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Research,development and industrial application of heat pipe technology in China


Applied Thermal Engineering 23 (2003) 1067–1083
www.elsevier.com/locate/apthermeng

Research, development and industrial application of heat pipe technology in China
Hong Zhang *, Jun Zhuang
Institute of Heat Pipe Technology, National Technology Research and Promotion Center for Heat pipe, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing 210009, PR China Received 6 January 2003; accepted 30 January 2003

Abstract This paper introduces some typical cases of industrial applications, which include the equipment for the waste heat recovery and the industrial process equipment. Carbon steel–water heat pipe technology, applied to air preheater and waste heat boiler, has been successfully used in many ?elds, such as waste heat recovery, energy conservation and environmental protection. Liquid metal high-temperature heat pipe technology has been extensively employed in the process equipment, for example, high-temperature hot air generators and heat extractors. Heat pipe technology also ?nds its use in chemical reactors including ammonia converters. The success of applications is founded on the basis of fundamental research of heat pipe technology, which includes the theoretical and experimental researches on the vapor–liquid two-phase ?ow and heat transfer inside the heat pipe, the heat transfer limits of heat pipes, the heat transfer enhancement with heat pipes, and researches on the material compatibility and life tests of heat pipes. Hie?cient heat pipe heat and mass transfer equipment is going to play a more and more important role in the various industrial ?elds. ? 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Heat pipe technology; Carbon steel–water heat pipe; Liquid metal heat pipe; Waste heat recovery; Industrial process; Heat pipe reactor

1. Introduction As a highly-e?ective heat transfer element, heat pipes have been gradually recognized, and are playing a more and more important role in almost all industrial ?elds. After more than 20 years of

*

Corresponding author. Tel./fax: +86-25-6637973. E-mail address: hzhang01@jlonline.com (H. Zhang).

1359-4311/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-4311(03)00037-1

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Table 1 Application of the heat pipe equipment in various industrial processes/set (NTRPC-HP) Type industrial ?eld Heat pipe gas– gas exchanger 119 36 20 20 195 372 Heat pipe Heat pipe steam generator gas–liquid exchanger 36 40 26 8 110 10 6 8 4 28 Separate type heat pipe exchanger 30 4 2 9 High-temperature heat pipe exchanger 3

Petroleum, chemical Metallurgy Construction material Power Sub-total Total

30

e?orts, China has successfully developed a series of heat pipe equipment, such as heat pipe gas– gas exchangers, heat pipe steam generators (waste heat boilers), high-temperature heat pipe (liquid metal heat pipe) steam generators, high-temperature heat pipe hot air furnaces, and has made remarkable progresses in the ?elds of metallurgical, petrochemical, chemical, power and construction material industries [1] on the basis of experimental and theoretical investigations. In this paper, the research work carried out in the National Technology Research and Promotion Center for Heat Pipe (NTRPC-HP) for the industrial applications of heat pipe technology. Examples of researches and applications are presented. Table 1 shows some applications of the heat pipe equipment in various industrial ?elds. The profound experimental researches on the heat pipe technology support its applications. These include the heat pipe waste heat recovery equipment for energy-saving and environmental protection, such as heat pipe air preheaters and heat pipe steam generators represented by carbon steel–water heat pipe technology, and the heat pipe industrial process equipment, such as highly-e?ective heat pipe exchangers, the high-temperature heat pipe hot air furnaces and high-temperature heat pipe heat extractors represented by liquid metal heat pipe technology, and the key industrial process equipment now being developed–– high-temperature and high pressure (480 °C and 32 MPa) heat pipe chemical reactors. All these demonstrate the broad prospects of application of heat pipe technology in the industries.

2. Some important features of application of heat pipe technology in industries [2] The extensive applications of heat pipe technology in industries with great prospects are based on the essential features of the heat pipe. These features, when combined with speci?c technical processes, bring into full play the superiority of the heat pipe technology and also solve the practical problems in industrial production in an e?cient and economic way. This is the key in the application of heat pipe technology in industries. The features include: (1) The high heat conduction performance of heat pipes. The heat pipe is a heat-conducting element with high heat transfer performance. It transfers heat via evaporation and condensation of the working ?uid in the fully-enclosed vacuum pipe, at a heat conductivity several times or even nearly 10,000 times better than that of good heat-conducting materials (copper, silver, etc.), hence the name ‘‘heat superconductor’’.

