the heat pipe serves without parallel as a heat transfer element and is capable of transferring large quantities of heat through its small sectional area remotely, dispensing with any additional motive power. the research and application of the heat pipe technology began in 1960's in the world, and in 1970's in china. initially, the heat pipe technology was primarily used for cooling electronic devices and applied for spacecrafts. till 1980's, the r & d of the heat pipe technology was re-focused on the energy saving and rational utilization, and gas-gas heatpipe heat exchanger, heatpipe waste heat boiler, and high temperature heatpipe steam generator were developed. thanks to simple structure, low price, and easy manufacturing, the carbon steel-water gravity heat pipe was widely applied.

　　now, the heat pipe and the heatpipe heat exchanger are generally used for high-efficiency heat transfer plants in the petroleum, chemical, motive power, metallurgical, building material and light industry sectors, and for cooling electronic devices, chips, notebook cpu, circuit control boards etc.

　　currently, micro heat pipes have been made in batches and on large scales. however, the heatpipe heat exchangers for waste heat recovery are required to be tailored according to special technological conditions and in-the-field operating environments.

i) operational principles of heat pipe

the heat pipe is a heat-transfer element with extremely high thermal conductivity and large heat transfer capacity, which accomplishes the heat transfer through the latent heat of phase change of the working fluid inside the sealed vacuum tube. for the typical gravity heat pipe, see picture i. as a negative pressure of 1 to 2×10-4pa is established in the heat pipe, the up end of the heat pipe releases heat, while the working media is condensed into the liquid. the condensed liquid, with the pull of gravity, will return to the hot side along the inner wall of the heat pipe and will be heated and vaporized again. in this way, the heat will be transferred from one end to the other successively. thanks to the heat transfer of phase change,the thermal resistance inside the heat pipe is very small,which results in a larger heat transfer

capacity at a smaller temperature difference. in terms of the gravity heat pipe, due to its simple structure, single-direction heat conductivity, and special heat transfer mechanism (the heat exchange between the hot fluid and the cold one is done outside the heat pipe), the heat transfer can be easily enhanced. moreover, as vacuum of 1.3×10-1～10-4pa is established inside the heat pipe, the working fluid is readily evaporated and boiled and the heat pipe is quick to start up. the heat-transfer capability of the heat pipe can rival and even exceed that of silver, copper and aluminum.

　　the heat pipe can be used individually or combinedly. the heatpipe heat exchanger consisting of heat pipes is characterized by such merits as high heat transfer efficiency, low flow resistance, compact structure, high reliability, and excellent maintenance economy, and is widely applied in the space technology, electronic, metallurgical, motive power, petroleum and chemical trades.

ii) types and fundamental structures of the heatpipe heat exchanger

　　the heatpipe heat exchanger is a surface heat exchanger with the hot fluid isolated from the cold fluid, featuring simple structure, high heat exchange efficiency, less metal consumption in the circumstances of transferring equal heat compared with other types of heat exchangers, low flow pressure loss, high reliability even under the condition of the failure and perforation of a single heat pipe, readily changeable heat transfer surface area on both cold and hot sides, and easy availability of dew-point corrosion control. thus, the heatpipe heat exchanger is applied more and more widely.

　　according to the forms of hot fluid and cold fluid, the heatpipe heat exchanger is sorted into gas-gas type, gas-liquid type, liquid-liquid type, and liquid-gas type. in accordance with the structures, it is sorted into integral type, separate type, rotary type and combinational type.

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