Transcript of "Feedwater heaters in thermal power plants"
IN THERMAL POWER PLANTS
A feedwater heater is a
power plant component
used to pre-heat water
delivered to a steam
In a steam power plant
(usually modeled as a
Rankine cycle), feedwater
heaters allow the feedwater
to be brought up to the
saturation temperature very
PURPOSE OF FEEDWATER HEATERS
Feedwater heaters serve three purposes in the
They provide efficiency gains in the steam cycle by
increasing the initial water temperature to the boiler, so
there is less sensible heat addition which must occur in
They provide efficiency gains by reducing the heat
rejected in the condenser, and they minimize thermal
effects in the boiler.
Steam is extracted from selected stages in the turbine to
shell and tube heat exchangers or to open feed water
heaters where the steam and feed water are in direct
FEED WATER HEATERS
In shell and tube or closed type feedwater
heaters the feedwater flows through the
tubes and the extracted steam condenses
on the shell side.
The condensed steam from each feedwater
heater drains successively to the next lower
pressure heater and is returned to the
feedwater through a heater drain pump or
through the condenser.
Low Pressure Heater: A heater located (with
regard to feedwater flow) between the
condensate pump and either the boiler feed
pump . It normally extracts steam from the low
High Pressure Heater: A heater located
downstream of the boiler feed pump. Typically,
the tube side design pressure is at least 100
KG/CM2, and the steam source is the high
HEATERS TO POWER PLANT CYCLE
The heating process by means of extraction
steam is referred to as being regenerative.
The feedwater heaters are an integral portion
of the power plant thermodynamic cycle.
Normally, there are multiple stages of
feedwater heating. Each stage corresponds to a
turbine extraction point.
The presence of the heaters in the cycle
enhances the thermal efficiency of the power
Horizontal: Most heaters are of this configuration. These
are the most stable in regard to level control ,although they
occupy more floor space. Disassembly is by means of either
shell or bundle removal. Most are floor mounted, although
some are mounted in the condenser exhaust neck.
Vertical, Channel Down: Although these conserve floor
space, the amount of control area available for liquid level
fluctuation is less. Disassembly is by shell removal.
Installation and removal may be more difficult than for
Vertical, Channel Up. These are the least frequently used.
Disassembly is by means of bundle removal. If a subcooling
zone is present, it must extend the full length of the bundle,
since the water must enter the bottom and exit at the top
end of the heater.
Condensing Zone: All feedwaters have this zone. All
of the steam is condensed in this area, and any
remaining non condensable gases must be removed.
A large percentage of the energy added by the heater
Subcooling Zone: The condensed steam enters this
zone at the saturation temperature and is cooled by
convective heat transfer from the incoming feedwater.
Desuperheating Zone: The incoming steam enters this
zone, giving up most of its superheat to the feedwater
exiting from the heater.
Both copper alloys & non-ferrous alloys are used for the LP
Heaters & HP Heaters tubes.
Copper alloys are used extensively in the LP Heaters tubes.
These alloys have got excellent thermal conductivity but on
the other hand these alloys have problems of copper carry
over & ammonia attack, which may require a complex boiler
cleaning after short intervals.
To avoid all the above problems, the stainless steel tubes
are invariably used for LP Heaters. Stainless steel is
unaffected at all operating conditions, except that, it is
susceptible to chloride induced stress corrosion.
Most common materials used for HP Heaters are carbon
steel, stainless steel & monel metal.
FEEDWATER HEATER EFFICIENCY
Two variables are used to monitor a
feedwater heaters efficiency.
The heater Terminal Temperature
Difference or TTD is a measure of how
close the outlet feedwater temperature is to
the feedwater heater saturation
The heater Drain Cooler Approach or DCA is
a measure of how close the heater drain
outlet temperature is to the feedwater inlet
IMPACT ON THERMAL PERFORMANCE
The impact of a 1°F (0.56°C) increase in
the TTD of the top heater is approximately
a 0.016% increase in heat rate. For the
other feed water heaters, a 1°F (0.56°C)
increase in TTD increases the heat rate by
If there is a 1°F (0.56°C) Increase in DCA,
the corresponding increase in heat rate is
0.005%. The impact can be less at part
ITEMS AFFECT PERFORMANCE OF
x Improper heater level can cause flashing in the drain
cooler section and tube damage.