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(2) The dual-wall heat exchange characteristics of the heat pipe are an important guarantee for the safe, reliable and long-term operation. With traditional one-wall heat-exchange equipment, the equipment should be stopped for repair even when only one heat exchange element is damaged. This is not the case with the heat pipe equipment. Even if there is damage to a single heat pipe in the equipment comprising heat pipe bundles, the two di?erent types of heat exchange ?uids will not be mixed, and therefore the overall heat exchange e?ect will not be a?ected. (3) The heat ?ux exchange and self soot-blowing characteristics of heat pipe are important technical guarantee to prevent dew-point corrosion and dust clogging in industrial equipment. It has been proved that such accidents as deterioration of equipment e?ciency or even forced outage due to clogging and dew-point corrosion of large power station boilers, various industrial waste heat boilers in high dust content environment and other heat exchange equipment in dusty environment can be prevented and avoided when they are replaced with heat pipe heat exchangers. (4) Separate type heat pipe technology has made heat exchange possible at long distance where mixing is not allowed and where multiple heat sources (or heat sinks) are used, and can therefore successfully overcome the di?culty in heating the gas and air at the same time for the blastfurnace ?ue gas in iron melting and metallurgical industries. (5) The homogeneous temperature and heat shielding performance of heat pipes can solve such problems as non-homogeneous temperature distribution in a chemical reactor and reaction deviating from optimum reaction temperature, the overheating decomposition due to uneven pipe wall temperature in petroleum crackers and heat dissipation for the nuclear reactor vessel body. (6) Liquid metal heat pipe technology has made high-temperature heat exchangers safer, more compact and more e?cient. The use of liquid metal heat pipes and reduction in material prices will make it possible to realize the continuous heat extraction in super high-temperature reaction equipment, such as continuous gas production in the coal gasi?cation, and new type heat pipe steam generators in nuclear power plants.

3. Industrial applications of carbon steel–water heat pipe technology On the basis of theoretical research, experimental study by simulating actual working conditions is a necessary step in realizing the industrial applications of heat pipe technology. With the issue of carbon steel–water heat pipe compatibility being solved, the cost of heat pipe equipment has greatly dropped. This, plus a number of superior properties of heat pipe technology, has enabled the extensive application of carbon steel–water heat pipe heat exchangers in industries. At present, heat pipe waste heat recovery equipment for energy-saving and environmental protection, such as heat pipe air preheater and heat pipe steam generator on the basis of the carbon steel– water heat pipe technology have become mature, and have found wide application in petroleum, chemical, metallurgical, power and building material industries. 3.1. Research and industrial application of carbon steel–water heat pipe steam generator In-depth studies have been made on the heat transfer performance of carbon steel–water heat pipes and its limit of heat transfer by many research fellows of heat pipe technology [4,5]. To realize industrial application and ensure e?cient, safe and reliable operation of the designed heat

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pipe equipment in the industrial applications, such issues for the carbon steel–water heat pipe as maximum heat transfer capacity, maximum acceptiable temperature, enhanced internal boiling heat transfer to eliminate local overheating and service life must be solved. Therefore, a series of researches have been conducted in laboratories. And on this basis, heat pipe steam generators for application in high-temperature and highly dusty conditions have been successfully developed. 3.1.1. Maximum heat transfer capacity of carbon steel–water heat pipe Fig. 1 is a test bench for single heat pipe performance simulating the condition that ?ue gas sweeps the heat pipe longitudinally [6], and the following researches have been conducted on the test system: (1) Maximum heat transfer capacity of a single heat pipe under testing conditions; (2) Variation in wall temperature of the heat pipe under changing conditions; (3) Verifying the correctness of theoretical calculation value.
Steam 2.5MPa boiler Steam

SaturatedWater

High-pressure water pump 1100~1300oC Air Combustion chamber Mixing chamber 500~970oC

Fuel & Air

Heat pipe

pump

Fig. 1. Schematic of test system for the single heat pipe performance.

Fig. 2. Wall temperature distribution of heat pipe.

