The Ljungström® Regenerative Air Preheater is more widely used than any other type of heat exchanger for comparable

service in the steam generating industry.  The reasons for this worldwide acceptance are its high thermal effectiveness,

proven performance and reliability, effective leakage control, compactness of its design, and its adaptability to most any

fuel burning process. It is both designed and built to operate over extended periods with durable, uninterrupted service.

Simplicity of the design also makes it easy and economical to maintain in operation and at scheduled outages.

Available in a broad range of sizes, arrangements, and materials, Ljungström® Regenerative Air Preheaters are custom

engineered to meet the specific requirements and operating conditions of a variety of applications including:

Electrical Power Generating Plants 

  Fluidized Bed Boilers 

Package and large industrial boilers

Hydrocarbon and chemical

processes

Waste incinerators and drying systems

Flue gas and other reheating systems

Overview & Technology

The major component of the Ljungström® air preheater is a continuously rotating cylinder, called the rotor, which is

packed with thousands of square feet of specially formed sheets of heat transfer surface which are commonly called

elements.

The preheater structure consists of hot and cold end center sections, connected by two main pedestals on either

end.The rotor is commonly supported on the cold end center section by a

support bearing and maintained upright by a guide bearing located in the hot

end center section.Sector sealing plates and axial seal plates are attached

the center sections and main pedestals respectively to form separate gas

and air passages through the heat exchanger.

The rotor is enclosed by housing panels connected to the main

pedestals to form the preheater casing, with the drive unit, cleaning device

mechanism, and sealing surface adjusters all located externally to be

readily accessible while the unit is in operation.  Upper and lower

connecting plates are attached to the preheater casing to form a transition from the preheater rotor and casing shape

to the clients ductwork connections.

As the rotor revolves, waste heat is absorbed from the hot exhaust

gas passing through one-half of the structure. This accumulated heat

is released to the incoming air as the same surfaces pass through

the other half of the structure. The heat transfer cycle is continuous

as the surfaces are alternately exposed to the outgoing gas and

incoming air streams.

The advanced sealing system of the Ljungström® air preheater is a

result of an evolution of devices and methods to develop a sealing

system that is capable of successfully controlling and minimizing air-

to-gas leakage. The design takes advantage of normal thermal

growths to achieve effective sealing with a minimum of maintenance

requirements.The integrated sealing system comprises of proximity

seals and stationary sealing surfaces arranged to inhibit leakage

from the air side of the air preheater to the gas side.

Design Principles & Accessibility

Efficient heat transfer surfaces, effective sealing arrangements,

uncomplicated design, and easy accessibility are among the many reasons

why the Ljungström® air preheater is so often selected.  Metallic heat transfer

surfaces are contained in the rotor that turns at 1 to 3 rpm, depending on the

size of the unit. The rotor housing and rotor have sealing members to form

separate gas and air passages through the heat exchanger. The rotor drive

unit, cleaning device mechanism, rotor bearing assemblies, and sealing

surface adjuster are all located externally and are readily accessible while

the unit is in operation. Heat transfer surfaces are visible through lighted

ports during operation.

A view of the internal structure of the Ljungström® air preheater shows

the rotor post with top and bottom trunnion headers. The sealing

surfaces for the radial and axial seals are also shown.

A completed rotor with radial and axial seals attached to each sector-

shaped compartment. The pin rack at the bottom of the rotor accepts

the drive mechanism and also serves as a circumferential bypass

sealing surface.

Modular™ & Semi-Modular™ Rotors

The Air Preheater Company surpassed its own record for minimum installation costs with the introduction of the

Modular™ and Semi-Modular™ rotors.  Installation time can be reduced by as much as 34% over conventional field-

assembled designs.

Heat transfer surfaces are preinstalled in full-sector

modules that are prefabricated at the manufacturing plant

with all structural welds completed under controlled

conditions.

Shipping cost savings are realized because the modules

serve as carriers for the individual heat transfer surface

containers.  In addition, handling costs and assembly

time at the installation site are substantially reduced.

