Furnace heat transfer

Input/Output Performance Parameters for Furnace Operation The term firing density is typically used to define the basic operational input parameter for fuel-fired furnaces. In practice, firing density is often defined as the input fuel feed rate per unit area (or volume) of furnace heat-transfer surface. Thus defined, the firing density is a dimensional quantity. Since the feed enthalpy rate Hf is... 

Basically, a steel ladle is a relatively small, refractory-lined vessel that during preheating behaves very much like a small furnace. Heat transfer from the preheater to the ladle is primarily by convection and radiation. The convection can be described using... 

When a deposit layer is formed, surface temperature is increased and wall emissivity decreased. However, the superposition of these effects on rjf is less than a pure summation since, by decreasing e, the net heat flux to the layer is reduced, thus retarding the increase of surface temperature to a certain extent. The simple well-stirred analysis, carried out for constant surface temperatures yields typically a drop of furnace efficiency rif of 5.5 percentage points corresponding to an increase of furnace exit temperatures of HO K for a decrease of from 0.8 to 0.4. Detailed 3-D furnace heat transfer calculations carried out for the same decrease of but allowing a variation of surface temperatures yield typically losses of rif only 3.5 percentage points corresponding to increase of Tgx of only 70 K. 

The 3-D furnace heat transfer model has been verified several times in the past and has successfully predicted performance of gas, oil, coal, and slurry fired boilers located all over the world (9-12). The generalized performance predictions of the impact of wall deposits on heat transfer is shown in Figure 13 for a number of p.f.. 

M. P. Mengtt and R. Viskanta, An Assessment of Spectral Radiative Heat Transfer Predictions for a Pulverized-Coal Fired Furnace, Heat Transfer—1986, 2, pp. 815-820,1986. 

To keep the temperature differences small within the load(s) across the furnace, heat transfer beneath the load from the gas blanket to piers and product must be kept relatively low. To minimize heat transfer from the gas stream, the thickness of the stream must be very small (8 to 12 in., or 200 to 300 mm), and the percentage of diatomic gases in the products of combustion must be low. Excess air will lower the percentage of diatomic gases and reduce the temperature drop of the gas sdeam under the load from the burner wall to the opposite wall. 

Concurrent Phase 2.2. As all of the solid heat-receiving surfaces in the furnace begin to absorb heat, their surface temperatures rise. The refractory surfaces, being poorer conductors, experience a more rapid rise in their surface temperature, and therefore become good re-radiators, helping to transfer more heat to the loads. This secondary radiation (fig. 5.5) has always been considered to be a major portion of all the heat transferred to the loads in furnaces operating above about 1400 F (760 C). Many people have ignored gas radiation, but it is a big factor in furnace heat transfer. 

A direct-fired tubular reactor is used in the thermal cracking of light hydrocarbons or naphthas for the production of olefins, such as ethylene (see Fig. PI.9). The reactants are preheated in the convection section of the furnace, mixed with steam, and then subjected to high temperatures in the radiant section of the furnace. Heat transfer in the radiant section of the furnace takes place through three mechanisms radiation, conduction, and convection. Heat is transferred by radiation from the walls of the furnace to the surface of the tubes that carry the reactants, and it is transferrcd through the walls of the tubes by conduction and finally to the fluid inside the tubes by convection [8]. 

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