The air preheater is a critical, yet often overlooked, component of the boiler combustion air system. Evaluating and optimizing a heater’s performance is difficult given how entwined it is with the entire combustion system and the lack of standardized calculation tools. Reducing leakage by using modern seal technology will improve combustion efficiency, maintain fan performance, and keep your downstream air quality control equipment operating within spec.
The regenerative air heater or air preheater (APH) on a large utility boiler often accounts for about 10% of the unit’s thermal efficiency. Its performance is so critical that just a 10F change in gas exit temperature can change the boiler efficiency by a quarter of a percent, representing hundreds of thousands of dollars per year in purchased fuel cost. For an aging fleet of boilers operating with ultra-low-NOx firing equipment, accurate measurement, control, and containment of the air and flue gas flowing through the APH is critical.
The major advantage of using an APH is that it is the least expensive heat-recovery device available that is capable of operating in the harsh environment of a fossil-fueled boiler’s flue gas exhaust. A major drawback of the regenerative APH is the undesired leakages that are inherent to its design.
Air leakage has the largest single effect on APH performance. Ignore the health of your APH long enough and you’ll soon experience some combination of corrosion, fouling, ammonium bisulfate plugging, increased auxiliary power consumption, and higher-pressure differentials that can limit combustion air fan operation.
On balanced-draft boilers, additional air in-leakage upstream of the APH from leaky penthouses, casings, and expansion joints quickly makes accurate control of excess oxygen problematic. Also, both APH leakage and upstream air in-leakage degrade air pollution control equipment performance by increasing the velocity of suspended fly ash and reducing electrostatic precipitator residence time. (See “Real-Time Monitoring System Measures Air In-Leakage,” August 2010 and “Air Heater Leakage: Worse Than You Think,” April 2006 in our archives at http://www.powermag.com.)
APH Design Fundamentals
A typical APH is illustrated in Figure 1. A regenerative air heater captures the heat in boiler exhaust gases by passing them over a heat-absorbing metallic element. In this most common design of a regenerative APH (Ljungström) the element is continually rotated so that it alternately contacts the hot gases (red arrow) and the cool inlet air (blue arrow) produced by the plant’s forced-draft (FD) fans. Typically, about 50% to 60% of the total heat content of the exit gases is captured and recycled, resulting in about a 10% improvement in boiler efficiency when compared to an identical unit without an APH.
Also shown on Figure 1 are the various leakage paths through the APH. Each leakage type is discussed in detail below.
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| 1. Circumferential leakage through an APH. The left blue line represents the bypass seal leakage around the air preheater into the warm airflow. The bottom blue line represents the bypass seal leakage (also called peripheral seals) passing the axial seals into the gas path. The red line on the right represents the bypass seal leakage passing around the air preheater (APH) into the cold gas flow. The top yellow arrows represent the hot radial seal leakage, while the bottom yellow arrows represent the cold radial seal leakage. Courtesy: Storm Technologies Inc. |
Sealing these dynamic structures is extremely difficult due to their large diameter (up to 60 feet across) and the large temperature difference between their hot and cold ends (about 400F). Together, these characteristics produce significant radial thermal expansion difference between the hot and cold sides of the APH’s rotor after unit start-up. It’s not uncommon for the outer edges of a large APH at operating temperature to droop or “turn down” by 3 inches or more compared with the cold condition (Figure 2).
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| 2. Thermal turndown. The typical APH may be as much as 60 feet in diameter. When the APH rotor is heated from a cold condition (blue), thermal expansion (yellow) can cause the rotor to droop or “turn down” up to 3 inches on the periphery. Knowing the amount of turndown is important when presetting the seal position before operation, because seal positions will change as the rotor warms to its operating temperature. Source: Storm Technologies Inc. |
This thermal distortion droop opens gaps in the sealing surfaces that separate the cold incoming air from the outlet gases as well as in the sealing surfaces around the circumference of the APH. Turndown changes the gaps between both the radial and circumferential seals and their respective sealing surfaces where gas and air bypass the rotor and heat exchange elements.
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