EFFECT OF NUMBER FEED WATER HEATERS AT BOILER THERMAL EFFICIENCY IN POWER PLANTS

1. 2015
CEPC
Hossam Ahmed Zein
[EFFECT OF NUMBER FEED
WATER HEATERS AT BOILER
THERMAL EFFICIENCY IN
POWER PLANTS]

2. 1
Improve Plant Heat Rate with
Feedwater Heater Control
08/01/2013 | Donald Hite
Meaningful, yet often hidden thermal performance losses occur in
feedwater heaters. These controllable losses frequently occur
because of poor-performing level controls. In this article, three
issues related to feedwater heater performance are discussed. First,
the large cost of a minute change in plant thermodynamic
performance is quantified. Next, the physical difficulties of
accurate level control are presented, as well as background on the
important role that feedwater heaters play within the plant steam
cycle. Finally, two short case studies are presented that quantify
the cost savings that accrued to plants that successfully updated
their feedwater heater level controls.
Cost of Heat Rate Deviation
Heat rate is a measurement used in the energy industry to calculate
how efficiently a power plant uses the heat content of the fuel,
expressed as the Btu of fuel heat required to produce a kilowatt-
hour of electricity in units of Btu/kWh. The heat rate of just the
steam turbine portion of the conventional power plant may also be
calculated by dividing the energy content of the steam entering the
steam turbine by the electricity generated. The turbine cycle heat
rate represents the combined performance of the turbine,
condenser, feedwater heaters, and feed pumps. The unit heat rate is
found by dividing the steam turbine heat rate by the boiler
efficiency.
If the plant operated without any losses, all of the fuel heat energy
would be converted into electricity and the unit heat rate would be

3. 2
3,412 Btu/kWh (the conversion factor between Btu and kWh at
100% efficiency). Although never achievable, the goal of the
performance engineer is to find ways to continuously reduce the
plant’s heat rate, because a lower heat rate (higher efficiency)
reflects more cost-efficient operation of the plant.
Calculating the annual fuel cost associated with slight deviations
from the plant’s target heat rate is always enlightening because the
impact is much more profound than expected. For example, if a
plant’s target heat rate is 12,000 Btu/kWh and the actual is a mere
0.1% higher than target or 12,011 Btu/kWh, what is the increase in
annual fuel cost?
The change in annual fuel cost ($/year) for a 1 Btu/kWh deviation
in heat rate can be calculated simply as HRD/BE x FC x CF x
UGC x T, where:
HRD = heat rate deviation (Btu/kWh)
BE = boiler efficiency
FC = fuel cost ($/million Btu)
CF = plant capacity factor
UGC = unit gross capacity (kW)
T = 8,760 hours per year
If we assume for a typical 500-MW coal-fired plant that BE = 88%, the fuel
cost is $2.01/MMBtu, CF = 85%, we will calculate the change in annual fuel
cost as 1/0.88 x 2.01/1,000,000 x 0.85 x 500,000 x 8,760 = $8,504/year. A
very small 11 Btu/kWh deviation means that the fuel cost will increase
$93,540 per year (see sidebar).

4. 3
General Guidelines for Heat Rate
Small changes in heat rate produce large changes in a plant’s
annual fuel bill. Additionally, small improvements in heat rate are
often very economic investments. Consider the following:
 A 1% improvement (reduction in heat rate) = $500,00 in annual
savings for a 500-MW plant.
 A 5 degree F reduction in final feedwater temperature increases heat
rate by 11.2 Btu/kWh, resulting in an average increase in annual fuel
cost of $59,230, for the typical 500-MW plant.
 The average U.S. coal-fired plant has a heat rate of ~12,000 Btu/kWh
(based on the higher heating value of coal) with a range of 9,000 to
12,000 Btu/kWh.
 The modern gas-fired combined cycle plant has a net heat rate of
~7,000 Btu/kWh, the best in the business (based on the lower
heating value of natural gas).
 Every plant has many opportunities to improve its heat rate
Feedwater Heater Operation
Feedwater heaters are used to heat feedwater before the water
enters the boiler. The higher the feedwater temperature, the less
fuel is required to produce the steam used to produce electricity in
the steam turbine. However, steam is extracted from different
locations on the steam turbine to heat the feedwater, which
increases the plant heat rate. The net effect of feedwater heating
using extraction steam is a reduction in the plant heat rate.
The typical coal-fired power plant has six to seven stages of
feedwater heating. The number of feedwater heaters is an
economic balance between the added capital cost (the price of a
typical feedwater heater is ~$1.2 million) and improved operating
efficiency over the life of the plant (Figure 1).