Full-sector modules are simply pinned (not welded) to the

rotor post to reduce field welding requirements.  This

method of attaching the sectors throughout the rotor assembly results in a discontinuous stress-free structure that

significantly reduces thermal distortion and its associated leakage.

Trisector Ljungström® Air Preheater

Semi-Modular™ Air Preheater

during construction

Designed for coal-fired applications, the trisector Ljungström® air preheater permits a single heat exchanger to

perform two functions: coal drying and combustion air heating.  Because only one gas duct is required, the need for

ductwork, expansion joints, and insulation is greatly reduced when compared with a separate air heating system. 

Equipment layout is simplified, less structural steel is needed to install the system, and less cleaning equipment is

required.

The duct arrangement of a trisector Ljungström® air

preheater shows the air and gas flows through the unit. 

The size and location of the primary air duct can vary,

depending on the flow and temperature requirements.

The design has three sectors -- one for flue gas, one for

primary air that dries the coal in the pulverizer, and one

for secondary air that goes to the boiler for combustion.

Horizontal-Flow and Industrial Size

Large horizontal-flow Ljungström® air preheaters

with rotors measuring 22.5 feet in diameter or

larger are supplied with full sector heat transfer

surface containers for all layers.  Unlike the shop-

assembled modular design for vertical flow, these

heat transfer surface containers are lowered into

the sector shaped compartments formed by the

diaphragms of the field-assembled rotor.  Smaller

horizontal-flow air preheaters are supplied with

multiple containers for each layer. Designed for

either vertical or horizontal flows, the industrial

Ljungström® air preheater is available in a full

range of sizes for gas flows of 20,000 to 780,000

lb./hr. The package Ljungström® air preheater is

completely factory-assembled.  The "S" and "R"

series Ljungström® air preheaters are similar to

the standard package unit, but are too large to

ship completely assembled.  Major components

are prefabricated to minimize field assembly.

A full sector heat transfer surface container

is lowered into the rotor of this large

horizontal flow Ljungström® air preheater,

measuring approximately 60 feet in

diameter.

Gas-to-Gas Heat Exchangers

The same design that is used in a Ljungström® Air Preheater can be used for another purpose in a power plant.

Downstream of the Ljungström® Air Preheater, there are several pieces of equipment designed to clean up the flue

gas before it exits the power plant through the stack. At each step, the flue gas loses some heat. If it loses too much,

by the time it reaches the stack it does not have enough energy to disperse properly. A power plant that has its

discharge settle as a cloud over a residential area is in serious trouble.

That is what a Ljungström® Gas-Gas heater is used to prevent. Located just before the stack, the unit salvages heat

from the flue gas just before it goes into the scrubber (which removes sulfurs) and then puts it back into the cleaned

flue gas. This provides the cleaned flue gas with enough heat to rise out of the stack and disperse.

Ljungström® Gas-Gas heaters tend to be shallow, as the amount of heat transferred is relatively small compared to

the air preheaters. There are also two other main diferrences.  The Ljungström® Gas-Gas heater operates in very low

temperatures. The combination of low temperature and sulfur content require the unit to be designed with the

consideration that there will be a significant amount of sulfuric acid condensation. For this reason, we used enameled

element and flake glass lining to prevent the equipment from corroding away.

An air preheater (APH) is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.

In particular, this article describes the combustion air preheaters used in large boilers found in thermal power stations producing electric power from e.g. fossil fuels, biomasses or waste.[1][2][3][4][5]

The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows

control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

Contents

[hide]

1 Types o 1.1 Tubular type

1.1.1 Construction features 1.1.2 Problems

1.1.2.1 Dew point corrosion o 1.2 Regenerative air preheaters

1.2.1 Rotating-plate regenerative air preheater 1.2.1.1 Construction features 1.2.1.2 Problems

1.2.2 Stationary-plate regenerative air preheater o 1.3 Regenerator

2 See also 3 References

[edit] Types

There are two types of air preheaters for use in steam generators in thermal power stations: One is a tubular type built into the boiler flue gas ducting, and the other is a regenerative air preheater.[1][2][6] These may be arranged so the gas flows horizontally or vertically across the axis of rotation.

