Heat pipes in modern heat exchangers

Applied Thermal Engineering 25 (2005) 1–19 www.elsevier.com/locate/apthermeng

Review

Heat pipes in modern heat exchangers Leonard L. Vasiliev

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A.V. Luikov Heat and Mass Transfer Institute, Academy of Science, 15 P.Brovka str., 220072 Belarus Minsk, Russia Received 16 June 2003; accepted 8 December 2003 Available online 28 August 2004

Abstract Heat pipes are very flexible systems with regard to effective thermal control. They can easily be implemented as heat exchangers inside sorption and vapour-compression heat pumps, refrigerators and other types of heat transfer devices. Their heat transfer coefficient in the evaporator and condenser zones is 103 – 105 W/m2 K, heat pipe thermal resistance is 0.01–0.03 K/W, therefore leading to smaller area and mass of heat exchangers. Miniature and micro heat pipes are welcomed for electronic components cooling and space two-phase thermal control systems. Loop heat pipes, pulsating heat pipes and sorption heat pipes are the novelty for modern heat exchangers. Heat pipe air preheaters are used in thermal power plants to preheat the secondary–primary air required for combustion of fuel in the boiler using the energy available in exhaust gases. Heat pipe solar collectors are promising for domestic use. This paper reviews mainly heat pipe developments in the Former Soviet Union Countries. Some new results obtained in USA and Europe are also included. 2003 Elsevier Ltd. All rights reserved. Keywords: Heat pipes; Heat pipe panels; Thermosyphons; Loop heat pipe; Pulsating heat pipe; Sorption heat pipes; Heat exchangers; Electronic cooling

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

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Conventional heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Miniature and microheat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

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Vapor-dynamic thermosyphons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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Tel./fax: +375-17-284-2133. E-mail address: [email protected] (L.L. Vasiliev).

1359-4311/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2003.12.004

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L.L. Vasiliev / Applied Thermal Engineering 25 (2005) 1–19 4.

Loop heat pipes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

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Aluminium heat pipe panels, pulsating 5.1. Heat pipe panels . . . . . . . . . . . 5.2. Pulsating heat pipes. . . . . . . . . 5.3. ‘‘Spaghetti’’ heat pipes . . . . . . .

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Sorption heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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Heat pipe application in thermoelectric coolers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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Heat pipe heat exchangers for air preheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

and ... ... ...

‘‘spaghetti’’ heat pipes . ................. ................. .................

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction Recent heat pipe (HP) R&D (conventional heat pipes, heat pipe panels, loop heat pipes, vapour-dynamic thermosyphons, micro/miniature heat pipes, sorption heat pipes, etc.) [1], oriented on different industry application is listed in this paper. Sorption technologies with heat pipe thermal control are efficient with an emphasis on implementation and integration of sorption machines in industry, as well as domestic, commercial, administrative buildings and transport for heating, cooling, air-conditioning, heat recovery and use of waste heat [2–5]. The heat pipe thermal control system is the key element of the heat pump, refrigerator, heat transformer, and gas and energy storage devices. Sorption machines with heat pipes as heat exchangers have some advantages such as short time cycle, improved compactness of cascading machines (less intermediate elements), increased coefficient of performance (COP) due to intercascaded heat recovery. Sorption machines with heat pipe thermal control can be used combined with solar energy, process heat recovering (waste heat), or a burning fuel as their driving energy source. In some cases there is a possibility to combine the application of different energy sources (solar/ gas, solar/electricity) in the same prototype of heat pump due to heat pipe application as heat exchangers. Actually an investigation of hydrocarbons boiling heat transfer related to applications of heat pipes and thermosyphons as thermal control systems in refrigeration technology, liquid hydrocarbons gasification, electronic components, fuel cells, etc. is important. Over the past decade, compact heat pipe heat exchangers for miniature heat pumps and refrigerators, gas and cold storage systems with dimensions in the order of centimeters are welcomed mainly due to their high thermal efficiencies, small size, low weight, and design flexibility. In many branches of industry, low temperature heads and small heat fluxes characterize operating conditions of heat pipe heat exchangers. This is also relevant to installations for hydrocarbon gasification. Possible applications of miniature two-phase heat exchangers could be space heating or cooling in vehicles or buildings, microprocessor cooling, fuel cells for transport and portable cooling devices. Multi-cascade machines based on heat pipe heat recovery have a low maintenance cost,