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In Fig. 2, the three curves represent the temperature distribution on the outer wall of the heat pipe and the maximum transferred power under three typical testing temperatures. The maximum transfer power measured is far below the theoretical value and values obtained in limit tests. Therefore the heat pipe is still safe when it is operating under the condition of maximum ?ue gas temperature of 700 °C. 3.1.2. Study on enhanced boiling heat transfer in carbon steel–water heat pipes [7,8] The carbon steel–water heat pipe has demonstrated great superiority as a heat transfer element of heat-exchange equipment, but there are still some problems in the actual application, such as entrainment limitation, serious unstable ?uctuation of wall temperature, narrow applicable temperature range, etc. Therefore, to enhance its internal performance and widen its application, temperature is still one of the imperative technical issues to be solved in the promotion of heat pipe technology. Inserting a coaxial perforated pipe (referred to as isocon in this paper) in the heat pipe is one of the e?ective and feasible methods to enhance its internal heat transfer. It converts the liquid pool boiling in the evaporating section into liquid boiling in narrow slots, and separates the vapor rising route in the evaporating section from the re?ux condensed liquid, to reduce the interaction between vapor and liquid, while not a?ecting the normal evaporation and condensation. The main mechanism of an isocon to enhance the boiling heat exchange is to expand the covering area of the liquid ?lm and to increase the disturbance of liquid around the bubbles. The testing apparatus is shown in Fig. 3. The testing pipe is a carbon steel–water thermosyphon, U34 ? 8 and 3000 mm in length, including an evaporating section of 1600 mm and a condensing section of 900 mm. The evaporating section is heated in a high-power silicon–carbon rod heating oven, and in the condensing section, water in the jacket can remove the heat. On the
Heat pipe Measure temp. tube Cooling water Taml Thermocouple Isocon

Pump

SD

Media Converter
5VDC. _ __ __ 1A + RX UP LINK LINK PWR LINK TX

Data system

Fig. 3. Heat pipe power-test apparatus.

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wall of the heat pipe, 15 pairs of thermocouples are ?xed by spot welding for temperature measurement, and the steam temperature in the pipe is measured by a thermocouple in the center pipe. The isocons used are sized as U27 ? 3, U25 ? 1 and U22 ? 1, with a porosity e respectively at 3%, 16% and 33%. The testing study results are as follows: (1) The structural dimensions of the isocon have a large bearing on the boiling heat transfer coe?cient. Within a certain range, the higher the porosity is, the better heat transfer e?ect, and within the range permitted by engineering manufacture, the smaller the pipe clearance and thinner liquid ?lm are, the better the heat transfer e?ect. (2) The optimum isocon structure: the pipe clearance is about 4.5 mm with a porosity of about 33%. With the best isocon, under the same power transfer conditions, the steam temperature in the heat pipe is reduced by 15–30 °C, and the internal heat transfer coe?cient in the heat pipe is increased remarkably, for about 2–3 times. (3) The dimensionless number formula for enhanced boiling heat transfer of isocon obtained by performing multi-element linear least square regression analysis on 330 testing points is as follows:
0:315 Nu ? 199:5Mb u?1:23 e0:08 Pr0:009 ?p=pa ?0:22

Testing range: the characteristic criteria parameter of micro-layer liquid ?lm Mb ? 2 ? 10?5 –4:67 ? 10?4 , the geometric characteristic criteria parameter of isocon u ? 0:42–0:46, porosity e ? 3%–33%, and the ratio of working pressure to ambient pressure in the thermosyphon p=pa ? 1:3–15:5; Pr ? 0:9–1:46. 3.1.3. Steam generator for high-temperature and high-dust content gases (waste heat boilers) The reliability of steam generators for high-temperature and high-dust content gases (waste heat boilers) has always been a highlighted issue in chemical industry as well as in other industries. The most typical of this type of equipment is the waste heat boiler for the high-temperature SO2 gas after the ?uidized-bed roasting in the sulfuric acid industry. A 125,000 t/a sulfuric acid plant produces SO2 gas at 277744 Nm3 /h, at an inlet temperature of 950 °C, with a dust content of about 250 g/m3 and a dew point temperature of 192–210 °C. The operation parameters are shown in Table 2. The best features of heat pipe type steam generators are compact structure, small volume, light weight and high safety and reliability. Its mass is only 1/3–1/5 of that of an ordinary tube waste heat boiler and its overall dimensions only 1/2–1/3 of the latter. The pressure loss after the ?ue gas passes the waste heat boiler is normally 20–60 Pa, and therefore the power consumption of the ID pump is also quite low. Damage of a heat pipe element will not a?ect the circulation in the steam system, and it is not necessary to shut down the system for repair. Therefore, safety and reliability of the system have been greatly raised. The heat pipe steam
Table 2 Operation parameters of heat pipe steam generator Cold side water, steam Flow rate Inlet temp./°C Outlet temp./°C Pressure/MPa 9.5 T/h 104 224 2.5 Hot side SO2 gas 277744 Nm3 /h 950–970 355 Micro pressure

