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A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S |
M A Y / J U N E
2 0 1 7
5 7
7
face area,
T
is the radiating surface temperature (K) of the
support ring, and
T
c
is the temperature (K) of the surround-
ing surface chamber wall. The Stefan-Boltzmann law also
applies to power losses at radiating surfaces. The equation
shows that energy loss is proportional to the fourth power of
the ring temperature (walls “A” and “B” in Fig. 3). Reducing
the temperature of wall “A” is a key factor in improving ener-
gy efficiency in the vacuum furnace.
Table 1 shows prior insulation test results and results
using the new board (last row in table). Figure 4 shows the
improvement in thermal efficiency for the new graphite
board compared with other modern designs.
CURRENT DESIGN VERSUS NEW DESIGN
Based on improved results from using the new graph-
ite board, Solar named the board HEFVAC for high efficiency
vacuum capabilities and applications, and will introduce a
new hot zone design that incorporates the more energy ef-
ficient insulating board. Until now, the foil/2-in. thick rayon
felt combination offered the best energy efficient graphite
vacuum furnace (Table 1 and Fig. 4). The new design offers
a significant improvement over the current design. The
HEFVAC lowers the ring temperature by nearly 200°F at a
hold temperature of 2250°F. At a hold temperature of 2000°F,
a 61% improvement in temperature loss occurs (Table 2).
Fig. 2 —
Schematic of test chamber showing extension for test
sample placement.
Fig. 3 —
Schematic of furnace wall cross section: Radiation heat
transfer from ring wall “A” to cold wall “B” largely contributes to
heat loss within the furnace hot zone.
TABLE 1 — COMPARISON OF PRIOR INSULATION
TEST RESULTS AND NEW BOARD RESULTS
Insulation type
Hold temperature, °F
1750 2000 2250
All metal (3 Mo, 2 SS)
551
650
733
Foil/2-in. kaowool
452
548
640
Foil/2-in. rayon
graphite felt
456
544
616
Foil/2-in. pan
graphite felt
490
574
659
Standard graphite
board (avg.)
517
572
622
New 2-in. board with
foil facing*
334
367
405
*New graphite board material from Graphite Machining Inc.
6
Fig. 4 —
Comparison of thermal eiciency of furnace insulation package.
FEATURE