5. 4
1. Efficient water heating. The feedwater heater uses steam extracted from
the steam turbine to heat the boiler’s incoming
water. The net effect of this process is improved boiler efficiency and a
lower plant heat rate. A typical steam plant will have
six or seven feedwater heaters.
Source: Magnetrol International
Feedwater heaters take advantage of the heat of condensation
(energy available from the change from saturated steam to
saturated liquid) to preheat water destined for the boiler. In simple
terms, the shell and tube heat exchanger directs feedwater to pass
through the tube side while extraction steam from the turbine is
introduced on the shell side (Figure 2).

6. 5
2. Three-step process. A standard high-pressure feedwater heater has
three sections: desuperheating, condensing, and
the drain cooler or subcooling. Low-pressure heaters are similar in design,
although they often can eliminate the
desuperheating zone. This method of feedwater heating is far more
efficient than relying on burning more fuel to bring water
up to temperature. Source: Magnetrol International
The feedwater heating process actually takes place in three distinct
steps. First, the desuperheating zone cools the superheated steam to
the point that the steam is saturated. Next, the condensing zone
extracts the energy from the steam/water mixture to preheat the
boiler feedwater passing through the tube side. Finally, a drain
cooler is incorporated to capture additional energy from the liquid.
The three heating processes all occur within a single heater.

7. 6
Reliable Level Controls Required
Arguably the most important aspect to feedwater heater
performance is precise and reliable level control under all
operating conditions. Accurate level control ensures the unit is
operating in the area of greatest efficiency (straight condensation)
so that optimum heat transfer occurs while preventing undue wear
and tear on the feedwater heater and other system components
(Figure 3).
3. On the level. Modernizing feedwater heater level controls allows
operators to better manage controllable losses while
significantly reducing maintenance costs. Torque tube displacers (shown in
the photo) are common in the industry and one
of the easiest to retrofit. Source: Magnetrol International
Aging level instrumentation will introduce measurement errors that
will reduce the effectiveness of a feedwater heater. In fact, errors
of plus or minus 3 or 4 inches in water level are commonplace in
the typical power plant. A feedwater heater experiencing
suboptimal water levels will experience reduced heating

8. 7
effectiveness. In turn, the boiler controls must fire the boiler harder
to replace the lost energy, burning more fuel, and increasing the
plant’s heat rate. If the heater condensate level is higher than
design, the active condensing zone is effectively decreased and
tubes in the heater that should be condensing steam are instead
subcooling the condensate, which is an energy loss to the system.
Some heater designs have the means to bypass the feedwater
around the heater when condensate levels fluctuate to the extremes
of its operating envelope. If condensate levels rise too high, there
is a possibility of water induction into the steam turbine, which can
have disastrous consequences. The feedwater heater may be
mechanically protected from these extreme events, but the
operating efficiency of the plant is certainly degraded.
A lower-than-acceptable level of condensate introduces excessive
amounts of high-temperature steam to the drain cooler, which
causes the condensate to flash to steam. The resulting damage to
the drain cooler section increases maintenance cost and
unscheduled downtime. In some cases, low heater levels can cause
a mixture of steam and water through the heater, producing a
reduction in heat transfer that also increases the plant heat rate.
Monitoring Performance
The primary parameters used to monitor individual heater
performance are the feedwater temperature rise, the terminal
temperature difference (TTD), and the drain cooler approach
(DCA). Three important measurements can be taken that will
provide information about the operating efficiency of a feedwater
heater (Figure 4):

9. 8
4.Performance measures. Feedwater performance is
quantified in three ways: the feedwater temperature rise
(the difference between the outlet and inlet
feedwatertemperatures), the terminal temperature
difference (the saturation temperature of the heating
steam and the feedwater outlet temperature), and the
drain cooler approach
(the temperature difference between the drain cooler
outlet and the feedwater inlet temperature). Source:
Magnetrol International
 Feedwater temperature rise is the difference between the feedwater
outlet temperature and the feedwater inlet temperature. A properly
performing heater should meet the manufacturer’s design
specifications, provided the level controls are up to the task.
 Terminal temperature difference (TTD) provides feedback on the
feedwater heater’s performance relative to heat transfer and is
defined as the saturation temperature of the extraction steam minus
the feedwater outlet temperature. An increase in TTD indicates a
reduction in heat transfer while a decrease is an improvement.
Typical ranges for TTD on a high-pressure heater with and without a
desuperheating zone are –3F to –5F and 0F, respectively. The TTD for
low-pressure heaters is typically around 5F. Steam tables and an
accurate pressure reading are required to complete this calculation.

