Superheating of Steam

P M V Subbarao

Professor

Mechanical Engineering Department

Recognition and Adaptation of Efficient Mode of Heat Transfer …..

Furnace Energy Balance

Water

walls

Economizer

Furnace

Enthalpy to be lost by hot gases:

FEGTadgaspgas TTcm ,

Capacity of Flue Gas

Total Thermal Power available with flue gas:

chimneyadgaspgas TTcm ,

Rate of steam production:

steam

chimneyadgaspgassteam h

TTcmm

,

s

1

2

3

4

5

6

2f

2s

4523 hhmhhm mreheatsteafmainsteam

Paths of Steam and Gas

Water walls

Drum

Economizer

DPNL SH

Platen SHT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A Boiler Furnace

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

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

Thermal Balance in Convective SH.

• The energy absorbed by steam

• The convective heat lost by flue gas

• Overall Coefficient of Heat Transfer, K

)( sup,sup,, inoutsteamconabs hhmQ

TFKAQ SHlosscon

,

ssc

sc

me

me

as

as

g hh

K11

1

Mean Temperature Difference

• The average temperature difference for parallel flow and counter flow is expressed as

min

max

minmax

lnTT

TTT

2minmax TT

T

• It is also called log mean temperature difference• When tmax /tmin > 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.

S1

S2

S1

S2

Inline

Staggered

Effectiveness Factor

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

oK

K

sg

O

hh

kK11

1

Thermal Balance in Super Heater.

• The energy absorbed by steam

• The convective heat lost by flue gas

• Overall Coefficient of Heat Transfer, U

)( sup,sup,, inoutsteamconabs hhmQ

TUAQ SHlosscon

,

Platen SH, U (W/m2 K) 120 – 140

Pendent SH, U (W/m2 K) 120 – 140

Convective SH, U (W/m2 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 – 60mm.• The design is similar to convective superheaters.

• Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.