Superheating of Steam P M V Subbarao Professor

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

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

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

3 2 s 2 f 5 4 2 1 6 s

Paths of Steam and Gas Drum Water walls Economizer

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

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 : S 1/d > 4. 5 • Longitudinal pitch : S 2/d > 3. 5. • The outside tube diameter : 32 – 51 mm • Tube thickness : 3 – 7 mm S 1 S 2

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 : S 1/d = 2 – 3. The longitudinal pitch : S 2/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 – 51 mm • Tube thickness : 3 – 7 mm S 1 S 2

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

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.

S 1 Inline S 2 S 1 Staggered S 2

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

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/m 2 K) 120 – 140 Pendent SH, U (W/m 2 K) 120 – 140 Convective SH, U (W/m 2 K) 60 – 80

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 – 60 mm. • The design is similar to convective superheaters. • Overall Heat Transfer Coefficient : 90 – 110 W/m 2 K.