on the conductivity of the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability.
The most common types of wicks that are used are as follows:
Sintered Powder :
This process will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. Very tight bends in the heat pipe can be achieved with this type of structure.
Grooved Tube :
The small capillary driving force generated by the axial grooves is adequate for low power heat pipes when operated horizontally, or with gravity assistance. The tube can be readily bent. When used in conjunction with screen mesh the performance can be considerably enhanced.
Screen Mesh :
This type of wick is used in the majority of the products and provides readily variable characteristics in terms of power transport and orientation sensitivity, according to the number of layers and mesh counts used.
3. WORKING
A metal cylinder is sealed with a fluid within it creating a closed system. One end of the tube is heated and the other is cooled. The heat source (the evaporator) causes the fluid to boil and turn to vapor (this is absorbing energy as heat). When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Once the vapor reaches the cold end of the tube (the condenser), the fluid changes phase again from vapor back to a liquid. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. This liquid returns to the hot (evaporator) end by means of a wick so that the liquid can
repeat the process. This process is capable of transporting heat from a hot region to a colder region. It requires no addition of external energy
Figure 3.1 : SECTIONED SIDE AND FRONT VIEW REPRESENTATION
Heat pipes have an effective thermal conductivity many thousands of 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.
4. SPECIFIC TYPES OF HEAT PIPES
4.1. FLAT PIPES
Flat heat pipes are just that; the orientation of the wick structure is designed so that the liquid is more evenly distributed to the top and the bottom of the plate.
Figure 4.1 : FLAT PIPE
The wick structure in a flat plate is a sintered metal; it is a metal powder that has been molded and heated until the metal has fused, creating a structurally stable metal with small pores within. Flat heat pipes produce a surface that has a relatively uniform temperature distribution and large surface area. These would be useful in the case where one needs to radiate heat uniformly instead of from a point source. The use of flat plates as wall components could be one possible application for heat pipe technology in the house.
4.2. THERMAL SWITCHES
Thermal switches in a heat pipe serve to prevent the pipe from working in certain cases. This can be accomplished by introducing a blockage, made possible in a variety of different ways. Methods would include freezing the fluid, placing a magnetically operated vane within the pipe which would block the vapor flow, or using a physical displacement block (which controls the amount of fluid in the reservoir and in the heat pipe by blocking the fluid from being transported by the wick).
Figure 4.2 : THERMAL SWITCHES
4.3. THERMAL DIODES
Another possible way to stop or control the heat transfer within the pipe would be by limiting the acting surface of the condenser by using an inert gas (this is the principle also behind variable conductance heat pipes). Thermal diodes allow the heat pipe to only work in one direction. In one example of a heat diode, if the location of the condenser and evaporator switch, the liquid becomes trapped in a reservoir whose wicks are not connected to the rest of the pipe. This makes it so that the liquid will not be able to travel down the length of the heat pipe until the condenser and evaporator switch again to heat the liquid to the gaseous phase so it can flow down the pipe once more.
Figure 4.3 : THERMAL DIODES
Another example of a thermal diode is when there is excess liquid in a reservoir within the heat pipe. When the evaporator and condenser are switched, the liquid in the reservoir becomes a vapor and condenses on the condenser. This large amount of fluid prevents any vapor from condensing at the other end of the heat pipe and therefore will only allow heat transfer in one direction.
Figure 4.4 : THERMAL DIODES WHEN THERE IS EXCESS LIQUID
5. APPLICATIONS
Heat pipe has been, and is currently being, studied for a variety of applications, covering almost the entire spectrum of temperatures encountered in heat transfer processes. Heat pipes are used in a wide range of products like air-conditioners, refrigerators, heat exchangers, transistors, capacitors, etc. Heat pipes are also used in laptops to reduce the working temperature for better efficiency. Their application in the field of cryogenics is very significant, especially in the development of space technology. We shall now discuss a brief account of the various applications of heat pipe technology
5.1. SPACE TECHNOLOGY
The use of heat pipes has been mainly limited to this field of science until recently, due to cost effectiveness and complex wick construction of heat pipes. There are several applications of heat pipes in this field like
¢ Spacecraft temperature equalization
¢ Component cooling, temperature control and radiator design in satellites.
¢ Other applications include moderator cooling, removal of heat from the reactor at emitter temperature and elimination of troublesome thermal gradients along the emitter and collector in spacecrafts.
5.2HEAT PIPES FOR DEHUMIDIFICATION AND AIR CONDITIONING
In an air conditioning system, the colder the air as it passes over the cooling coil (evaporator), the more the moisture is condensed out. The heat pipe is designed to have one section in the warm incoming stream and the other in the cold outgoing stream. By transferring heat from the warm return air to the cold supply air, the heat pipes create the double effect of pre-cooling the air before it goes to the evaporator and then re-heating it immediately.
Activated by temperature difference and therefore consuming no energy, the heat pipe, due to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature, increasing the moisture removal capability of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can be achieved at higher thermostat settings, which results in net energy savings. Generally, for each 1 F rise in thermostat setting, there is a 7% savings in electricity cost. In addition, the pre-cooling effect of the heat pipe allows the use of a smaller compressor.
5.3. LAPTOP HEAT PIPE SOLUTION
Heat pipe technology originally used for space applications has been applied it to laptop computer cooling. It is an ideal, cost effective solution. Its light weight (generally less than 40 grams), small, compact profile, and its passive operation, allow it to meet the demanding requirements of laptops.
For an 8 watt CPU with an environmental temperature no greater than 40°C it provides a 6.25°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90°C or less.
One end of the heat pipe is attached to the processor with a thin, clip-on mounting plate. The other is attached to the heat sink, in this case, a specially designed keyboard RF shield. This approach uses existing parts to minimize weight and complexity. The heat pipe could also be attached to other physical components suitable as a heat sink to dissipate heat.
Because there are no moving parts, there is no maintenance and nothing to break. Some are concerned about the possibility of the fluid leaking from the heat pipe into the electronics. The amount of fluid in a heat pipe of this diameter is less than 1cc. In a properly designed heat pipe, the water is totally contained within the capillary wick structure and is at less than 1 atmosphere of pressure. If the integrity of the heat pipe vessel were ever compromised, air would leak into the heat pipe instead of the water leaking out. Then the fluid would slowly vaporize as it reaches its atmospheric boiling point. A heat pipeâ„¢s MTTF is estimated to be over 100,000 hours of use.
5.4. CPU WORK STATIONS
Heat pipes have become widely used to cool the CPU's of computers due to the fact that they can be manufactured at such a small scale. They act as heat sinks for the processors and other components of computers that generate substantial heat. The heat pipe solutions for thermal control at this level are a component and overall systems requirement. Not only do the heat pipes take on a different configuration with multiple heat pipes and cooling fins, but also airflow becomes the critical design factor. Heat pipes designed to move 75 watts are usually flat with fin stacks from three to six inches, in many cases with fins mounted on each side of the CPU input pad
5.5. FLEXIBLE SOLUTIONS
Heat pipes are manufactured in a multitude of sizes and shapes. Unusual application geometry can be easily accommodated by the heat pipeâ„¢s versatility to be shaped as a heat transport device. If some range of motion is required, heat pipes can even be made of flexible material.
Two of the most common are:
Constant Temperature: The heat pipe maintains a constant temperature or temperature range.
Diode: The heat pipe will allow heat transfer in only one direction.
Figure 5.2 : HEAT PIPES IN DIFFERENT SIZES AND SHAPES
5.6. MEGA FLATS
Flat heat pipes are typically used for cooling printed circuit boards or for heat leveling to produce an isothermal plane. Mega flats are several flat heat pipes sandwiched together.
Some of the flat heat pipes manufactured are:
XY Mega Flats: Surface maintained within .01° F isothermal with concentrated load centers.
6" X 6" Mega Flat: Dissipated 850 watts from a printed circuit board.
Figure 5.3 : DIFFERENT MEGA FLATS
Weight Reduction Mega Flats:
Standard - aluminum construction.
Lightweight - ½ the weight of aluminum.
Very light weight - 1/3 the weight of aluminum.
6. CONCLUSION
The cost of heat pipes designed for laptop use is very competitive compared to other alternatives. Cost is partially offset and justified by improved system reliability and the increased life of cooler running electronics. Heat pipes, in quantity, cost a few dollars each while an entire cooling system will cost between $5 - $10 in production quantities, depending on the final design. Standard design products are available to reduce cost even further. Heat pipe manufacture has been a difficult area to compete in. Simple in concept, but difficult to apply commercially, the heat pipe is a very elusive technology & holds the key to the future of heat transfer & its allied applications.
7. REFERENCES
Andrews, J; Akbarzadeh, A; Sauciue, I.: Heat Pipe Technology, Pergammon, 1997.
Dunn, P.D.; Reay, D.A.: Heat Pipes, Pergammon, 1994.
heatpipe.com.
cheresources.com.
indek.com

