I. Feedwater heater
A. What’s feedwater heater?
Components in which fluid exiting
the condenser is preheated by extracted steam from turbines
B. Why feedwater heater?
o To reduce thermal shock in boiler
Thermal shock
is a force that is caused by difference of expansion of solid (usually metals)
because of large difference of the temperature on its part
- Causes bending and cracking of the solid. Can be
prevented/mitigated by
- Reducing temperature difference (Applicable in
this system)
- Using high conductivity materials (Applicable,
but costly)
- Forcing the temperature increase/decrease to
become gradual (not feasible
To reduce boiler load (except for startup phase) = By preheating the working fluid, fuel consumption is reduced
Disadvantages :
- More components, more initial cost, and more
maintenance cost
- Steam bled from turbine is used to preheat the
fluid instead of producing work, which in turn, decreasing the work output
- Adding components through which the fluid flows
increase the loss by friction and heat loss to surrounding
Key consideration: Optimization
of advantages
C. What are the variations of
feedwater heater?
Open Feedwater heater
Steam
bled from turbine is allowed to mix with saturated water output of condenser.
Advantages
- Faster heat rate between steam and saturated
water, because of direct contact of both fluids
- Direct contact heat exchanger is generally less
expensive compared to other type (especially S&T HE), and easier maintenance
process
Disadvantages
- Because of the pressure difference in all three
state of turbine, condenser, and boiler, additional pump is required for each
feedwater, which means, additional initial cost and maintenance cost
Closed Feedwater Heater
Steam bled
from turbine does not mix with saturated water output of condenser. In
practice, shell and tube heat exchanger is commonly used
Advantage
- Fewer pumps required (only necessary in case of
forward fed feedwater heaters). Because the non-direct contact heat exchanger, pressure
difference between turbine and condenser is irrelevant
- Technology of S&T HE, which is commonly used
in this case, is already mature, and still continue to develop
Disadvantage
- Utilizing a steam trap valve, a throttling
device that only allows liquid to pass. While cheaper and needs less
maintenance than a pump, this device produce more irreversibility while
producing no work.
- HE used in this system (especially S&T type)
is expensive, and the maintenance is not as easy as its direct contact
counterpart
Steam Trap
When
the steam enter the system (ex : closed feed water)
and heated the water inside pipe of CFW it
will condenses and produce condensate and reduce heat contact area, we need
remove it and need the valve to prevent leakage of steam
Steam traps are a type of
automatic valve that filters out condensate (i.e. condensed steam) and
non-condensable gases such as air without letting steam escape.
II. Deaerator
What's deaerator?
A
device that is used to dissolved gasses from working fluid?
What’s the importance of using
deaerator?
It
helps to prevent corrosion by removing, mainly, dissolved oxygen (main
corrosive agent)
and
CO2 (secondary corrosive agent, by carbonic acid)
Oxygen is removed until at most
7ppb (0.005 cm3/L). CO2 is usually completely
removed)
How's deaerator work?
By heating the
working fluid. Rate of dissolved oxygen released from the water is proportional
to the temperature. At saturation temperature at a given pressure, almost all
oxygen is released
By
adding oxygen scavenger (sodium sulfide (N2SO3) to Sodium sulfate (N2SO4))
Sodium sulfide is more reactive
to oxygen compared to water. One of the disadvantages of this method is the
sedimentation of sodium sulfate at the bottom of de-aerated water storage tank.
What are the variations of
deaerator?
· A. Tray type (Cascade Type Deaerator)
This type
includes a vertical domed deaeration section mounted on top of horizontal
cylindrical vessel which acts as de-aerated water storage tank.
The
purpose of the cascaded trays within the deaeration section is similar to fill
in cooling tower, which is to increase the heat transfer of water, which in
turn, increasing the heat transfer rate. Boiler feedwater enters the
deaeration section on top of the device; while heating steam enters through
submerge pipes in the de-aerated water storage tank. As mentioned above,
solubility of oxygen in the water decreases with increase of temperature.
B. Spray type
The spray type deaerator consists
only of a vessel that serves both as deaeration section and as de-aerated water
storage tank.
In this type, instead of using
trays to increase the heat transfer area, sprinkler nozzle is used. The main
goal is still to increase the temperature of the boiler feedwater.
In both of these deaerator types,
some things that need to be considered for choosing are :
- Price. Generally, tray type deaerator is more
expensive and harder to design compared to spray type deaerator
- ·Efficiency. Efficiency of deaerator can be calculated
by the same method of heat exchanger efficiency. This depends on, among other thing, the size
of the deaerator.
Some engineers also added vent
condenser at the exit of the removed gasses, to filter some steam/water that
might escape the deaeration process. This addition, of course, reduce the need
of makeup water.