L.L. Vasiliev / Applied Thermal Engineering 25 (2005) 1–19

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are reliable and may require low driving temperatures [6]. Sorption technologies are convenient for adsorption refrigeration and ice making systems [7,23]. The fishing industry in tropical and developing countries is often an important part of the food and income supply. A large proportion of fishing in these countries is operated by small-scale fishing vessels, mostly open boats with no refrigeration on board and fish is often not iced onboard. It is widely and publicly recognized that post-harvesting losses are 20–30% in these countries due to improper handling of the fish mainly because there is no on-board chilling. This results in a loss of food supply, loss of export revenues and improper utilization of a limited natural resources. Heat pipe R&D was realised to perform the coupling between the heat pump and refrigerator topping and bottoming cycles, stimulate the heat and mass transfer in solid sorber designs, solar refrigerators to heat sorbers and to cool condensers using heat pipe systems [9–14,17–21].

2. Conventional heat pipes Conventional heat pipes (Fig. 1) with sintered metal powder wick inside saturated with liquid are convenient as heat transfer devices [9]. A very important feature of the HP is the ability to transport a large amount of energy over its length with a small temperature drop by means of liquid evaporation at the HP evaporator (heat source), vapour condensation at the condenser (heat sink) and liquid movement in the opposite direction inside a wick by capillary force. Essential is the possibility to change the direction of heat flow along the HP in time and to use HPs for cooling and heating alternately. If the HP envelope and the HP wick have a small heat capacity there is a basic possibility to realize the transient operation of HP in solid sorption machines when there is a necessity to heat and to cool the sorbent bed as quickly as possible. For example, a simple one-stage resorption heat pump (Fig. 2) with a heat pipe thermal control system is composed only of two reactors filled with NiCl2 /‘‘Busofit’’ and BaCl2 /‘‘Busofit’’ [13], saturated with ammonia. The active carbon fiber ‘‘Busofit’’ is an efficient sorbent material with high gas permeability, being easily impregnated by salts. The device is functioning in gaseous phase with coefficient of performance COP ¼ 1.44. The heat pump, Fig. 3 has two adsorbers (sorbent beds) 1 and 2, connected with condenser 3 and evaporator 4 through four valves 5–8. The sorbent beds are heated and cooled by HPs 13 and 14, which have HP evaporators and condensers in series. The HP evaporators are heated by electric cartridge heaters 11–12. When one adsorber is heating, the second is cooling by the liquid

Fig. 1. Conventional heat pipe schematic. (1) HP case, (2) porous wick, (3) vapour channel, (4) vapour, and (5) liquid.

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Fig. 2. Resorption two reactors cooler with heat pipe thermal control.

Fig. 3. Sorption heat pump with two reactors and heat pipe thermal control. (1,2) Adsorbers; (3) condenser; (4) evaporator; (5–8) valves; (9–10) liquid heat exchangers on the outer surface of the heat pipe condensers; (11,12) heat pipe evaporators with the electric heaters on its outer surface; (13,14) copper–water heat pipes; (15) expansion valve; (16,17) reversing valve; (18) liquid pump; (19) liquid flow meter; and (20) thermostat.

heat exchanger. The HP condensers 9–10 are cooled by the liquid (oil or water) loop connected to a liquid pump 18. The system of liquid loops gives us the possibility to accomplish the regeneration through the use of a heat transfer liquid loop [8]. 2.1. Miniature and microheat pipes Now, one of the directions of heat pipes development is miniature heat pipes (mHP), both for passive systems for electronics cooling and for use in refrigerating machines. High-performance mHPs have been designed and manufactured in the Luikov Institute, Belarus since 1998. Optimisation of the new copper sintered powder wick in miniature heat pipes with outer diameter 4


