4.4. Muffle furnace (electric resistance)
Figure 6 shows a typical electric resistance muffle furnace. The electric heating elements are similar to those found in domestic electric ovens. They are independent of the atmosphere in which they operate. Therefore they can be placed within the muffle chamber itself, resulting in a higher operating efficiency compared with the gas heated muffle furnace, which more than offsets the higher energy cost for electricity compared with gas.
The advantages and limitations of this type of furnace are as follows:
Advantages
• Uniform heating of the work.
• Accurate temperature control.
• Ease of fitting automatic control instrumentation.
• High temperature stability.
• Full atmosphere control.
• Comparatively easy maintenance.
Limitations
• Higher energy source costs.
• Lower maximum operating temperatures, as above 950◦C to 1000◦C the life of the resistance elements is low.
Figure 6 Electrically heated muffle furnace: (a) the electric resistance furnace; (b) heating element