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The application of heat pipes to electronics
Vacuum News frequency of the first dye laser This relationship results m an output frequency in the vacuum ultrav=olet and permits wavelength select=on by tunmg the frequency of the second dye laser. Although the conversion efficiency ts presently small--about a hundredth of a per cent--the output power In the ultraviolet is
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The application of heat pipes to electronics Heat popes transfer heat wtth temperature gradients some hundreds of times smaller than any known solid conductor. They operate on a closed bi-phase cycle using no motive power other than the heat source itself. The heat is carrted as latent heat of evaporation by mass transfer in the vapour phase. T w o phase heat transfer pre-dates the tndustrial revolut=on Closed loop, bi-phase systems, such as the steam engine and refngerator, over the last hundred years have affected the ctvihzed world. The unique feature of the heat pipe, first patented m 1942, is the use of surface tens0on, generating captllary pressure, to return the condensed liquid to the evaporator. Th=s patent, Essued to R S Gaugler, relates to the cooling of an ice box. The term "Heat Pipe' was first used in a paper by G M Grover et al, reporting work carried out under the auspices of the US Atomic Energy Commisston. This work, entitled "Structures of Very Htgh Thermal Conductance" was directed towards the use of sodtum filled heat pipes for thermomc generators, and heralded a period of extens=ve research into apphcations in space. Although the efficiency of electron=c components has steadily tmproved since their inceptton, the higher packing density and more stringent enviromental requtrements have resulted m thermal problems of increasing difficulty. Consequently early cons=deratton =s n o w given to the thermal design of many advanced electrontc equipment projects. In many cases the more conventional techniques are found to be inadequate, and designs embodying heat pipes are gaining popularity. In the mare, electromcs apphcattons are more potenttal than current, a situation whtch Js changmg rapidly as engineers become better acquainted with design parameters, and as an increasing demand leads to lower prices
General d e s c r i p t i o n . A heat ptpe is a sealed tube containing flutd and a wick. One area of the tube is heated, causing the flutd to evaporate, another area is cooled, causing the vapour to condense. The vapour carrtes the heat, as latent heat from the heated area to the coiled area. The Ifqu~d ts recirculated from the condenser to the evaporator by the capdlary actmn of the "'w=ck". Gravity ards thts capillary action when the evaporator end =s downward and opposes captllary actton when the evaporator end is upward. In stmple versions of the heat ptpe, the " w i c k " ~s uniform throughout the length of the tube, and the roles of evaporator and condenser can be interchanged. Performance parameters. The design parameters of heat p=pes may generally be expressed as performance hmtts. An explanatton of those bmtts is g=ven m the following sections, and some typtcal values in Table 1. The overall thermal performance of a heat transfer system embodymg a heat ptpe is usually determmed by the heat sink, when the heat pipe =s operating withm those bruits. Thus heat p=pes for a design may be s=mply specified m
260
several hundred m~lliwatts, more than adequate for most photochemical and spectroscoptc work. The effictency, output power, and tuning range should all mcrease substantially w=th further development. IBM Ltd Circle number 39 on Reader Enquiry Service card
Table 1. Data refers to boding pomt temperature and atmosphertc pressure. It ts only approx=mate Bothng point (K) He N NH 3 Rll Rl13 Acetone Methanol Ethanol Water Toluene Thermex Hg Rb Cs K Cd Na Zn Mg Lf Ag
4 77 240 297 321 329 338 351 373 384 530 630 961 963 1033 1038 1165 1180 1380 1603 2485
Ax=al L~mttmg Radial flux superheat flux (W/cm 2) (K) (W/cm 2) .01 10 109 12 8 34 49 24 450 22 25 1620 250 124 450 3760 2270 11400 5900 8250 7000
11 22 18 17 15 1.8 21 3.9 50 2.3 3.4 526 26 91 29 151 57 219 192 110 377
1 09 03 0.4 03 08 11 17 10.0 08 11 13000 2000 5000 3000 4500 9000 6500 6000 2000 10000
Capdlary nse (cm) 2 17 72 22 1.8 3.9 39 4 2 10 0 44 5.0 49 62 36 14.9 12 0 25 1 19.3 56 9 86 9 87
terms of those performance limtts, and w=thm them, heat ptpes may be considered as near tsothermal conductors of heat. The matenals from whtch a heat ptpe is made, the container, the w~ck, and the working fluid, must be compattble. Decompos=tlon of the working flu=d, either by d=rect chemical actton w=th the wtck or contamer, or by thetr catalyttc actton, to form a non-condensible gas, ~s the usual cause of a short working hfe. A well reported example ~s the reaction between stainless steel and water, which generates hydrogen, stopping the operatton of the heat ptpe. The range of flutds for cryogenic heat pipes ts bmited, those that can be used suffer from the low performance inherent m the phystcs of low temperatures. Although hKgh temperature heat ptpes enjoy outstanding performance, the workmg flutds must be of the utmost purtty, and the present methods of construct=on are not eas=ly adapted to the product=on line. Both cryogentc and hquid metal heat p~pes are expensive and the demand low. Pnces range from £500 to £3000. The demand for cheaper heat ptpes m the more moderate temperature range is increasing, and m some measure this market requirement can, and is being met by tubular forms w=th s=mple woven w=re mesh w=ck structures W o r k i n g pressure. The vapour instde the tube ts in equilibrium with the hquid, so that the pressure ts the saturated vapour pressure corresponding to the temperature at whtch the heat ptpe