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mm and length of 200 mm was carried out. The maximum heat transfer rate for these HPs is almost 50 W [14]. Software was developed and used for prediction of round and flat heat pipes (including mHP) characteristics [15]. Heat pipe family qualified geometry is: circular tube diameter 4–25 mm, flat heat pipe thickness 2–20 mm, length 0.1–0.8 m, wall thickness 0.2–1.0 mm. Pipe material––copper 99.95% purity, wick––copper sintered powder, wire mesh and wire bundle with thickness 0.2–0.8 mm. Transport capacity 10–500 W. Water, methanol and propane are used as working fluids. The heat pipe mathematical model developed includes heat pipe parameters: Input: heat pipe geometric parameters; capillary structure parameters, working fluid properties; material properties; and heat flow. Output: maximum heat flow rate Qmax along the heat pipe vs. the temperature of the working fluid; capillary and boiling limits; heat pipe axial temperature profile, temperature drop between the evaporator and condenser. The results of numerical modelling were verified by the experimental data with an accuracy of ±10%. A new type of miniature heat pipe is currently being investigated by the NASA Glenn Research Center, USA [28]. It is called the microloop heat pipe in silicon. The principle application will be electronic cooling at the chip level. The heat pipe evaporator is constructed of silicon, such that there will be little thermal interface resistance between the source of the heat generation, the computer chip junction, and the working fluid. The ultimate heat sink could be a box-level spacecraft thermal bus or even the spacecraft radiator, depending on transport length and compensation chamber size. The device utilizes a coherent porous silicon (CPS) wick that provides small effective pore radii. This new technology is a type of microelectro mechanical systems (MEMS) process that allows one to ‘‘drill’’ a pattern of micron-sized holes in a silicon wafer. The review of microheat pipes was published in [29].

3. Vapor-dynamic thermosyphons Vapor-dynamic thermosyphons [16] and loop heat pipes (LHP) [30] can provide the coupling between topping and bottoming sorption cycles (Fig. 2) [8,30]. The direct coupling ensures the operating temperatures in both cycles more favourable from the thermodynamic point of view since temperature drops are definitely smaller compared with conventional heat exchangers. Such thermosyphons (Fig. 4) have a low thermal resistance, their length can reach some meters and they have the ability to transport energy to sorbent media being heated by hot gases, or flames. In vapour-dynamic thermosyphons vapour and liquid flows are separated by the wall, heat transfer is realized in the gap between the inner and outer tubes. Vapour condensation is performed on the inner surface of the outer condenser tube. The vapour-dynamic thermosyphons in order to avoid the flooding limit and increase the maximum performance have tube separator inside used as vapour channel and a two-phase coaxial annular channel around this separator where the vapour condensation is produced with high efficiency [16,17]. Vapour-dynamic thermosyphons can transport up to 10 kW and more for several meters distance with its thermal resistance R ¼ 0:03–0:05 K/W of heat, which is difficult to acheive in conventional thermosyphons located horizontally.

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Fig. 4. Water/SS ‘‘vapour dynamic’’ thermosyphon. (1) Electric heater; (2) mini-boiler; (3) condenser; (4) feeding liquid tube; (5) vapour passage; (6) trap for gas; (7) water heat exchanger; (a) water; (b) vapour; and (c) non-condensable gas NCG.

The advantages of this thermosyphon are: • high heat transfer performance due to the vapour and liquid flow separation, there are no interface friction losses; • low thermal resistance of thermosyphon; • vapour flow in the co-axial gap push the non-condensable gases to the gas trap, that means the thermosyphon is eager to work with non-condensable gases inside (Fig. 5); • the ability of the thermosyphon to transfer the heat over a long distance in a horizontal direction, which is difficult to achieve by conventional thermosyphons.

Fig. 5. Thermal resistance R of ‘‘vapour-dynamic’’ thermosyphon as a function of heat input Q. (1) Water; (2) HCFC 22; (3) water with air; and (4) HCFC 22 with an air in the gas trap.


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Fig. 6. Schematic diagram of the vapour-dynamic thermosyphon with two condensers and two valves for sorption heat pump. (1) Adsorber, (2) valve, (3) liquid pipe, (4) vapour channel, (5) electric heaters, and (6) mini-boiler.

An example of vapour-dynamic thermosyphon with two condensers is shown in Fig. 6. The feature of this system applied to the sorption heat pump thermal control is the periodical switching on and off of two condensers, while the evaporator is operating continuously, [18,19]. 4. Loop heat pipes Capillary pumped loops (CPL), and loop heat pipes (LHP), Fig. 7 are an attractive alternative for heat regulation [30]. In the LHP the capillary pumped evaporator (Figs. 8 and 9) is used instead of a boiler. Such an evaporator is more flexible from the point of view of its orientation space and is more compact. In the LHP there is a possibility to use an evaporator above the condenser. In the LHP the vapour flows through the vapour channels towards the condenser and the liquid goes back the evaporator due to the capillary pressure head of the porous wick. In the near future an LHP should be used as thermal control devices in scientific and telecommunication

Fig. 7. Loop heat pipe with two evaporator/condensers, liquid and vapor lines.

Fig. 8. Evaporator/condenser of a loop heat pipe.

