Heat Pipes

 

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 idea of heat pipes was first suggested by R.S.Gaugler in 1942. However, it was not until 1962, when G.M.Grover invented it that its remarkable properties were appreciated & serious development began.
It consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. A heat pipe is similar to a thermo siphon. It differs from a thermo siphon by virtue of its ability to transport heat against gravity by an evaporation-condensation cycle with the help of porous capillaries that form the wick. The wick provides the capillary driving force to return the condensate to the evaporator. The quality and type of wick usually determines the performance of the heat pipe, for this is the heart of the product. Different types of wicks are used depending on the application for which the heat pipe is being used.

Configuration of Heat Pipe
Basic Element of the Heat Pipe
? Container (closed and vacuum)
? Working fluid
? Wick (capillary construction)

Principles of Operation
A heat pipe is a hermetically sealed evacuated tube normally containing a mesh or sintered powder wick and a working fluid in both the liquid and vapor phase.
When one end of the tube is heated the liquid turns to vapor absorbing the latent heat of vaporization. The hot vapor flows to the colder end of the tube where it condenses and gives out the latent heat. The re-condensed liquid then flows back through the wick to the hot end of the tube. 
      Since the latent heat of evaporation is usually very large, considerable quantities of heat can be transported with a very small temperature difference from one end to the other.
      The vapor pressure drop between the evaporator and the condenser is very small; and, therefore, the boiling ? condensing cycle is essentially an isothermal process. Furthermore, the temperature losses between the heat source and the vapor and between the vapor and the heat sink can be made small by proper design. Therefore, one feature of the heat pipe is that it can be designed to transport heat between the heat source and the heat sink with very small temperature losses.
      The amount of heat that can be transported as latent heat of vaporization is usually several orders of magnitude larger than can be transported as sensible heat in a conventional convective system with an equivalent temperature difference. Therefore, a second feature of the heat pipe is that relatively large amounts of heat can be transported with small lightweight structures.
      The performance of a heat pipe is often expressed in terms of equivalent thermal conductivity. The huge effective thermal conductivity of the heat pipes can be illustrated by the following.
      A tubular heat pipe using water as the working fluid and operated at 150 ?C would have a thermal conductivity several hundred times that of a copper bar of the same dimensions.
      A heat pipe using lithium as the working fluid at a temperature of 1500 ?C will carry an axial heat flux of 10 - 20 kW/cm2
By suitable choice of working fluid and container materials it is possible to manufacture heat pipes for use at temperatures ranging from - 269 ?C to in excess of 2300 ?C.


 

Applications of the Heat Pipe
     The heat pipe has been, and currently is being, studied for a wide variety of applications, covering almost the complete spectrum of temperatures encountered in heat transfer processes. The applications range from the use of liquid helium heat pipes to aid target cooling in particle accelerators, to cooling systems for state-of-the-art nuclear reactors and potential developments aimed at new measuring techniques for the temperature range 2000 ? 3000 ?C.
Broad Areas of Application

In general the applications come within a number of broad groups, each of which describes a property of the heat pipe. These groups are:

  • Separation of heat source and sink
  • Temperature flattening
  • Heat flux transformation
  • Temperature control
  • Thermal diodes and switches

The high effective thermal conductivity of a heat pipe enables heat to be transferred at high efficiency over considerable distances. In many applications where component cooling is required, it may be inconvenient or undesirable thermally to dissipate the heat via a heat sink or radiator located immediately adjacent to the component. For example, heat dissipation from a high power device within a module containing other temperature ? sensitive components would be effected by using the heat pipe to connect the component to a remote heat sink located outside the module. Thermal insulation could minimize heat losses from intermediate sections of the heat pipe.
The second property listed above, temperature flattening, is closely related to source ? sink separation. As a heat pipe, by its nature, tends towards operation at a uniform temperature, it may be used to reduce thermal gradients between unevenly heated areas of body. The body may be the outer skin of a satellite, part of which is facing the sun, the cooler section being in shadow. Alternatively, an array of electronic components mounted on a single pipe would tend to be subjected to feedback from the heat pipe, creating temperature equalization.
The third property listed above, heat flux transformation, has attractions in reactor technology. In thermionic, for example, the transformation of a comparatively low heat flux, as generated by radioactive isotopes, into sufficiently high heat fluxes capable of being utilized effectively in thermionic generators has been attempted
The fourth area of application, temperature control, is best carried out using the variable conductance heat pipe. This can be used to control accurately the temperature of devices mounted on the heat pipe evaporator section. While the variable conductance heat pipe found its first major applications in many more mundane applications, ranging from temperature control in electronics equipment to ovens and furnaces.
As with any other device, the heat pipe must fulfill a number of criteria before it becomes fully acceptable in applications in industry. For example, in the die-casting and injection molding the heat pipe has to be:

  • Reliable and safe
  • Satisfy a required performance
  • Cost ? effective
  • Easy to install and remove.

