Saving Fuel by Preheating Combustion Air

To determine how much fuel can be saved by preheating air, read %available heat from figure 5.1 with and without preheated air, and use equation 5.7. In rare cases, fuel also can be preheated, but not if the fuel contains hydrocarbons that may crack when heated and deposit on the heat transfer surfaces. Preheating fuel usually is not justifiable if the fuel has a heating value greater than about 350 Btu/ft (13 MJ/m ). 

Example 5.2 A furnace is needed to melt 25 000 pounds of aluminum per hour from cold to 1450 F, which requires 505 Btu/pound, or 25 000 x 505 = 12 625 000 Btu/hr heat to the load. It is estimated that the wall, storage, opening, and watercooling losses are estimated as 1 000 000 Btu/hr. Thus, the heat need or required available heat = 12 625 000 -P 1 000 000 = 13 625 000 Btu/hr. 

If it is then decided to add an air preheater to accomplish heat recovery, the required gross heat input to the furnace will equal required available heat or heat need (%available heat/100) = 13 625 000 h- (48%/10) = 28 400 000 gross Btu/hr. A security factor of at least 25% should be used therefore, the design input should be (28.5 kk Btu/hr) (1.25) = 35.6 gross kk Btu/hr. 

Added benefits from preheating combustion air are faster burning, resulting in a hotter burner wall, and lower flue gas exit temperature. The desired prompt heat release is difficult to evaluate. An interesting facet of the available heat charts (figs. 5.1 and 5.2) is that the curves jc-intercepts (where available heat is zero) are theoretical adiabatic flame temperatures, or hot-mix temperatures mentioned earlier. For the 

Counterflow types deliver the highest air preheat temperature, but parallel flow types protect the recuperator walls from overheating. Therefore, the hot flue gases are often fed first to a parallel flow section and then to a counterflow section to benefit from both advantages. 

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