CHAPTER 1 INTRODUCTION 1.0 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 thermosyphon. It differs from a thermosyphon 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. 1
Transcript
CHAPTER 1
INTRODUCTION
1.0 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 thermosyphon. It differs from a thermosyphon 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.
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1.1 HISTORY OF HEAT EXCHANGER PIPES
As the story goes, it was in 1963 when a Los Alamos National Research Laboratory engineer named George Grover demonstrated the first heat pipe. Heat pipe technology was borrowed from simple heat conducting pipes used by English bakers 100 years ago.
Since 1963, heat pipes progressed and modern applications of this technology range from miniature heat pipes for cooling processors inside laptop computers, to groups of half inch diameter and five feet long pipes that will be used in NASA spacecraft, to pipes of two inch diameters (or more) which are used to cool injection molds used in plastic forming. The lengths of the pipes can vary from inches to 24 feet or more.
A lithium filled heat pipe developed at Los Alamos in the mid 1980s transferred heat energy at a power density of 23 kilowatts per square centimeter. If we consider that the heat emitted from the sun's surface is roughly six kilowatts per square centimeter, we begin to realize the enormous heat transferring capacity of the heat pipe.
Fig: Heat Pipe
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1.2 TYPES OF HEAT PIPES:-
Thermosyphon- gravity assisted wickless heat pipe. Gravity is used to force the condensate back into the evaporator. Therefore, condenser must be above the evaporator in a gravity field.
Leading edge- placed in the leading edge of hypersonic vehicles to cool high heat fluxes near the wing leading edge.
Rotating and revolving- condensate returned to the evaporator through centrifugal force. No capillary wicks required. Used to cool turbine components and armatures for electric motors.
Cryogenic - low temperature heat pipe. Used to cool optical instruments in space.
Flat Plate- much like traditional cylindrical heat pipes but are rectangular. Used to cool and flatten temperatures of semiconductor or transistor packages assembled in arrays on the top of the heat pipe.
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Fig 1.2- Flat Plate Heat Plate
Micro heat pipes- small heat pipes that are noncircular and use angled corners as liquid arteries. Characterized by the equation: rc /rh³1 where rc is the capillary radius, and rh is the hydraulic radius of the flow channel. Employed in cooling semiconductors (improve thermal control), laser diodes, photovoltaic cells, medical devices.
Variable conductance- allows variable heat fluxes into the evaporator while evaporator temperature remains constant by pushing a non- condensable gas into the condenser when heat fluxes are low and moving the gas out of the condenser when heat fluxes are high, thereby, increasing condenser surface area. They come in various forms like excess-liquid or gas-loaded form. The gas-loaded form is shown below. Used in electronics cooling.
Capillary pumped loop heat pipe- for systems where the heat fluxes are very high or where the heat from the heat source needs to be moved far away. In the loop heat pipe, the vapor travels around in a loop where it condenses and returns to the evaporator. Used in electronics cooling.
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Heat pipes were developed especially for space applications during the early 60´ by the NASA. One main problem in space applications was to transport the temperature from the inside to the outside, because the heat conduction in a vacuum is very limited.
Hence there was a necessity to develop a fast and effective way to transport heat, without having the effect of gravity force. The idea behind is to create a flow field which transports heat energy from one spot to another by means of convection, because convective heat transfer is much faster than heat transfer due to conduction Nowadays heat pipes are used in several applications, where one has limited space and the necessity of a high heat flux.
Of course it is still in use in space applications, but it is also used in heat transfer systems, cooling of computers, cell phones and cooling of solar collectors. Especially for micro applications there are micro heat pipes developed as for cooling the kernel of a cell phone down. Due to limited space in personal computers and the growing computational power it was necessary to find a new way to cool the processors down.
By means of a heat pipe it is possible to connect the processor cooling unit to a bigger cooling unit fixed at the outside to cart of the energy. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. One common example of a heat exchanger is the radiator in a car, in which the heat source, being a hot engine-cooling fluid, water, transfers heat to air flowing through the radiator (i.e. the heat transfer medium).
The effect of Globalization has impact Heat Pipe Manufactures in India to a mass effect. The Government has decided to put up new plants for more pipe products and also job awareness.
One of the automobile industry set up in India in year 2007 ‘TECH-ED EQUIPMENT COMPANY’ namely has decided to improve and develop conventional HEAT PIPES in upcoming 2011.One of the products include Manufacturer & exporter of heat pipe, shell & tube heat exchanger model:ht19, fluidised bed apparatus, composite wall with water, finned tube heat exchanger, cooling towers & heat exchangers, heat pipes, radiations.
