Gas Circulation in Furnaces

The lower the temperature to which the material is to be heated, the greater the necessity for thorough gas circulation in the heating chamber, especially if loads are placed compactly in the furnace or oven (e.g., piled or coiled material that is to be heated rapidly and uniformly. (See sec. 6.6.)... 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

Indirect heating through a refractory vail, as in the Destrugas process, is very slov. It takes about 20 hours to heat a slovly descending layer of refuse up to the desired temperature of 950 - 1050°C. This results in a lov capacity per unit reactor volume. Even inductive heating of steel balls, admixed to the refuse, as in early WSL-experiments, does not sufficiently accelerate the heating process. Better results are obtained by forced circulation of pyrolysis gas, as in the Lambiotte vood carbonization furnace and in the WSL cross-flov reactor. 

In Fig. 33.5, the level in the separator and the pressure in the gas circulation system are the most important indicators of material contents. In addition, the pressure in the furnace is indicative of the material contents of the furnace. 

FIG. 12-31 Circulating gas impinging on a large solid object in perpendicular flow, in a roller-conveyor Furnace. 

Gas or oil fuel is burned in a furnace or boiler (heat exchanger) to heat air or water, or to make steam that carries the heat absorbed from the combustion process to the rooms in the house, as shovra in Figure 2. This IS accomplished by circulating the warm air through ducts directly to the rooms, or by circulating hot water or steam through pipes to baseboard hot water con-... 

Coil boilers are available as fully automatic package units, typically ranging in size from 15 to 300 boiler hp (500 lb/hr-10,000 lb/hr). Most designs employ forced circulation and a single, continuous spiral, helical-wound coiled tube that hangs inside the furnace. The coil is subjected to intense heat release from a gas or oil burner. 

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