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Air Preheaters and Spares
What is an Air Preheater?
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.
Purpose of Preheaters
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 conveyed to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the conveyance system and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).
Types of Air Preheaters
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. These may be arranged so the gas flows horizontally or vertically across the axis of rotation.
1. Tubular Type Air Preheater
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.
2. Rotating-Plate Regenerative Air Preheater
The rotating-plate design (RAPH) 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. 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. 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.
What ASPL can do
We introduce ourselves as one of the leading Engineering Company, manufacturing spares for Air preheaters like all APH replacement parts, basket elements both non Enamelled elements and Enamelled elements, Seals, Tbars, speed reducers etc. We have installed in our works at Chennai the machines required for manufacturing all types of APH baskets.
For us to quote budgetary for basket or seal job, we need the following items for each requirement:
Number of air heaters
Size and type of air heater (i.e. 32.5 VI)
Is it a modular air heater or not
Number of diaphragms (12, 24, 36 or 48)
Air preheater Contract number (if known)
Individual air preheater number (if known)
For a basket quote, we need to know which layers to be quoted (hot, intermediate, cold, etc.):
For Each Individual Layer we need to know:
o Profile of element (i.e. CU, DL, DN, DU, DL7, DN7, DU7, NF6)
o Element material type (i.e. CorTen Grade A, B, C; ASTM A606 type 4; ASTM A606 type 2; ASTM 1008; etc.)
o Gauge of material
o Depth of element layer
o Enameled (Y/N)
Drawings with dimensions needed for a basket quote:
Module Assembly Drawing (if a modular air heater)
Rotor Assembly Drawing
Basket Arrangement Drawings for each layer to be quoted
If we are quoting seals, we need:
To know the rotation of the air heater from the hot side (gas side).
The Radial Seal Assembly Drawing with dimensions.
As soon as you can return this information for each requirement, we can prepare a budgetary quote.
If you have a specific requirement, please send us your enquiry, mentioning complete description of the items required for Air Preheater.
Download the Brochure here: Heating Elements
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