Now You're all on my ground, f***wit or not... here they are, all the
thermal properties of air and helium @ atm. pressure and T=20°C or 100°C.
Use fixed width font to read the table. Data from a professional chemical
process simulation SW.

================================================== ============================ STREAM ID AIR-COLD AIR-HOT HELIUM-COLDHELIUM-HOT NAME PHASE VAPOR VAPOR VAPORVAPOR ----- TOTAL STREAM ----- RATE, KG-MOL/HR 1.000 1.000 1.0001.000 K*KG/HR 2.897E-02 2.897E-02 4.003E-034.003E-03 TEMPERATURE, C 20.000 100.000 20.000100.000 PRESSURE, KG/CM2 1.020 1.020 1.0201.020 MOLECULAR WEIGHT 28.972 28.972 4.0034.003 -------- VAPOR --------- SPECIFIC GRAVITY (AIR=1.0) 1.000 1.000 0.1380.138 CP, KCAL/KG-C 0.241 0.242 1.2251.225 DENSITY, KG/K*M3 1189.231 933.844 164.207129.012 Z (FROM DENSITY) 0.9998 1.0002 1.00041.0004 TH COND, KCAL/HR-M-C 0.02209 0.02726 0.127620.15315 VISCOSITY, CP 0.01792 0.02162 0.019690.02322It is to be pointed out that, when using these gases as a coolant, theirdensity is as important as their MASS specific heats, ie. we need toconsider the specific heat per unit VOLUME. Doing so, with reference tostandard conditions (20°C - 1 atm), we can see what follows:Air: 0.241 kcal/kg * 1.189 kg/m3 = 0.287 kcal/m3 (@ cond.)Helium: 1.225 kcal/kg * 0.164 kg/m3 = 0.201 kcal/m3 (@ cond.)Briefly, it takes (+/-) the same CUBIC METERS of air or helium to take awaythe same calories. The differences a1 - Helium thermal conductivity is six times higher, Cp is five timeshigher, density is seven times smaller: therefore, the overall heat transfercoefficient (OHTC) is much larger when using helium (say very roughly 6 * 5/ 7 = 4.3 times). This means that small, delicate objects that must beeffectively cooled but cannot be finned or sunk in a liquid coolant NEEDhelium. Using air they would simply run too hot.2 - Helium is as chemically inert as it can be, while air, and particularlymoist air, can corrode thin wires or the even thinner insulating lacquers,in the long run and at high temperature.AFAIK helium was used in some types of graphite-moderated nuclear reactorsas a kernel coolant to withstand the terrific heat release per element unitsurface, before liquid sodium proved to be an even better fluid in terms ofOHTC (but I don't think that a molten caustic metal at some 200°C would beso good to cool a recording lathe's coil...)I don't know how much this all has to do with tubes, indeed...CiaoFabio"Stewart Pinkerton" ha scritto nel messaggionews:uufa3157uhsmneq1of4sunsn2n6j4mvck3@4 ax.com... On Mon, 14 Mar 2005 12:40:12 +1100, "Phil Allison" wrote:"Iain M Churches" "Phil Allison" ** The cutter head may need cooling - but Helium is not a goodcoolant. The cutting stylus is in fact *heated* . The cutting stylus is indeed heated. The cutter head coil is cooled as required with helium. This is a fact which is well documented.** How bizarre - cold air would do. Helium has much higher heat capacity than air, and much lower mass. It is an excellent (and inert) cooling medium, and is used in many industrial processes. -- Stewart Pinkerton | Music is Art - Audio is Engineering