1. P M V Subbarao Professor Mechanical Engineering Department
Superheating of Steam
P M V Subbarao
Professor
Mechanical Engineering Department
Recognition and Adaptation of Efficient Mode of Heat Transfer …..

2. Furnace Energy Balance
Enthalpy to be lost by hot gases:
Water walls
Economizer
Furnace

3. Capacity of Flue Gas Total Thermal Power available with flue gas:
Rate of steam production:

4. 3 5 2s 2f 4 2 1 6 s

5. Paths of Steam and Gas
Drum
Water walls
Economizer

6. Thermal Structure of A Boiler Furnace
DPNL SH
Platen SHTR R H T
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCW
pump
Bottom ash
stack
screen
tubes

7. Convective Superheater (Pendant)
Convective super heaters are vertical type (Pendant ) or horizontal types.
The Pendant SH is always arranged in the horizontal crossover duct.
Pendant SH tubes are widely spaced due to high temperature and ash is soft.
Transverse pitch : S1/d > 4.5
Longitudinal pitch : S2/d > 3.5.
The outside tube diameter : 32 – 51mm
Tube thickness : 3 – 7mm S1 S2

8. Convective Superheater (Horizontal)
The horizontal SH are located in the back pass.
The tubes are arranged in the in-line configuration.
The outer diameter of the tube is 32 – 51 mm.
The tube thickness of the tube is 3 – 7 mm.
The transverse pitch : S1/d = 2 – 3.
The longitudinal pitch :S2/d = 1.6 – 2.5.
The tubes are arranged in multiple parallel sets.
The desired velocity depends on the type of SH and operating steam pressures.
The outside tube diameter : 32 – 51mm
Tube thickness : 3 – 7mm S1 S2

10. Thermal Balance in Convective SH.
The energy absorbed by steam
The convective heat lost by flue gas
Overall Coefficient of Heat Transfer, K

11. Mean Temperature Difference
The average temperature difference for parallel flow and counter flow is expressed as
It is also called log mean temperature difference
When Dtmax / Dtmin > 1.7, the average temperature may be expressed as:
Generally, the flow direction of the flue gas is perpendicular to the axes of tubes.
If number of bends are more than four, the flow can be treated as counter or parallel flow.

12. S1 S2
Inline S1
Staggered S2

13. Effectiveness Factor
The ratio of the heat transfer between fouled and clean tubes.

14. Thermal Balance in Super Heater.
The energy absorbed by steam
The convective heat lost by flue gas
Overall Coefficient of Heat Transfer, U
Platen SH, U (W/m2 K)
120 – 140
Pendent SH, U (W/m2 K)
Convective SH, U (W/m2 K)
60 – 80

15. Reheater
The pressure drop inside reheater tubes has an important adverse effect on the efficiency of turbine.
Pressure drop through the reheater should be kept as low as possible.
The tube diameter : 42 – 60mm.
The design is similar to convective superheaters.
Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.