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TECHNICAL SEMINAR REPORT ON HEAT PIPES

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SUBMITTED BY
MD. QUAMRUL HODA (06J51A0328)
DEPARTMENT OF MECHANICAL ENGINEERING
MANNAN INST OF SCI & TECH
Aloor Village,Chevella Mandal. Ranga Reddy District, A.P

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ABSTRACT
The heat pipe is a device having a high thermal conductance which utilizes the transport of a vapour and rejection of latent heat to achieve efficient thermal energy transport. The theory of heat pipes is well developed. Their use in applications involving temperatures in the cryogenic regime, and with development units running as high as 2000 degrees C, shows that they can function over a large part of the temperature spectrum. Applications in spacecraft, electronics and die casting are but few of the uses for these devices
[HEAT PIPES]
A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors.
INDEX
1. INTRODUCTION
2. DESIGN CONSIDERATION
CONTAINER
WORKING FLUID
WICK OR CAPILLARY STRUCTURE
3. WORKING PRINCIPLE
4. SECTIONED SIDE AND FRONT VIEW REPRESENTATION
5. SPECIFIC TYPES OF HEAT PIPES
6. APPLICATIONS
7. FLEXIBLE SOLUTIONS
8. CONCLUSION
1.INTRODUCTION
A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors.
A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid. The length of the heat pipe can be divided into three parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container.
2.DESIGN CONSIDERATIONS
The three basic components of a heat pipe are :
1. The container.
2. The working fluid.
3. The wick or capillary structure.
CONTAINER
The function of the container is to isolate the working fluid from the outside environment. 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.
Selection of the container material depends on many factors. These are as follows:
¢ Compatibility (both with working fluid and external environment)
¢ Strength to weight ratio
¢ Thermal conductivity
¢ Ease of fabrication, including welding, machineability and ductility
¢ Porosity
¢ Wettability
Most of the above are self-explanatory. A high strength to weight ratio is more important in spacecraft applications. The material should be non-porous to prevent the diffusion of vapor. A high thermal conductivity ensures minimum temperature drop between the heat source and the wick.
WORKING FLUID
A first consideration in the identification of a suitable working fluid is the operating vapour temperature range. Within the approximate temperature band, several possible working fluids may exist, and a variety of characteristics must be
examined in order to determine the most acceptable of these fluids for the application considered.
The prime requirements are:
¢ compatibility with wick and wall materials
¢ good thermal stability
¢ wettability of wick and wall materials
¢ vapor pressure not too high or low over the operating temperature range
¢ high latent heat
¢ high thermal conductivity
¢ low liquid and vapor viscosities
¢ high surface tension
¢ acceptable freezing or pour point
The selection of the working fluid must also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe like, viscous, sonic, capillary, entrainment and nucleate boiling levels.
In heat pipe design, a high value of surface tension is desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force. In addition to high surface tension, it is necessary for the working fluid to wet the wick and the container material i.e. contact angle should be zero or very small. The vapor pressure over the operating temperature range must be sufficiently great to avoid high vapor velocities, which tend to setup large temperature gradient and cause flow instabilities.
A high latent heat of vaporization is desirable in order to transfer large amounts of heat with minimum fluid flow, and hence to maintain low pressure drops within the heat pipe. The thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid viscosities.
WICK OR CAPILLARY STRUCTURE
It is a porous structure made of materials like steel, alumunium, nickel or copper in various ranges of pore sizes. They 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 produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt.
Fibrous materials, like ceramics, have also been used widely. They generally have smaller pores. The main disadvantage of ceramic fibres is that, they have little stiffness and usually require a continuos support by a metal mesh. Thus while the fibre itself may be chemically compatible with the working fluids, the supporting materials may cause problems. More recently, interest has turned to carbon fibres as a wick material. Carbon fibre filaments have many fine longitudinal grooves on their surface, have high capillary pressures and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fibre wicks seem to show a greater heat transport capability.
The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. 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. Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid.
The maximum capillary head generated by a wick increases with decrease in pore size. The wick permeability increases with increasing pore size. 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. The overall thermal resistance at the evaporator also depends on the conductivity of the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability.
The most common types of wicks that are used are as follows:
Sintered Powder :
This process will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. Very tight bends in the heat pipe can be achieved with this type of structure.
Grooved Tube :
The small capillary driving force generated by the axial grooves is adequate for low power heat pipes when operated horizontally, or with gravity assistance. The tube can be readily bent. When used in conjunction with screen mesh the performance can be considerably enhanced.
Screen Mesh :
This type of wick is used in the majority of the products and provides readily variable characteristics in terms of power transport and orientation sensitivity, according to the number of layers and mesh counts used.
3. WORKING PRINCIPLE
A metal cylinder is sealed with a fluid within it creating a closed system. One end of the tube is heated and the other is cooled. The heat source (the evaporator) causes the fluid to boil and turn to vapor (this is absorbing energy as heat). When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Once the vapor reaches the cold end of the tube (the condenser), the fluid changes phase again from vapor back to a liquid. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. This liquid returns to the hot (evaporator) end by means of a wick so that the liquid can
repeat the process. This process is capable of transporting heat from a hot region to a colder region. It requires no addition of external energy.
4.SECTIONED SIDE AND FRONT VIEW REPRESENTATION
Heat pipes have an effective thermal conductivity many thousands of 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.
5.SPECIFIC TYPES OF HEAT PIPES
FLAT PIPES
Flat heat pipes are just that; the orientation of the wick structure is designed so that the liquid is more evenly distributed to the top and the bottom of the plate.
The wick structure in a flat plate is a sintered metal; it is a metal powder that has been molded and heated until the metal has fused, creating a structurally stable metal with small pores within. Flat heat pipes produce a surface that has a relatively uniform temperature distribution and large surface area. These would be useful in the case where one needs to radiate heat uniformly instead of from a point source. The use of flat plates as wall components could be one possible application for heat pipe technology in the house.
THERMAL SWITCHES
