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ASAC- Excess Air Decreases Combustion Efficiency


Excess air wastes money from your operations by decreasing firetube combustion efficiency in two ways:

  • The sensible heat used to warm the excess air to exhaust temperature is lost out the stack.
  • Excess air of only 16.2% (3.4% excess oxygen) lowers the flame temperature about 260 degrees, thus reducing the temperature difference between the flame and the process so that less heat is transferred per unit area of firetube. Figure 1 illustrates the effect of excess air on overall efficiency.

Typically, firetubes with uncontrolled secondary air operate with stack exhaust gas temperatures of 600 to 1100 degrees and excess air of 100 to 600 percent (11 to 18.2% excess oxygen) depending on the process loads and the diameter of the firetube. This operating envelope covers efficiencies ranging from less than 10 to 60%, with large diameter firetubes tending to the low end of the efficiency range. The orange shaded area of Figure 1 illustrates the typical efficiency range for natural draft firetubes with uncontrolled secondary air.

Large Firetubes Are Prone To Low Combustion Efficiencies
Field performance tests have been conducted on various sizes and types of equipment to determine typical combustion efficiencies. Large diameter firetubes generally exhibit lower efficiencies than small diameter firetubes. This is because a large firetube has the ability to draw excess air, due to stack draft-in proportion to its cross-sectional area (a function of its diameter squared), whereas the ability to deliver energy from the firetube to the process is in proportion to the surface area (a function of the diameter). Therefore large firetubes are able to draw huge quantities of excess air and, if left uncontrolled, are prone to low efficiencies. Field tests of large firetubes yielded combustion efficiencies ranging from 8 to 40% depending on operating mode and service. Generally, an efficiency of 25 to 35% is normal for large diameter firetubes.

Field tests of large firetubes with uncontrolled secondary air have yielded combustion efficiencies ranging from 8 to 40% depending on operating mode and services.

Controlling Excess Air Increases Combustion Efficiency
The temperature of the stack exhaust is indicative of the energy wasted by inefficient combustion. Obviously, any heat contained in the exhaust is not used in heating the process, and cooling the exhaust temperature is a primary goal in attaining increased efficiency. The best way to cool the stack exhaust and to increase efficiency is by controlling the excess air in a narrow band of about 10 to 25% (2.1 to 4.5% excess oxygen). Reducing the excess air to this range will typically cool the exhaust gases to about 400 to 600 degrees and increase the overall efficiency to 75 to 80% (higher heating value basis) as illustrated by the blue region in Figure 1.

Off-On Firetube Operations Waste Valuable Heat
In addition to operating with uncontrolled secondary air, many firetubes operate in off-on (snap acting) mode. As shown in Figure 2, during the off cycle, the stack continues to draft ambient air into the firetube, thereby acting as a negative heat exchanger to the process. Large quantities of heat are lost from the process during the off cycle. Equipment designed with multiple firetubes suffer this negative effect if one or more of the firetubes is idle. Therefore, control of the flow of air into the combustion chamber while the firetube is idle is also an important objective in attaining the highest efficiency for a natural draft firetube.


FIGURE 1 - Source: Process Heat tip Sheet #2, U.S. Department of Energy, DOE/GO-102002-1552

Off-On Firetube Operations Waste Valuable Heat
 


FIGURE 2 - Effect of off-on operations on firetube efficiency

 

 

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Portions of the process described are covered by U.S. Patents: 5,766,313. 6,238,461. 6,364,933. 6,551,379. 6,984,257. 7,531,030. 7,905,722 and patents pending ASAC and VRSA are patent pending.