Reducing and maintaining low air preheater leakage is vital to minimize the fan horsepower required to move the air and gas flows through the air preheater. It also serves to reduce the dilution effect and corrosion potential of the leaving gas stream due to mixing with colder air at the air inlet temperature. Seals can wear due to sootblowing, corrosion, erosion, and contact with the static sealing surfaces on start up and/or shutdown.  Seal wear and seal settings should be checked at least once per year so that seals can be reset to proper clearances or replaced should they exhibit excessive wear. Sealing plate surfaces may also wear due to contact with the seals and erosion, and they may also become out of level and out of plane. Seal plate wear should also be repaired as soon as detected, and plate alignments should be verified every 3-5 years and reset as soon as the need is detected. Radial Seals and Sector Plates Radial seals and sector plates are located at the hot and cold ends of the air preheater.  The radial seals are attached to the diaphragms, which separate the individual rotor compartments, and as the rotor turns the seals pass in close proximity to the sector plates located between the air and gas sides of the air preheater. The purpose of these seals is to reduce the area available for leakage from the air to the gas side between the rotor and the air preheater housing. Axial Seals and Sealing Plates Axial seals, used in conjunction with by-pass seals, minimize leakage passing radially around the rotor shell. The axial seals are mounted on the outside of the rotor shell and seal against the axial seal plates mounted on the air preheater housing. Duplex™ Sealing Duplex™ sealing is a system which provides reduced leakage through modifications to the rotor. Instead of a single leaf seal sweeping each sealing plate at any given instant, there are two, which together create a moving plenum across the sealing surface.  This is analogous to a double orifice - there is a double restriction to flow crossing from one side of the Ljungström® air preheater to the other. 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%.  Duplex™ sealing is normally applied to the radial and axial seals together but can be applied to either one separately, and in almost all cases this system can be retrofitted to existing air preheaters. Modulating Active Leakage Control System (LCS) The function of the active LCS is to minimize the direct air to gas leakage that occurs when the hot end radial seal clearance increases as the rotor approaches its operating temperature and the hot end radial seals move away from the hot end sector plates.  The system includes a drive system to move each sector plate up and down, a rotor position sensor to determine the position of the rotor and a control system to automatically operate the drives. Ee-Zee™ By-Pass Seals and Rotor T-bar The bypass seals and rotor T-bar, which are used in conjunction with axial seals and axial seal plates, differ from circumferential seals in that they only reduce the quantity of air flowing over the outer rim of the rotor as the task of reducing axial leakage is handled by the axial sealing system.  The bypass seals are mounted on the housing and seal against the rotor mounted T-bar.  Ee-Zee™ bypass seals are pre-assembled seals that reduce installation cost without sacrificing reliability or life. Circumferential Seals and Circumferential Sealing Ring Circumferential seals are mounted on the rotor and the sealing ring connected to the housing.  The circumferential seals and sealing ring prevent air and gas from bypassing the heating surface through the space between the rotor and the housing shell. They also prevent air and gas from flowing axially around the rotor. F-Flex™ Seals Hot end radial seal clearance increases as the rotor approaches its operating temperature and the hot end radial seals move away from the hot end sector plates.  When this occurs, leakage in this area also increases, which can account for half or more of the total air preheater leakage.  APC F-Flex™ seals are specifically designed for base load operated preheaters, and incorporate a tapered flexible leaf which can be set to a negative clearance in the cold condition, and which will extend in the hot condition to operate as a standard proximity seal.  The taper reduces the drag on the rotor drive during startup and minimizes seal and sector plate wear. Rotor Post and Static Seals Rotor post seals prevent leakage between the ends of the rotor post and the air preheater housing. The static seals prevent leakage between the hot and cold end sector plates and the hot and cold end center sections.