Also ‘STAR SCIENTIFIC INSTRUMENTS’ has similar plans to invest money on advancement in HEAT PIPES mechanism. The improve products are Supplier and manufacturer of pipes which includes heating pipes, water pipes, metal pipes, steel pipes, stainless steel pipes, pattern pipes, mild steel pipes, motor pipes and water steel pipes.
Other noteworthy manufacturers include:
1. Indo Anuska ltd.
2. Vedom CO.
3. Multi Tech.
4. Thermosys
5. Fin Tubes Manufacture Co.
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1.4 OBJECTIVE OF THE PROJECT WORK
The main objectives of the project are as follows:
1. To design a low cost Heat Exchanger Pipe.
2. Experimental Study of Heat Pipes.
3. To determine the cooled water temperature with respect to hot water temperature.
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CHAPTER 2REVIEW OF LITERATURE
2.0 INTRODUCTION
— Heat pipes are two-phase heat transfer devices with high effective thermal conductivity. Due to the high heat transport capacity, heat exchanger with heat pipes has become much smaller than traditional heat exchangers in handling high heat fluxes.
With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectranics, energysaving in HVAC systems for operating rooms, surgery centers, hotels, cleanrooms etc, temperature regulation systems for the human body and other industrial sectors.
Development activity in heat pipe and thermosyphon technology in asia in recent years is surveyed. Some new results obtained in Australia and other countries are also included.
2.1 REVIEW OF LITERATURE
2.1.1 History
As a highly-effective heat transfer element, heat pipe shave gradually recognized, and are playing a more and more important role in almost all industrial fields. A heat pipe is an evaporation-condensation device for transferring heat in which the latent heat of vaporization is exploited to transport heat over long distances with a corresponding small temperature difference.
The heat transport is realized by means of evaporating a liquid in the heat inlet region (called the evaporator) and subsequently condensing the vapour in a heat rejection region (called the condenser) closed circulation of the working fluid is maintained by capillary action and /or bulk forces. The heat pipe was originally
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invented by Gaugler of the General Motors Corporation in 1944, but did not truly garner any significant attention within the heat transfer community until the space program resurrected the concept in the early 1960's. An advantage of a heat pipe over other conventional methods to transfer heat such an a finned heat sink, is that a heat pipe can have an extremely high thermal conductance in steady state operation.
Hence, a heat pipe can transfer a high amount of heat over a relatively long length with a comparatively small temperature differential. Heat pipe with liquid metal working fluids can have a thermal conductance of a thousand or even tens of thousands of times greater than the best solid metallic conductors, silver or copper.
There are generally at least five physical phenomena that will limit, and in some cases catastrophically limit, a heat pipe ability to transfer heat. They are commonly known as the sonic limit, the capillary limit, the viscous limit, the entrainment limit and the boilling limit.
2.1.2 HEAT PIPES IN HVAC SYSTEMS
The application of heat pipes for heat recovery in cold climates is widely recognised. With advancement of heat pipes with a low air pressure drop, made possible by loop configurations, heat recovery applications can be extended to milder climates and still pay for themselves.
A new possibility is 'cooling' recovery in summertime, which is now economical enough to be considered. The application of heat pipes to increase the dehumidification capacity of a conventional air conditioner is one of the most attractive applications. By using dehumidifier heat pipes, one can decrease the relative humidity in the conditioned space (typically by 10%) resulting in noticeably improved indoor air quality and reduce power demand. Heat pipe also promise to improve greatly indoor air quality, and at the same time help conserve energy.
Wasim SAMAN examined the possible use of a heat pipe heat exchanger for indirect evaporative cooling as well as heat recovery for fresh air preheating. Thermal performance of a heat exchanger consisting of 48 thermosyphons
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arranged in six rows was evaluated. The tests were carried out in a test rig where the temperature and humidity of both air streams could be controlled and monitored before and after the heat exchanger. Evaporative cooling was achieved by sparying the condenser sections of the thermosyphons.
The parameters considered include the wetting arrangement of the condenser section, flow ratio of the two streams, initial temperature of the primary stream and the inclination angle of the thermosyphons.
Their results showed that indirect evaporative cooling using this arrangement reduces the fresh air temperature by several degrees below the temperature drop using dry air alone [1]. Humidity control is a never-ending war in tropical hot and humid built environment. Heat pipes are passive components used to improve dehumidification by commercial forced-air HVAC systems. They are installed with one end upstream of the evaporator coil to pre-cool supply air and one downstream to re-heat supply air.
This allows the system's cooling coil to operate at a lower temperature, increasing the system latent cooling capability. Heat rejected by the downstream coil reheats the supply air, eliminating the need for a dedicated reheat coil. Heat pipes can increase latent cooling by 25-50% depending upon the application. Conversely, since the reheat function increases the supply air temperature relative to a conventional system, a heat pipe will typically reduce sensible capacity. In some applications, individual heat pipe circuits can be controlled with solenoid valves to provide improved latent cooling control.