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Fig. 4. Heat pipe steam generator for sulfuric production.

generator used for this process was put into operation successfully in one trial in November 1996 on the basis of the experimental study. The site conditions are as shown in Fig. 4. It has been running up to now without and maintenance and repair. This fully indicates the operation reliability of a heat pipe steam generator under severe conditions. 3.2. Heat pipe air preheaters 3.2.1. Investigation on heat pipe gas–gas heat exchangers The research results on gravity type heat pipe heat exchangers and on the internal and external heat transfer coe?cient of the heat pipes were published as early as in 1986 [9]. Investigations were conducted on heat pipe gas–gas heat exchangers of 20 di?erent structure types of 26–32 mm in diameter and 1.2–2 m in length under 300 di?erent operation conditions. Fig. 5 shows the photo of the testing apparatus. This testing system permits performance tests on heat pipe heat exchangers at any tilting angle between 0° and 90°. The investigation results are: Correlated formula for heat transfer coe?cient of integral heat pipe heat exchangers:  0:25  0:1999  0:2093  0:2470  ?0:1365 St Sl tf d 0:6249 1=3 Prw Pr Nu ? 0:01334Re tf Pr d0 St hf The pressure drop across the heat exchanger:

Fig. 5. Photo of the performance test system of a heat pipe gas–gas heat exchanger.

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Dp ? f

L ? G2 max qgdev
?0:2803



f ? 0:456Re

lw l

0:14 

St do

2:135 

Sl St

?0:6469 

tr hr

0:8191

3.2.2. Heat pipe air preheaters A typical heat pipe air preheaters is shown in Fig. 6. Its best features are simple structure, high heat transfer e?ciency and convenient adjustment of heat exchange area ratio of the cold side to hot side, thereby e?ectively avoiding acid and dew point corrosion. Table 3 gives the main parameters of a large heat pipe air preheater used in the primary reformer of a large fertilizer plant [12]. The practical operation results show that the heat recovered by the heat pipe heat exchanger proper is equivalent to about 1 ton of diesel oil per hour, with apparent economic e?ciency, and a demonstrative role in the energy-saving transformation for large fertilizer plants. 3.3. Design and standards for heat pipe heat exchangers The design for a heat pipe heat exchanger mainly comprises of heat transfer calculation and structural design. In thermodynamic calculation, mainly the overall heat transfer coe?cient is

Fig. 6. Photo of heat pipe gas–gas heat exchanger.

Table 3 Parameters of heat pipe air preheater Pipe size/mm Heat exchanger size Flow rate/Nm3 /h Inlet temp./°C Outlet temp./°C Pressure drop/Pa Heat recovery/kW U51 ? 4.5, L ? 6000, 1914 pieces Height 6.4 m, Length 2.4 m, Inlet width 13.7 m, Outlet width 10.37 m Flue gas Air 238,000 195,860 297.7 54.8 171.2 228.7 580 280 43.1 GJ/h (11,970 kW)

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calculated, the overall heat transfer area is determined on the basis of average temperature difference and heat load, to determine the number of heat pipes needed, and then the ?uid pressure drop and heat pipe transfer power are veri?ed. Although in diversi?ed types of structures, the structural design for heat pipe heat exchangers mostly fall in the scope of conventional design, following the relevant provisions in ‘‘Safety supervision regulations on pressure vessels’’ issued by the State Ministry of Labor and in national standards GB150-1999 ‘‘Steel pressure vessels’’ and GB151-1999 ‘‘Steel tube-shell type exchangers’’. For the design, manufacture and inspection of heat pipes, the National Technological Supervision Administration of China has formulated jointly with relevant departments some standards of People?s Republic of China, such as ‘‘Technical speci?cations for carbon steel–water heat pipes’’ and ‘‘Technical speci?cations for carbon steel–water heat pipe heat exchangers for gas–gas heat exchangers’’, and they will soon be put into implementation. At present, some standards made by certain enterprises are being implemented, such as the enterprise standard of Jiangsu Province––Su Q/B-25-86 ‘‘Technical speci?cations for carbon steel–water gravity heat pipes’’ [10] and heat pipe standard of Liaoning Province––Liao Pan Q34-88, RH-YQS boiler heat pipe economizer [11], etc.