Thermal switches in a heat pipe serve to prevent the pipe from working in certain cases. This can be accomplished by introducing a blockage, made possible in a variety of different ways. Methods would include freezing the fluid, placing a magnetically operated vane within the pipe which would block the vapor flow, or using a physical displacement block (which controls the amount of fluid in the reservoir and in the heat pipe by blocking the fluid from being transported by the wick).
THERMAL DIODES
Another possible way to stop or control the heat transfer within the pipe would be by limiting the acting surface of the condenser by using an inert gas (this is the principle also behind variable conductance heat pipes). Thermal diodes allow the heat pipe to only work in one direction. In one example of a heat diode, if the location of the condenser and evaporator switch, the liquid becomes trapped in a reservoir whose wicks are not connected to the rest of the pipe. This makes it so that the liquid will not be able to travel down the length of the heat pipe until the condenser and evaporator switch again to heat the liquid to the gaseous phase so it can flow down the pipe once more.
Another example of a thermal diode is when there is excess liquid in a reservoir within the heat pipe. When the evaporator and condenser are switched, the liquid in the reservoir becomes a vapor and condenses on the condenser. This large amount of fluid prevents any vapor from condensing at the other end of the heat pipe and therefore will only allow heat transfer in one direction.
6.APPLICATIONS
Heat pipe has been, and is currently being, studied for a variety of applications, covering almost the entire spectrum of temperatures encountered in heat transfer processes. Heat pipes are used in a wide range of products like air-conditioners, refrigerators, heat exchangers, transistors, capacitors, etc. Heat pipes are also used in laptops to reduce the working temperature for better efficiency. Their application in the field of cryogenics is very significant, especially in the development of space technology. We shall now discuss a brief account of the various applications of heat pipe technology
SPACE TECHNOLOGY
The use of heat pipes has been mainly limited to this field of science until recently, due to cost effectiveness and complex wick construction of heat pipes. There are several applications of heat pipes in this field like
¢ Spacecraft temperature equalization
¢ Component cooling, temperature control and radiator design in satellites.
¢ Other applications include moderator cooling, removal of heat from the reactor at emitter temperature and elimination of troublesome thermal gradients along the emitter and collector in spacecrafts.
HEAT PIPES FOR DEHUMIDIFICATION AND AIR CONDITIONING
In an air conditioning system, the colder the air as it passes over the cooling coil (evaporator), the more the moisture is condensed out. The heat pipe is designed to have one section in the warm incoming stream and the other in the cold outgoing stream. By transferring heat from the warm return air to the cold supply air, the heat pipes create the double effect of pre-cooling the air before it goes to the evaporator and then re-heating it immediately.
Activated by temperature difference and therefore consuming no energy, the heat pipe, due to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature, increasing the moisture removal capability of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can be achieved at higher thermostat settings, which results in net energy savings. Generally, for each 1 F rise in thermostat setting, there is a 7% savings in electricity cost. In addition, the pre-cooling effect of the heat pipe allows the use of a smaller compressor.
LAPTOP HEAT PIPE SOLUTION
Heat pipe technology originally used for space applications has been applied it to laptop computer cooling. It is an ideal, cost effective solution. Its light weight (generally less than 40 grams), small, compact profile, and its passive operation, allow it to meet the demanding requirements of laptops.
For an 8 watt CPU with an environmental temperature no greater than 40°C it provides a 6.25°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90°C or less.
One end of the heat pipe is attached to the processor with a thin, clip-on mounting plate. The other is attached to the heat sink, in this case, a specially designed keyboard RF shield. This approach uses existing parts to minimize weight and complexity. The heat pipe could also be attached to other physical components suitable as a heat sink to dissipate heat.
Because there are no moving parts, there is no maintenance and nothing to break. Some are concerned about the possibility of the fluid leaking from the heat pipe into the electronics. The amount of fluid in a heat pipe of this diameter is less than 1cc. In a properly designed heat pipe, the water is totally contained within the capillary wick structure and is at less than 1 atmosphere of pressure. If the integrity of the heat pipe vessel were ever compromised, air would leak into the heat pipe instead of the water leaking out. Then the fluid would slowly vaporize as it reaches its atmospheric boiling point. A heat pipeâ„¢s MTTF is estimated to be over 100,000 hours of use.
CPU WORK STATIONS
Heat pipes have become widely used to cool the CPU's of computers due to the fact that they can be manufactured at such a small scale. They act as heat sinks for the processors and other components of computers that generate substantial heat. The heat pipe solutions for thermal control at this level are a component and overall systems requirement. Not only do the heat pipes take on a different configuration with multiple heat pipes and cooling fins, but also airflow becomes the critical design factor. Heat pipes designed to move 75 watts are usually flat with fin stacks from three to six inches, in many cases with fins mounted on each side of the CPU input pad
7.FLEXIBLE SOLUTIONS
Heat pipes are manufactured in a multitude of sizes and shapes. Unusual application geometry can be easily accommodated by the heat pipeâ„¢s versatility to be shaped as a heat transport device. If some range of motion is required, heat pipes can even be made of flexible material.
Two of the most common are:
Constant Temperature: The heat pipe maintains a constant temperature or temperature range.
Diode: The heat pipe will allow heat transfer in only one direction.
HEAT PIPES IN DIFFERENT SIZES AND SHAPES
MEGA FLATS
Flat heat pipes are typically used for cooling printed circuit boards or for heat leveling to produce an isothermal plane. Mega flats are several flat heat pipes sandwiched together.
Some of the flat heat pipes manufactured are:
XY Mega Flats: Surface maintained within .01° F isothermal with concentrated load centers.
6" X 6" Mega Flat: Dissipated 850 watts from a printed circuit board.
DIFFERENT MEGA FLATS
Weight Reduction Mega Flats:
Standard - aluminum construction.
Lightweight - ½ the weight of aluminum.
Very light weight - 1/3 the weight of aluminum.
8.CONCLUSION
The cost of heat pipes designed for laptop use is very competitive compared to other alternatives. Cost is partially offset and justified by improved system reliability and the increased life of cooler running electronics. Heat pipes, in quantity, cost a few dollars each while an entire cooling system will cost between $5 - $10 in production quantities, depending on the final design. Standard design products are available to reduce cost even further. Heat pipe manufacture has been a difficult area to compete in. Simple in concept, but difficult to apply commercially, the heat pipe is a very elusive technology & holds the key to the future of heat transfer & its allied applications.