Commercial
news
The application of heat pipes to electronics Heat popes transfer heat wtth temperature gradients some hundreds of times smaller than any known solid conductor. They operate on a closed bi-phase cycle using no motive power other than the heat source itself. The heat is carrted as latent heat of evaporation by mass transfer in the vapour phase. T w o phase heat transfer pre-dates the tndustrial revolut=on Closed loop, bi-phase systems, such as the steam engine and refngerator, over the last hundred years have affected the ctvihzed world. The unique feature of the heat pipe, first patented m 1942, is the use of surface tens0on, generating captllary pressure, to return the condensed liquid to the evaporator. Th=s patent, Essued to R S Gaugler, relates to the cooling of an ice box. The term "Heat Pipe' was first used in a paper by G M Grover et al, reporting work carried out under the auspices of the US Atomic Energy Commisston. This work, entitled "Structures of Very Htgh Thermal Conductance" was directed towards the use of sodtum filled heat pipes for thermomc generators, and heralded a period of extens=ve research into apphcations in space. Although the efficiency of electron=c components has steadily tmproved since their inceptton, the higher packing density and more stringent enviromental requtrements have resulted m thermal problems of increasing difficulty. Consequently early cons=deratton =s n o w given to the thermal design of many advanced electrontc equipment projects. In many cases the more conventional techniques are found to be inadequate, and designs embodying heat pipes are gaining popularity. In the mare, electromcs apphcattons are more potenttal than current, a situation whtch Js changmg rapidly as engineers become better acquainted with design parameters, and as an increasing demand leads to lower prices
General d e s c r i p t i o n . A heat ptpe is a sealed tube containing flutd and a wick. One area of the tube is heated, causing the flutd to evaporate, another area is cooled, causing the vapour to condense. The vapour carrtes the heat, as latent heat from the heated area to the coiled area. The Ifqu~d ts recirculated from the condenser to the evaporator by the capdlary actmn of the "'w=ck". Gravity ards thts capillary action when the evaporator end =s downward and opposes captllary actton when the evaporator end is upward. In stmple versions of the heat ptpe, the " w i c k " ~s uniform throughout the length of the tube, and the roles of evaporator and condenser can be interchanged. Performance parameters. The design parameters of heat p=pes may generally be expressed as performance hmtts. An explanatton of those bmtts is g=ven m the following sections, and some typtcal values in Table 1. The overall thermal performance of a heat transfer system embodymg a heat ptpe is usually determmed by the heat sink, when the heat pipe =s operating withm those bruits. Thus heat p=pes for a design may be s=mply specified m
260
several hundred m~lliwatts, more than adequate for most photochemical and spectroscoptc work. The effictency, output power, and tuning range should all mcrease substantially w=th further development. IBM Ltd Circle number 39 on Reader Enquiry Service card
Table 1. Data refers to boding pomt temperature and atmosphertc pressure. It ts only approx=mate Bothng point (K) He N NH 3 Rll Rl13 Acetone Methanol Ethanol Water Toluene Thermex Hg Rb Cs K Cd Na Zn Mg Lf Ag
4 77 240 297 321 329 338 351 373 384 530 630 961 963 1033 1038 1165 1180 1380 1603 2485
Ax=al L~mttmg Radial flux superheat flux (W/cm 2) (K) (W/cm 2) .01 10 109 12 8 34 49 24 450 22 25 1620 250 124 450 3760 2270 11400 5900 8250 7000
11 22 18 17 15 1.8 21 3.9 50 2.3 3.4 526 26 91 29 151 57 219 192 110 377
1 09 03 0.4 03 08 11 17 10.0 08 11 13000 2000 5000 3000 4500 9000 6500 6000 2000 10000
Capdlary nse (cm) 2 17 72 22 1.8 3.9 39 4 2 10 0 44 5.0 49 62 36 14.9 12 0 25 1 19.3 56 9 86 9 87
terms of those performance limtts, and w=thm them, heat ptpes may be considered as near tsothermal conductors of heat. The matenals from whtch a heat ptpe is made, the container, the w~ck, and the working fluid, must be compattble. Decompos=tlon of the working flu=d, either by d=rect chemical actton w=th the wtck or contamer, or by thetr catalyttc actton, to form a non-condensible gas, ~s the usual cause of a short working hfe. A well reported example ~s the reaction between stainless steel and water, which generates hydrogen, stopping the operatton of the heat ptpe. The range of flutds for cryogenic heat pipes ts bmited, those that can be used suffer from the low performance inherent m the phystcs of low temperatures. Although hKgh temperature heat ptpes enjoy outstanding performance, the workmg flutds must be of the utmost purtty, and the present methods of construct=on are not eas=ly adapted to the product=on line. Both cryogentc and hquid metal heat p~pes are expensive and the demand low. Pnces range from £500 to £3000. The demand for cheaper heat ptpes m the more moderate temperature range is increasing, and m some measure this market requirement can, and is being met by tubular forms w=th s=mple woven w=re mesh w=ck structures W o r k i n g pressure. The vapour instde the tube ts in equilibrium with the hquid, so that the pressure ts the saturated vapour pressure corresponding to the temperature at whtch the heat ptpe