4. Industrial application of liquid metal heat pipes (high-temperature heat pipes) With the continual advancement of heat pipe technology, heat pipe type industrial process equipment have been developed and applied, making the heat pipe equipment not only just for recovery of waste heat, but also indispensable highly-e?cient heat transfer equipment in some industrial processes. In industrial applications, the heat exchange temperature sometimes is as high as 900–1000 °C, and obviously, carbon steel–water heat pipes cannot be used under such conditions. Therefore, research on industrial applications of liquid metal heat pipes (high-temperature heat pipes) has been conducted. Nanjing University of Chemical Technology successfully developed a high-temperature heat pipe steam generator in 1990 [13]. It is used for recovery of high-temperature waste heat in small fertilizer plants, and has been operating up to now. Subsequently, it developed a high-temperature heat pipe gas–gas heat exchanger, which was awarded the State Invention Prize in 1996. For large scale industrial promotion and application, the ‘‘research on heat transfer characteristics of liquid metal heat pipe and equipment’’ is conducted as ?nanced by the State Planning Commission, to solve such issues as heat transfer characteristics, safety and economy of liquid metal heat pipe and equipment. 4.1. Research foundation for industrial application of liquid metal heat pipes 4.1.1. Research on heat transfer characteristics of liquid metal heat pipes and equipment (1) Research on heat transfer performance and limits of liquid metal heat pipes: According to the requirements of research, highly-e?ective high-temperature heat pipes were developed, and a testing system for single-pipe performance for high-temperature heat pipes was set up as shown in Fig. 7. The experimental results show: the heat transfer ability of a single highly-e?ective hightemperature heat pipe exceeds or equal to 40 kW, the pipe size: U57 ? 3.5 ? 2800 mm, and the calculation formula for heat transfer resistance within a high-temperature heat pipe is obtained.

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Fig. 7. High-temperature heat pipe power-test apparatus.

(2) Testing research on overall heat transfer characteristics and the e?ciency of high-temperature heat pipe heat exchangers: An experimental system for overall heat transfer characteristics of gas– gas high-temperature heat pipe heat exchangers is set up by simulating the actual industrial conditions (Fig. 8), and the following results are obtained (the recommended applicable range of Re number being 1900–20,000). The dimensionless number formulas for calculating the heat transfer coe?cient of a hightemperature heat pipe heat exchanger:   kh 1=3 Re0:6256 Prh ah ? 0:1098 h d   kc 1=3 ac ? 0:0938 Re0:6367 Prc c d

Fig. 8. Schematic of test system for high-temperature heat pipe exchange performance.

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The dimensionless number formulas for calculating the friction coe?cient of pressure drop: f ? 1:991Re?0:144358 4.1.2. Research on safety in industrial application of liquid metal heat pipes With the continual expansion of application of high-temperature liquid metal sodium heat pipes, safety has become a primary issue to be solved. For the application of sodium heat pipes in steam generators, theoretical analysis and experimental studies on the sodium–water reaction in sodium heat pipes were conducted [15] with a simulation testing system for sodium–water reaction to simulate the conditions of pipe wall rupture in the condensing section of the high-temperature sodium heat pipe steam generators, as shown in Fig. 9. The pressure pulse peak values of sodium– water reaction were measured to ?nd its regularity of reaction. It can be seen from the experimental results that there is certain regularity in the sodium–water reaction in a heat pipe. The reaction proceeds in two phases. The ?rst phase is intense explosion reaction, but the pressure does not reach the peak value during this phase; while the second phase is a slow reaction, equivalent to a small scale leakage (50 mg/s–10 g/s) in the sodium–water reaction in a fast-neutron reactor, and the reaction rate is less than 1 g/s. So the following conclusions are reached: (1) The sodium–water reaction in a heat pipe is a quantitative sodium–water reaction in a limited space, and when the heat pipe is damaged to become an open system, the pressure rise in the pipe is suppressed. (2) The rise of temperature and pressure resulting from the sodium–water reaction in the heat pipe is related to the quantity of sodium ?lled, and its pressure and temperature rise is accomplished within 10 s to several minutes. (3) The result of single heat pipe leakage in the sodium heat pipe steam generator is only the failure of a single heat pipe, not a?ecting the operation of the adjacent pipes and the system.

Fig. 9. Test system of sodium–water reaction in heat pipe.

