Page 48 - Boiler plant and distribution system optimization manual

Energy Management Basics for Boilers and Systems 33

The efficiency improvement for each· one The efficiency improvement for each 10 de-
percent change in excess air varies with the stack grees F change in stack temperature varies with
temperature (Fig 3.2). To estimate efficiency the excess air level (Fig 3.3). To estimate efficiency
change, multiply the factor (left) corresponding change, multiply the factor (left) corresponding
to the stack temperature times the change in ex- to the excess air level times each 10°F change in
cess air. stack temperature.
Example, what will the efficiency change be
with a stack temperature of 500°F if the excess Example: What will the efficiency change be
air is reduced by 50%? The factor from Fig 3.2 at with an excess air level of 60% if the stack tem-
500°F is .075; this multiplied by 50% excess air re- perature is reduced by 100°F? The factor from Fig
duction is a 3.75% improvement in efficiency. 3.3 at 60% excess air is 0.325; this multiplied by
100°F/10 a 3.25% improvement in efficiency.

Figure 3.2, Efficiency change(%) for each 1% change in excess air at
various stack temperatures.