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5

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Heat transfer coef., 10 ,W/ m2 K

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4

3

2

1 2

3

4

5 4

6

2

Heat flux, 10 ,W/m

Fig. 9. Heat transfer coefficient as a function of a heat flux in LHP evaporator.

satellites (efficient and flexible thermal link between dissipative elements and radiators). From a thermal modeling point of view, on the one hand the simple spreadsheet analysis is only convenient at the LHP, or thermal subsystem level. On the other hand, a complete thermohydraulic analysis tool could be heavy and time-consuming for system level modeling (ESATAN-FHTS, SINDA-FLUINT). LHPs with flexible transport lines are needed in some cryogenic applications as thermal links between cryocoolers and the cooled components for a variety of reasons such as vibration isolation, increased thermal transport distance, thermal diode function, multiple components, etc. One additional technical requirement for cryogenic LHPs is that they should be capable of cooling down from a room temperature, which is above the critical temperature of the working fluid, to the operating cryogenic temperature with only the condenser end being cooled by a cryocooler. This requirement results in a difference in the configuration of cryogenic LHPs compared to room-temperature LHPs. The LHP evaporator can be applied as an element of refrigerator contour or as a condenser too (Figs. 10 and 11) [11,12]. The inner part of this evaporator is made of Ti (or Ni) sintered powder wick with a central tube for liquid flow and vapour flow channels on the inner surface of an SS tube or on outer surface of the wick. This evaporator has two separate tubes, the vapour tube and the liquid tube. These two tubes are separated by Ti sintered powder wick as capillary barrier, and the vapour tube is maintained at higher temperature than the liquid tube. The LHP sintered wick structure of the evaporator has a porosity of 45%: the length of the unit is 280 mm, the outer diameter––38 mm, evaporator body material––stainless steel (SS), maximum diameter of pores––10 lm, medium diameter of pores 3–5 lm, capillary pressure head 0.4 bar and the porous wick thickness––8 mm. Evaporators are compatible with water, ammonia, methanol, ethanol, acetone and methane. The maximum heat flow rate of the evaporator is 1500 W, the thermal resistance of the evaporator R, ¼ 0.06 K/W. Such a design of the condenser and evaporator has some advantages with the comparison of the shell and tube condensers and evaporators. These systems are considerably lighter and smaller.


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Fig. 10. Relation between the vapour temperature Tv and Q for different heat transfer direction and the HP inclination angle u ¼ 90 (evaporator above the condenser); Tcool ¼ 10; (1,2) different directions of heat transfer.

Fig. 11. Relation between the vapour temperature Tv and Q for different heat transfer direction and the HP inclination angle u ¼ 0 (horizontal position of HP); Tcool ¼ 8; (1,2) different directions of heat transfer.

The performance of the evaporator depends on the transport properties of the wick, i.e., permeability, thermal conductivity as well as structural characteristics of the wick: homogeneous or heterogeneous porous system, narrow or wide size distribution of the pores. The most convenient wick structure needs to have large pore diameter layer with high thermal conductivity from the vapour side and small pore diameter layer with low thermal conductivity from the liquid side. The LHP condenser is usually a tube-in-tube (coaxial) type, or it is made of a SS envelope with narrow passages (Figs. 12 and 13), or with a porous structure (Fig. 8). Fig. 12 shows the condenser’s basic elements. They are: an aluminium or SS shell with liquid cooling tube inside and capillary mini grooves in the vapour channels on the inner surface of the

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Fig. 12. Schematic of the element of a condenser with narrow passages and arteries the porous body is used as a liquid accumulator.

Fig. 13. The specific heat flux effect on heat transfer in the condenser with narrow passages and arteries.

outer tube or on the surface of the aluminium body; capillary arteries for liquid film formed on the grooved surface of the shell metal (or sintered metal) body; narrow passage slots connecting the arteries to the surface for vapour condensation. The aluminium condenser has the following parameters: inner diameter––16 mm, outer diameter––32 mm, number of vapour channels––12 mm, diameter of capillary arteries––1 mm, width of narrow passages about 0.05 mm and width of triangular grooves about 1 mm [18].