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heat pipe

BY
JIBAN JYOTI SAMANT
REGD No-S701223801

---

INTRODUCTION


A HEAT PIPE IS A DEVICE THAT CAN QUICKLY TRANSFER HEAT FROM ONE POINT TO ANOTHER.

THEY ARE ALSO REFFERED TO AS THE “SUPERCONDUCTORS’’ OF HEAT AS THEY POSSESS AN EXTRA ORDINARY HEAT TANSFER CAPACITY AND WITH ALMOST NO HEAT LOSS

ABSTRACT

A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. 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 vaporization. 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. Heat pipes have an effective thermal conductivity many thousands of times that of copper. Heat pipes can be built in almost any size and shape
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CONTENTS

CHAPTERS PAGE NO.

1. INTRODUCTION 1

2. HEAT PIPE BASICS 3

3. COMPONENTS OF A HEAT PIPE 6

4. SHAPES AND DESIGNS OF HEAT PIPES 8

5. HEAT PIPES FOR INLET AIR COOLING 10

6. HPHE FOR WASTE HEAT RECOVERY 12

7. CONCEPTUAL SCHEME 16

8. CONCLUSIONS AND FUTURE WORK 19

9. REFERENCES 21






1. INTRODUCTION

Engine Air Pre-cooling

The inlet air temperature of a compressor has a considerable effect
on the engine performance, especially during hot summer climates when high
atmospheric temperatures cause reduction in net power output. After-coolers are
used to cool the air delivered by the compressor for improving engine power
output, but the mass flow rate of air to the turbocharger compressor cannot be
increased by means of after-cooling.

Several methods have been suggested to cool the compressor
inlet air. This is known as engine air pre-cooling. Heat pipes can be a possible
solution to the air inlet temperature cooling. It is proposed that the evaporator of
the heat pipe heat exchanger will remove the heat from the inlet air, while the
heat rejection from the condenser section to the surroundings can be achieved by
a number of ways. One of the ways is by the utilization of waste heat.

A proposed scheme is to use a turbocharger with an engine air
pre-cooling system and a waste heat recovery system making use of the efficient
heat transfer characteristics of the heat pipes.