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Table 4 Experimental parameters No. 1 2 3 4 5 Size U38 ? 6 U38 ? 6 U32 ? 3 U32 ? 3 U32 ? 3 Material 12CrMoV 12CrMoV 1Cr18Ni9Ti 15CrMo 15CrMo Working ?uid Na K Na Na K Fluid weight 55 50 85 90 30 Filling ratio (%) 28 28.8 43.7 46.3 17.5 Time/h 2100 2100 2020 1940 450 Vapor temp./°C 500 480 450 480 450

4.1.3. Research on compatibility of low-alloy steel–liquid metal heat pipes [16] For the promotion and application of high-temperature heat pipe heat exchangers in industries, equipment cost is one of the key factors. In this research, two low-alloy steel materials, 15CrMo and 12CrMoV, were used for a 2000 h life comparison test with stainless steel, and the purpose is to ?nd a heat pipe which is simple in manufacture process, stable in performance and low in cost for the operation temperature range of 450–600 °C, so as to provide a basis for the industrial application of low-alloy steel high-temperature thermosyphons. The main test parameters are given in Table 4. All ?ve heat pipes are 720 mm long, the evaporating section being 400 mm and condensing section 270 mm. They were heated using electric heating furnace, with the upper ends cooled naturally by air. The research results are: (1) Corrosion of alkali metals (sodium and potassium) to the internal wall surface of low-alloy steel heat pipes is mainly caused by the activation of oxygen carried by impurities and the re?uxing and washing of liquid media, and the corrosion in the condensing section of the heat pipe is more serious than that in the evaporating section. After operation for 2000 h, the corrosion pits in the condensing section is generally below 60 lm. The corrosion rates are: 0.131–0.219 mm/a with 12CrMoV, 0.174–0.262 mm/a with 15CrMo and 0.394–0.438 mm/a with 1Cr18Ni9Ti. (2) Under high oxygen content condition, the corrosion type of alkali metals on the metal materials is basically identical. The corrosion by alkali metals on 15CrMo is mainly homogeneous physical dissolution, and an even loose corrosion layer is formed on the inner wall of the heat pipe. Their corrosion on 12CrMoV is mainly local cracking corrosion, which develops irregularly. And the corrosion behavior of 1Cr18Ni9Ti is selective dissolution at crystal boundary, resulting in gradual separation of crystalline grains. As oxygen in impurities is the key factor a?ecting the compatibility of heat pipes, by improving the present heat pipe manufacture process and alkali metal puri?cation process and increasing the vacuum of heat pipes can e?ectively reduce the corrosion of alkali metals on the shell materials of heat pipes, and prolong the service life of heat pipes. 4.1.4. Simulated optimization research on high-temperature heat pipe heat exchangers [17] A high-temperature heat pipe heat exchanger is a combined heat pipe heat exchanger comprising heat pipes ?lled with di?erent working ?uids. It can be divided into three sections according to di?erent working ?uids in the pipes: high-temperature section, mediate temperature section and low temperature section. The high-temperature section normally consists of sodium and potassium heat pipes, the intermediate temperature section normally of naphtha heat pipes and the low-temperature section of carbon steel–water heat pipe. As the internal working temperature ranges suitable for these three types of pipes are not well linked, while the temperature

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?eld of cold and hot ?uids outside the pipes is continuous, the design for the linking parts for di?erent temperatures in a high-temperature heat pipe heat exchanger is complicated. In addition, there is a great di?erence in the inlet and outlet temperature of the cold and hot media in a hightemperature heat pipe heat exchanger, and therefore it cannot be simply solved with conventional physical means. In literature [17], a simulated optimization calculation method is proposed and it has successfully solved the problem of simulation calculation for high-temperature heat pipe heat exchangers, providing a powerful theoretical basis for structural optimization of high-temperature heat pipe heat exchangers. From the viewpoint of practical engineering applications, it has greatly reduced the cost of high-temperature heat pipe heat exchangers, and laid a fairly solid theoretical foundation for the further development and application of high-temperature heat pipe heat exchangers. 4.2. Example of industrial application of liquid metal heat pipe technology 4.2.1. High-temperature heat pipe hot air furnace [18] With the development of ?ne chemical industry, higher requirements have been raised on the spray drying technology for powder materials, which require a hot air of 450–600 °C or even higher temperature in many applications. It is quite di?cult to heat the air to such a temperature range with conventional heat exchange equipment. If the ?ue gas of fuel is used directly, pollutants may be carried with it, rendering the product quality not up to the speci?cation. Fig. 10 shows a new heat pipe high-temperature hot air furnace (put into operation in February, 1997). Its heat transfer capacity is 1163 kW and its parameters are as shown in Table 5. This unit uses coal as raw material to get hot air free of any contamination. The water content of the dried
950 ~ 850 ?C Flue gas 470 ~ 530 ?C Air

Furnace

160 ~ 170 ?C

Fig. 10. High-temperature heat pipe hot air furnace.