5. Aluminium heat pipe panels, pulsating and ‘‘spaghetti’’ heat pipes 5.1. Heat pipe panels Another alternative to the conventional heat pipe is an aluminium (multi-channel) heat pipe panel (Fig. 14) with propane as a working fluid to cool the low temperature sorbers of heat pumps


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Fig. 14. Aluminium multi-channels pulsating heat pipe panel with propane as a working fluid and silica gel monolithic sorption bed on its finned surface.

and refrigerators [21]. The main parameters of flat heat pipe panels are: HP width––70 mm, HP height––7 mm, HP length––700 mm, evaporator length––98 mm, condenser length––500 mm, mass––0, 43 kg. HP thermal resistance R ¼ 0:05 K/W, evaporator heat transfer coefficient a ¼ 8500 W/m2 K, condenser heat transfer coefficient a ¼ 2500 W/m2 K. Heat pipe panels are convenient as thermal control systems for the electronic components, heat pumps and refrigerators with efficient heat recovery between different sorption cycles. The working fluid (hydrocarbons) dynamic movement is stable with liquid filling ratio near 0.6 of the heat pipe volume. The propane as a good alternative to water for such heat pipes enables a continuous motion due to the interplay between the driving and restoring forces. 5.2. Pulsating heat pipes Pulsating heat pipes (PHPs) [31] have also emerged as interesting alternatives to conventional heat pipes. PHPs have complicated thermohydrodynamic operational characteristics. In fact, it is rare to find a combination of such events and mechanisms like bubble nucleation and collapse, bubble agglomeration and pumping action, flow regime changes, pressure/temperature perturbations, dynamic instabilities, metastable non-equilibrium conditions, flooding or bridging etc., all together contributing towards the thermal performance of a device. Recent literature suggests that important milestones have been achieved in characterization of these devices [32,33]. The pulse thermal loop (PTL) is one of several oscillatory thermal transport cycles under development that are receiving attention as a potential semi/passive, high-power, high flux heat transport device. The PTL is unique in that it is capable of generating driving pressures in excess of many mechanically pumped loops. Capillary forces do not limit the PTL and it is capable of transferring high heat loads over long distances and against significant resistance (i.e. gravity, small tube diameters, etc.). An analytical and experimental investigation is underway as part of NASA’s Advanced Technology Development Program to identify the performance limits of the pulse thermal loop as a function of system scale [28] (Fig. 15). 5.3. ‘‘Spaghetti’’ heat pipes The small diameter (3 mm) bendable SS ‘‘spaghetti’’ heat pipes are similar to pulsating heat pipes, but have a compact condenser and large surface evaporator. An example of a ‘‘spaghetti’’

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Fig. 15. Two configurations of pulsating heat pipe: (i) open loop an (ii) closed loop.

heat pipe filled with ammonia, shown in Figs. 16 and 17 [11,12,18,19] is disposed inside the refrigerator chamber in such a way that food can be kept within the refrigerating temperature range as uniformly as possible. The ‘‘spaghetti’’ heat pipe is thermally linked with an evaporator of the sorption refrigerator (heat pipe condenser) and has a good thermal contact with this evaporator [18,19]. The ‘‘spaghetti’’ heat pipe panel is used as uniform temperature sheets inside the cold chamber connected with cold store unit (sorption refrigerator, cold accumulator, or dry ice box).

Fig. 16. Schematic of the ‘‘spaghetti’’ heat pipe panel: (1) condenser of the heat pipe, (2) evaporator of the adsorption refrigerator, (3) porous structure, and (4) heat pipe evaporator.


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Fig. 17. ‘‘Spaghetti’’ heat pipe panels inside the refrigerating chamber.

Such types of heat pipes have no capillary structure inside and are functioning under the oscillating motion of the two-phase ammonia due to a big difference between the liquid and vapor density under the heat load. The driving force of ‘‘spaghetti’’ heat pipes is the pressure force generated by the liquid boiling and vapor generation at high temperature zones (lower part of the refrigerator cabinet), non-equilibrium state between vapor and liquid and vapor bubbles collapse in the upper cold part of the panel. Vapor plugs (bubbles) push the liquid plugs to the cold part of the unit, where vapor bubbles collapsed with the increasing of pressure difference between vapor and liquid. Heat pipe thermal resistance is R ¼ 0:006 K/W. Heat pipe length is 1 m, and heat pipe width is 0.5 m.

