1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-15a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Ammonia-Charged Aluminum Heat Pipes with Extruded Wicks Practice: Use heat pipes, preferably aluminum heat pipes charged with anhydrous ammonia, in s
2、pacecraft and instrument thermal control applications. This practice enhances the control and flow of heat generated within the spacecraft.Programs that Certify Usage: This practice has been used on OAO-C, ATS-F, IUE, HST.Center to Contact for Information: GSFCImplementation Method: This Lessons Lea
3、rned is based on Reliability Practice No. PD-ED-1209; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Heat pipes use the latent heat of vaporization of a working fluid to transfer heat efficiently at a nearly constant temperature. This character
4、istic can be used to control the temperature of spacecraft components and systems. The Goddard Space Flight Center (GSFC) has chosen ammonia-charged aluminum heat pipes for most near-room temperature (200K to 350K) applications. The axial groove aluminum pipe is the design of choice, because it is e
5、asy to design and relatively easy to fabricate. The aluminum container and axial grooves are extruded in one process. At the operating Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-temperature of unmanned spacecraft, ammonia has the most favorable
6、thermodynamic properties that make it an excellent heat pipe working fluid. Anhydrous ammonia is compatible with the aluminum heat pipe body and wick if proper care is taken in the manufacturing process.Implementation Method:All heat pipes have three physical elements in common. These include an out
7、er container, a small amount of working fluid, and a capillary wick structure. In addition to these basic components, heat pipes may also include gas reservoirs (variable conductance/diode heat pipes) and liquid or gas traps (diodes). Functionally, the heat pipe consists of three sections: evaporato
8、r, condenser section, and adiabatic regions. The evaporator section is mounted to the heat-producing components, while the condenser is thermally coupled to a heat sink or radiator. The adiabatic section allows heat to be transferred from the evaporator to the condenser with very small heat losses a
9、nd temperature drops. Figure 1 depicts the basic heat pipe.refer to D descriptionD Heat pipes can operate in the fixed conductance, variable conductance, or diode mode. The fixed conductance heat pipe can transfer heat in either direction and operates over broad temperature ranges, but has no inhere
10、nt temperature control capability. Constant conduction heat pipes allow isothermalization of shelves, radiators and structures; spread heat from high heat dissipating components; and conduct heat away from heat producing devices embedded within instruments and satellites. In the variable conductance
11、 heat pipe (VCHP), a small quantity of non-condensable gas (NCG) is loaded into the heat pipe. The VCHP can be used to control the temperature of equipment Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-within very narrow limits; control is possible
12、 to less than 1K by using careful design techniques. This is accomplished by controlling the location of the NCG/vapor interface within the condenser end of the heat pipe, thereby varying the active length of the condenser and causing a modulation in the condenser heat rejection capability. Temperat
13、ure control of the attached device is achieved by an active feedback system consisting of a temperature sensor at the heat source and a controller for a heater at the NCG reservoir. The heater causes the gas in the reservoir to expand, thus moving the gas/vapor interface. Diode heat pipes permit hea
14、t to flow in one direction and inhibit heat flow in the opposite direction.Specific benefits of heat pipes are: 1) heat pipes have enormously more heat transfer capability than other methods on a weight and size basis, 2) heat pipes permit configuration flexibility in contact areas with heat sources
15、 and heat sinks, 3) heat can be transported over considerable distances with insignificant temperature drop, 4) capillary pumping in the wick is generated by the heat transfer process and requires no other power or moving parts to pump the condensate, and 5) heat pipes operate satisfactorily in a ze
16、ro gravity environment.The choice of working fluid is dictated by several considerations, including operating temperature, latent heat of vaporization, liquid viscosity, toxicity, chemical compatibility with container material, wicking system design, and performance requirements. Figures 2 and 3 and
17、 Table 1 depict some of the above characteristics for several fluids. The highest performance from a heat pipe is obtained by utilizing a working fluid that has a high surface tension (s), a high latent heat (l), and a low liquid viscosity (n1). These fluid properties are contained in the parameter
18、N1the Liquid Transport Factor. Figure 4 is a plot of N1for five typical heat pipe working fluids. These data are used as selection criteria for heat pipe working fluids. Once an application is defined, the heat pipe designer reviews the requirements and selects the best working fluid. Below the free