10. 9
 Drain cooler approach (DCA) temperature is the temperature
difference between the drain cooler outlet and the feedwater inlet
temperatures. The DCA infers the condensate levels present within a
feedwater heater. An increasing DCA temperature difference
indicates the level is decreasing, whereas a decreasing DCA indicates
a rise in level. A typical value for DCA is 10F.
Instrument-Induced Errors and Heat Rate
Although there are a number of physical anomalies that degrade
heater performance, this section focuses on issues tied in some way
to inadequate level control resulting in a below-design final
feedwater temperature. The problems can range from something as
simple as inaccurate or fluctuating readings across several
instruments that leave the “real” level in question to those that
justify taking a feedwater heater out of service. Regardless of the
severity, the intention is to show the ripple effect that poor
feedwater heater level control has on overall boiler and turbine
cycle efficiency (increase in net unit or turbine cycle heat rate).
Here are two primary sources of instrument-induced errors:
 Drift (mechanical or electronic). Drift is usually associated with aging
instrumentation, moving parts, or is intrinsic to the design, such as
with torque tube/displacers. With this technology, calibration
between shutdowns is often necessary to achieve reasonable
accuracy and prevent nuisance deviation alarms between multiple
level transmitters. Responsiveness to rapid level changes can also be
slow due to dampening effects fundamental to the principle of
operation.
 Vulnerable measurement technology. Many measurement
technologies are vulnerable to changing process conditions such as
shifts in specific gravity and/or the dielectric constant of the media
related to variations in process pressures and temperatures. Certain
technologies also cannot provide accurate level from startup to
operational temperatures without applying external correction
factors, or the specified accuracy is only realized at operational
temperatures. In general, technologies that fall in this category

11. 10
include differential pressure, magnetostrictive, radio frequency
capacitance, and torque tube/displacers.
Lower-than-expected final feedwater temperature occurs when a
feedwater heater is taken out of service due to unreliable level
input to the control system or when the level is too high or low. If
the condition is a result of high feedwater heater level, the operator
would note a decrease in feedwater heater temperature rise, a
decreasing DCA temperature difference, and an increasing TTD.
The inverse is true if feedwater heater levels are too low. In either
of the scenarios, risk of damage to hardware increases, heat
transfer is impaired, and feedwater to the economizer is not at the
required temperature. There are two probable operator responses to
a low final feedwater temperature, neither of which is desirable:
 Overfire the boiler to increase temperature (level too high/low or out
of service). This action will increase fuel consumption and emissions,
as well as increase the gas temperature exiting the furnace. The
increased gas temperature will increase the reheat and superheat
sprays. Also, the steam flow through the IP and LP stages of the
steam turbine will increase (when the heater is out of service),
causing flashing and potential damage to the drain cooler section and
possibly cause thermal damage to the tubes.
 Open emergency drains to lower level (level too high). This option
causes an immediate loss of plant efficiency and can cause damage
to hardware if water is inducted into the extraction line. The worst
case occurs when the water is inducted back into the steam turbine,
causing a possible catastrophic failure. The best case is that the
turbine water induction protection (TWIP) trips the unit, shutting the
heater down and causing expensive loss of production and other
related costs.
Case Studies
The case studies cover two key topics relative to feedwater heater
performance. The first details the increased annual fuel cost
associated with an off-design final feedwater heater temperature at

12. 11
a 500-MW coal-fired plant. Although this particular situation does
not fall into an extreme case warranting a heater bypass, it
exemplifies how seemingly minor trade-offs in level control often
have a large impact on unit heat rate (Table 1).
Table 1. Case study 1. This table displays the results of off-
design final feedwater heater temperature at a 500-MW coal-fired
plant. Based on low outlet temperature, the heat rate rose 47
Btu/kWh, adding $243,000 annually to the plant’s cost of fuel.
Source: Magnetrol International
The second case study brings to light the day-to-day operational
risks and costs that ineffective or aging instrumentation
technologies have on the bottom line (Table 2). At this plant, the
feedwater heaters were replaced in 2002, but the original
instrumentation (1966-vintage pneumatic level controls and sight
glass) was reused. The unreliable instrumentation caused feedwater
heater level fluctuations that intermittently caused a bypass of all
the LP heaters as part of the turbine water induction protection and
placed the unit at risk of unexpectedly tripping offline.

13. 12
Table 2. Case study 2. Cost justification for replacing aging level
controls technology due to excessive bypassing of LP heaters.
Source: Magnetrol International
Note that the case studies do not take into account additional
emissions cost, effects on boiler and turbine efficiencies, overfiring
conditions, lost production, and other related factors, discussed
earlier.
In both case studies, the return on investment for modernizing the
instrumentation on the plant’s feedwater heaters fell in the 1.0- to
1.5-year time frame.
—Contributed by Donald Hite, regional manager Southeast
Magnetrol International and Orion Instruments.