6. Sorption heat pipes The sorption heat pipe (SHP) is a novelty and combines the enhanced heat and mass transfer in conventional heat pipes with sorption phenomena of a sorbent bed. Sorption heat pipe could be used as a sorption heat transfer element and be cooled and heated as a heat pipe [3,20,34]. The sorption heat pipe (Fig. 18) has a sorbent bed (adsorber/desorber and evaporator) at one end and a condenser and evaporator at the other end. The basic principle of the sorption heat pipe operation is:

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Fig. 18. Sorption heat pipe. Longitudinal cross section, [3]. (1) Vapour channel; (2) porous sorption structure; (3) finned surface of heat pipe evaporator/condenser; (4) porous wick inside heat pipe; (5) porous valve; (6) heat pipe low temperature evaporator with porous wick; (7) working fluid accumulated inside the evaporator; and (8) cold box with thermal insulation.

Phase 1. At the beginning of the heat pipe functioning it is necessary to desorb a sorption structure (2) (Fig. 18) of the heat pipe due to absorption of the heat of a heat source. During desorption of a sorbent bed the working fluid vapor (1) needs leave a porous structure (2) and be condensed in the heat pipe evaporator/condenser (3). The vapor is generated inside the porous structure of a sorbent bed, the vapor pressure is increasing, and the vapor flow enters the condenser and is condensed, releasing heat to the surroundings. Part of the cold working fluid is filtered through the porous valve (5) and enters the evaporator due to the pressure drop between the hot part of a heat pipe and the evaporator. The other part of the working fluid is returned to the sorbent bed due to capillary forces of the wick (4) and increases the procedure of sorbent bed heating by the heater, following the microheat pipes phenomena inside the sorbent bed. When desorption of the sorbent structure is accomplished, the source of energy is switched off, the pressure in the sorbent bed decreased and the working fluid is accumulated inside the evaporator. Phase 2. After Phase 1 the porous valve (5) is opened and the vapor pressure inside the heat pipe is equalises following the procedure of the liquid evaporation inside the porous structure of the evaporator (6). During the liquid (7) evaporation in the evaporator (6), the cold generation is available inside the cold box (8). When the liquid evaporation is accomplished and the sorbent bed is saturated with the vapor, a porous valve is closed and the sorbent bed begins to be cooled with the help of the heat pipe condenser (3). Phase 2 is completed. The experimental set-up is shown in Fig. 19. The sorption heat pipe is insensitive to some ‘‘g’’ accelerations and it is suggested for space and ground applications. This system is composed of a loop heat pipe (LHP), or capillary pumped loop (CPL) and a solid sorption cooler. The LHP can be transformed into an SHP with the same evaporator, but the SHP has one or more sorption beds. The SHP extends the limits of two-phase thermal control and ensures a successful electronic components cooling even in very harsh environmental conditions (ambient temperature 40 C, or more) and also ensure deep cooling of space sensors, down to the triple point of the hydrogen (Fig. 20). 7. Heat pipe application in thermoelectric coolers It is necessary to note that heat pipes can be applied with success in other types of refrigerating machines, for example in thermoelectric refrigerators [1]. Properties of heat pipes here are used to transfer and to transform heat flow. Sometimes the area of surface of heat input and heat output


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Fig. 19. Schematic of sorption heat pipe experimental set-up. (I) Heat pipe evaporator, (II) heat pipe condenser, (III) liquid cooler, (IV) sorption canister. (1) Heat pipe envelope, (2) capillary-porous wick, (3) vapor channels disposed along the heat pipe envelope, (4) liquid compensation chamber inside the porous wick, (5) electric heater, (6) sorption canister, (7) sorption structure thermal control system, (8) heat pipe condenser, (9) liquid cooler, (10) valve, (11) regulated valve, (12) pressure sensor, (13) vacuum sensor, (14) valve for the fluid charging, and (11) 19 thermocouples.

need be not identical. So, the Peltier element size is small, and the surface of heat output should be large or have a specific form (Fig. 21). The combination ‘‘heat pipe––Peltier element’’ can be used in systems for processor cooling and in medical devices, for example in cryo-surgery. A device for local hypothermia with a heat pipe-based instrument was developed. It has successfully passed tests and is recommended for introduction to medical practice [22].

8. Heat pipe heat exchangers for air preheating Since the end of the 1970s there are some publications related to heat pipe heat exchangers [24]. A large number were published in the USSR [24–26]. The heat pipe or two-phase thermosyphon

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Fig. 20. Heat transfer coefficient in the LHP and SHP evaporator as a function of time t. LHP evaporator is functioning in the time 0–3000 s interval, SHP evaporator in working in the 3000–3500 s time interval.