— Check operation of automatic controls and level
— Check for possible tube leaks in feedwater heater.
— Vent valves may not be set up properly.
x Improper extraction line pressure drops.
— Possible problem with extraction line check valve.
x Tube fouling due to corrosion affects the heat transfer in
the heater .
— Clean tube bundles
x Continuous vent orifice plugging.
x Channel pass partition/gasket leak.
Heat balance diagram -660 mw
Super critical thermal power plant
Feedwater heaters in red
Efficiency Improvement as an
Emissions Control Strategy
Feedwater heaters are designed into
the turbine cycle to improve efficiency
(lower heat rate).
As the heat rate decreases (heat rate
improves), the amount of fuel for the
same generation also goes down. Of
course with less fuel burned,
emissions are lowered.
BOILER OPERATION WITHOUT HP
Some plants have been designed for peaking by
allowing the removal of HP heaters out of service but
there is a significant heat rate penalty for this.
Even if the boiler was designed for a low feedwater
temperature condition,the increased gas temperature
and weight through the superheater from higher firing
to compensate for the lower feedwater heat input
increases tube and support temperatures.
The high temperature ferritic tubes are life limited by
creep, as are ferritic to austenitic dissimilar welds and
support or alignment casting attachment welds.
Low feedwater temperature operation results in closer
approach to tube "alarm" temperatures, and may
require load reduction to stay within temperature
Coal flow requirement for a
250 MW Thermal power plant
(with and without HP heaters)
BMCR TMCR 80%
250 200 150 250
179 166 136.5 106.5 178.4
0.664 0.6825 0.71 0.7136
Coal requirement is more if both HP heaters are out
Safety Issue during Tube Leaks
As per HEI (14), section 6.1.2 (b), a
feedwater heater shell-side safety valve
shall be sized to pass the flow from a clean
break in one tube resulting in the flow from
If a heater is not isolated promptly when a
tube leak occurs, more tubes may be
damaged resulting in a higher flow into the
shell than the safety can relieve. Obviously
as the heater becomes more susceptible to
tube leaks, the risk of this scenario
The majority of feedwater
heaters are U-tube type.
Consequently, it is very
difficult to replace damaged
tubes unless they are
located on the periphery of
the tube bundle. The most
common practice is to plug
both the inlets and outlets
of damaged tubes.
Once tube plugging begins,
a feedwater heater’s
performance starts to
AFFECTS OF TUBES PLUGGED
Tubes plugged in HP feedwater heaters will increase
the pumping power requirements.
On a unit that has a steam driven boiler feed pump,
the turbine extracts more steam which is typically
exhausted into the condenser.
These turbines are generally less efficient than the
main turbine, add extra heat loading to the condenser
and therefore decrease efficiency.
A motor-driven boiler feed pump will use more power,
again increasing heat rate.
In some cases, this extra power is not available so
maximum generation will be reduced.
Tubes plugged in LP feedwater heaters will also
increase pumping power requirements.
HP HEATER PARAMETERS(210 MW)
S.N PARAMETER UNIT DESIGN
1 BLEED STEAM PRESS KG/CM2 17.03 37.52
2 BLEED STEAM
DEG C 431.4 336
3 INLET FEED WATER
DEG C 164.2 197.8
4 OUTLET FEEDWATER
DEG C 197.8 240
5 DRAIN TEMP DEG C 170.9 207
6 DRIP IN TEMP DEG C 199
7 SATURATION TEMP DEG C 203.1 245
8 FW TEMP RISE DEG C 37 42.2
9 TTD DEG C 3.1 5.00
FEEDWATER HEATERS IN 500 MW
To increase the cycle efficiency, the condensate
and feed water is preheated in stages of low pressure
& high pressure heaters .
Extraction no. 1 is taken from LP turbine casing
and connected to LP heater no. 1 which is under
vaccum at full load.
Extraction no. 2 and 3 are taken from LP turbine
casing and connected to LP heater no. 2 and 3
Extraction nos. 4 and 5 are taken from I. P.
Cylinder and supply steam to the deaerator and H. P.
heater no. : 5A & 5B respectively.
Extraction no. : 6 is taken off the cold reheat line and
supplies steam to H. P. heater no. : 6A & 6B.