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An air preheater (APH) is a general term to describe any device designed to preheat the combustion air used in a fuel-burning furnace for the purpose of increasing the thermal efficiency of the furnace.

In particular, this article describes the combustion air preheaters for the large fuel-burning furnaces used to generate steam in thermal power plants. The air preheater increases the steam generator's thermal efficiency by preheating the combustion air with heat recovered from the hot combustion flue gases (see the adjacent diagram).

Types
The two most often used categories of air preheaters in thermal power plants are regenerative air preheaters and tubular air preheaters.

Regenerative air preheaters
The two most common types of regenerative air preheaters are


 * The rotating-plate regenerative air preheater, often referred to as a RAPH. The RAPH was invented by Fredrik Ljungström, who was a Swedish engineer, and is also called a Ljungström air preheater.
 * The stationary-plate regenerative air preheater, often referred to as a Rothemuhle because that is the German town whre the original manufacturer produced them for many years.

Regenerative air preheaters may also be categorized as recuperators, which are special types of heat exchangers designed to recover or reclaim heat in order to reuse or recycle it.

Rotating-plate regenerative air preheater
The rotating-plate air preheater (RAPH) consists of a central rotating-plate element installed within a casing that is divided into sectors. There are three basic designs for the rotating-plate element:


 * The bi-sector design has two sectors.
 * The tri-sector design has three sectors.
 * The quad-sector design has four sectors.

In the tri-sector design, the steam generator's hot flue gas flows through the largest sector (usually spanning about half the cross-section of the casing) and transfers some of its heat into the heat-absorbing material within the rotating wheel element. The cooled flue gas is then routed to further treatment in dust removal and other equipment before being vented from the flue gas stack. Ambient air is blown through the second, smaller sector by a centrifugal fan and absorbs heat from the heated material as it rotates through that smaller sector. The heated air then flows into the steam generating furnace as combustion air. The third sector is the smallest one and it heats a portion of the ambient air which is then routed into the coal pulverizers and is used to transport the coal-air mixture to coal burners. Thus, the total air heated in the RAPH provides: heated primary combustion air, heated air to remove moisture from the pulverised coal and carrier air for transporting the pulverised coal to the coal burners. Since the flue gas pressure is lower than the pressure of the air being heated, there is some small leakage (between the sectors) of flue gas into the air.

The bi-sector design is used in thermal power plants burning fuels (such as oil or gas) that do not require pulverizing or removal of moisture and therefore have need for heated air other than for combustion air.

The quad-sector design has a large sector heated by flue gas and three air-heating sectors: one is for the combustion air and that sector is flanked by two smaller air sectors. In applications such as circulating fluid bed (CFB) combustion systems where the differential between the air pressure and the flue gas pressure is even higher than in a conventional coal-fired steam generator, flue gas pressure, such a design is ideal since it acts to reduce the leakage of air into the flue gas. The rotating wheel element rotates quite slowly (around 3-5 revolutions per minute) to allow optimum heat transfer first from the hot exhaust gases to the element and then, as it rotates, from the element to the air in the other sectors.

Construction features
The heat-absorbing material in the rotating wheel element consists of vertical corrugated plates pressed into steel baskets, with sufficient space between the plates for the flue gas to pass through. The plates are corrugated to provide more surface area for the heat to be absorbed and also to provide needed rigidity. The baskets are designed to be replaceable as needed.

The vertical shaft that rotates the wheel is supported on thrust bearings at the lower end lubricated with an oil bath that is cooled by water circulating in coils inside the oil bath. Cooling of the bottom end of the shaft is needed since that is where the hot flue gas enters the preheater. The top end of the shaft has a simple roller bearing to hold the shaft in a vertical position.

Radial supports and cages for holding the corrugated plate baskets in position are attached to the rotating shaft. Radial and circumferential seal plates are also provided to minimise leakage of flue gas or air between the sectors.

For cleaning of the baskets while in operation, steam jets are provided to blow any fly ash (deposited by the flue gas) into an ash hopper below the preheater.

The rotating shaft is driven by a motor and gearing. To avoid uneven thermal expansion and contraction resulting in damage to the rotating wheel, the rotation must be started before starting the steam generator and must also be kept in rotation for some time after the steam generator is shut down.

The baskets of corrugated plates are subject to abrasive and corrosive wear from the fly ash and corrosive gases in the flue gas. Hence frequent replacements are required and new baskets are always kept on hand and ready for use.

Stationary-plate regenerative air preheater
The heat absorbing element in this type of regenerative air preheater is stationary rather than rotating. Instead, the air ducts in the preheater are rotated so as to alternately expose sections of the heating absorbing element to the upflowing air.

The hot flue gas enters at the top of the preheater and flows down through those exposed sections of the stationary heat-absorbing element that are not blocked by the rotating air outlet ducts, thus heating those sections of the stationary element. As the air ducts slowly rotate around, they pass over the heated sections and the incoming air is heated as it flows upward through those heated sections.

As indicated in the adjacent drawing, there are rotating inlet air ducts (inside the outer casing) at the bottom of the stationary heat absorbing element as well as the rotating outlet air ducts at the top of the stationary element.

The basic heat transfer principals of the stationary-plate regenerative preheater are the same as for the rotating-plate regenerative preheater. The table below provides a comparision of some design parameters between the rotating-plate and stationary-plate preheaters:

Tubular type
Tubular air preheaters may have a number of configurations:


 * 1) A bundle of vertical tubes through which the flue gas flows downward (see adjacent diagram) and exchanges heat with ambient air flowing horizontally across the exterior of the tubes. Baffles are usually provided so that the air flows across the tubes a number of times. As shown in the adjacent diagram, the air flow across the tubes three times and is referred to as 3-pass tublular air preheater.
 * 2) The same as (1) above except that the flue gas flows upward rather than downward.
 * 3) A bundle of horizontal tubes through which the air flows and exchanges heat with the hot flue gas flowing downward across the tube bundle. In some designs, there may be three separate horizontal tube bundles one above the other. The air enters the lower tube bundle from the right-hand side, exits on the left-hand side and then enters the middle tube bundle on the left-hand side and exits on the right-hand side. Finally, the air enters the upper tube bundle on the right-hand side and exits on the left-hand side. In essence, such a design is similar to the 3-pass design of (1) above except that the air is in the tubes rather than outside the tubes.

The tubular preheater ductings for cold and hot air require more space and structural supports than a rotating preheater design. Further, due to dust-laden abrasive flue gases, the tubes outside the ducting wear out faster on the side facing the gas current. Many advances have been made to eliminate this problem such as the use of ceramic and hardened steel.

Many new circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) steam generators are currently incorporating tubular air heaters offering an advantage with regards to the moving parts of a rotary type.

Dew point corrosion
Dew point corrosion occurs for a variety of reasons. The type of fuel used, its sulfur content and moisture content are contributing factors. However, by far the most significant cause of dew point corrosion is the metal temperature of the tubes. If the metal temperature within the tubes drops below the acid saturation temperature, usually at between 190°F (88°C)and 230°F (110°C), but sometimes at temperatures as high as 260°F (127°C), then the risk of dew point corrosion damage becomes considerable.