GAS AIR HEATER & ITS LEAKAGE

NITCHITSU-LUGAT ROTARY REGENERATIVE AIR PREHEATER

Gas Air Heater (CH8.2/1.175+0.3C):

Introduction: The Air Preheater at Hub is a Regenerative type Rotary heat exchanger in which flue gases and air passes through a rotating cylinder/rotor filled with heat transfer surfaces (heating elements).The flue gas heat the rotating elements, which in turn preheat the air before

it is fed to the boiler.

Contd

• The higher the exit flue gas temperature the higher will be the boiler heat loss / efficiency loss. Therefore air pre heater is to recover the sensible heat from the boiler flue gas exiting the economizer which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas.

• Heating combustion air can raise boiler efficiency about 1% for every 22°C in temperature increase of combustion air.

• It may be Bi-Sector , Tri Sector etc. At Hub GAH is a bi-sector containing

air and flue gas .The rotor is supported at both ends with a roller Bearing.

GAH AT HUB SPECIFICATIONS:MANUFACTURER NITCHITSU-LUGAT AIR PRE-HEATER

NUMBER 2SETS /BOILER

SIZE CH8.2 /1.175 + 0.3C

HEATING SURFACE:

GAS SIDE : 17554 m2

AIR SIDE: 17554 m2

INLET GAS TEMPERATURE 350 OC

OUTLET GAS TEMPERATURE 152 OC

INLET AIR TEMP 74OC

OUTLET AIR TEMP 309OC

DUCT ARRANGEMENT GAS OVER AIR

AIR LEAKAGE 7 % (MCR)

HOT END LAYER:

MATERIAL: MILD STEEL

ELEMENT DEPTH 1175 mm

THICKNESS: 0.5mm

EXCESS AIR LEAVING BOILER 5.00%

COLD END LAYER:

MATERIAL: CRLS

ELEMENT DEPTH 300 mm

THICKNESS: 1mm

PRESSURE DROP

AIR SIDE 100 mm Aq

GAS SIDE 120 mm Aq

BEARINGS 2 SELF ALLIGNING ROLLER BEARING

Components:

Hot End Element

Cold End Element

Sector Plate

Radial Seals

Circumferential Seals

Hub Seals

Hot End/Cold End Elements:

Heating Element Types

Hot End Elements: (DU Profile): The DU (double Undulated design is widely used in hot and intermediate elements. The common characteristic of DU elements is the alternating stacking of undulated elements sheets with sheets that contain both undulations and notches

Heating Elements Types:

CU Element: CU (Corrugated Undulated) elements are a compact highly thermal efficient element but with correspondingly high pressure drop characteristics as well.

Circumferential Seals

Circumferential seals are located on the entire circumference of the air heater rotor, on both the hot end and cold end of the air heater.

Radial Seals:

Radial seal leakage is the mass of inlet air that leaks through the air heater seals into the gas outlet stream.

Heating Elements:

Mechanical Consideration: There are many different options when it comes to choosing a heat exchanger element design. Each configuration has its own unique pressure drop and heat transfer characteristics. For example, an element that is designed to achieve maximum heat transfer in a limited amount of space (depth) may also have a higher overall pressure drop than an element design that requires a greater depth to achieve the equivalent heat transfer. and vice versa.

GAH leakages:

As a rule of thumb For a horizontal-shaft APH, approximately two-thirds of air leakage occurs at the CE. Air heater leakage can occur from different paths.

• Path 1: Normal air flow path

• Path 2: Normal gas flow path

• Path A: Ambient FD fan leaking directly to the

APH Gas Outlet

• Path B: Pre-heated FD fan air flow short circuiting

the APH

• Path C: Ambient FD fan air leaking around AH.

• Path D: Hot gas exiting boiler

Pressure Differential:

There is a Pressure Differential Exist between the FD fan outlet air and the incoming Flue Gas .FD fan air Being pressurized tries to find a way towards lower pressure side. That is why there is a inherent Leakage exist in Rotary regenerative Gas air heater.

Path A:

Cold Radial Leakage

Path B

Hot Radial Seal Leakage

Path C:

Circumferential Seal Leakage

Path D:

Circumferential Leak

GAH Seal Leakages

Impact On Plant Performance:

• Air heater responsible for at least 10% of a units thermal efficiency.

• Excessive leakage can deteriorate net unit efficiency as well as reduce power generation.

• GAH leakage Increases heat rate.

Its major drawback of regenerative air heater is the undesired leakage that is inherent in its design.

Also the GAH leakage will cause more power consumption as more volume of a air is required from FD fan.

Power Consumption (FD) Vs Volumetric Flow

Causes Of Leakage:

• Major causes of GAH leakage is the large temperature difference between its hot and cold end elements. These opposing temperature gradients work together to produce a significant radial thermal expansion difference between the hot and cold sides of the air heaters rotor after unit start up. Due to this inherent thermal distortion, it's not uncommon for the outer edges of a large air heater at operating temperature to experience a significant "droop" (or "turn down"). The distortion caused by this thermal turndown (which can be as much as 4 inches on some rotors) changes the gaps between the seals and the sealing surfaces as the rotor warms to operating temperature, and is the most significant contributor to air heater leakage. This phenomenon must be accounted for when setting the seals at a cold state, That’s why it is stated that

Seals are to be set, not only to be fitted.

Turn Down:

Clearance between the radial seal and stationary sector plate is zero on hot end .while on cold end this clearance is zero at the inner circumference and 10mm at the outer circumference.

Top View

Cold

Contd

• Non Uniform Thickness of Sector Plate .Resulting in gap between

Radial Seal and plate due to Corrosion.

• Improper functioning of Sliding foots.

• Due to temperature fatigue a permanent deformation may

occur causing undesirable seal gaps causing excess leakage.

At Cold condition the gaps between the sector plate and the

Radial seal plate at the sector plate corner and the hub side are not the same.

Therefore the radial seals are fit firstly at the location where

Gap is minimum.

Sliding Foots:

Top View

How to reduce Leakage

• Whenever there is unevenness and distortion in the sealing surfaces (sector plates), as is commonly found in older air heaters. They lead to “distortion gaps” which are significant contributor to the high leakage that are mostly found in old air heaters.

• If rotor Expansion , casing expansion matches with the seal clearances then leakage would be minimize.

• Actual Droop Measurement. (How much Hot end Moves Toward the Cold end from the edges ?).

• Using Sacrificial Radial Seal to measure the Actual expansion during the Unit Operation.

• Recommended angles of the Radial Seal Plates are according to our requirement./or not?