1.2 Effect of Ambient Air Temperature on performance of engines

. The turbocharger performance and operation, like gas turbines, strongly depends on the ambient air temperature. As the pressure and temperature of the ambient air changes, so does the air density. As the air density increases, as the air mass flow rate increases, which allows the compressor to move a higher mass of air for the same volumetric flow rate. Turbochargers are designed to work at temperatures that optimize the operation of the coupled turbocharger and engine system. During warmer months, a turbocharger may work at off-design conditions because of lower air density, which then moves the operating point away from the point of peak performance.



fig 1.1 Schematic of turbocharger assembly


fig 1.2 A typical performance map for a compressor




2. HEAT PIPE BASICS

2.1 What are heat pipes?

A heat pipe is a device that efficiently transports heat from its one
end to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors.

Heat input at the evaporator vaporizes the working fluid and this vapor travels to the condenser section. Here the latent heat is rejected via condensation. The vapor of the working fluid condenses and the condensate returns to the evaporator by means of capillary action. A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid.

The length of the heat pipe can be divided into 3 parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container. Fins may be attached to the evaporator and the condenser portion to increase heat transfer rate depending upon the application.

fig 2.1 Cut out section of a heat pipe


2.2 Operating principle

A heat pipe operates on a closed two phase cycle. As previously mentioned, there is liquid-vapor equilibrium inside the heat pipe. When the heat is supplied to the evaporator, this equilibrium breaks down as the working fluid evaporates.The generated vapor is at a higher pressure than the liquid and it travels to the condenser section through the vapor space provided. Vapor condenses giving away its latent heat of vaporization to the heat sink. The capillary pressure created in the menisci of the wick pumps the condensed fluid back to the evaporator section.

The cycle repeats and the heat is continuously transported from evaporator to condenser in the form of latent heat of vaporization. When heat is applied to the evaporator, the liquid recedes into the pores of the wick and thus the menisci at the liquid-vapor interface are highly curved. At thecondenser end, the menisci at the liquid-vapor interface are nearly flatduring the condensation. The difference in the curvature of menisci driving force that circulates the fluid against the liquid and vapor pressure losses and body forces such as gravity (if there is an adverse tilt with respect to the ground).


fig 2.2 Heat transfer mechanism in a heat pipe
fig 2.3 Driving forces in the wick structure


3. COMPONENTS OF A HEAT PIPE

3.1 Working fluid

Every heat pipe application has a particular temperature range in which the heat pipe needs to operate. As a rule of thumb, the useful range extends from the point where the saturation pressure is greater than 0.1 atm and less than 20 atm. Below 0.1 atm, the vapor pressure limit may be approached while above 20atm, the container thickness must be increased to a point where the heatpipe operation is limited by the increased thermal resistance.

Water is the most economical and efficient working fluid. In some applications, however, a mixture called glycol water is used as the working fluid.


3.2 Wick structure


The important requirements of a wick structure are listed below:


•Should be compatible with the wick and container material
•High latent heat
•High thermal conductivity
•High surface tension
•Low liquid and vapor viscosities
•Wettability of wick and wall materials



The wick structure in a heat pipe facilitates liquid return to the evaporator from the condenser. The main purposes of the wick are to generate the capillary pressure, and to distribute the liquid around the evaporator section of heat pipe. Figure 6 shows various wick structures. The most commonly used wick structure is a wrapped screen wick. At this point, we will define a parameter called ‘Mesh Number’ which is used to specify a particular wrapped screen wick. It is defined as the number of wires per linear inch, counted from the centerof any wire to a point exactly one inch distant, including the fractional distance between wires thereof. For example a mesh number of 60 per inch can beinterpreted as 60 × 60 mesh wires per inch. A mesh could be square or rectangular mesh.. Obviously, higher mesh number represents a finer grid.

.
fig 3.1 Various wick structures


4. SHAPES AND DESIGNS
The most frequently used shapes on heat pipes are listed below. Keep in mind that many other shapes can be easily tooled.
4.1 Rounds
Simple Bend
d 2 3 4
R 1.5 2 2.6

Return Loop
d 2 3 4
D 8 10 12

Offset
d 2 3 4
l 8 12 14

"h" can be anything

Loop (flat)
d 2 3 4
D 4 5 6



Motion Loops



d ± 0.76 mm
D ± 1.0 mm
l ± 1.0 mm

d3 4 * *
D 3.6 5 6



4.2 Mini Flat

Easy Bend
± 0.5 mm
t 1.3 1.6 2.4
R 0.8 1.0 1.2

Hard Bend t = 2.3 ± 0.5 mm
W 6.2 8..7
R* 12 15


t = 2.0 mm thick ± 0.5 mm
W 3.6 5.1 8.8
R* 4 11 15

t = 1.6 mm thick ± 0.5 mm
W 2.8 4.5 7.0
R* 3 4 15

Change Plane Center

Change Plane Flush


: any angle 0° to 90°
t: any mini flat thickness





5. HEAT PIPES FOR INLET AIR COOLING.
5.1. HEAT EXCHANGER

The solid model of a finned-tube heat pipe heat exchanger is shown in Fig 4.1 that gives a general idea of a heat pipe heat exchanger concept.Staggered arrangement of tubes is shown in this figure. The upper portion of theheat exchanger is the condenser section while the lower portion is the evaporator,placed in the inlet air duct. The inlet air passes through this staggered bank oftubes. The condenser section is shown above the evaporator because we are using gravity assisted heat pipes.



fig 5.1 Heat Pipe Heat Exchanger(gravity assisted)



fig 5.2. Schematic for inlet air cooling and
condenser section heat removal


The ‘Part 1’ illustrated in Fig 4.2 essentially removes the heat from the air. This is achieved by the evaporator section of the HPHE. This heat lost by the ambient air is the heat gained by the condenser section of the HPHE and we will address this heat removal problem as the ‘Part 2’ shown in Figure 13. Figure 14 shows different ways of achieving heat rejection to the surrounding (Part 2). The heat removal by implementing a waste heat recovery system will be discussed in detail in the next chapter.




6. HPHE FOR WASTE HEAT RECOVERY
6.1 Research work on waste heat recovery using heat pipes
Interesting research work has been done to utilize heat pipes in a waste heat recovery application. Akyurt et al. (1994), in their paper on performance of a heat pipe based on waste heat recovery and utilization for a gas-turbine engine, present results of a modeling and simulation study on combined performance of a power system. In this model, water in a steel type two-phase thermo siphon loop extracts waste energy from the stack of the gas turbine and delivers it to the generator of an aqua-ammonia absorption chiller.