Primary applications are limited to hot and humid climates and where high levels of outdoor air or low indoor humidity are needed. Hospitals, supermarkets and laboratories are often good heat pipe applications. Tucker mentioned that for many years, heat pipe heat exchangers (HPHEs) with two-phase closed thermosyphons, as show in Fig. 1, have been widely applied as dehumidification enhancement and energy savings device in HVAC systems.
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Fig 2.1.2(a) A typical heat pipe heat exchanger (HPHE) applied in HVAC Systems.
Literature review indicated that research work related to energy recovery using HPHE carried out in subtropical climates is hardly found. Niu et al.studied a HVAC system combining chilled ceiling with desiccant cooling for maintaining the indoor air humidity within a comfort zone and to reduce the risk of water condensation on chilled panels.
The results reveal that chilled ceiling combined with desiccant cooling might conserve up to 44% of primary energy use compared to a conventional constant volume all-air system. In a separate study, Zhang et al. conducted a research on energy consumption for conditioning ventilation air and the annually performance of a membrane-based energy recovery ventilator (MERV) in Hong Kong.
The results indicated that approximately 58% of the energy needed for cooling and heating fresh air might be saved yearly with an MERV, while only roughly 10% of the energy might be saved via a sensible-only energy recovery ventilator (SERV).
In a similar study, Zhang et al. conducted a study on a thermodynamic model built with an air moisture removal system incorporated a membrane-based total heat exchanger to estimate the energy use annually. The outcomes suggested that the independent air moisture removal could save 33% of primary energy.
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Yat H. Yau studied an 8-row thermosyphon-based heat pipe heat exchanger for tropical building HVACsystems experimentally.
This research was aninvestigation into how the sensible heat ratio ( SHR) of the 8-row HPHE was influenced by each of three key parameters of the inlet air state, namely, dry-bulb temperature, relative humidity and air velocity . On the basis of his study, it is recommended that tropical HVAC systems should be installed with heat pipe exchangers for dehumidification enhancement. The HPHE evaporator section functions as a pre-cooler for the AC system and the condenser section as a reheating coils as shown in Fig. 2.
Fig.2.1.2(b) Simple schematic diagram for HVAC model running with a HPHE [2]
By doing this the cooling capacity for the original system is re-distributed so that latent cooling capability of the conventional cooling coil is enhanced. In hot and humid tropical climates, the moisture removal capability of the chilled water coil in the HVAC systems can be enhanced if the supply air is precooled before reaching the chilled water coil. For instance, a typical HVAC system at average ambient condition of 320C and 58% relative humidity (RH) with total cooling load at 58.5 kw can save 14.4 kw if HPHE is added into the HVAC system as shown in Fig. 3.
2.1.3 HEAT PIPE HEAT EXCHANGER (HPHE) IN INDUSTRIAL SYSTEMS
Since the 1970s, HPHE have been extensively applied in many industries such as energy engineering, chemical engineering and metallurgical engineering as waste heat recovery systems. One of the important applications of heat pipes as HPHE is the recovery of heat from exhaust gases in industrial plants. As exhaust gases enter the surrounding, not only waste energy but also damage the environment.
Due to the high transport capacity, heat exchangers with heat pipes are smaller than traditional heat exchangers in handling high heat fluxes .S.H.Noie investigated the thermal performance of a heat exchanger consisting of 90 thermosyphons arranged in six rows in a test rig. The variable parameters which were being altered were the air velocity and the inlet temperature to the evaporator section.
A computer program was developed to analyze the thermosyphon heat exchanger using the ε – NTU method. In order to verify its accuracy and conformity, the experimental results were compared to those predicted by the simulation program. The temperature across the evaporator section was varied
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in the range of 100-250 0C while the inlet temperature to condenser section was nearly constant 25 0C. Distilled water was used as the working fluid with a fill ratio of 60 % of the evaporator section length. The air face velocity ranged from 0.5 to 5.5 m/s and the heat input into the evaporator section was varied between 18 and 72 kw using electric heating elements.
The overall effectiveness of the thermosyphon heat exchanger obtained from experiments varied between 37% and 65%. The experimental results showed the minimum effectiveness of the thermosyphon took place at Ch = Cc. Therefore, equal value of air face velocities in evaporator and and condenser sections should be avoided.
Research and applications of water-based HPHE for heat recovery have been reported in some works but appear still limited in comparison with organic based fluids. As an extension of the work by Than et al., experimental testing on more water-based, HPHE configurations and comparison with the prediction by the effectiveness- NTU method have been carried out and the results are presented. They showed effectiveness increases with the number of rows, the increase is about 30% .from 2 to 4 rows and 10-20% from 4 to 6 rows.