Obviously, each application must be studied in its own right, and the criteria vary considerably. A feature of the molding processes, for example, is the presence of high frequency accelerations and decelerations. In these processes, therefore, heat pipes should be capable of operating when subjected to this motion, and this necessitates development work in close association with the potential users.

Die casting and Injection Molding
Die casting and injection molding processes, in which metal alloys or plastics are introduced in molten form into a die or mould and rapidly cooled to produce a component, often of considerable size and complexity, have enabled mass production on a considerable scale to be undertaken. The production rate of very small plastic components may be measured in cycles per second, while alloy castings such as covers for car gearboxes may be produced at upwards of one per minute. Aluminum zinc and brass are the most common metals used in the die-cast components, but stainless steel components may now be made using this technique.
The removal of heat during the solidification process is the most obvious requirement, and nearly all dies are water-cooled. However, difficulties are sometimes experienced in taking water-cooling channels to inaccessible parts of the die. A common solution is to use the inserts made of more highly conducting material such as molybdenum, which conducts the heat away to more remote water-cooling channels. Furthermore, it is often inconvenient to take water-cooling to movable or removable nozzles, sprue pins, and cores.
Possibly a more important aspect of die cooling is the need to minimize thermal shock, thus ensuring a reasonable life for the components. With quite large temperature differences between the molten material and the cooling water, which must be tolerated by the intervening die, the life of the die can be shortened. What these parts clearly require is a means of rapidly abstracting heat from their working surfaces at a temperature more nearly approaching that of the molten metal.
Two more thermal problems may be mentioned. In some processes it may be necessary or desirable to heat parts of the die to ensure continuous flow of the molten material to the more inaccessible regions remote to the injection point. To obtain the subsequent rapid solidification, a change from heating to cooling is required in a minimum amount of time to keep cycle times as short as possible.
The heat pipe in its simple tubular form has properties that make it attractive in two areas of application in dies and moulds. Firstly, the heat pipe may be used to even out temperature gradients in the die by inserting it into the main body of the die, without connecting it to the water-cooling circuits.
Probably the most important application is in assisting heat transfer between the die face and the water-cooling path in areas where hot spots occur.
Cooling of Electronic Components
At present the largest application of heat pipes in terms of quantity used is the cooling of electronic components such as transistors, other semiconductor devices, and integrated circuit packages.
There are two possible ways of using heat pipes:

  • mount the component directly onto the heat pipe, and
  • mount the component onto a plate into which heat pipes are inserted.

Spacecraft
Heat pipes, certainly at vapor temperatures up to 200 ?C, have probably gained more from developments associated with spacecraft applications than from any other area. The variable conductance heat pipe is a prime example of this ?technological fall-out?. In the literature can be found details about the following types of application:

  • Spacecraft temperature equalization
  • Component cooling, temperature control and radiator design
  • Space nuclear power sources
  • Moderator cooling
  • Removal of the heat from the reactor at emitter temperature. (Each fuel rod would consist of a heat pipe with externally attached fuel).
  • Elimination of troublesome thermal gradients along the emitter and
    collector.

Energy Conservation
The heat pipe, because of its effectiveness in heat transfer, is a prime candidate for applications involving the conservation of energy, and has been used to advantage in heat recovery systems, and energy conversion devices.
Energy conservation is becoming increasingly important as the cost of fuel rises and the reserves diminish, and the heat pipe is proving a particularly effective tool in a large number of applications associated with conservation.
There are a large number of techniques for recovering heat from exhaust air or gas streams or from hot water streams. Details and explanations about heat pipe heat exchangers can be found in this material. Also, a lot of details can be found visiting the Web pages belonging to heat pipe manufacturers presented in this chapter.
Features of heat pipe heat exchangers that are attractive in industrial heat recovery applications are:
? No moving parts and no external power requirements, implying high reliability.
? Cross-contamination is totally eliminated because of a solid wall between the hot and cold fluid streams.
? Easy to clean.
? A wide variety of sizes are available, and the unit is in general compact and suitable for all.
? The heat pipe heat exchanger is fully reversible ? i.e. heat can be transferred in either direction.
? Collection of condensate in the exhaust gases can be arranged, and the flexibility accruing to the use of a number of different fin spacing can permit easy cleaning if required.

The application of heat pipe heat exchangers fall into three main categories:
1. Recovery of waste heat from processes for reuse in the same process or in another, e.g. preheating of combustion air. This area of application is the most diverse and can involve a wide range of temperatures and duties.
2. Recovery of waste heat from a process to preheat air for space heating.
3. Heat recovery in air ? conditioning systems, normally involving comparatively low temperatures and duties.

