POWER May 2015 - 57

FUNDAMENTALS
condensers are less efficient overall, but are
necessary when the power plant is located
in a region where suitable water is at a premium.
As utility-scale air-cooled condensers
are rare, this article instead focuses on watercooled
condensers.
Condenser Performance
For a given steam turbine system, the performance
of the condenser is largely a factor of
how efficiently heat can be transferred from
the steam to the cooling water. The faster
that heat can be transferred, the faster the
turbine exhaust steam can be condensed into
liquid water.
One metric of this condensation efficiency
is the condenser pressure, often
measured as vacuum. A condenser that can
pull a stronger vacuum not only results
in a higher cycle efficiency but also more
potential power output. For example, a
power plant with a feedwater flow rate of
2.5 million pounds per hour and condenser
pressure of 3 inches Hg would see a heat
rate improvement of nearly 6% if station
engineers could reduce the condenser pressure
to 1 inch Hg. This improvement alone
could satisfy most of the Environmental
Protection Agency's (EPA's) much-debated
heat rate improvement goals for existing
coal-fired power plants.
Tube Fouling. Condenser heat transfer
2. Cleanliness is next to profitability. Regular cleaning (indicated here with the
vertical bars) of a condenser can greatly improve its heat transfer ability, which results in a lower
condenser pressure, lower condenser terminal temperature difference, and lower heat rate
penalty. Source: Charles Trumble, Black & Veatch
● Condenser shell selected actual pressure ● Condenser terminal temperature difference
● Btu loss condenser BP
water or river water in their circulating
water system, fouling can take the form of
mud, organic slime, and marine plants and
animals. For example, the Madras Atomic
Power station in India has found no fewer
than 48 different types of marine organisms
living within its cooling water systems, ranging
from barnacles and snails to tube worms
and sea anemones. One wonders if the owners
have considered installing glass windows
in their condenser cooling water tunnels, and
This improvement alone could satisfy
most of the EPA's much-debated heat rate
improvement goals for existing coal-fired
power plants.
is greatly hampered by fouling of the heat
transfer tubes. In addition, long-term fouling
deposits can increase tube corrosion by
allowing corrosive chemical reactions to
" hide " under the deposits.
While in rare cases scale deposits can
form on the steam side of the condenser
tubes, most tube fouling occurs on the water
side of the tubes. Fouling can further be subdivided
into inorganic and organic causes. Inorganic
fouling is often composed of calcium
carbonate and calcium phosphate, but it can
also include manganese deposits, clay, and
fine rock silt. In some cases, pieces of rotting
or degrading cooling towers can become
trapped within the condenser tubes or upon
the surface of the tubesheet.
In the case of power plants that use seaMay
2015 | POWER
charging admission as an aquarium.
Freshwater cooling systems tend to experience
much less biofouling, and for closedloop
systems biofouling is often controlled
by regular chlorination. Most slime-producing
bacteria are killed at chlorine levels
greater than 0.5 ppm to 1.0 ppm for an hour,
although high levels of chlorine can also
damage wooden components and structures
in cooling towers.
The effects of condenser fouling can be
serious. In one instance a nuclear power plant
lost 50% of its generating capability due to a
biological film within its condenser tubes.
Many power engineers believe that prevention
is the best medicine for condenser
fouling, and chemical treatment, such as
reducing the pH of the circulating water by
www.powermag.com
adding small quantities of sulfuric acid, is a
common practice. Once the fouling has accumulated,
however, some sort of physical
cleaning will be required. Several cleaning
methods are available for condensers, both
online and offline (Figure 2). Using plastic
or metal brushes to clean the tubes can
yield a benefit of approximately 0.1 inches
Hg in vacuum, while a more involved mechanical
cleaning can reduce condenser
pressure by 0.6 inches Hg or more. An average
improvement in condenser vacuum
of 0.35 inches Hg can result in a 30 to 70
Btu/kWh heat rate improvement, resulting
in an annual fuel, operations, and maintenance
savings of $60,000 for a 500-MW
power plant (Figure 3).
Electrical Efficiency. Condenser systems
need either circulating water pumps to supply
them with cooling water or fans to supply
fresh air to air-cooled tubes. (Because most
air-cooled condensers are relatively new devices,
they tend to already have fairly modern
motors and controls; thus, the potential for
savings is less.)
Water pumping systems tend to require
from 8 to 12 hp/MW of generating capability.
The main circulating water pumps
and hotwell pumps combined can use
from 0.5% to 1.0% of the total unit power,
and old or inefficient pumps and motors
can result in a significant heat rate and
cost penalty.
A 2009 study by ABB (Power Generation:
Energy Efficient Design of Auxiliary Systems
in Fossil-Fuel Power Plants) estimates that
in a typical 500-MW power plant, retrofitting
the circulating water and hotwell motors
with variable-frequency drives could result in
an annual savings of more than 10,000 tons
of CO2
, more than $350,000 in avoided fuel
and operations costs, and a payback period of
57

http://www.powermag.com

POWER May 2015

Table of Contents for the Digital Edition of POWER May 2015