19、zing point of water and above about 200K, ammonia is an excellent working fluid. Regardless of the fluid chosen, minimum purity must be at least 99.999 percent. A careful analysis of the purity of the ammonia should be obtained from an independent laboratory prior to use.Provided by IHSNot for Resal
20、eNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD refer to D descriptionD Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Table 1: Comparison of Latent Heat to Specific Heat for Typi
21、cal Heat Pipe Fluids FLUID PROPERTIESFLUIDBOILING POINT KLATENT HEAT kJ/kg hfgSPECIFIC HEAT kJ/kg-K cpRATIO K hfg/cpHelium 4 23 4.60 5 Hydrogen 20 446 9.79 46 Neon 27 87 1.84 47 Oxygen 90 213 1.90 112 Nitrogen 77 198 2.04 97 Argon 87 162 1.14 142 Propane 231 425 2.20 193 Ethane 184 488 2.51 194 Meth
22、ane 111 509 3.45 147 Toluene 384 363 1.72 211 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Acetone 329 518 2.15 241 Heptane 372 318 2.24 142 Ammonia 240 1180 4.80 246 Mercury 630 295 0.14 2107 Water 373 2260 4.18 541 Benzene 353 390 1.73 225 Cesiu
23、m 943 49 0.24 204 Potassium 1032 1920 0.81 2370 Sodium 1152 3600 1.38 2608 Lithium 1615 19330 4.27 4526 Silver 2450 2350 0.28 8393 refer to D descriptionDThe outer container usually consists of a metal tube to provide mechanical support and pressure containment. The chosen design and processing of t
24、he container are extremely important in selecting the metal, because they can affect the useful life of the heat pipe. In addition, a compatibility must exist between the pipe material and the working fluid. For heat pipes, working fluid/container compatibility issues encompass any chemical reaction
25、s or diffusion processes occurring between the fluid and wall/wick materials that can lead to gas formation and/or corrosion. Table 2 lists the Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-compatibilities of several metals and working fluids. Alon
26、g with the metal/fluid compatibility, other considerations in the metal selection are ease of working the material, extrusion capability of the material, and its weldability. Proper container cleaning and heat pipe processing procedures are of extreme importance, since residual contamination within
27、the heat pipe may also lead to gas generation. Steps must also be taken to ensure the purity of the fluid charge; trace amounts of water in ammonia can lead to a reaction with the aluminum container and the formation of hydrogen gas. Chi reference 1 and B this design probably is the most commonly us
28、ed for space application.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD In addition, X-ray certification of all welds at the end caps and fill tube is required to ensure good weld penetration and the absence of voids. The hea
29、t pipe container must be pressure tested to at least twice its maximum expected operating pressures (MEOP) prior to filling reference 3. Other qualification procedures include performance tests at adverse tilt angles to demonstrate proper wick function, and gas pocket tests performed with the heat p
30、ipe in the reflux mode. Heat pipes should be handled with care, especially those that contain ammonia or other high vapor pressure fluids. They should be treated as any other pressure vessel, and appropriate safety precautions must be exercised. Exposure to ammonia vapor can cause severe irritation
31、to eyes and other mucous membranes. Exposure to ammonia liquid can cause severe burns to the skin. Whenever possible, heat pipes should be stored in a cold, dry environment. This will inhibit any internal chemical reactions which produce non-condensable gas. Technical Rationale:Spacecraft applicatio
32、ns to date have been for heat pipes operating between 200K and 350K. Consequently, a working fluid whose freezing and boiling points encompass this temperature range Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-and has a high latent heat, a low vi
33、scosity, and high heat transport capability must be selected. GSFC has selected ammonia as an appropriate working fluid whose fluid properties meet these criteria. However, for safety reasons, the toxicity of ammonia precludes its use in manned environments such as the shuttle cabin reference 4. GSF
34、C has selected aluminum alloys, such as 6061 and 6063, for the container material of the heat pipe because of their long-term compatibility with ammonia (see Table 2); heritage; ability to have an extruded axial groove wick structure; ease of fabrication, shaping, and configuring; good thermal compa
35、tibility with aluminum radiators and heat sinks; and weldability characteristics.References:1. Chi, S. W., “Heat Pipe Theory and Practice,“ Hemisphere Publishing Corp., New York, 19762. Brennan, P. J., and Kroliczek, E. J., “Heat Pipe Design Handbook,“ B ultimately, the heat pipe may cease to functi
36、on. Failure to certify welds at the end caps and the fill tube could result in improper or defective welds permitting leaks or catastrophic failure of the pressure vessel. For long-term space missions, working fluids in the appropriate temperature range, such as methanol and water, exhibit an incomp
37、atibility with aluminum, and should not be used.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-15a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
