Gas Air Heater

Gas Air Heater is main component to increase efficiency of combustion by increase secondary air temperature to find perfect combustion.

If air heater got trip, ID Fan inlet temperature get high and can not hold for long time. If ID Fan trip, the furnace pressure get unstable and MFT is possible happen if in-experience or nervous of operator who can't handle that situation.

The Ljungstrom tri-sector air heater (50% gas, 35% secondary air and 15% primary air) as shown in Figure above, is a regenerative type air heater. The heat removed from the outgoing flue gas heats incoming primary and secondary air. The use of preheated air speeds up the combustion process, improves combustion, and increases the over all unit efficiency by reclaiming some of the waste heat from the flue gas leaving the boiler.


The air heater absorbs waste heat from flue gas. It then transfers this heat to incoming cold air by means of continuously rotating heat transfer elements of specially formed metal plates. Thousands of these high efficiency elements are spaced and compactly arranged within sector-shaped compartments of a radically divided cylindrical shell called the rotor. The housing surrounding the rotor is provided with duct connections at both ends, and is adequately sealed by radial circumferential sealing members. This forms an air passage through one-half of the heater, and a gas passage through the other.

As the rotor revolves the mass of elements alternately through the gas and air passages, heat is absorbed by the element surfaces passing through the hot gas stream. As these same surfaces are carried through the air stream, they release the stored up heat to the air. This greatly increases the temperature of the incoming combustion air.

The air heater consists of the heat transfer surface, and the primary rotor driving mechanism, which consists of the motor, coupling, speed reducer, and pinion gear. Auxiliary air drive systems are also provided with each air heater. Each air heater rotor is bottom supported by a self-aligning thrust bearing assembly and top guided by a spherical roller bearing assembly. Oil baths lubricate both bearing assemblies.

Main Electric Motor.The driving force for turning the rotor is applied to its outside edge. The pin rack mounted on the rotor shell, is engaged by a pinion gear connected to a 30 Hp drive motor through a set of reduction gears. The speed reduction gears are enclosed in a housing partially filled with oil for lubrication. The operator is provided a sight glass on the housing for visual observation of the proper oil operating level.

Air Drive (emergency drive).



Coupling.
This animation below that the air heater motor is in service so that the speed rotor is high enough to dis-engange the over-running clutch.


This animation below that the air heater motor is trip so that the speed rotor is slow down and low enough to engange the over-running clutch.


Each air heater has also has an emergency air motor and a maintenance air motor.
The air motor is activated by de-energizing (opening) the electric solenoid valve in the air supply line. The air motor will stop when the electric motor is re-started. The emergency air motor will not rotate the air heater from a dead stop.
The air operated, maintenance motor is activated by the operator when the air heater is proven not to be running. The air motor is activated by energizing (opening) the electric solenoid valve in the air supply line.


To control the leakage resulting from the difference in the pressure levels of the fluid streams the air heater is furnished with a sealing system.
The sealing systems are comprised of radial seals, circumferential seals or by pass seals, axial seals, and rotor post seals.

The air heaters are furnished with radial seals (leakage control system) which are attached to the hot and cold end or each rotor diaphragm. These seals are initially set at specified clearances from the sector plates. The clearances are such that during operation there will be a minimum clearance between the seals and sector plates. If the Unit get blackout and no electric power, the operator should retract all leakage control system plate to avoid air heater element touching that.

LCS (Leakage Control System) have three pieces, put to place between (1) Gas & Primary Air, (2) Gas & Secondary Air and (3) Primary & Secondary Air. Every LCS has timer (every 2 hours) to searching Primary Sensor to making small gap between air heater rotor and LCS sector plate. After LCS sector plate touching the primary sensor, the sector plate will be move retract as long 80s. LCS sector plate also retract for 80s if the air heater current getting high (27 Amp). If primary sensor doesn't work, signal sensor fault will shown to initiate sector plate to mandatory retract.

The air heater also has axial seals. These seals are attached to the outside circumference of the rotor in line with each rotor diaphragm and extend from the hot to the cold end. Axial seal plates, which are adjustable, are attached to the inside of pedestals which make up a portion of the rotor housing and are in line with the outboard end of the sector plates and extend from the hot to the cold end. Initially the axial seal plates are set such that there is a specified clearance between the axial seals and the plates.

During operation, thermal distortion of the rotor will reduce these clearances to a minimum. In addition to the axial seals, the air heater is furnished with stationary bypass seals. These seals are attached to angles, which in turn are secured to the hot and cold end connecting plates. These seals are initially set such that during operation there will be minimum clearance between the seals and sealing surfaces mounted at the periphery of the rotor at both hot and cold end.
The function of these seals is to limit the by passing of the rotor by the fluid streams through the space between the rotor and the rotor housing. They contribute little to the leakage control system.

Finally, rotor post seals are attached to the circumference of the rotor post at both hot and cold end. They operate with an established clearance from circular sealing rings.
During normal operation soot blowing of the heat transfer surface of the air heater will be required. The air heater is supplied with a steam and air soot blower at the cold end.

Adopted to Babcock and Wilcox training.

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http://idpowerstation.blogspot.com/2009/04/low-nox-burner.html