Air heater

An air heater (also referred to as air preheater or regenerative air heater) is a common component of large boilers with the purpose of increasing its overall efficiency. It can be used alone, to replace a recuperative air heater, or in conjunction with a steam coil, all three with the purpose of increasing efficiency. The regenerative air heater functions through heating air which his being drawn in by the forced draft fan using the exhaust from the boiler. Through utilizing the heat from the exhaust and decrease the energy lost with the exhaust, the regenerative air heater increases the energy input to the boiler.^[1]

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Classifications

The two main classes of regenerative air heaters are Ljungström and Rothemühle.^[2] The main difference between the two arises from the stationary mechanics of the Rothemühle heater compared to the rotation of the heat transfer elements within the Ljungström heater.

The Ljungström is the most common type of regenerative air heater used.^[1] The combustion air being taken in is separated from the exhaust air in the cylindrical shell. As the heating element rotates, the surface area between the two gasses increases, which results in a higher overall heat transfer. This then increases the efficiency of the boiler combustion process.

The Rothemühle is less common than the Ljungström due to its complexity. Instead of having a rotating heating element, a moving duct directs the flow of each gas so that they have more contact due to increased surface area. The end result of each system is the same: increased surface area between the two gasses causes less heat to be lost and efficiency to be increased.^[1]

Operational Concerns

There are three main operational concerns to be considered when using a regenerative air heater of any kind: corrosion, erosion, and plugging and cleaning.

Corrosion

Corrosion occurs when the exhaust passing through the system loses heat too quickly. The sulfur dioxides and sulfur trioxides in the exhaust condense, combine with the hydrogen in the air, and form sulfuric acid (H₂SO₄). This acid can eat away at the heating element material. In order to reduce the effects of corrosion, several precautions must be taken. First, the systems temperature must be constantly monitored. Second, one can take precautions in the manufacturing of the system. Specifically, making the heating element out of stainless steal reduces the effects. Also, thicker heating elements, or ones with a resistant coating, are less affected by correction. ^[1]

Erosion

Similar to corrosion, erosion occurs due to the toxins in the exhaust gas. Specific, the exhaust carried fly ash, ash from the burning of the coal which was light enough to be carried by the air current. This fly ash crashes into the heating element causing erosion over time. There are several precautions one can take to reduce erosion. The system can be designed to minimize the velocity of the gas so the effects of erosion are reduced. Also, periodic cleaning of the system can decrease the effects of erosion as well as increase the functionality of the system. Ash which plugs the system not only erodes the heating element, but it also reduces the heat transfer, thus lowering efficiency. ^[1]

Advantages and disadvantages

There are several advantages and disadvantages of regenerative air heaters over steam coils and recuperative air heaters. Both recuperative air heaters and steam coils serve the same purpose as a regenerative air heater. Regenerative air heaters have moving parts while recuperative air heaters do not. Unlike regenerative and recuperative air heaters steam coils use auxiliary steam from the boiler instead of exhaust gas from the boiler to preheat the combustion air.

Recuperative

Compared to regenerative, recuperative air heaters are considerably more cumbersome and large. As their size increases as does their efficiency. So, due to space limitations, recuperative systems tend to be limited in their efficiency. However, recuperative air heaters require a much lower initial financial investment and result in lower leakage than regenerative.

Steam coils

Steam coils are even less efficient than recuperative and require the highest operating cost of all three options. However, they are the smallest in size and the cheapest option.

Regenerative

Regenerative systems, while the most commonly used, do have their drawbacks. Specifically, they have the highest leakage of all three systems as well frequent maintenance requirements. Their advantages over the other systems result in their popularity. They are compact, efficient, and easier to repair than the other system.

References

1. ^ ^{a b c d e} Babcock, and Wilcox. Steam: It's generation and Use. 40 th ed. Barberton, Ohio: The Babcock & Wilcox Company, 1992
2. ^ Drbal, Lawrence, Patrica Boston, Kalya Westra, and R. Bruce Erickson. Power Plant Engineering. Norwell, Massachusetts: Kluwer Academic Publishers, 1996