Table 5 Parameters of high-temperature heat pipe hot air furnace Flue gas Flow rate/Nm3 /h Inlet temp./°C Outlet temp./°C Heat recovery 4300–4900 850–950 150–170 1163 kW Air 6000–6500 20 470–530

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Steam Saturated water

Flue gas
~ 700 ?C

645 ?C

High temp. heat pipe

Fig. 11. Schematic of high-temperature heat pipe heat extractor. Table 6 Parameters of high-temperature heat pipe heat extractor Cold side saturate water Flow rate/Nm /h Inlet temp./°C Outlet temp./°C Pressure drop/Pa Heat recovery/kW
3

Hot side ?ue gas 10,000 Nm3 /h 700 645 220

20,000 kg/h 253 253 3800 2438

product can fall below 2%, and product quality reaches or even exceeds the international standards. The increase in hot air outlet temperature depends on the materials used in the hightemperature zone. With a small amount of NiCr high-temperature alloy steel as the heat pipe material, high-temperature hot air with a temperature above 800 °C can be obtained. 4.2.2. High-temperature heat pipe heat extractors [19] A high-temperature heat pipe heat extractor for ?ue gas from catalytic regenerator is as shown in Fig. 11.The heat in the catalytic regenerator ?ue gas is extracted using high-temperature heat pipes to ensure that the temperature of the ?ue gas entering the next stage of turbine is below or equal to 645 °C. This equipment is now operating in a petrochemical plant and it has met all the expected objectives. It is still operating well after practical operation for one and a half year, completely solving all previous problems. Its main technical parameters are shown in Table 6. This research result has fully proved the successful application of the high-temperature heat pipe technology in high-temperature heat extraction, and it has extensive application ?elds and very high economic values for promotion.

5. Research and development of heat pipe chemical reactors under high-temperature and high pressure With its superior characteristics, heat pipe technology is playing a more and more important role in the waste heat recovery, energy-saving and environmental protection units and in indus-

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trial process equipment. To reform the chemical reactor, key equipment in industrial processes with the heat pipe technology will not only be bene?cial to energy optimization of the reactor itself. More importantly, it is good to chemical reaction to raise the output and yield, thus bringing the industrial production equipment level onto a new step. Our university has been engaged in the research and development of heat pipe chemical reactors since the beginning of the 1990s with the ?nancing from the Science and Technology Commission of Jiangsu Province, and has got achievements in laboratory study, pending further industrialized study. The following is a brief presentation on a heat pipe chemical reactor ‘‘in?nitely approaching’’ the optimal reaction temperature under high-temperature and high-pressure conditions (480 °C, 32 MPa). 5.1. Concept of a heat pipe chemical reactor ‘‘in?nitely approaching’’ the optimal reaction temperature Research was conducted on the ammonia converter, the key equipment in fertilizer production. The heat pipe technology is adopted to develop a heat pipe chemical reactor in?nitely approaching the optimal reaction temperature on the basis of the research results on the heat transfer characteristics of vapor–liquid two-phase ?ow in the helical tube of the loop heat pipe evaporator [22], to optimize the temperature distribution on the reactor bed and increase the net ammonia yield without increasing the ?ow resistance. The main concept is: the evaporator of the loop heat pipe is placed in the converter using the adiabatic reaction and indirect heat exchange structure, to divert the heat out of the converter to produce steam, so that the reaction temperature approaches the most suitable value for reaction, thereby increasing the net ammonia yield. Speci?cally, the structure is divided into ?nite section model and in?nite approaching model, with the operation curves as shown in Fig. 12. With the so-called ?nite section model, there are ?nite and ?xed number of catalyst layers in the converter, and this model is suitable to ammonia converters with adiabatic reaction and indirect heat exchange between sections, as well as axialradial ammonia converters; the in?nite approaching model is suitable to axial ammonia converters, in which the loop heat pipes are placed in the catalyst layers, and the reaction heat is extracted while the reaction is going on, so that the temperature of the catalyst layers can in?nitely approach the optimal reaction temperature curve.

(1)

Temperature / ?C

(2)

Temperature / ?C

Fig. 12. Operation curves: (1) Finite sections model; (2) In?nite approach model.

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Fig. 13. Main interface pages of simulation calculation.