Fig6.1.Schematic arrangement for waste heat recovery and utilization system



6.2. Working of Absorption Chillers
Absorption chillers use heat instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.
In the chiller, refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant revaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device. The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water.
Compared with mechanical chillers, absorption chillers have a low coefficient of performance (COP = chiller load/heat input). However, absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat. Vapor compression chillers, by contrast, must be motor- or engine-driven.
Low-pressure, steam-driven absorption chillers are available in capacities ranging from 100 to 1,500 tons. Absorption chillers come in two commercially available designs: single-effect and double-effect. Single-effect machines provide a thermal COP of 0.7 and require about 18 pounds of 15-pound-per-square-inch-gauge (psig) steam per ton-hour of cooling. Double-effect machines are about 40% more efficient, but require a higher grade of thermal input, using about 10 pounds of 100- to 150-psig steam per ton-hour.
A single-effect absorption machine means all condensing heat cools and condenses in the condenser. From there it is released to the cooling water. A double-effect machine adopts a higher heat efficiency of condensation and divides the generator into a high-temperature and a low-temperature generator.
6.3 Condenser Section Cooling with PCM

When any material is heated two things happen. First the material stores energy, which is called sensible heat. Second, the material gets warmer. When the material melts, it changes from a solid to a liquid phase. A large amount of heat is stored at the melting point. The temperature does not increase during the melting process but the heat addition continues during this process. The energy stored in the material is called latent heat.
The amount of energy stored at melting point is called the heat of fusion. When the material freezes, then the latent heat isreleased. This latent heat can be effectively utilized in cooling-heating applications by using phase change materials (PCM).
In the conceptual model discussed so far, the heat pipe heat exchanger is to be used for inlet air cooling of turbocharger compressor. Its condenser section can be cooled by using an absorption chiller.
In desert areas like Arizona and Nevada, the difference between day and night time temperatures is large. During summer, the outside day time temperature can be as high as 105°F while the night time temperature may drop to 50-70°F. These natural conditions can be used to cool the condenser without installing an absorption chiller. Phase change materials may play an important role in achieving this purpose.
6.3.1. Conceptual Scheme for Condenser Section Cooling with PCM
Fig 6.2 shows a conceptual scheme for condenser side cooling with a phase change material. The PCM, in solid state initially, is contained in a box around the heat pipes and is direct contact with the heat pipe surface.
During day operation, the heat pipe will reject its heat to the PCM. The phase change material is adiabatically insulated from the surrounding atmosphere or else it would gain heat from hot outside air. Thus, it can accept heat for at least 8 hours.


During the night, when the outside air temperature drops to 50-60°F, the adiabatic insulation is removed and the PCM is exposed to the outside air. This exposure to cool air for approximately 10-12 hours helps solidify the PCM and prepares it for the next cycle.





fig 6.2. Condenser Section Cooling with PCM



7. CONCEPTUAL SCHEME
7.1 Scheme for Inlet Air Cooling and
Waste Heat Recovery using HPHE
Figure 6.1 shows a conceptual layout of the inlet air cooling waste heat recovery system. For this layout, the HPHE #1 is placed in the inlet air duct to the turbocharger compressor while the HPHE #2 is placed in the turbine exhaust duct for the recovery of waste heat. This system will work as follows.



fig 7.1. Conceptual Scheme for Inlet Air Cooling And Waste Heat Recovery using HPHE







The turbine exhaust will pass through the evaporator section of the HPHE # 2.Condenser section of this HPHE will be placed in the hot water circuit of the indirect type of absorption chiller. The water will gain the heat from the HPHE and will be fed to the generator of the absorption chiller. The chilled water
generated by the absorption chiller will be circulated through the condenser section of the HPHE #1. This is basically a shell and tube heat exchanger arrangement. Depending upon the type of absorption chiller, the cooling liquid will be either glycol water (if it is an aqua-ammonia absorption chiller) or just chilled water (if it is a Li-Br absorption chiller). The heat gained by the cooling liquid is essentially the heat removed by the evaporator section of the HPHE #1,from the inlet air to the compressor.
In the proposed system, the air to be cooled will pass through a staggered bank of finned heat pipes, placed in the air duct, somewhere between the air filter housing and the turbocharger compressor inlet. This cooled air will then enter the compressor. Before the compressed air is supplied to the engine, it will be cooled by the after-cooler. The hot exhaust gas will then expand in the turbine. The turbine exhaust in turn will again pass through the HPHE # 2.

7.2. Cost of a HPHE

Generally, heat pipe heat exchanger units available in the market are produced for air to air heat recovery applications. A prototype of the conceptual model described in this thesis needs to be developed upon availability of funds. An approximate cost estimate for a heat pipe heat exchanger was obtained from Advanced Cooling Technologies Inc. (Lancaster, Pennsylvania) a heat pipe manufacturing company.




According to the previous models for the approximate cost of a HPHE unit will be $3500. Another cost estimate was obtained from Innergytech Inc (Canada) which was based on the air to air recovery units that this company manufactures. According to these specifications, the cost of ThermogainTM( A registered trade name of Innergytech Inc) is $1950.00.

7.3. Advantages

• Very high thermal conductivity of heat pipes results in efficient heat transfer.

• Apart from the kinetic energy, heat contained in the exhaust gases is also made use of, resulting in high fuel efficiency.

• Very efficient in arid climatic conditions where there is a great difference between the day and night temperatures.

• Precludes the engine from climatic variations



7.4. Disadvantages

• High cost ( as high as $ 1950 - $ 3500 )

• The technology is still under research.