Fig. 21. Heat pipe––Peltier element combination.

device is an important concept in heat exchangers, which can be used in different branches of industry such as metallurgy, power, oil-refining, glass, etc. It is possible to identify three main application areas for the waste heat recovery equipment: (1) use of process waste heat for preheating process supply air; (2) use process waste heat for space heating and air-conditioning; (3) recovery of exhaust heat from an air conditioning system in a commercial or domestic building for preheating supply air. The heat pipe heat exchanger used for gas–gas heat recovery is essentially a bundle of finned heat pipes assembled like a conventional air-cooled heat exchanger. The heat pipe in the heat exchanger can be divided in to three parts: evaporator, adiabatic section and condenser (Fig. 22). Passing hot flue gases over the evaporator causes the working fluid to boil and the vapors to flow to the cold end of the tube. Cold air flowing over the condenser in counter flow direction condenses the vapors releasing latent heat that heats the air. Inside a heat pipe, boiling and condensing heat transfer mechanisms transfer heat. For these mechanisms, heat transfer can proceed at extremely high rates as compared to conduction and/or convection. Wall grooves or wicks are used inside the heat pipes to improve wall wetting and heat transfer. As the heat transfer temperature range in the air preheater is 370–140 C, a single fluid cannot be used in all the heat pipes along the flow direction [24]. Water and toluene, normally, has been used as a working fluid inside the heat pipe for low temperature range (30–200 C). Dowtherm and naphthalene have been used for higher temperature range [25,27,35,36].


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Fig. 22. Heat pipe air preheater [27].

A heat pipe air preheater consists of two ducts with a common wall. Individual heat pipe tubes extend through the common wall across both ducts. Hot flue gases flow through one duct while cold combustion air flows through other duct. The tubes are usually seal welded or gasketed in some fashion at the common wall to prevent air leakage between the flue gas and air sections. The ends of the tubes are free to expand or contract within the duct casing. Providing fins can extend the individual tube surfaces and compact units can be designed. Heat transfer in a heat pipe air preheater can be calculated on the basis of thermal resistance circuit and the design is checked for critical heat transport limitations. Heat pipe heat exchangers have slightly lower efficiencies than some other gas–gas heat recovery systems, notably the heat wheel, but in common with tubular and plate recuperators have the advantages of zero crosscontamination, brought about by the presence of a splitter plate which effectively seals the inlet and exhaust ducts, and the fact that there are no moving parts, including pumps. 9. Conclusions A short review of heat pipe R&D contains mainly data from FSU 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. References [1] L.L. Vasiliev, State-of-the-art on heat pipe technology in the Former Soviet Union, Appl. Thermal Eng. 16 (7) (1998) 507–551. [2] R.E. Critoph, The use of thermosyphon heat pipes to improve the performance of a carbon–ammonia adsorption refrigerator, in: Procceedings IV Minsk International Seminar ‘‘Heat Pipes, Heat Pumps, Refrigerators’’, 12–15 September 2000, Minsk, Belarus, pp. 35–41.