5.2. Control and process simulation calculation for optimal reaction temperature for heat pipe ammonia converters The software for simulation calculation was developed, with its main interface pages as shown in Fig. 13. The master program of the simulation calculation is to solve in parallel the TeM rbH Ps;eB (Gemkin-Bezev) kinetic equations and thermal equilibrium di?erential equations using an accurate di?erentiation method under given conditions (pressure, gas amount and gas composition, etc.), to get the temperature and ammonia content distribution in the catalyst layers, then the results are compared with the most suitable temperature curve and the set conditions, to adjust the calculation parameters and obtain the optimized results. The results show that the net ammonia yield is over 19% in all cases, 6 percentages higher than the present values in ammonia converters (about 13%). This proves that heat pipe type ammonia converter with in?nite approaching optimal reaction temperature model is worthy further development and promotion, with broad prospects.

6. Concluding remarks There are two main subjects in the research, development and application of heat pipe technology in industries: one is technology extension and application. This needs to standardize and systematize the present fairly mature heat pipe products, and to normalize their design, manufacture and quality inspection, so that they become conventional equipment in industrial production, thereby further widening the application of heat pipe technology. The second is research and development. This needs to bring into full play the characteristics of the heat pipe technology, and to further develop new type high-e?ciency heat and mass transfer equipment by integrating with other areas of sciences, to bring changes to some traditional equipment and to improve the safety, reliability and e?ciency of systems.

H. Zhang, J. Zhuang / Applied Thermal Engineering 23 (2003) 1067–1083

1083

References
[1] J. Zhuang, H. Zhang, Heat Pipe Technology and Engineering Application, Chemical Industry Press, June 2000. [2] J. Zhuang, H. Zhang, Prospect of heat pipe technology for year 2010, Chemical Engineering and Machinery 25 (1) (1998) 44–49. [4] L.M. Huang, The performance limits of a vertical two-phase closed thermosyphon, Master Thesis, Nanjing University of Chemical Technology, June 1986. [5] J. Zhuang et al., Applied research on heat pipes, Journal of Nanjing Institute of Chemical Technology (2) (1979). [6] J. Zhuang, R.H. Dong, et al., Application of heat pipe steam generators in sulfuric acid production, Chemical Engineering and Machinery 25 (2) (1998) 88–91. [7] L. Zhang, Study on the enhancing internal heat transfer of carbon steel–water heat pipe, Master Thesis, Nanjing University of Chemical Technology, June 1995. [8] S.M. Sun, Study on the enhancing internal boiling heat transfer of carbon steel–water thermosyphon, Master Thesis, Nanjing University of Chemical Technology, June 1997. [9] Y.S. Pei, Optimal design for heat pipe heat exchanger and study on the coe?cient of convective heat transfer outside of heat pipe, Master Thesis, Nanjing University of Chemical Technology, June 1986. [10] Technology Speci?cation for Carbon Steel–Water Gravity Heat Pipe, Su Q/B-25-86, Approved by Jiangsu Province Standard Bureau. [11] R.D. Zhao, K. Lang, et al., Discuss on foundation of Liaoning Province?s carbon steel–water heat pipe standard, in: Proceeding of 4th China Heat Pipe Conference, P262–214, August 1994. [12] Z.R. Sun, B. Yang, Application of large size heat pipe air preheater in 1st converter of large chemical fertilizer, in: Proceeding of 4th China Heat Pipe Conference, P396–400, August 1994. [13] J. Zhuang, Application of heat pipe technology in small scale chemical fertilizer, Design Technology of Small Scale Nitrogenous Fertilizer 3 (1993) 35–41. [15] G.Y. Zhang, Some fundamentals on high temperature heat pipe steam generator, Doctor Thesis, Nanjing University of Chemical Technology, December 1999. [16] W.W. Zhou, An investigation on the compatibility and heat transfer performance of low alloyed steel–alkali metal thermosyphon, Master Thesis, Nanjing University of Chemical Technology, June 1996. [17] D. Chen, Study on simulation optimal for high temperature heat pipe heat exchanger, Master Thesis, Nanjing University of Chemical Technology, February 2002. [18] H. Zhang, L.G. Teng, et al., Heat pipe techniques applied to the production of K12 , Energy Research and Utilization 56 (4) (1998) 13–18. [19] B. Yu, Heat pipe heat exchanger of catalyze cracking reactor, in: Proceeding of 4th China Heat Pipe Conference, P220–223, September 1998. [22] J. Zhuang, H. Zhang, et al., Separate-type heat pipe ammonia converter, Patent No. ZL 99228340.9.


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