Chemical Engineering November 2010 - 60

Feature Report
Engineering Practice
Steam Generation
Thermodynamics
Brad Buecker
Kiewit Power Engineers Co.
A
lthough proper water treatment
and conditioning for combinedcycle
units and standalone
steam generators are vitally important
for reliability in the chemical
process industries (CPI), a well maintained
and operated steam generator
can also save the facility much money
with regard to efficient performance.
This article outlines some of the primary
thermodynamics behind steam
generator heat transfer.
Thermodynamic definitions
The word thermodynamics conjures
up visions of complex mathematics
to many people, even to some with
technical backgrounds. Yet, relatively
simple formulas from thermodynamics
can be used to explain much about
steam generator fundamentals.
Thermodynamics is built around
three laws, which are sometimes jokingly
defined as, " You can't get something
for nothing, " (first law) and " You
can't break even " (second and third
laws). In actuality, the first law is that
of conservation of energy. It says that
energy used within a system is neither
created nor destroyed but only transferred.
The classic energy equation for
a simple system (defined as a control
volume in textbooks) [1, 2] is:
(1)
Where,
Q = Heat input per unit time
Ws = Shaft work, such as that done
by a turbine per unit time
Efficiency improvements can quickly be identified
with back-of-the-envelope calculations
m*
2 = Mass flow out of the system
per unit time
m*
(m*
1 = Mass flow into the system per
unit time
2V2
V = Fluid velocity
2 - m*
1V1
energy
gz2 - gz1 = Change in potential
energy
u2 = Internal energy of the exiting
fluid
u1 = Internal energy of the entering
fluid
P2υ2 = Flow work of fluid as it exits
the system (P = pressure, υ =
specific volume)
P1υ1 = Flow work of fluid as it enters
the system
dEc.v./dt = Change in energy within
the system per unit time
While this equation may look complicated,
it can be easily understood
through a few definitions and simplifications.
First, in many systems and
especially steam generators, potential
and kinetic energies are minor compared
to other energy changes and can
be neglected. Second, in a steady flow
process such as a steam generator, the
system does not accumulate energy,
so dEc.v./dt is zero. Removing these
terms from Equation (1) leaves the internal
energy of the fluid (u) plus its
flow work (Pυ) capabilities. Scientists
have combined these two terms into
the very useful property known as enthalpy
(h). Enthalpy is a measure of
the available energy of the fluid, and
enthalpies have been calculated for
a wide range of steam and saturated
liquid conditions. These values may
44 ChemiCal engineering www.Che.Com november 2010
2)/2 = Change in kinetic
be found in the standard ASME steam
tables, where saturated water at 0°C
has been designated as having zero
enthalpy. Using these simplifications
and definitions, the energy equation
for steady state operation in a turbine
reduces to:
(2)
But this equation represents the ideal
scenario, and here is where the second
law steps in. Among other things, the
second law describes process direction.
A warm cup of coffee placed on a
kitchen table does not become hotter
while the room grows colder. Human
beings grow old, and so on.
A foundation of the second law is
the concept of the Carnot cycle, which
says that the most efficient engine
that can be constructed operates with
a heat input (QH) at high temperature
(TH) and a heat discharge (QL) at low
temperature (TL), in which
(3)
This equation represents a theoretically
ideal engine. In every process
known to humans, some energy losses
occur. These may be due to friction,
heat escaping from the system, flow
disturbances, or a variety of other factors.
Scientists have defined a property
known as entropy (s), which, in its
simplest terms, is based on the ratio of
heat transfer in a process to the temperature
(Q/T), where T is in Kelvin.
In every process, the overall entropy
change of a system and its surroundings
increases. So, in the real world,
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