Preservation of Permafrost
One of the largest contracts for heat pipes was placed with McDonnell Douglas Corporation by Alyeska Pipeline Service Company for nearly 100,000 heat pipes for the Trans ? Alaska pipeline.
The function of these units is to prevent thawing of the permafrost around the pipe supports for elevated sections of the pipeline. Diameters of the heat pipes used are 5 and 7.5 cm, and lengths vary between 8 and 18 m.
The system developed by McDonnell Douglas uses ammonia as the working fluid, heat from the ground being transmitted upwards to a radiator located above ground level.
Details and photographs of Trans?Alaska Pipeline can be found at this link:

Snow Melting and Deicing

An area of application, and one in which work in Japan has been particularly intense, has been the use of heat pipes to melt snow and prevent icing.
The operating principle of the heat pipe snow melting (or deicing) system is based upon the use of heat stored in the ground as the heat input to the evaporators of the heat pipes.

Heat Pipe Inserts for Thermometer Calibration
Heat pipe inserts have been developed at IKE, Stuttgart, for a variety of duties, including thermocouple calibration. The heat pipes are normally operated inside a conventional tubular furnace. The built-in enclosures provide isothermal conditions, a necessary pre-requisite for temperature sensor calibration. The isothermal working spaces can also be used for temperature sensitive processes, such as fixed-point cell heating, crystal growing and annealing.

High Temperature Heat Pipe Furnace

Under contract from the European Space Agency, IKE developed a high temperature heat pipe surface, for materials processing in a micro gravity environment in the temperature range 900 to 1500 Degree C

Miscellaneous Heat Pipe Applications
To assist the reader in lateral thinking, a number of other applications of heat pipes are listed below.

  • Heat pipe roll-bond panels for warming bathroom floors (Japan)
  • Heat pipe-cooled dipstick for cooling motor bike engine oil (Japan)
  • Passive cooling of remote weather station equipment (Canada)
  • Cooling of drills (Russia)
  • Thermal control of thermoelectric generators (USA)
  • Cooling of gas turbine blades (Czech Republic)
  • Thermal control of electric storage heaters (Byelorussia, UK)
  • Cooling of semi-automatic welding equipment (Russia)
  • Deicing fish farms and ornamental ponds (Romania)
  • Heating heavy oil in large tanks (Romania)
  • Cooling of soldering iron bit (UK)
  • Cooling of bearings for emergency feed water pumps (UK)
  • Cooling of targets in particle accelerators (UK)
  • Isothermalisation of bioreactors (China)
  • Cooling of snubber pins in the synthetic fiber industry (UK)
  • Thermal control in electric batteries Dehumidifiers (USA)
  • Car passenger compartment heating
  • Domestic warm air heaters (USA)

Structure, Design and Construction

A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium at both ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid, (or coolant), chosen to match the operating temperature. Some example fluids are water, ethanol, acetone, sodium, or mercury. Due to the partial vacuum that is near or below the vapor pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase. Having a vacuum eliminates the need for the working gas to diffuse through another gas and so the bulk transfer of the vapor to the cold end of the heat pipe is at the speed of the moving molecules. The only practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end.
Inside the pipe's walls, an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. This is typically a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.
A heat pipe is not a thermo siphon, because there is no siphon. Thermo siphons transfer heat by single-phase convection.
Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapour pressure is low.
The materials chosen depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2?4 K) to mercury (523?923 K) & sodium (873?1473 K) and even indium (2000?3000 K) for extremely high temperatures. The vast majority of heat pipes for low temperature applications use some combination of ammonia (213?373 K), alcohol (methanol (283?403 K) or ethanol (273?403 K)) or water (303?473 K) as working fluid. Since the heat pipe contains a vacuum, the working fluid will boil and hence take up latent heat at well below its boiling point at atmospheric pressure. Water, for instance, will boil at just above 273 K (0 degrees Celsius) and so can start to effectively transfer latent heat at this low temperature.
The advantage of heat pipes is their great efficiency in transferring heat. They are a much better heat conductor than an equivalent cross-section of solid copper. A heat flux of more than 230 MW/m? has been recorded (nearly four times the heat flux at the surface of the sun).[1]
Active control of heat flux can be affected by adding a variable volume liquid reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section. Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the area available for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design.
A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal diode. This heat pipe will transfer heat in one direction, acting as an insulator in the other.
By limiting the quantity of working fluid in a heat pipe, inherent safety is obtained. Water expands 1600 times when it vaporizes. In water containing heat pipe if the water is limited to a 1600th of the volume of the heat pipe, the pressure within the pipe up to 100 C is limited to one atmosphere. Calculations can be made to ensure that the pressure is within the limits of the pipe strength at the highest possible working temperature of the device.

 


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