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[3] L.L. Vasiliev, V.M. Bogdanov, USSR patent 174411 ‘‘Heat Pipe’’, B.I. No. 24, 30.06.1992, 1992. [4] C. Kren, C. Schweigler, F. Ziegler, Efficient Li–Br absorption chillers for the European air conditioning market, in: ISHPC’02 Proceedings of the International Sorption Heat Pump Conference, Shanghai, China, 24–27 September 2002, pp. 76–83. [5] L.L. Vasiliev, L.E. Kanonchik, A.G. Kulakov, A.A. Antuh, NaX zeolite, carbon fibre and CaCl2 /ammonia reactors for heat pumps and refrigerators, Journal of Adsorption, USA (2) (1996) 311–316. [6] B. Spinner, D. Stitou, P.G. Grini, Cascading sorption machines: new concepts for the power control of solid–gas thermochemical systems: towards sustainable technologies, in: Proceedings of the Absorption Heat Pump Conference, Montreal, Canada, vol. 2, 17–20 September, 1996, pp. 531–538. [7] R.Z. Wang, Adsorption refrigeration research in SJTU, in: Proceedings of IV Minsk International Seminar ‘‘Heat Pipes, Heat Pumps, Refrigerators’’, Minsk, Belarus, 4–7 September 2000, pp. 104–114. [8] V.A. Babenko, L.E. Kanonchik, L.L. Vasiliev, Heat and mass transfer intensification in solid sorption systems, Enhan. Heat Transfer 5 (1998) 111–125. [9] L.L. Vasiliev, L.E. Kanonchik, A.A. Antoukh, A.G. Kulakov, I. Rosin, Waste heat driven solid sorption coolers, SAE Technical Paper 941580, 24th International Conference on Environmental Systems and 5th European Symposium on Space Environmental Control Systems, Friedrichshafen, Germany, 20–23 June 1994. [10] L.L. Vasiliev, L.E. Kanonchik, A.A. Antuh, A.G. Kulakov, V.K. Kulikovsky, Waste heat driven solid sorption coolers containing heat pipes for thermo control, J. Adsorp. (USA) (1) (1995) 303–312. [11] L.L. Vasiliev, D.A. Mishkinis, A.A. Antuh, A.G. Kulakov, L.L. Vasiliev Jr., Heat pipe cooled and heated solid sorption refrigerator, in: Proceedings of the 19th International Congress of Refrigeration, vol. IIIa, Holland, 1995, pp. 200–208. [12] L.L. Vasiliev, D.A. Mishkinis, A.A. Antukh, A.G. Kulakov, L.L. Vasiliev Jr., Solid sorption machines on heat pipes, in: Proceedings of the International Forum ‘‘Heat and Mass Transfer––96’’, Minsk, Belarus, 1996, pp. 183– 192. [13] L.L. Vasiliev, D. Nikanpour, A. Antukh, K. Snelson, L. Vasiliev Jr., A. Lebru, Multisalt––carbon chemical cooler for space application, in: Proceedings of the International Sorption Heat Pump Conference, Munich, Germany, 24–26 March 1999, pp. 579–584. [14] L.L. Vasiliev, A. Antukh, V. Maziuk, A. Kulakov, M. Rabetsky, L. Vasiliev Jr., Oh Se MiN, Miniature heat pipes experimental analysis and software development, in: Proceedings of the 12th International Heat Pipe Conference ‘‘Heat Pipes Science, Technology, Application’’, Moscow–Kostroma–Moscow, Russia, 19–24 May 2002, pp. 329–335. [15] V. Maziuk, A. Kulakov, M. Rabetsky, L. Vasiliev, M. Vukovic, Miniature heat––pipe thermal performance prediction tool––software development, Appl. Thermal Eng. 21 (2001) 559–571. [16] L.L. Vasiliev et al., Heat transfer Device, US Patent No. 455966, November 26, 1985. [17] L.L. Vasiliev, L.A. Kanonchik, F.F. Molodkin, M.I. Rabetsky, Adsorption heat pump using carbon fiber/NH3 and heat pipes, in: Proceedings of the 5th IEA Heat Pump Conference, Toronto, Canada, 22–26 September 1996, pp. 35–43. [18] L.L. Vasiliev, D.A. Mishkinis, A.A. Antukh, L.L. Vasiliev Jr., Solar–gas solid sorption refrigerator, J. Adsorp. 7 (2001) 149–161. [19] L.L. Vasiliev, D.A. Mishkinis, A.A. Antukh, L.L. Vasiliev Jr., Solar–gas solid sorption heat pump, Appl. Thermal Eng. 21 (2001) 573–583. [20] L.L. Vasiliev, L.L. Vasiliev, Jr., Two phase thermal control system with a loop heat pipe and solid sorption cooler, SAE Technical Paper Series 2000-01-2492, 2000. [21] L.L. Vasiliev, Sorption machines with a heat pipe thermal control, in: Proceedings of the International sorption heat pump conference, Shanghai, China, 24–27 September 2002, pp. 408–413. [22] L.L. Vasiliev, A.S. Zhuravlyov, F.F. Molodkin, V.V. Khrolenok, S.L. Adamov, A.A. Turin, Medical heat pipe instrument for local cavitary hypothermia, in: Proceedings of the 1st International Seminar ‘‘Heat Pipes, Heat Pumps, Refrigerators’’, Minsk, Belarus, 12–15 September 1995, pp. 104–114. [23] H.F. Smirnov, B.V. Kosoy, Refrigerating heat pipes, Appl. Thermal Eng. 21 (2001) 631–641. [24] L.L. Vasiliev, Heat Pipe Heat Exchangers, Nauka i Technika, Minsk, 1981 (in Russian). [25] L.L. Vasiliev, V.G. Kiselev, Yu.N. Matveev, F.F. Molodkin, Heat Pipe Heat Exchangers, Nauka I Technica, Minsk, 1987 (in Russian).


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