Now, TME offer also includes the so-called heat pipes for excess heat dissipation from Wakefield-Vette. They are manufactured by a renowned company, which belongs to American corporation Wakefield-Vette. Although heat pipes seem to be quite a new solution, in fact their history dates back to as early as 1942, when the first patent for the use of heat pipes in cooling systems was filed by General Motors.
What are heat pipes and how do they work?
Heat pipes, which are characterized by very high efficiency, are basically very simply operated 2-phase devices without any internal parts or components. They use a cycle of evaporation and condensation of the operating medium to transfer excess heat from one end to the other. The key here is the use of evaporation, convection and then condensation of the medium with which the tube is filled. Heat pipes are characterised by great efficiency in comparison with solutions based on thermal conductivity and have a wide range of operating temperatures – from a few Kelvins (cryogenic applications) to more than one and a half thousand degrees Celsius (space and aviation technologies). Hence, their scope of application is extensive and includes, naturally, refrigeration and heating technologies, chemical industry, advanced construction engineering, food industry, shipbuilding, aviation and space technology and – first and foremost, probably the best known of all applications – electronics.
Heat pipe – principle of operation
The principle of operation of a heat pipe is slightly different for each of its two basic types, which are gravity heat pipes – the so-called thermosiphons – and heat pipes with a capillary structure, usually called wick heat pipes. In the former, the circulation of the operating medium – it is about the return of the condensate to its original place – takes place as a result of gravity. On the other hand, in heat pipes with a capillary structure, the circulation of the medium is based on the capillary action. However, regardless of which type of heat pipe we are dealing with, the operating pattern is roughly the same. In the first stage, the end of the heat pipe which receives the heat (evaporator section) raises the temperature of the operating medium until it changes from liquid to gaseous state, turning into steam. The difference in temperature and pressure (convection force) between the warmer and cooler end of the tube creates a stream of steam directed towards the cooler end of the pipe, which acts as a condenser. This is where the steam condenses, transferring the stored latent heat energy to the pipe material, which then transfers it to its environment. In the final stage of the cycle, the condensed vapour returns in the form of drops of liquid to the evaporator section and does so either by gravity – simply by flowing down – or by capillary forces, acting thanks to the porous structure of the inner walls of the heat pipe.
How heat pipes work:
- The operating medium absorbs heat during the transition to a volatile state.
- The steam moves along the tube to the area with a lower temperature.
- Steam condenses back into liquid and dissipates heat.
- The medium is absorbed by the wick structure.
- The medium returns through the capillary microstructure to the end of the pipe which has a high temperature.
- Natural or forced air flow dissipates excess heat to the environment.
Heat pipe – how to choose the right solution?
This could be the end of the line, but it is important to note the issues that have been omitted so far: the pressure inside the pipe and the type of operating medium, which could be water or other liquid. By regulating the pressure properly, it is possible to influence for each medium the value of the temperature at which the medium is converted to volatile state and of the so-called dew point, i.e. the value of temperature difference at which the steam in contact with the structure of the pipe in the condensation section is effectively converted to the liquid state. Choosing the operating medium – which can be helium, hydrogen, nitrogen, acetone, potassium, liquid silver and many others – is also important, because the right medium should be chosen depending on the conditions in which the heat pipe will work. By comparison, heat pipes filled with ammonia work well in the temperature range from about 200 to almost 400 degrees Kelvin, while space technology has to use mediums that work well in the temperature range of 1000-2500 degrees Kelvin. That is why NASA, ESA or Space X projects use specialist heat pipes filled with liquid lithium or silver, among others.
Heat pipe – types and design
From the point of view of the functions performed in the process of circulating heat transfer from one area to another, there are three parts within the heat pipes themselves, commonly referred to as sections. The first and second sections are the evaporation section and the condensation section already described. Between them there is another, middle one, called adiabatic. It is where the free, parallel and mutually undisturbed movement of the steam and liquid phase takes place. The steam moves from the evaporation section to the condensation section in the entire volume of the tube, while the condensed liquid flows down smooth walls (thermosiphons) or moves inside a porous capillary microstructure of the inner wall.
While the structure of gravity heat pipes (thermosiphons) is simple, capillary heat pipes are a much more complex. Manufacturers use different materials to produce the core and the capillary structure, where the latter can be made from ceramic materials, glass fibres, sintered metallic powder or wire mesh, among others. In fact, it is a matter of obtaining a wick structure that facilitates the return of the liquid to the evaporator, which is not easy. Creating a proper microstructure is the most complicated stage in the manufacturing process of a heat pipe, especially if it is to be a composite wick, i.e. consisting of at least two different materials.Homogeneous wicks – made of one material – usually have the form of longitudinal grooves parallel to the pipe axis. They are manufactured simultaneously with the heat pipe itself, while composite wicks are manufactured in at least two or three stages. However, regardless of whether it is a grooved, mesh, mixed (grooved and mesh), sintered or rolled wick, its structure has a key impact on the capillary action of the liquid, which takes place by overcoming internal flow resistance due to capillary pressure. These two forces, which try to withstand each other, act differently on different types of operating liquid. That is why manufacturers, such as Wakefield-Vette had to carry out numerous detailed tests and trials for each individual medium, thanks to which they developed structures in which the increase in capillary forces – when the pore or mesh core size is gradually reduced – is greater than the increase in friction forces. In the case of Wakefield-Vette products, the vast majority of the heat pipes offered are composite wicks, sintered from copper powder, which during the sintering process creates a porous structure resembling a sponge. Thanks to the appropriate size of micropores, these pipes effectively transfer the operating liquid after condensation, both vertically, diagonally and horizontally, hence the high rating of their performance on the market.
A separate issue is the material and construction of the body of the heat pipe itself, which is made of different types of metals, ceramics or glass – depending on the purpose of the pipe. The material of the body of the heat pipe must withstand the internal pressures that occur during normal operation of the heat pipe and not react with the operating medium (risk of corrosion) as well as have a high thermal conductivity coefficient, so that the whole system can perform its role effectively. Coming back to operating liquids, it has to be mentioned that not only should they not react chemically with the material of the pipe body and the wick itself, but they must also have thermal stability, low viscosity of liquid and steam at high surface tension and, of course, a high thermal conductivity coefficient.
Heat pipe – how to connect them with each other?
Connecting heat pipes involves another aspect, which is flattening them. It is often necessary to fit the system to the desired shape, to insert it into the gap in which it will be installed, or simply to increase the contact surface of the pipe for better heat absorption. The price for fitting a pipe to the place where it is to operate – by flattening it – is its reduced heat capacity and cross-sectional area. In such situations, the efficiency of a flattened pipe is similar to that of a fully round one, but with a significantly smaller diameter. What is worse, the greater the diameter of the initial heat pipe, the more noticeable the reduction of its thermal capacity after flattening. In the case of bending heat pipes there is also a slight reduction in their capacity, which further deteriorates, if the bending radius goes below 4 or 3 times the diameter of the pipe itself. In extreme situations the flow of steam and heat may be cut off, which effectively prevents the heat pipe from functioning.
Connecting heat pipes to plates and heat exchangers is mainly about maximising the contact surface while respecting the previously mentioned guidelines for flattening and bending. In most cases the heat pipes are inserted into ducts arranged on the plate in order to maximise their contact surface. The matching heat pipe can be fixed in such a duct by soldering or using thermal epoxy resin. The heat pipe can also be clamped between two plates by means of matching grooves which are connected to each other. In this type of clamping configuration, the thermally conductive paste can be used to increase the contact between the heat pipe and the plates and at the same time reduce the thermal resistance at the contact point.