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Chapter 7
Steam Conditioning
Introduction
Power producers have an ever-increasing need to
improve efficiency, flexibility, and responsiveness
in their production operations. Changes resulting
from deregulation, privatization, environmental
factors, and economics are combining to alter the
face of power production worldwide. These
factors are affecting the operation of existing
power plants and the design of future plants
resulting in a myriad of changes in the designs
and operating modes of future and existing power
plants. 3-
Competing in today’s power market requires
heavy emphasis on the ability to throttle back
operations during non-peak hours in order to
minimize losses associated with power prices
falling with demand. These changes are
implemented in the form of increased cyclical
operation, daily start and stop, and faster ramp
rates to assure full load operation at daily peak
hours.
Advanced combined cycle plants are now
designed with requirements including operating
temperatures up to 1500°F, noise limitation in
urbanized areas, life extension programs,
cogeneration, and the sale of export steam to
independent customers. These requirements
have to be understood, evaluated, and
implemented individually with a minimum of cost
and a maximum of operational flexibility to assure
profitable operation.
Great strides have been made to improve heat
rates and increase operational thermal efficiency
by the precise and coordinated control of the
temperature, pressure, and quality of the steam.
Most of the steam produced in power and process
plants, today, is not at the required conditions for
all applications. Thus, some degree of
conditioning is warranted in either control of
pressure and/or temperature, to protect
downstream equipment, or desuperheating to
enhance heat transfer. Therefore, the sizing,
selection, and application of the proper
desuperheating or steam conditioning systems are
critical to the optimum performance of the
installation.
Thermodynamics of Steam
Highly superheated steam, (i.e. 900 - 1100°F) is
usually generated to do mechanical work such as
drive turbines. As the dry steam is expanded
through each turbine stage, increasing amounts of
thermal energy is transformed into kinetic energy
and turns the turbine rotor at the specified speed.
In the process, heat is transferred and work is
accomplished. The spent steam exits the turbine
at greatly reduced pressure and temperature in
accordance with the first law of thermodynamics.
This extremely hot vapor may appear to be an
excellent source for heat transfer, but in reality it is
just the opposite. Utilization of superheated steam
for heat transfer processes is very inefficient. It is
only when superheated steam temperatures are
lowered to values closer to saturation that its heat
transfer properties are significantly improved.
Analysis has shown that the resultant increase in
efficiency will very quickly pay for the additional
desuperheating equipment that is required.
In order to understand why desuperheating is so
essential for optimization of heat transfer and
efficiency, we must examine the thermodynamic
relationship of temperature and the enthalpy of
water. Figure 7-1 illustrates the changes of state
that occur in water over a range of temperatures,
at constant pressure, and relates them to the
enthalpy or thermal energy of the fluid.