• Heat rejection to the atmosphere may not
be practically obtained in all situations












8. CONCLUSIONS AND FUTURE WORK

8.1 Considerations

The considerations in using heat pipes are;
•Installation cost of the indirect fired absorption chiller is higher and it is not readily available in the low capacity units (less than 7 tons). The smallest unit commercially available is 10 tons. However, if this method of cooling is applied to an entire compression station, then the extra capacity is not an issue
•The concept to insert the waste heat recovery HPHE condenser directly into the generator instead of hot water circuit is dismissed since it would require significant design changes.
•While direct-fired absorption chillers are commercially available with an installation cost of about $5,000, these chillers require a continuous supply of natural gas. If these units are installed, then the second HPHE for waste heat recovery is of no use, but in turn the waste heat is not utilized.
•Cooling of the condenser section of heat pipe with PCM is not practical at this
time because of the long hours of operation, the cost associated with the purchase of PCM, and the insulating properties of paraffins may provide resistance to the heat flux.

8.2. Overall Conclusion
The overall conclusion is that a heat pipe air cooling system could be installed that would increase the density of the air entering the compressor. In warmer climates, this system would preclude the engine from operating under high ambient conditions, since the ambient air temperature would be maintained constant throughout the warmer seasons. If a heat pipe system is to be installed, at present the best technology is with a heat recovery absorption unit.


Of course, if extra cooling capacity is available via evaporative cooling systems in dry climates, this extra capacity could be used to cool the heat pipes, significantly lowering the equipment cost.

8.3. Future Work
Future work could be developing a prototype of an inlet-air cooling HPHE based on the cost estimation given in the previous chapter. If the PCM slurry cooling is to be done, the research work must be done on micro-encapsulated PCM slurry cooling.


9. REFERENCES
1. Ait-Ali, Mohand A, 1996, “Optimum Power Boosting of Gas Turbine Cycles With Refrigerated Inlet Air and Compressor Intercooling” Proceedings of the 1996 International Gas Turbine and Aeroengine Congress & Exhibition, Jun 10-13,Burmingham, UK, 96-GT-393

2. Anderson, William G., Hoff, Sandra, Winstanley, Dave ,1993, “Heat pipe cooling of turbo shaft engines”, American Society of Mechanical Engineers, GT-220, 12p

3. Faghri Amir, 1995, Heat Pipe Science and Technology, Taylor & Francis.

4. Peterson G.P., 1994, An Introduction to Heat Pipes, John Wiley and Sons, Inc.


5. http://lanl.gov/orgs/pa/science21

[attachment=8184]

INTRODUCTION

Heat Pipes Basics
RELATIVE AMPLITUDES


OPERATING PRINCIPLE


Heat transfer mechanism in a heat pipe
Driving forces in the wick structure
COMPONENTS OF A HEAT PIPE
Working fluid


Wick structure
Various wick structures
SHAPES AND DESIGNS
Simple Bend
MINI FLAT
HPHE FOR WASTE HEAT RECOVERY
Schematic arrangement for waste heat recovery and utilization system

CONCEPTUAL SCHEME
Advantages 
Very high thermal conductivity of heat pipes results in efficient heat transfer.
Apart from the kinetic energy, heat contained in the exhaust gases is also made use of, resulting in high fuel efficiency.
Very efficient in arid climatic conditions where there is a great difference between the day and night temperatures.
Precludes the engine from climatic variations
Disadvantages
High cost ( as high as $ 1950 - $ 3500 )
The technology is still under research.
Heat rejection to the atmosphere may not be practically obtained in all situations
Conclusion
The overall conclusion is that a heat pipe air cooling system could be installed that would increase the density of the air entering the compressor. In warmer climates, this system would preclude the engine from operating under high ambient conditions, since the ambient air temperature would be maintained constant throughout the warmer seasons. If a heat pipe system is to be installed, at present the best technology is with a heat recovery absorption unit. Of course, if extra cooling capacity is available via evaporative cooling systems in dry climates, this extra capacity could be used to cool the heat pipes, significantly lowering the equipment cost.

