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.
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:
Â¢ Compatibility (both with working fluid and external environment)
Â¢ Strength to weight ratio
Â¢ Thermal conductivity
Â¢ Ease of fabrication, including welding, machineability and ductility
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:
Â¢ compatibility with wick and wall materials
Â¢ good thermal stability
Â¢ wettability of wick and wall materials
Â¢ vapor pressure not too high or low over the operating temperature range
Â¢ high latent heat
Â¢ high thermal conductivity
Â¢ low liquid and vapor viscosities
Â¢ high surface tension
Â¢ 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 )
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.
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
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
Â¢ Spacecraft temperature equalization
Â¢ Component cooling, temperature control and radiator design in satellites.
Â¢ 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 40Ã‚Â°C it provides a 6.25Ã‚Â°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90Ã‚Â°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 .01Ã‚Â° 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 - Ã‚Â½ the weight of aluminum.
Very light weight - 1/3 the weight of aluminum.
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.
Andrews, J; Akbarzadeh, A; Sauciue, I.: Heat Pipe Technology, Pergammon, 1997.
Dunn, P.D.; Reay, D.A.: Heat Pipes, Pergammon, 1994.