Comparison between the 4-row and 2X2-row modules shows the latter has higher effectiveness especially at flow rate ratio>1, suggesting that the gap between the 2-row modules serves to break up the thermal boundary layer in the air passages between the fins.
The feasibility of using heat pipe heat exchangers for heating applying automotive exhaust gas is studied by F. Yang et al. [17]. Practical heat pipe heat exchanger was set up for heating a large bus. Simple experiments were carried out to examine the performance of the heat exchanger. It was shown that the experimental results, which indicate the benefit of exhaust gas heating, are in good agreement with numerical results.
A closed-ended oscillating heat-pipe (CEOHP) air-preheater for energy thrift in a dryer was investigated by S. Rittidech et.al.[18]. The CEOHP air-preheater design employed cooper tubes: thirty –two set of capillary tubes with an inner diameter of 0.002 m, an evaporator and a condenser length of 0.19 m, and each of which has eight meandering turns. The evaporator section was heated by hot-gas, while the condenser section was cooled by fresh air.
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In the experiment, the hot-gas temperature was60,70 or 80 0C with the hot-gas velocity of 3.3 m/s. The fresh air temperature was 30 0C. Water and R123 was used as the working fluid with a filling ratio of 50%. It was found that, as the hot-gas temperature increases from 60 to 80 0C, the thermal effectiveness slightly increases. If the working fluid changes from water to R123, the thermal effectiveness slightly increases. The designed CEOHP air-preheater achievesenergy thrift.
An experimental study was carried out for the heat transfer characteristics and the flow patterns of the evaporator section using small diameter coiled pipes in a looped heat pipe (LHP) by Jie Yi et.al. Two coiled pipes: the glass pipe and the stainless steel pipes were used as evaporator section in the LHP, respectively. Flow and heat transfer characteristics in the coiled tubes of the evaporator section were investigated under the different filling ratios and heat fluxes.
The experimental results show that the combined effect of the evaporation of the thin liquid film, the disturbance caused by pulsation and the secondary flow enhanced greatly the heat transfer and the critical heat flux of the evaporator section.
Wangnipparnto et.al. investigated a numerical method to analyze the thermosypon heat exchanger with and without the presence of electrohydrodynamics. The proposed model was capable of handling both balanced and unbalanced thermosyphon heat exchangers.
For the balanced thermosyphon heat exchanger, the calculated results of heat transfer rate for water and R-134a agreed well with experimental data. For the unbalanced thermosyphon heat exchangers, it was found that the performance improvement increased with the ratio of m´e / m´c when electrohydrodynamics was applied at the condenser alone.
2.1.4 CONCLUSION
A short review of heat pipe applications and technology contains mainly data from some country in Asia which testifies, that heat pipes are very efficient heat transfer devices, which can be easily implemented as thermal links and heat exchangers in different systems to ensure the energy saving and environmental protection.
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Chapter -3Design and Development of a Heat Exchanger Pipes
3.0 INTRODUCTION
The design of Heat Pipe is a science to design the various components involved in an optimum manner to utilize in our day to day use.
3.1 Selection of parameter for selecting of Heat Pipe
I. Investigating and determining the following operational parameters:-
a. Heat load and geometry of the heat source.
b. Possible heat sink location, the distance and orientation relative to the heat source.
c. Temperature profile of heat source, heat sink and ambient
d. Environmental condition (such as existence of corrosive gas).
II. Selecting the pipe material, wick structure, and working fluid:-
a. Determine the working fluid appropriate for your application
b. Select pipe material compatible to the working fluid
c. Select wick structure for the operating orientation
d. Decide on the protective coating.
III. Determining the length, size, material and shape of the heat pipe:-
A particular working fluid can only be functional at certain temperature ranges. Also, the particular working fluid needs a compatible vessel material to prevent corrosion or chemical reaction between the fluid and the vessel. Corrosion will damage the vessel and chemical reaction can produce a non-condensable gas.
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Heat pipes are not functional when the temperature of the pipe is lower than the freezing point of the working fluid. Freezing and thawing is a design issue, which may destroy the sealed joint of a heat pipe when place vertically. Proper engineering and design can overcome this limitation.
IV. Determining the common wick structure:-
There are four common wick structures used in commercially produced heat pipes; groove, wire mesh, powder metal and fiber/spring. Each wick structure has its advantages and disadvantages. There is no perfect wick. Refer to Fig. 2 for a brief glance of actual test performance of four commercially produced wicks. Every wick structure has its own capillary limit. The groove heat pipe has the lowest capillary limit among the four, but works best under gravity assisted conditions where the condenser is located above the evaporator.
