The double sealing principle
In an air preheater, radial leakage typically accounts for around 70% of the total leakage. The Howden VN double seal system, which has been operating successfully for more than 30 years in both new installations and retrofit projects, substantially reduces these losses. Double sealing can usually be achieved by increasing the number of radial walls or by widening the sector plate. The result in both cases is that there are two or more radial seals under the sector plate at all times and the seal leakage is reduced by a factor of 1/√2.
The advantages of a fixed sector plate
Sealing systems that depend on articulated parts that are moved by actuators and controlled by sensors are extremely vulnerable in hazardous environments.
Factors such as erosion from ash and corrosive atmospheres greatly impair their durability, and significant maintenance is required to keep them working at acceptable levels of availability.
By eliminating the need for moving parts, the fixed sector plate offers a far more reliable long-term control of leakage. Thermal capping of the rotor is accommodated by the parabolic design of the bottom sector plate, which offers maximum adaptability to the radial seals.