Reliable Rankine Cycle : One OFWH & Many CFWHS….

A truly Concurrent Design ……

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

6

5

4

3

21

DCGSC

6 5 4 3 2 1

DC

GSC

Gas Release Mechanism in Deaerator

The Mechanical Deaerator

• Corrosion of iron or steel in boilers or boilers feed water piping is caused by three fundamental factors:

• 1. Feedwater temperature

• 2. Feed water ph value

• 3. Feedwater oxygen content Temperature and ph value influence the aggressiveness of corrosion.

• The higher the temperature, and the lower the pH value the increased aggressiveness of the feedwater.

• The dissolved oxygen content of the feedwater is a large factor in determining the amount of corrosion that will take place.

• The presence of oxygen, and other non-condensable gases, in the feedwater is a major cause of corrosion in the feedwater piping, boiler, and condensate handling equipment.

• Deaeration is based on two scientific principles. • The first principle can be described by Henry's Law. • Henry's Law asserts that gas solubility in a solution decreases as

the gas partial pressure above the solution decreases. • The second scientific principle that governs deaeration is the

relationship between gas solubility and temperature. • Easily explained, gas solubility in a solution decreases as the

temperature of the solution rises and approaches saturation temperature.

• A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater.

• Correct deaerator operation requires a vessel pressure of about 20 – 30 kPa above atmospheric, and

• a water temperature measured at the storage section of 50C above the boiling point of water at the altitude of the installation. 

• There should be an 45 – 60 cm steam plume from the deaerator vent, this contains the unwanted oxygen and carbon dioxide. 

• The following parameters should be continuously monitored to ensure the correct  operation of the deaerator.

• Deaerator operating pressure.

• Water temperature in the storage section.

Deaerator Principles

• Deaeration is the mechanical removal of dissolved gases from the boiler feedwater.

• There are three principles that must be met in the design of any deaerator.

• 1. The incoming feedwater must be heated to the full saturation temperature, corresponding to the steam pressure maintained inside the deaerator .

• This will lower the solubility of the dissolved gases to zero.

• 2. The heated feedwater must be mechanically agitated.

• This is accomplished in a tray deaerator by first spraying the water in a thin film into a steam atmosphere.

• Creating a thin film reduces the distance the gas bubble has to travel to be released from the water.

• Next, the water is cascaded over a bank of slotted trays, further reducing the surface tension of the water.

• This allows for the removal of any gases not liberated by the initial spraying.

• 3. Adequate steam supply must be passed through the water, in both the spray section and the tray section to sweep out the gases from the water.

Principle of Operation of A Dearator

Anatomy of A Dearator

De-Aerators (Parallel Flow)

De-Aerator (Counter Flow)

Sequence of FWHs

HP CFWHs – one OFWH – LP CFWHs

Energy Balance for ith – HP - CFWH

1,

1

1,1,,

1,

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1,1,

2,1,

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1

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2,2,

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D

Deaerator as Ith OFWH

BFP

IbISG hym ,&

1,1

&1

If

I

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1

1

&

Ic

I

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1,

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1,1,

1, 1

Ic

I

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I

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Loss of steam in Deaerator, yloss

1,

1

1,1,

1

,,

1

Ic

I

jjSGIbISGIf

I

jjSG

IfSGvapsatlossSG

hymhymhym

hmhym

Energy Balance for (I+1)th – LP - CFWH

2,1

&1

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I

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1,1 &

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1,1 &

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1,1

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1

1

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hymhyym

Bled steam

DS

T

L

-TTD

DCC

CDC

Steam

FW

Condensate

Feedwater heater with Drain cooler and Desuperheater

-TTD=Terminal temperature difference

C=Condenser

DC=Drain cooler

DS=Desuperheater

DS

Bled steam

Feedwater T

L

+ TTD

DCC

CDC

Steam

FW

Condensate

Feedwater heater with Drain cooler

FW

34.535

35.536

36.537

37.538

38.539

39.540

40.541

0 1 2 3 4 5 6

No of Feed water Heaters

Eff

icie

nc

y(%

)

Regeneration cycle

Reheat-Regeneration cycle

Improvement in efficiency due to reheating in a reheat-regeneration cycle

Pmax=8 MPa,480oC , Pc=0.04 MPa

Comparision of Actual data (210MW) with Simulated results using MC

1.6

71

4.0

57

0.0

68

0.2

78

0.1

51

0.7

14

0.2

39

0.0

91

0.0

44

0.7

14

1.6

71

4.0

57

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1 2 3 4 5 6

Extraction Points

Pre

ss

ure

(M

Pa

)

Actual

Simulated(MC)