3.2 DESIGN OF HEAT EXCHANGER PIPES
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In the designing of heat pipes for the present project work, the following components were chosen in order to make the system efficient and economical.
3.2.1 DETAILS OF CONSTRUCTION
(a) Design Considerations
The three basic components of a heat pipe are:
I. the container II. the working fluid
III. the wick or capillary structure IV. the digital thermometerV. copper bars
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, machine ability 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
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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.
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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 of materials like steel structure made, aluminum, 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
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the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability.
Working
Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of 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.
Fig 3.0- Container.
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 (APC)". 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.
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
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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.
3.2.2 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.
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3.3 RESEARCH DESIGN AND ITS UTILIZATION
With the recent improvements in Heat Pipes, it has attracted many utility systems for providing cooling process in wide variety of applications, which are as under:
3.3.1 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. (See photo of inside of laptop computer)
Fig 3.3.1- Laptop Processor
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.
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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 pipe’s MTTF is estimated to be over 100,000 hours of use.
3.3.2 NOTEBOOK AND MOBILE PCs THERMAL CONTROL
Heat pipes have proven to be the excepted means of providing thermal control in notebook and Mobil PCs systems. Heat pipes can move and dissipate CPU generate heat selectively throughout the system without affecting temperature sensitive components. Low wattage heat pipes (under 20 watts) have standardized input plates to the heat pipe. The connection to the heat exchanger via the heat pipe can have any number of configurations to accommodate component placement, multiple power ranges and fan options.
3.3.3 CPU WORK STATIONS
The heat pipe solutions for thermal control at this level is 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. Input pads are standard using stand-offs, transition sockets, and bolster plates on the bottom of the PC board. The spring clips used on the fan/heat sink combination won’t work here. Airflow management is important in the overall efficiency of the heat pipe and should be calculated along with the intended heat pipe design.
3.3.4 WORK STATIONS 75- 100 WATTS
Thermal solutions are normally designed with multiple heat pipes, dedicated airflow and maximum input area. Fins stacks typically extend over both sides of the CPU. Input attachment to the CPU is with stand-offs, transition sockets or bolster plates.
3.3.5 500 MHz OPERATING SYSTEMS
This group uses two thermal products, heat pipes to transfer the CPU heat (100 to 300 watts) and a second internal or external cooling source. Input power is
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generated from multiple CPUs and components with single or multiple heat pipes. Cooling temperatures on the output range from -0° C to - 40° C. This system requires thermal isolation because of dewpoint considerations.
Fig 3.3.5- 500 MHz OPERATING SYSTEMS
3.3.6 Flexible Solutions
Heat pipes are manufactured in a multitude of sizes and shapes. Unusual application geometry can be easily accommodated by the heat pipe’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.
3.3.7 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.
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3.3.8 Some of the flat heat pipes manufactured:
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.
Fig 3.3.8- Flat Heat Pipes.
Weight Reduction Mega Flats:
1. Standard - aluminum construction.
2. Lightweight - ½ the weight of aluminum.
3. Very light weight - 1/3 the weight of aluminum.
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3.3.9 Cost Effectiveness of Heat Pipes
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.
Fig3.3.9- Heat Flow through a Pipe
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The heat transfer through a pipe is dependent on the thickness of the pipe and isolation layers. The thickness of the pipe and layers can be defined by the radius of layers i.e. R1, R2,..., Rn. The thermal conductivity of layers are, 1, 2,..., . The fluid within the pipe is at temperature Tin, and the heat transfer coefficient from fluid to the wall is in. The temperature and heat transfer coefficient for the fluid outside the pipe are Tout and out. By using Fourier's law of conduction and Newton's law of cooling, it can be shown that for a steady state heat transfer:
Advanced Cooling Technologies, Inc. (ACT) has worked extensively on heat pipes product reliability. This paper covers the following aspects related to heat pipe product reliability.
I. General Quality
II. Suitable Working Fluid/ Material Systems
(a). Documented Compatibility (b). ACT’s Life Test Data
III. Heat Pipe Performance Limits
IV. Shock and Vibration
V. Acceleration
VI. Frozen Start Up
VII. Thermal Cycling
VIII. Summary
I. General Quality
Heat pipes are proven, reliable, heat transfer devices which have been used in applications from laptop cooling to satellite thermal control. A well controlled manufacturing process is critical to fabricating reliable, long life heat pipes. Common causes of heat pipe degradation include leaks between inside and outside of the heat pipe and gas generation within the heat pipe envelope. Even very small leaks in the envelope material, at a joint, or at the seal of the fill tube, may cause degradation over time. Internal gas generation is the result of chemical reactions caused by either an incompatible fluid/material system or contaminants from improper cleaning and processing. Eliminating these manufacturing problems requires heat pipe specific manufacturing knowledge and experience, and the capability of implementing that knowledge in a consistent manufacturing process.
