Where: Tg = Final stack flue temperature o

Tce = Flue gas air preheater recommended average cold end temperature

Ta = Ambient air temperature

C) Continue the line to the curve for a stack temperature rise of 500°F and read the current combustion efficiency to be 81.4%.

STEP 3: Determine the proposed boiler combustion efficiency using the same figure.

D) Repeat steps A through C for the proposed combustion efficiency assuming the new exhaust stack temperature conditions. Read the proposed combustion efficiency to be 85.0%.

STEP 4: Determine the fuel savings.

E) Percent fuel savings = [(new efficiency) - (old efficiency)]/(new efficiency)

Percent fuel savings = [(85.0%) - (81.4%)]/(85.0%) Percent fuel savings = 4.24%

F) Fuel savings =(current fuel consumption) x (percent fuel savings) Fuel savings = (1,032,460 therms/yr) x (4.24%) Fuel savings = 43,776 therms/yr


As with the earlier example, this analysis methodology assumes that the results of the combustion analysis and boiler load are constant. Obviously this is an oversimplification of the issue. Because the air-to-fuel ratio (excess air level) is different for different boiler loads, a more thorough analysis should take this into account.

Figure 5.7 Combustion efficiency curve for stack temperature reduction example.

Additional considerations in flue-gas heat recovery include:

1. Space availability to accommodate additional heating surface within furnace boundary walls or adjacent area to stack.

2. Adequacy of forced-draft and/or induced-draft fan capacity to overcome increased resistance of heat-recovery equipment.

3. Adaptability of soot blowers for maintenance of heat-transfer-surface cleanliness when firing ash-and soot-forming fuels.

4. Design considerations to maintain average cold-end temperatures for flue gas/air preheater applications in cold ambient surroundings.

5. Modifications required of flue and duct work and additional insulation needs.

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