What You Should Know About Furnaces

1) Beware the Multiple Simultaneous Specs gambit!

    All OTHER web sites quote what the capabilities of their furnaces are in terms of maximum temperature, pressure, atmosphere, etc.  The problem is that these are not simultaneous capabilities - for instance, their furnace can't do the maximum possible temperature in air.  Be sure that your important specifications can be met in a single furnace.

2) There are very few "All Purpose" furnaces.

    The way furnaces are heated constrain what they can be used for.  For electrically heated furnaces there are a wide variety of heater types.  Some can only be used in air, some can only be used in vacuum or inert gas.   

3) Don't overspecify your furnace!

    Furnace cost is very sensitive to the maximum use temperature.  The general cost break temperatures are around 1200°C, 1600°C, and 1800°C.  For instance a furnace specified for 1250°C will cost significantly more than one specified for 1200°C because a different heating system must be used.  If you don't need a furnace in the higher temperature range, don't order it!  

4)  Beware Vacuum Specifications!

    The vacuum around your sample is not the vacuum that your vacuum gauge measures!  The vacuum around your sample at temperature is determined by the vapor pressure of the materials in your sample and those surrounding it.  These vapors condense before they are measured by the pressure sensor, which never measures them.  Be careful that these vapors don't contaminate your sample or your process.  

5)  Beware Furnace Vacuum Seals!

    Elevated temperature (above 200^oC) seals are relatively fragile as well as hard and expensive to do.  Know where and what your furnace seals are.  O-Ring seals must be kept cool.  Welds in stainless steel will fail at their weakest point when they are thermally cycled too often.  Ceramic seals must not be overstressed.  Many manufacturers of vacuum tube furnaces do not do a proper job sealing to the tube....

6)  Furnaces are Never Turnkey Systems

    Furnaces cannot be operated without understanding how the furnace interacts with your sample.  Temperatures are never totally accurate or uniform.  The time history of your sample is not the same as what you have programmed the furnace to do.  Safety systems can never take care of all hazardous situations.  Furnace system utilities almost always interact with the utility supplies. 

COST FACTORS

1)  Cost Approximations

** For electrically heated furnaces, higher temperature means more expensive, where the temperature ranges are approximately: 1) cheapest - up to 1200°C, 2) more expensive - up to 1600°C, 3) more expensive - up to 1800°C, 4) you need lots of $ (except at Thoughtventions) - above 1800°C.

** Furnace cost scales with the size of the furnace.

** Vacuum furnaces are significantly more expensive than atmosphere furnaces

** High pressure furnaces are very expensive.

** Systems are much more expensive than furnaces.

2)  Power supplies are expensive.

    Furnaces usually use a lot of power.  The cost of that power is usually large, but there is a big initial cost (in the equipment) due to the way the power is delivered.  Electric power is relatively cheap, but high current, low voltage power is expensive because of the transformers required.  High temperature furnaces usually require this type of power.

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Last updated: July 2015