ACT excels in meeting both requirements. First, ACT has a relatively large number of engineers with hundreds of years of combined experience in heat
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pipe design, fabrication and testing, for both high volume and custom applications. Second, ACT’s quality system is certified to ISO9001 and AS9100 standards for terrestrial and aerospace product manufacturing, respectively. Our quality system has passed all audits to date with a perfect, 100% score, indicating a strong and consistent performance in implementing the quality system in the manufacturing practice. As a result, ACT’s heat pipe products are reliable and have been used in numerous, mission critical, space, military and commercial systems.
II. Suitable Working Fluid / Material Systems
A heat pipe material system includes the envelope material, the wick material, the working fluid, and any braze, solder or weld filler materials used in sealing the heat pipe. ACT works with a variety of heat pipe material systems ranging from low temperature Aluminum/Ethane heat pipes operating at -100˚C to high temperature Haynes/Sodium heat pipes operating at 1,100˚C. Documented Compatibility
Two major results of material incompatibility are corrosion and generation of non-condensable gas (NCG). If the wall or wick material is soluble in the working fluid, mass transfer is likely to occur between the condenser and evaporator, with solid material being deposited in the latter. This will result in either local hot spots or blocking of the pores of the wick.
NCG generation is the most common indication of a heat pipe failure. As the NCG accumulates in the heat pipe condenser section, it gradually blocks the heat transfer area, consequently degrading the heat pipe performance. Table 1 shows well documented compatibility data for low temperature working fluids.
Two of the most reliable and most proven heat pipe material/fluid systems are copper/water and aluminum/ammonia. Copper/water is the standard for terrestrial electronics cooling, and aluminum/ammonia is the standard for satellite thermal control.
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III. ACT(Advanced Cooling Technique)’s Life Test Data
ACT has been developing new material systems for emerging applications. ACT maintains a large number of life test heat pipes made of various material/fluid combinations. Some of our intermediate temperature fluid test results are summarized in Table 2. As shown, titanium and super alloys can be used as the envelope material for certain intermediate temperature fluids in the temperature range of 450 to 700K (177 to 427°C). For instance, AlBr3 showed very strong compatibility with C22, C2000, and B3.
ACT has also performed extensive testing of high temperature water heat pipes4. While copper/water heat pipes have been extensively used in the temperature range of 20 to 150°C, they are not suitable for applications requiring operation beyond 150°C. With the vapor pressure rising with temperature, copper is not an ideal envelope material because of its low yield strength and high density. ACT has run life tests to prove the compatibility of stronger envelope materials for high temperature water heat pipes.
IV. Shock and Vibration
ACT has substantial experience in designing, fabricating and testing heat pipe assemblies to various shock and vibration loadings. ACT has in house mechanical shock and vibration test equipment as shown in Figure 2. ACT’s heat pipes and loop heat pipes have been tested to diverse shock and vibration conditions including:
- 4,500 lbf force sustained vibration loads
- Up to 9,000 lbf shock loads
- 0-3,000 Hz Frequency Range
- Over 100g's peak acceleration
- Vibration: Sine, Random, Sine on Random, Random on Random
ACT’s shock test rig can produce a peak acceleration of Gpk=123g. The vibration capabilities include a frequency of 267 Hz, Grms=24g, and Gpk=45g. Examples of shock and vibration test profiles are shown in Figure 3.
Fig IV. (a) Shock Test Table (Left). (b) Vibration Test Table (Right).
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Fig IV. (a) Shock Amplitude vs. Time. (b) Acceleration vs. Time
Testing has confirmed that vibration loading has little or no impact on the performance of ACT’s heat pipes. Shock and vibration testing showed no evidence of overstress or fatigue on the heat pipes or solder joints.
V. Acceleration
As long as the wick’s capillary force is greater than the pressure drops and the acceleration loading, the heat pipe will perform properly under various acceleration loadings. However, extremely large adverse acceleration loadings may overwhelm the wick’s capillary capability, de-priming the wick or eventually causing the wick to dry out. If the acceleration is for short durations, the wick structure will re-prime and the thermal transient may be within an allowable range.
An alternative approach will be required if the transients are for longer durations. If the axis and direction of acceleration are known the heat pipes can be configured such that acceleration helps return the condensed fluid “gravity aided”. If the acceleration axis is unknown heat pipes can be arranged in pairs so that regardless of the acceleration vector one heat pipe will always be “gravity aided”.
