Furnaces direct-/indirect-fired

Direct-fired combustion equipment is that in which the flamt of combustion are used to achieve the desired result by r and convection. Common examples include rotary kilns and open-hearth furnaces. Indirect-fired combustion equipment is that in wh e... 

A furnace may be direct-fired or indirect-fired. The indirect-fired is known as a muffle furnace, and in such furnaces the combustion gases are separated from the stock being heated to prevent contamination. 

Indirectly fired furnaces find applications in metallurgy, paint enameling, the pharmaceutical industry, and other situations where it is necessary to control the chemistry of the furnace atmosphere [186]. An important aspect of the heat treatment of metals, for example, is the effect of the furnace atmosphere on the stock being heated. In most cases, the need is to minimize or eliminate completely the undesirable effects of furnace gases, such as oxidation or decarbonization. Both directly [187,188] and indirectly [189,190], natural-gas-fired furnaces have been analyzed, but the details cannot be included here. 

Furnace Classification by Direct-Fired or Indirect-Fired... 

The heating capacity of furnaces that are equipped with flame-in-tube muffles (radiant tubes) is limited by the heat that can be radiated from the tubes. The heating capacity of an indirect-fired furnace is less than that of a direct-fired furnace having... 

Stack annealing furnaces are bell-type furnaces in which stacked coils of steel wire or strip are heated to about 1250 F (680 C), copper heat treated at 500 to 900 F (2.60 to 480 C) (see figure 3.12). They may be direct fired or indirect fired, depending on the materials being annealed. Cover annealing furnaces have a gas-tight inner cover or bell within the bell furnace in which a prepared atmosphere is circulated by a base fan. Radiant tubes may be used instead of an inner cover. (See fig. 3.22.)... 

Unless otherwise specified in this book, furnaces and ovens are assumed to be direct fired. Indirect-fired units use radiant tubes or muffles to protect the load from contact with the poc. 

The raw materials for synthetic graphite are petroleum coke and coal tar pitch. The formed shapes from petroleum coke and pitch are fired in baking furnaces at 1,200 °C, and then they are installed in graphitation furnaces (direct or indirect... 

Thermal procedures for soil treatment include (1) fluidized>bed furnaces (2) chain-grate furnaces (3) direct-fired drum furnaces (rotary kilns) with afterburners (4) indirect-fired drum furnaces (pyrolysis furnaces) with afterburners and (5) plasma high-temperature technology. 

Indirect or direct fired heaters are widely used in the process industries. Heat loss is kept to a minimum by refractory coatings on the furnace wall. Any material in the fuel that is corrosive or forms excess soot has to be avoided. Usually 20-25% excess air is required for fuel oil vs. 5-10% for gaseous fuel, hence the latter is more economic. Energy in the exit flue gas not used to heat water or a product can be recovered by heat exchangers that generate additional steam or preheat the entering air. 

There are two types of air heaters which can be used in a spray-drying system, e.g., direct air heaters and indirect heaters. Direct air heaters, such as direct gas or oil fired furnace, can be used whenever the contact between combustion gas and spray is acceptable. When products of combustion of fossil fuels cannot contact with the spray, an indirect heater, such as indirect steam air heater, indirect gas, or oil-fired heater, is recommended. Interested readers can find more details about it in the literature (Matsers 1991). 

If the burner firing rate is increased, the gas volume and temperature increases thus, the gas flow velocity increases. The cumulative heat transfer from hot gases to loads (directly, and indirectly via refractory to loads) is a function of time. Higher velocity shortens the time for heat transfer to be accomplished within a given flow path length (furnace size) thus, the gases remain at higher temperature. 

Fired shapes are loaded in graphitization furnaces. In the Acheson process (indirect graphitation), the electroconductive media is carbon filler, and the shapes are heated due to the electroresistivity of the filler. In direct graphitization, the filler is used only to make the reduction atmosphere and to prevent heat losses. The electroconductive media are the shapes of carbon cathode blocks. They are placed lengthwise to the current leads and the movable transformer is switched to the leads, so the hot zone of the furnace is also moving (Fig. 2.29). 

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