• Heat recovery boilers, also known as waste heat recovery boilers or heat recovery steam generators (HRSGs).

• waste heat boilers are used to cool waste gas streams from a given inlet temperature to a desired exit temperature for further processing purposes. A common application for an HRSG is in a combined cycle power station .

• where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine.

•The output of the HRSG is solely dependent on the performance and load of the gas turbine.

Fire Tube Boiler

Water Tube Boiler

• In this type of boilers, the hot gas stream which could be at very high pressure flows inside the tubes, while low pressure saturated steam is generated outside the tubes.

• They could be of single or of multi-gas pass design.

• Water tube boilers are more difficult to design compared to fire tube boilers,

• due to the complex arrangement or disposition of heating surfaces such as superheaters , evaporators and

economizers.

• Due to the higher heat transfer coefficients associated with gas flow over the tubes , water tube boilers requires less surface area and hence the gas pressure drop can be lower than in a fire tube boiler.

Unfired

Auxiliary fired

• unfired HRSG is selected when the plant steam requirements are such that the energy in the exhaust gases is adequate . • HRSGs in combined cycle plants are unfired

• Typical gas temperature entering the HRSG ranges from 800° to 1050°F, depending on the gas turbine used.

• Supplementary firing of exhaust gases is done to raise the temperature of the gas stream entering the boiler

• To a maximum of 1700°F, so that additional steam can be generated without major modifications to the unfired boiler design, HRSG in cogeneration plants are fired

• The 1700°F limit is set by the design of the casing

• The efficiency of the HRSG system improves with firing

Natural Forced

Once-through

• Natural circulation units have vertical tubes and horizontal gas flow orientation .

• In natural circulation units, the difference in density between water and steam drives the steam–water mixture through the evaporator tubes and risers and back to the steam drum.

• The forced circulation HRSG uses

horizontal tubes and gases flow in

the vertical direction.

• In forced circulation units, a pump

is used to drive the

steam–water mixture

through the horizontal

evaporator tubes.

• A once-through HRSG (called an OTSG) does not have a steam drum like a natural or forced circulation unit

• Once-through units can have

either a horizontal or

vertical gas flow path.

• In once-through designs, there is no circulation system. Water enters at one end and leaves as steam at the other end of the tube bundle.

Superheater

Evaporator

Economizer

• The most important component would of course

• In the evaporation circuit the water is heated to as close to saturation temperature as is possible. •This process changes the water from liquid to vapor or steam.

•Evaporator sections are where the boiling process or steam

generation occurs. As heat energy is absorbed by water from the gas stream, the water temperature increases.

D-Frame evaporator layout

O-Frame evaporator layout.

A-Frame evaporator layout.

I-Frame evaporator layout.

Horizontal tube evaporator layout.

• The major function of super-heater is to increase the pressurized water temperature above the steam saturation temperature for use in the steam turbine.

• Types of Superheaters:

Vertical tube superheater Horizontal tube superheater I-frame superheater

An economizer’s function is to increase water temperature close to the Saturation Temperature, known as Approach Temperature. Approach Temperature is designed to ensure maximum heat energy absorption efficiency and operational flexibility.

Economizer sections are composed of extended or finned tubesurface banks.

Selecting the tube material and size to use in a HRSG design is really a matter of experience.

Segmented Fins:These are usually one of the two types shown

below.   High Frequency

Continuously Welded  Standard Frequency

Spot Welded

Solid Fins:

These are the most popular fins for modern HRSG’s. High Frequency

Continuously Welded

Stud Fins:These are used generally when the fuel is No. 6 or

higher. Resistance

Welded

 

Enthalpies : = at (T = Tsat -15) & P = at 97% dryness fraction & Pb = at 97% dryness fraction & PbGas temperature distribution :Heat balance on high pressure superheater:

Heat balance on high pressure evaporator :

Heat balance on intermediat superheater :

• Heat balance on intermediat evaporator :

• Heat balance on economizer:

• Heat balance on low pressure superheater :

• Heat balance on low pressure superheater :

Over all heat transfer coefficient :

• For gas side

67.0

25.0

5.0

321 **

49259

49259

2

PP

a

g

o C

KCG

T

T

dof

lfdofCCCh

0.351 Re*0.25C

SlfeC /25.02 *65.035.0

tL ssNb eeC /15.03 *)*8.07.0(7.0

2

For steam side:

• Pressure drop calculation in tube:

Do Outer diameter 38.1(mm)

Di Inner diameter 33.8836(mm)

T Tube thickness 2.1082 (mm)

Np Number of passes for all tubes 60

Npeh Number of pass for high pressure economizer

100

Vg Gas velocity 15 (m/s)

Vst Steam velocity 25(m/s)

Vw Water velocity 1(m/s)

Nf Number of fins per meter 310

Sf Fin spacing 2.4(mm)

Lf Fin height 15(mm)

Tf Fin thickness .8(mm)

ηf Fin efficiency 85%

Sl Lognitudnal pitch 83(mm)

St Transverse pitch 83(mm)

• Assumptions :

Temperature distribution:

Tshh 612.2 (c)

Tevh 585.7405 (c)

Tshi 518.3584 (c)

Tevi 466.7537 (c)

Teco 308.4764 (c)

Tevl 266.2291 (c)

Tst 150.3862 (c)

• High pressure superheater specifications:

Heat gain 12.173 MW

Total surface area 20421 (m2)

Total number of tubes 1634

Number of passes 60

Tube length 8 (m)

Pressre drop 72000 (Pa)

Material Carbon steel .25% carbon

Intermediate pressure superheater specifications:

Heat gain 31 MW

Total surface area 64977 (m2)

Total number of tubes 1189

Number of passes 60

Tube length 4 (m)

Pressre drop 41000 (Pa)

Material Carbon steel .25% carbon• High pressure economizer specifications:

Heat gain 11.179 MW

Total surface area 78000 (m2)

Total number of tubes 2926

Number of passes 100

Tube length 16 (m)

Pressre drop 396 (Pa)

Material Carbon steel .25% carbon

Intermediate pressure economizer specifications:

  Heat gain 5.0769 MW

Total surface area 17440 (m2)

Total number of tubes 1666

Number of passes 60

Tube length 6.5 (m)

Pressre drop 91.5 (Pa)

Material Carbon steel .25% carbon• Low pressure superheater specifications:

Heat gain 3.1804 MW

Total surface area 15173 (m2)

Total number of tubes 1294

Number of passes 60

Tube length 7 (m)

Pressre drop 7400 (Pa)

Material Carbon steel .25% carbon