Table 85 Waste Heat Streams from Reheat Furnace Fired with Natural

Composition

Exhaust Stack: Cooling Product:

Combustion Products Water Steel Castings

Temperature (°F) Flow rate

Enthalpy rate (Btu/hr) Percent fuel energy rate

2200 57,000,000 cfh 265,800,000 64

191 36,000 gph 39,300,000 9

2000 50 tons/hr 34,900,000 8

heat recovered

Figure 8.13 Temperature distribution in recuperator.

¿stack gas

h comb. air, out h comb. air, in h stack gas, in — h stack gas, out

different. The annual heat recovery for each fuel was assumed to be proportional to the total consumption of each fuel, so that the heat recovered was found as heat recovered

ft3air ft3gas

6gal yr

Figure 8.13 Temperature distribution in recuperator.

also become the high limits for the combustion air, which in turn fixes the maximum allowable enthalpy for the combustion air. Thus the maximum rate of heat recovery is also fixed. Otherwise, the final temperatures of the two stream are based on an optimization of the economic opportunity. This is so because increased heat recovery implies increased recuperator area and thus increased cost. It may also imply higher combustion air temperatures with the resultant increase in fan operating costs and additional investment costs in high-temperature burners, combustion air ducts, and larger fans. In this case we assume that a 100°F temperature difference will occur between the preheated combustion air and the stack gases leaving the recuperator. Equation 8.14 cannot be used directly because the volume rates of fuel and air required are reduced with recuperation. However, if the air/fuel ratio is maintained constant, then Qstack/gas/Qcomb.air remains almost constant; then equation 8.14 can be written

This equation can be solved with the help of data from Figure 8.9 and the temperature relationship x 2040.8 ^^ x (24.6 -1.2)^ gal oil v ' ft3

This is an energy savings of

6.3 x 1011

1.39 x 109 x 1030 + 4.43 x 106 x 131,500 The predicted cost savings is

$1,308,900 for natural gas 950,200 for No. 2 fUel oil

$2,259,100 total

The complete retrofit installation is estimated to cost less than $2,000,000, and the payback period is less than one year. Two points must be emphasized. The entire retrofit installation must be well engineered as a system. This includes the recuperator itself as well as modifications and/or replacement of burners and fans, and the system controls. Only then can the projected system life span be attained and the capital payback actually realized. The cost of lost product must also be factored into the economic analysis if the installation is planned at a time that will cause a plant shutdown. Economics may dictate a delay for the retrofit until the next scheduled or forced maintenance shutdown.

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