I have noticed when examining performance of various heating elements in different atmospheres that in a vacuum maximum temperatures are lower than the in air temperatures. Why is this?

(Typical heating element alloys are Nickel-Chromium and Iron-Chromium-Aluminum)

As Olin Lathrop says below the evaporation of the element becomes significantly increased in a vacuum. I found this interesting quote in an old book:

This idea of working in a vacuum brought with it new difficulties as regards the construction of a furnace. The ordinary platinum resistance furnace proved of little value, as, owing to diminution of pressure, the volatility of the platinum foil, at high temperatures, was increased to such an extent that it was destroyed in a short time.

  • $\begingroup$ no convective cooling without air to convect $\endgroup$ – ratchet freak May 22 '15 at 11:48
  • $\begingroup$ @ratchetfreak That only explains why it would take less power to heat the element to a given temperature, not why the maximum rated temperature would be lower. $\endgroup$ – Chris Mueller May 22 '15 at 12:08
  • $\begingroup$ @WallacePark Can you please post an example link to the type of elements you are referring to? $\endgroup$ – Chris Mueller May 22 '15 at 12:09
  • $\begingroup$ Lower pressure = lower melting/boiling point maybe? $\endgroup$ – SF. May 22 '15 at 16:28

One reason is because in vacuum there is no convection to help remove the heat. All heat loss from the element is either by radiation, or via conduction thru the supports and wires. Technically this only means lower maximum heater power, but that may be dumbed down to temperature.

Another reason is due to evaporation of the heating element. At high enough temperatures, this can matter. With 1 atm ambient pressure, evaporation is greatly reduced at low partial pressures, like heater element metals have at glowing temperatures. In vacuum, even a little pressure is greater than ambient (because ambient is theoretically 0), so there can be significantly faster evaporation, reducing the life of the heating element.


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