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:

SIZES OF CRUCIBLES

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