GENERAL TECHNICAL DATA

COMBUSTION SPACE AND FLUE PASSAGES
The efficiency and the gas input are also effected by the insulation type and thickness, the degree of "tightness" (no air leaks), the flue size and placement and control of secondary air (if any).

The total air/gas mixture that can be fed into the combustion space will vary according to the mixture pressure developed by the burner design. The mixture pressure is the combination or result of the combustion air and gas pressures produced before the burner tip. Forced draft burners develop a high mixture pressure and it is possible to size the combustion space for approximately 150-250,000 Btu/ft3 (4.5-7.5 MJ/litre) if using this burner type. Atmospheric burners would be restricted to about 20-40,000 Btu/ft3 (.6-1.2 MJ/litre) for low gas pressure torches to 50-90,000 Btu/ft3 (1.5-2.7 MJ/litre) for higher pressure types.

A package burner using a fan blower that may develop a lower air pressure would require the free space to be sized for approximately 65-150,000 Btu/ft3 (2-4.5 MJ/litre).

Although there are several other factors that will dictate the flue size, a rough estimate for an atmospheric burner is 5-7,500 Btu/inch2 (.8-1.2 MJ/cm2). A forced draft type burner flue exit will vary from 15,000 Btu/inch2 (2.5 MJ/cm2) for a pot furnace to 30,000 Btu/inch2(5 MJ/cm2) for an oven type. It is important not to exceed a heat transfer rate of approximately 7,000 to 10,000 Btu/hour per square foot of exposed pot surface for long pot life.

SIMPLIFIED FORMULA TO CALCULATE HEAT INPUT
The specific heat of the metal x weight of the product x temperature to reach x efficiency of the process. The last figure is the most difficult to estimate accurately however well designed, insulated furnaces with a separate flue and sealed burner can be expected to be approx. 30% efficient. Open type processes could be only 10-15% efficient.

Other relevant factors are the losses likely through the insulation, the flue losses due to excess air, energy conversion losses and the heat required to maintain a constant state. This can indicate the turn down required for the burner and the type of burner to be used.

RELEVANT PROCESS HEATING DATA:

 METAL AVERAGE SPECIFIC HEAT APPROX. MELTING POINT DENSITY
kj/kg.K Btu/lb.F oC oF kg/m3 lb/ft3
Aluminium 0.95 0.22 660 1220 2700 170
Brass 0.42 0.10 1250 2280 8450 530
Bronze 0.46 0.11 1180 2150 8000 500
Cast Iron 0.50 0.13 1260 2300 7100 450
Copper 0.42 0.10 1080 1980 8930 560
Gold 0.13 0.03 1060 1900 20000 1200
Iron 0.50 0.13 1500 2800 7850 490
Lead 0.15 0.03 330 630 11400 710
Magnesium 1.04 0.25 650 1200 1730 108
Silver 0.25 0.05 960 1760 10500 655
Solder 0.16 0.04 220 420 9300 580
Steel 0.50 0.12 1400 2550 7800 490
Tin 0.25 0.05 230 450 7200 455
Zinc 0.40 0.09 420 780 7100 445

SIZES OF CRUCIBLES

CRUCIBLE NUMBER

APPROX. HEIGHT (mm)

APPROX. DIAMETER (mm)

BRASS (kg) (APPROX.)

ALUMINIUM (kg) (APPROX.)

20

280

210

30

9

25

280

225

36

11

30

295

235

44

14

35

305

250

50

16

40

320

260

58

18

50

350

285

75

23

80

400

320

117

36

100

425

345

146

45

150

465

375

220

68

200

510

410

295

90

250

545

440

365

115

300

575

465

440

135

400

620

500

585

180

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Copyright Australian Combustion Services Pty Ltd 1997-2004