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VI. Frozen Start Up
Many military and commercial applications specify temperatures ranging from -45˚C to + 70˚C. Water heat pipes are typically used in these applications because of their proven reliability and capability. The heat pipes must be able to operate with full capacity at the higher end of the temperature range to provide the required cooling. Frozen start up can be an issue if the system thermal mass and heat transfer are such that the fluid in the evaporator is thawed and vaporized by the heat input, travels to the condenser and freezes there. This could result in the depletion of fluid in the evaporator, eventually shutting down the heat pipe.
This is a system design issue and not typically a heat pipe limitation. There are four ways to address this issue. First, design the system so that frozen start up is not an issue. In other words, the input power and vapor transport are sufficient to thaw the entire system. Second, use active controls such as turning off fans to limit heat transfer in freezing conditions. Third, design in a secondary heat transfer mechanism so that the heat pipes are not needed to prevent device from overheating in freezing ambient conditions. Fourth, add a predetermined amount of NCG to the heat pipe to ensure “orderly” freezing and thawing. Options one and three are typical in most assemblies by default, but can be assured through analysis and testing.
VII. Thermal Cycling
Heat pipes utilize a wick structure to transport the liquid working fluid from the condenser to the evaporator. When properly made, the working fluid fully saturates the wick without making a puddle of excess fluid. With the fluid completely contained within the wick, it is not able to bridge the gap across the inside diameter of the heat pipe. This allows multiple freeze thaw cycles to occur without heat pipe deformation. A variety of working fluids may be used which directly affects the freezing temperature of the heat pipe. ACT routinely subjects heat pipes to thermal cycling to meet customer requirements. Typical freeze thaw tests are conducted from temperatures ranging from -20 to +20°C and -45 to +125°C. ACT has tested heat pipes up to 1,200 cycles, but 50-300 cycles are a more standard practice. Heat pipes may be thermally cycled prior to installation into assemblies. Heat pipe assemblies are also thermally cycled in assembled units to assure system level performance. Below are three examples:
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• Heat Pipes. ACT conducted tests to collect data on heat pipe thermal cycling survivability. The data set for these experiments used both fabricated flattened and bent 4mm heat pipes as well as 0.25” diameter copper water heat pipes. Heat pipes were exposed to as many as 1200 freeze thaw cycles without deformation or performance degradation.
• AlSiC HiK Plates. This project developed an innovative low-CTE heat spreader by embedding heat pipes into AlSiC plates. These plates showed similar effective thermal conductivity before and after 100 freeze/thaw cycles from -55°C to 125°C.
• Aluminum HiK Plates. In this project, copper water heat pipes are soldered into aluminum plates. Prior to fabrication, the heat pipes are screened by being exposed to 300 cycles from -20°C to +20°C. Once the assemblies were fabricated, the plates were exposed to an additional 50 cycles from -40°C to +75°C in two different orientations , (100 cycles total) to assure freeze/thaw survivability.
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3.5 OBSERVATION
3.5.1 Table
Serial No. Boiling Water temp(F) Cooled Water temp(F) Time(mins)
1 102.4 100.1 5
2 104.0 102.2 8
3 106.5 104.5 11
4 108.0 106.3 13
5 109.0 106.7 15
6 110.5 108.7 17
7 111.9 109.4 19
8 113.2 111.4 21
9 114.3 114.1 23
10 115.2 115.7 25
11 116.1 115.9 27
12 118.0 117.9 29
13 118.9 117.7 31
14 119.7 118.0 33
15 120.7 118.2 35
16 122.2 119.6 37
17 124.7 121.8 39
18 127.3 124.7 41
Table 3.5.1:- Data obtained during conducting the experiment.
Here the experiment is performed on 28th APRIL 2012 due to which we got the cooled water temperature as well as effective temperature with respect to boiled water temperature with the help of a digital thermometer.
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The set-up of the experiment is shown below:
Fig3.5.1: Experimental Set-Up Of Heat Pipes.
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3.5.2 GRAPH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180
20
40
60
80
100
120
140
ORIGINAL TEMPERATUREFINAL TEMPERATURE
Fig3.5.2: Bar- Chart.
Here a bar Graph is plotted between Original Temperature (boiled water temperature) and Final Temperature(cooled water temperature) as shown above.
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Also another Graph is plotted where difference of Hot water and cooled water is taken on y-axis and time t(in mins) is taken on x-axis which is shown below:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170
5
10
15
20
25
30
35
40
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DIFF BETWEEN HOT WATER AND COLD WATER(T)TIME(MINS)
Fig3.5.2: Graph between time and temperature.
Here we calculated the effective temperature(how much degree is cooled with the help of 6 copper bars) by difference of boiled water temperature and cooled water temperature. However , if we use copper bars more than 6, let’s say 40 then we will get more temperature difference.
