Ljungström® air preheaters

The Ljungström® air preheater (APH) is more widely used than any other type of combustion air preheater in the power industry, because of its compact design proven performance and reliability, and its fuel flexibility.

It is robustly designed and built to operate over extended periods, providing uninterrupted service. Design simplicity makes it easy and economical to maintain. Ljungström® air preheaters are available in sizes, arrangements, and materials, to meet the specific requirements and operating conditions for all applications.


At the heart 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.

The rotor is supported between upper hot and lower cold end center sections, which are connected by main pedestals on each end.  The rotor is supported on the cold end center section by a support bearing and maintained upright by a guide bearing attached to the  in the hot end center section.  Sector sealing plates and axial seal plates are attached the hot and cold end structures and main pedestals respectively to form separate gas and air flow paths through the rotor.

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 center sections and casing to form a transitional flow path from the rotor and casing shape to the clients ductwork connections.

As the rotor revolves, waste heat is absorbed from the hot flue 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.

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. 

Heat transfer surfaces

Alstom has continually led the industry with our research & development on Ljungström® heat exchanger heat transfer surfaces.  Our advanced designs and configurations provide unsurpassed thermal performance with exceptional cleanability, operation flexibility and lower pressure losses.

Leakage control

The advanced sealing system designs of the Ljungström® air preheater are a result of over 90 years of experience in successfully controlling and minimizing air-to-gas leakage. The designs take advantage of normal thermal growths to achieve effective sealing with a minimum of maintenance requirements.  Semi- and fully- automatic sealing system options are designed to control or minimize the direct air-to-gas leakage that is caused by thermal gradients in the rotor and housing structures.

In applications where air-to-gas pressure differentials are appreciable, double radial and axial seals are used to reduce direct leakage by up to 30%.  The selection of the sealing system is made taking into consideration the air preheater size, rotor diameter and the operating conditions of the boiler.

Rotor designs

Alstom has developed a variety of rotor designs to complement the power plant project resources.  We currently offer a number of rotor designs, each with different degrees of shop/field


We are able to supply air preheater arrangements to cover all possible applications and offer the boiler designer maximum flexibility, with the most common arrangements being Bi-Sector, Tri-Sector and Quad-sector designs.