Reliable Rankine Cycle : One OFWH

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Reliable Rankine Cycle One OFWH Many CFWHS.

• P M V Subbarao
• Professor
• Mechanical Engineering Department
• I I T Delhi

A truly Concurrent Design
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DC
GSC
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GSC
DC
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Gas Release Mechanism in Deaerator
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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.

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• 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.

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• 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.

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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.

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• 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.

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Principle of Operation of A Dearator
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Anatomy of A Dearator
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De-Aerators (Parallel Flow)
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De-Aerator (Counter Flow)
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Sequence of FWHs

• HP CFWHs one OFWH LP CFWHs

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Energy Balance for ith HP - CFWH
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Deaerator as Ith OFWH
BFP
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Loss of steam in Deaerator, yloss
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Energy Balance for (I1)th LP - CFWH
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Steam

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FW
C
DC
DS
Condensate
Bled steam
-TTD
T
-TTDTerminal temperature difference
CCondenser
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DC
DCDrain cooler
DSDesuperheater
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Feedwater heater with Drain cooler and
Desuperheater
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Steam

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FW
C
DC
Condensate
Bled steam
TTD
Feedwater
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C
DC
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Feedwater heater with Drain cooler
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Pmax8 MPa,480oC , Pc0.04 MPa
Reheat-Regeneration cycle
Regeneration cycle
Improvement in efficiency due to reheating in a
reheat-regeneration cycle
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