3.6 ADVANTAGES
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The advantages of Heat Pipes is as under:
Less temperature difference needed to transport heat than traditional materials (thermal conductivity up to 90 times greater than copper for the same size) resulting, in low thermal resistance.
Heat pipes enable devices with higher density heat dissipation requirements and greater reliability.
Low cost i.e it is more cheap and reliable and efficient.
Proven alternative to conventional methods of electronics cooling.
3.7 GALLERY
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3.8 Conclusion
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The above analysis presented the constructional and economic aspects of Heat Pipes. Thus, to ensure the feasibility and economy of usage of Heat Pipes we need to adopt proper design of Heat Pipes depending upon the instructions introduced by the government.
ACT has experienced engineers to design, analyze, and integrate heat pipe based thermal solutions for a wide range of applications. Expertise includes designing optimal heat pipes based on proven compatible fluids, analyzing heat pipe limitations, and manufacturing heat pipes to the highest quality standards. ACT also has extensive testing capabilities including shock, vibration, acceleration, and freeze/thaw tests. ACT has designed, manufactured and delivered heat pipe products for numerous commercial, military and aerospace systems.
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CHAPTER 4
CONCLUSION
4.0 Introduction
This chapter deals with the observation as observed while conducting the experiment of Heat Pipes. Also table are done as well as graphs are plotted for it. This section mainly consists of advantages, disadvantages , applications , results and discussion and lastly future aspect of the Heat Exchanger Pipes. and its applications.
4.1 FUTURE WORK
Advanced Cooling Technologies, Inc. (ACT) has worked extensively on heat pipes product . ACT has been developing new material systems for emerging applications. ACT maintains a large number of life test heat pipes made of various material/fluid combinations. ACT has designed, manufactured and delivered heat pipe products for numerous commercial, military and aerospace systems in India.
It has provided solutions to a number of systems which are as under:
1. Laptop.2. Notebook and Mobile PC’s.3. CPU Work Stations.4. WORK STATIONS 75 TO 100 WATTS.5. 500 MHz OPERATING SYSTEMS.etc
So it is necessary to manufacture more Heat Exchanger Pipes with more advancement so that it is readily available and comes with less cost so that more customers will get attracted to it. It must sustain itself from adverse circumstances and should have longer life. Hence it must be reliable .
Futhermore, it is necessary to manufacture such products which will provide benefit to the nation .So, Govt. must provide more funds to achieve it. There should be a nationwide manufacture unit to manufacture such pipe products so that it will benefit as many people as required.
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Hence expertise should be able to design optimal heat pipes based on proven compatible fluids, analyzing heat pipe limitations, and manufacturing heat pipes to the highest quality standards.
4.2 Discussion
By means of knowledge about the theory of heat pipes, one is now able to carry out a basic calculation of a heat pipe. Beginning with a definition of the problem, including the temperature difference and the heat flow which has to be transported, one can define all the demands on a heat pipe.
Going through all the steps by an example shows the major steps:
- Defining the temperature working range (0°C-100°C).
- Choosing a fluid based on (Water).
- Choosing a material which works with water (aluminum, nickel, steel), depends also on economic or weight issues.
- Checking if one does not reach any of the main limitations.
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4.3 CONCLUSION
All in all it is necessary to understand all the basic theories of heat and mass transfer to understand the working principle of a heat pipe. On a first look a heat pipe seems to be a very easy tool to transport energy, but if one looks closer, it is a very complex heat and mass transfer process which takes place in a heat pipe. First of all one has convective heat transfer in the adiabatic transport range, and one has convection through porous materials also. The second major point is mass transfer due to vaporization and condensation, also through porous media. Furthermore there are capillary effects, pressure effects and heat conduction effects involved, which creates a complex structure of heat transfer, where a lot of knowledge is involved. And all of these points can be treated as a own problem, from this follows that a complete understanding of all involved processes needs more time and space than it is available for this project report.
With the help of this chapter we came to know about the future aspect of Heat Exchanger Pipes. Hence different programs should be initiated by the government. This will not only reduce the burden for the conventional way but also help in development.
2. Intermediate Temperature Fluids Life Tests - Experiments, William Anderson, et al., 2007 International Energy Conversion Engineering Conference, St. Louis, MO, June 2007.
3. Intermediate Temperature Fluids Life Tests - Theory, Calin Tarau, et al., Space Technology and Applications International Forum (STAIF), Albuquerque, NM, February 11 - 15, 2007.
4. High Temperature Titanium-Water and Monel-Water Heat Pipes, William Anderson, et al., 2006 International Energy Conversion Engineering Conference, San Diego, CA, June 2006.
5. High-Temperature Water Heat Pipes, David Sarraf and William Anderson, IMAPS International Conference on High Temperature Electronics, Santa Fe, NM, May 15 - 18, 2006.
