The document discusses a regenerative air preheater (GAH) used at a power plant. It contains specifications for the GAH such as its size, heating surface areas, temperatures, and pressure drops. The GAH uses a rotating cylinder filled with heating elements to transfer heat from flue gases to combustion air. It helps increase boiler efficiency. However, leakage of air through the seals is an inherent issue. The document examines various paths and causes of leakage, including thermal expansion differences between the hot and cold ends of the rotor. It notes the importance of properly setting seals to account for this "turn down" and minimizing gaps between sealing surfaces. Excessive leakage reduces efficiency and increases heat rate and power consumption.

1. GAS AIR HEATER & ITS LEAKAGE
NITCHITSU-LUGAT ROTARY REGENERATIVE AIR PREHEATER

2. 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.

3. 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.

4. 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

5. Components:
 Hot End Element
 Cold End Element
 Sector Plate
 Radial Seals
 Circumferential Seals
 Hub Seals

6. Hot End/Cold End Elements:

7. 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

8. 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.

9. 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.

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

11. 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.

12. 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

13. 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.

14. Path A:
Cold Radial
Leakage

15. Path B
Hot Radial Seal Leakage

16. Path C:
Circumferential Seal Leakage

17. Path D:
Circumferential Leak

18. GAH Seal Leakages

19. 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.

20. Power Consumption (FD) Vs Volumetric Flow

21. 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.

22. 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.
Cold
Top View

23. 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.

24. Sliding Foots:
Top View

25. 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?