THANK YOU

[attachment=11206]
HEAT PIPES
INTRODUCTION
A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors. A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid. The length of the heat pipe can be divided into three parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container.
Basic components of a heat pipe
The basic components of a heat pipe are
1. The container
2. The working fluid
3. The wick or capillary structure
Container
The function of the container is to isolate the working fluid from the outside environment. It has to be there for leak proof, maintain the pressure differential across the walls, and enable transfer of thermal energy to take place from and into the working fluid.
The prime requirements are:
1. Compatibility (Both with working fluid and External environment)
2. Porosity
3. Wettability
4. Ease of fabrication including welding, machinability and ductility
5. Thermal conductivity
6. Strength to weight ratio
Working fluid
The first consideration in the identification of the working fluid is the operating vapor temperature range. Within the approximate temperature band, several possible working fluids may exist and a variety of characteristics must be examined in order to determine the most acceptable of these fluids for the application considered.
The prime requirements are:
7. Compatibility with wick and wall materials
8. Good thermal stability
9. Wettability of wick and wall materials
10. High latent heat
11. High thermal conductivity
12. Low liquid and vapor viscosities
13. High surface tension
Wick
The wick structure in a heat pipe facilitates liquid return from the evaporator from the condenser. The main purposes of wick are to generate the capillary pressure, and to distribute the liquid around the evaporator section of heat pipe. The commonly used wick structure is a wrapped screen wick.
Operating principle
Figure shows the working principle of a heat pipe. Thermal input at the evaporator region vaporizes the working fluid and this vapor travels to the condenser section through the inner core of heat pipe. At the condenser region, the vapor of the working fluid condenses and the latent heat is rejected via condensation. The condensate returns to the evaporator by means of capillary action in the wick.
As previously mentioned there is liquid vapor equilibrium inside the heat pipe. When thermal energy is supplied to the evaporator, this equilibrium breaks down as the working fluid evaporates. The generated vapor is at a higher pressure than the section through the vapor space provided. Vapor condenses giving away its latent heat of vaporization to the heat sink. The capillary pressure created in the menisci of the wick, pumps the condensed fluid back to the evaporator section. The cycle repeats and the thermal energy is continuously transported from the evaporator to condenser in the form of latent heat of vaporization. When the thermal energy is applied to the evaporator, the liquid recedes into the pores of the wick and thus the menisci at the liquid-vapor interface are highly curved. This phenomenon is shown in figure. At the condenser end, the menisci at the liquid-vapor interface are nearly flat during the condensation due to the difference in the curvature of menisci driving force that circulates the fluid against the liquid and vapor pressure losses and body forces such as gravity.
Experimental Procedure
The heat pipe construction is as follows. A copper tube of suitable length is cleaned thoroughly with suitable cleaning agents. Screen mesh acts as a wick is wound around a coil in layers and inserted into the copper tube intact. It is then closed by end caps at both ends. Thermocouples are equally spaced at various positions of the heat pipe. The mica sheet is wound over the evaporator region of the heat pipe since mica is a good electrical insulator and a thermal conductor. A heating coil is wound over the mica sheet in a uniformly spaced manner. The two end of the heating coil are connected to the electric power input. A few centimeter thick cover of glass wool is provided over the entire region of the heat pipe over the glass wool covering, the heat pipe is covered with thick PUF insulation which is normally provided n automobiles.
The heat pipe is evacuated to a pressure of -1.36atm for about 2hours using a vacuum pump. The heat pipe is tested for holding the vacuum for about twelve hours. After vacuum test,R-12 working fluid is filled in the heat pipe for specified pressure which can be indicated by the pressure gauge.
The coolant water supply is provided to the heat pipe and can be controlled by a valve. The thermocouples on the heat pipe are connected to the temperature scanner. A voltmeter is connected in parallel to the dimmerstat. Dimmerstat is supplied with ac current. The temperature scanner is connected to an electric power inlet through a voltage stabilizer.
The ambient pressure and ambient temperature are noted. The heat pipe evaporator region heating coil is connected to the electric power inlet. Coolant water is supplied to the condenser coolant chamber. The dimmerstat initially is at no-load condition. The load on the dimmerstat is varied very slowly till the required power is obtained. Power can be calculated using the equation P=VI cosФ, where cosФ is the power factor, (0.8 for A.C supply).
Heat pipe test rig
The copper tube heat pipe of 25.4 mm inner diameter and thickness employs a five layered 100x100 brass screen mesh wick. The length of the evaporator, adiabatic and condenser sections are 100, 50 and 150 mm respectively. The temperature of the heat pipe are measured using a copper-constantan T-type thermocouples arranged at ten positions equally spaced along a line on the periphery of the heat pipe. Additionally, two thermocouples are provided to measure the temperature of coolant inlet and outlet temperatures. The evaporator region is heated by an electric heating coil wound over a mica sheet. The condenser region is cooled using coolant flowing through condenser coolant chamber. Electric power input is varied by using dimmerstat. The thermocouples are connected to the 16-channel temperature scanner.
Experiment
The experimental heat pipe is initially at room temperature. The coolant water enters the condenser cooling chamber at this temperature and coolant is allowed to flow at a particular flow rate. The initial pressure reading is to be noted from the pressure gauge connected at the evaporator end. The ambient thermocouple temperatures are noted using thermocouples. Initially, the dimmerstat is to be kept at no-load condition. The load on the dimmerstat is slowly varied till it reaches the required value. The electric power is supplied to the electric heating coil which is wound over the evaporator section. The temperature at each position on the heat pipe can be measured by using the thermocouples connected at equal intervals. The initial temperature readings are taken in steps of 2 minutes and in later stages the time interval increases to 5 minutes. After 30-35 minutes the system will reach the steady state conditions.

[attachment=11961]
INTRODUCTION
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.
A heat pipe is a device that efficiently transports thermal energy from its one point to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the Good solid conductor
A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid. The length of the heat pipe can be divided into three parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container
CONSTRUCTION
It consists of a sealed hollow tube using thermo conductive metal such as such as copper or aluminum.. 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 coolant and the pipe with vapor phases of the working fluid.
Heat Pipe is thermal transfer devices that are capable of transferring heat seal hundred times faster than conventional methods.
Heat Pipe Structure: A traditional heat pipe is a hollow cylinder filled with a vaporizable liquid.
A. Heat is absorbed in the evaporating section.
B. Fluid boils to vapor phase.
C. Heat is released from the upper part of cylinder to the environment; vapor condenses to liquid phase
D. Liquid returns by gravity to the lower part of cylinder (evaporating section)
Heat Pipe is thermal transfer devices that are capable of transferring heat seal hundred times faster than conventional methods.
Heat Pipe Structure: A traditional heat pipe is a hollow cylinder filled with a vaporizable liquid.
A. Heat is absorbed in the evaporating section.
B. Fluid boils to vapor phase.
C. Heat is released from the upper part of cylinder to the environment; vapor condenses to liquid phase
D. Liquid returns by gravity to the lower part of cylinder (evaporating section)

I am designing a project using TEC's and need to know if a heat pipe will be the proper way to move the heat away form the TEC. I need to vent the heat 8 to 10 inches from the source. It also needs to be flexible. Can I use the heat pipes to move the heat to the heat sink that far away with out liquid cooling? Also would a flat heat plate with water (abundant and non toxic) be able to radiate the heat properly in a closed environment with out exposure to the ambient air? Or will the heat just be competing with the cooling effect of the heat removal in the first place? I'm a DIY'er and have little knowledge as of yet. I have an idea for an experiment that needs to be a light weight Peltier TEC, no more than 20 mm, drop at least 20 to 30 degrees from ambient air temperature with out creating 200 degree heat sink. I am not trying to create frost, just cooling. Can a TEC be used with a rheostat controller to adjust the input voltage to control the amount of thermal heat exchange and thus control the amount of heat produced? There by allowing me to use flexible heat pipes with out liquid to move the heat to a relocated heat sink? Also would it help to stack them?
Thanks for any help.

thanksssssssssssssssssssssssssssssssssssss you