Question 9.2: A muffle furnace is to be designed for silicon carbide heate......
A muffle furnace is to be designed for silicon carbide heaters.
Size of heating space (nominal) 28 (W) × 22 (H) × 120 (D) cm
Maximum temperature 1400°C
Atmosphere Air
Estimated power 15 kW
Power supply 380 V, 3 phase
Heating elements to be situated at the top across the width.
Now determine:
Number of heating elements required.
Size of elements.
Current and resistance of elements.
Transformer required.
The width of the chamber is 28 cm. The heating element chosen must have a hot (central) zone about 28 cm.
According to Figure 9.8 the permissible surface loading (P) at 1400°C and in air atmosphere is
P = 6 W/cm²
Element temperature ~ 1,475°C
Total heating surface required
= \frac{1,5000}{6} = 2,500 cm²
For equal distribution in three phases the number of elements must be divisible by 3.
For trial consider 18 elements so that power per element will be
=\frac{15,000}{18} = 833 W/element
Hot surface for each element
=\frac{2,500}{18} = 138.89 ~ 140 cm²
Referring to Table 9.7 it can be seen that the nearest resistor has a heating length 305 mm for furnace width 279 mm.
The surface area required per element is 140 cm² . The nearest area of the heating zone is 152 cm² for a 15.8 mm diameter rod.
We have no information about the thickness of the insulation. There are four options available. They are 76, 127, 166, 191, and 229 mm. Assume a wall thickness of about 200 mm.
The nearest size is 191 mm.
The element chosen has following specifications:
Heating length 305 mm
Furnace width 279 mm
Element diameter 15.8 mm
Radiating surface 15201 mm²
Resistance 1.63 ohm
Let us try a combination of six elements in parallel. The total resistance in each phase will be
Current = \sqrt{5000 / 0.271}=136 \ \mathrm{~A} / \text { phase }
Voltage = 136 × 0.271 = 36.8 V/Phase
kVA = 5 kW
Primary voltage 380/\sqrt{3} = 220 V
The transformer required is 380 V, 3 ph, 30 amp-primary; and 20, 30, 40, 50, 80 V taps on secondary with current 200 A, kVA ~ 20–25
Table 9.7 Extract from Specification Data for Silicon Carbide Heaters (Norton)
| Effective heating length (mm) | Width of furance chamber (mm) | Diameter | Nominal radiating surface (mm^2) | Thickness of furance wall (mm) | Overall length (mm) | Nominal resistances (ohm) |
| – | – | 9.5 | 9,120 | 76 \\ 127 \\ 165 \\ 76 | 483 \\ 584 \\ 600\\ 438 | 4.55\\ 4.62\\ 4.87\\ 3.24 |
| – | – | 11.1 | 10,639 | 127 \\ 165 \\ 191 \\ 229 \\ 76 | 584\\ 600\\ 711\\ 787 \\ 483 | 3.40\\ 3.43\\ 3.46\\ 3.50\\ 2.50 |
| 305 | 279 | 12.7 | 12,161 | 127 \\ 165 \\ 191 \\ 229 \\ 76 | 584\\ 600\\ 711\\ 787 \\ 508 | 2.54\\ 2.57 \\ 2.59\\ 2.62\\ 1.57 |
| – | – | 15.8 | 15,201 | 127 \\ 165 \\ 191 \\ 229 \\ 76 | 610\\ 686\\ 737\\ 813 \\ 508 | 1.60\\ 1.62\\ 1.63\\ 1.65\\ 1.07 |
| – | – | 19 | 18,239 | 127 \\ 165 \\ 191 \\ 229 \\ 191 | 610\\ 686\\ 737\\ 813\\ 787 | 1.08\\ 1.10\\ 1.11\\ 1.12\\ 0.063 |
| 25.4 | 24,323 | 229 \\ 343 | 864\\ 1,092 | 0.064\\ 0.066 |