Another type of air preheater is the regenerator used in iron or glass manufacture.

[edit] Tubular type

[edit] Construction features

Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the preheater tubes, transferring heat from the exhaust gas to the air inside the preheater. Ambient air is forced by a fan through ducting at one end of the preheater tubes and at other end the heated air from inside of the tubes emerges into another set of ducting, which carries it to the boiler furnace for combustion.

[edit] Problems

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.

[edit] Dew point corrosion

Dew point corrosion occurs for a variety of reasons.[7][8] 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.

[edit] Regenerative air preheaters

There are two types of regenerative air preheaters: the rotating-plate regenerative air preheaters (RAPH) and the stationary-plate regenerative air preheaters (Rothemuhle).[1][2][3][9]

[edit] Rotating-plate regenerative air preheater

Typical Rotating-plate Regenerative Air Preheater (Bi-sector type)[10]

Principle function for the Ljungstrom regenerative preheater.

The rotating-plate design (RAPH)[2] consists of a central rotating-plate element installed within a casing that is divided into two (bi-sector type), three (tri-sector type) or four (quad-sector type) sectors containing seals around the element. The seals allow the element to rotate through all the sectors, but keep gas leakage between sectors to a minimum while providing separate gas air and flue gas paths through each sector.

Tri-sector types are the most common in modern power generation facilities.[11] In the tri-sector design, the largest sector (usually spanning about half the cross-section of the casing) is connected to the boiler hot gas outlet. The hot exhaust gas flows over the central element, transferring some of its heat to the element, and is then ducted away for further treatment in dust collectors and other equipment before being expelled from the flue gas stack. The second, smaller sector, is fed with ambient air by a fan, which passes over the heated element as it rotates into the sector, and is heated before being carried to the boiler furnace for combustion. The third sector is the smallest one and it heats air which is routed into the pulverizers and used to carry the coal-air mixture to coal boiler burners. Thus, the total air heated in the RAPH provides: heating air to remove the moisture from the pulverised coal dust, carrier air for transporting the pulverised coal to the boiler burners and the primary air for combustion.

The rotor itself is the medium of heat transfer in this system, and is usually composed of some form of steel and/or ceramic structure. It rotates quite slowly (around 3-5 RPM) to allow optimum heat transfer first from the hot exhaust gases to the element, then as it rotates, from the element to the cooler air in the other sectors.

[edit] Construction features

In this design the whole air preheater casing is supported on the boiler supporting structure itself with necessary expansion joints in the ducting.

The vertical rotor is supported on thrust bearings at the lower end and has an oil bath lubrication, cooled by water circulating in coils inside the oil bath. This arrangement is for cooling the lower end of the shaft, as this end of the vertical rotor is on the hot end of the

ducting. The top end of the rotor has a simple roller bearing to hold the shaft in a vertical position.

The rotor is built up on the vertical shaft with radial supports and cages for holding the baskets in position. Radial and circumferential seal plates are also provided to avoid leakages of gases or air between the sectors or between the duct and the casing while in rotation.

For on line cleaning of the deposits from the baskets steam jets are provided such that the blown out dust and ash are collected at the bottom ash hopper of the air preheater. This dust hopper is connected for emptying along with the main dust hoppers of the dust collectors.

The rotor is turned by an air driven motor and gearing, and is required to be started before starting the boiler and also to be kept in rotation for some time after the boiler is stopped, to avoid uneven expansion and contraction resulting in warping or cracking of the rotor. The station air is generally totally dry (dry air is required for the instrumentation), so the air used to drive the rotor is injected with oil to lubricate the air motor.

Safety protected inspection windows are provided for viewing the preheater's internal operation under all operating conditions.

The baskets are in the sector housings provided on the rotor and are renewable. The life of the baskets depend on the ash abrasiveness and corrosiveness of the boiler outlet gases.

[edit] Problems

The boiler flue gas contains many dust particles (due to high ash content) not contributing towards combustion, such as silica, which cause abrasive wear of the baskets, and may also contain corrosive gases depending on the composition of the fuel. For example, Indian coals generally result in high levels of ash, sulfur and silica in the flue gas. The wear of the baskets therefore is generally more than other, cleaner-burning fuels.

In this RAPH, the dust laden, corrosive boiler gases have to pass between the elements of air preheater baskets. The elements are made up of zig zag corrugated plates pressed into a steel basket giving sufficient annular space in between for the gas to pass through. These plates are corrugated to give more surface area for the heat to be absorbed and also to give it the rigidity for stacking them into the baskets. Hence frequent replacements are called for and new baskets are always kept ready. In the early days, Cor-ten steel was being used for the elements. Today due to technological advance many manufacturers may use their own patents. Some manufacturers supply different materials for the use of the elements to lengthen the life of the baskets.

In certain cases the unburnt deposits may occur on the air preheater elements causing it to catch fire during normal operations of the boiler, giving rise to explosions inside the air preheater. Sometimes mild explosions may be detected in the control room by variations in the inlet and outlet temperatures of the combustion air.

Schematic of typical stationary-plate regenerative air preheater

[ed it ] Stationary-plate regenerative air preheater

The heating plate elements in this type of regenerative air preheater are also installed in a casing, but the heating plate elements are stationary rather than rotating. Instead the air ducts in the preheater are rotated so as to alternatively expose sections of the heating plate elements to the upflowing cool air.[1][2][3]

As indicated in the adjacent drawing, there are rotating inlet air ducts at the bottom of the stationary plates similar to the rotating outlet air ducts at the top of the stationary plates.

Stationary-plate regenerative air preheaters are also known as Rothemuhle preheaters, manufactured for over 25 years by Balke-Dürr GmbH of Ratingen, Germany.

[edit] Regenerator

Main article: regenerator

A regenerator consists of a brick checkerwork: bricks laid with spaces equivalent to a brick's width between them, so that air can flow relatively easily through the checkerwork. The idea is that as hot exhaust gases flow through the checkerwork, they give up heat to the bricks. The airflow is then reversed, so that the hot bricks heat up the incoming combustion air and fuel. For a glass-melting furnace, a regenerator sits on either side of the furnace, often forming an integral whole. For a blast furnace, the regenerators (commonly called Cowper stoves) sit separate to the furnace. A furnace needs no less than two stoves, but may have three. One of the stoves is 'on gas', receiving hot gases from the furnace top and heating the checkerwork inside, whilst the other is 'on blast', receiving cold air from the blowers, heating it and passing it to the blast furnace.

A Leakage Control System That Uses Thermal Growth to Maintain Sealing Efficiency

The advanced sealing system of the Ljungström® air preheater is a result of an evolution of devices and

methods to develop a sealing system that is capable of successfully controlling and minimizing air-to-gas

leakage. The design takes advantage of normal thermal growths to achieve effective sealing with a minimum of

maintenance requirements.

Along the radial and axial length of each rotor diaphragm are attached light-gauge leaf-type seals. During operation

these seals pass in close proximity to the sealing plates that are located within the air preheater housing. Two radial

sealing plates (sector plates) are located at each end of the rotor. Two axial sealing plates are diametrically opposite

each other in line with the sector plates. Collectively, these create a sealing "zone" between the air and gas streams.

To prevent air and gas from bypassing the rotor through the small space between the rotor and housing,

circumferential bypass seals are provided. Ask for complete descriptive literature covering Ljungström® air preheater

sealing systems.

Double Radial and Axial Seals Reduce Direct Leakage

In most applications where air-to-gas pressure differentials are sufficiently high, radial and axial sealing effectiveness

is further enhanced by a double sealing system that creates a moving plenum across the sealing surface. The

formation of this intermediate-pressure plenum between the air and gas streams serves to reduce the air-to-gas

pressure differential by a factor of nearly two. This decrease in pressure differential can reduce direct leakage by as

much as 30 percent. The double sealing system can also be applied to existing air preheaters.

Automatic Radial Sealing Control

The actuated sealing system minimizes the direct air-to-gas leakage that is caused by changes in hot end radial seal

clearance when the rotor approaches its operating temperature. It automatically adjusts the radial sealing plates as it

responds to changes in seal clearance detected by a rotor position sensor.