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PROPERTIES OF STEAM AND WATER
INTRODUCTION
The process by which we convert water into steam and
use the steam to turn a propulsion shaft encompasses the generation and
expansion phases of the steam cycle. A
study of the properties of water and steam at these critical phases is necessary
to understand the steam cycle. This
lesson defines terms associated with these properties and processes, and
explains the use of steam tables to calculate the work and efficiency created by
steam.
REFERENCES
Basic Thermodynamics
Understanding Thermodynamics
The Laws of Thermodynamics: A Very Short Introduction (Very Short Introductions)
INFORMATION
A.
Basic Thermodynamic Terms
1.
Enthalpy (h), measured in
British thermal units per pound (mass), or BTU/lbm, represents the total energy
content of steam. It expresses the
internal energy and flow work, or the total potential energy and kinetic energy
contained within a substance. The
advantage of enthalpy is that we can
express in one term all of the energy in a substance which is due to its
pressure and temperature. Enthalpy
values are used to represent the energy level of steam entering a
turbine, a value useful for determining turbine efficiency.
By superheating steam, we can add enthalpy to steam without raising the
pressure of the steam. For example,
steam at 620 psig and 850°F can do more work in a turbine than steam that is
620 psig and 650°F.
2.
Entropy (s), measured in BTU/lbm-°R,
represents the unavailability of energy (°R=Rankine temperature scale where 0°R
= absolute zero and 460°R = 0°F).
The second law of thermodynamics states that when heat is transferred
from high temperature to low temperature regions, some of the heat will be
rejected and not converted into mechanical work.
Entropy is a measure of how much heat must be rejected to a lower
temperature receiver at a given pressure and temperature.
a.
A complex explanation of the mathematical significance of the definition
of entropy is unnecessary. It is a
term which attempts to describe the universe’s tendency to evenly distribute
all mass and energy throughout space. Processes
which produce entropy are possible and those which destroy entropy are
impossible.
b.
Bodies with a high temperature will, when brought in contact with a body
of a lower temperature, always cause heat to transfer from the hot body to the
cold body. This will lower the
internal energy of the hot body and raise the internal energy of the cold body.
This is the principle that guides the design and operation of all naval
heat exchangers. For example, a
main engine lube oil cooler directs hot lube oil over cool seawater piping, so
that the hot lube oil will transfer some of its heat to the cooler seawater.
If left together indefinitely, the property of entropy would cause the
heat from the lube oil to be equally distributed between the oil and the water,
so that both would have the same temperature.
c.
Entropy would not be important except for the fact that the purpose of
any engine is to collect, transfer, and use energy.
Thus, in a steam plant for example, it is not possible to add energy to
water, boil it and transmit the resulting high energy steam across the
relatively cooler engineroom without some of that energy being lost.
Some of this energy will always be lost through system conditions such as
ineffective pipe lagging, piping leaks, and dirty or fouled tubes which retard
heat transfer. Operators must
constantly attempt to minimize the effects of these conditions to maximize plant
efficiency and reduce fuel and water costs.
3.
A working fluid is a substance
which receives, transfers and transmits energy in a thermodynamic system.
In most systems, the working substance is a fluid (liquid, vapor or gas).
In a steam system, water is the working fluid.
4.
Density (r), measured in lbm/ft,
represents the mass of a substance per unit volume, or how tightly packed the
molecules are. The more molecules
packed in a given space, the more dense
the material. The density of water
in a given location of the boiler is critical to the steam generation process
because relatively dense feedwater will naturally push a less dense steam/water
mixture through the boiler generating tubes.
5.
Specific volume (vSP),
measured in ft3/lbm, represents the space occupied per unit mass of a
substance. It is the mathematical
inverse of density. Most
engineering equipment is designed for size and strength taking into
consideration the specific volume of the intended working fluid.
6.
Specific weight (g), measured
in lbf/ft3, represents the weight of a substance per unit volume. This is the density of a substance acted upon by gravity.
The pressure of a fluid at the bottom of a storage tank is a direct
function of the height of the fluid in the tank and the specific weight of the
feedwater. This resultant pressure
is an important shipboard consideration with respect to providing a minimum
suction pressure for a pump below the tank to move the fluid through a system.
7.
The state of a working fluid
refers to the physical properties it possesses at a particular pressure,
temperature and volume. If each of
these are known with respect to a substance, the state of the substance is
known. The substance can be a
subcooled, saturated, or superheated solid, liquid, or gas.
Many systems operate the working fluid with very specific
temperature/pressure relationships. Water
is subcooled in the condensate and feed phases of the steam cycle to allow it to
be pumped, saturated in portions of the generation and feed phases for natural
flow or for maintaining proper chemistry, and superheated in the expansion phase
to extract maximum work from the steam to turn a propulsion turbine.
8.
A thermodynamic process is any
process which changes the state of the
working fluid. These processes can
be classified by the nature of the state change that takes place. Common types
of thermodynamic processes include the following:
a.
A reversible process is an ideal process where the working fluid
returns to its original state by conducting the original process in the reverse
direction. For a process to be
reversible, it must be able to occur in precisely the reverse order.
All energy that was transformed or distributed during the original
process must be capable of being returned to its exact original form, amount and
location. Reversible processes do
not occur in real life.
b.
An irreversible process is any process which is not reversible.
All real life processes, such as the basic steam cycle, are irreversible.
c.
An adiabatic process is a state change where there is no transfer
of heat to or from the system during the process.
Because heat transfer is relatively slow, any rapidly performed process
can approach being adiabatic. Compression
and expansion of working fluids are frequently achieved adiabatically with pumps
and turbines.
d.
An isothermal process is a state change in which no temperature
change occurs. Note that heat
transfer can occur without causing a change in temperature of the working fluid.
In the DFT, auxiliary exhaust heats incoming condensate, then condenses
to liquid and falls to the bottom of the tank. Throughout this process, the
temperature of the auxiliary exhaust remains constant at 246-249°F.
e.
An isobaric process is a state change in which the pressure of the
working fluid is constant throughout the change.
An isobaric state change occurs in the boiler superheater, as the heat of
the exiting steam is increased without increasing its associated pressure.
9.
A thermodynamic cycle is a
recurring series of thermodynamic processes through which an effect is produced
by the transformation or redistribution of energy.
In other words, it is a series of processes repeated over and over again
in the same order. Thermodynamic
cycles contain five basic elements: (1) a working fluid, (2) an engine, (3) a
heat source, (4) a heat receiver, and (5) a pump.
All thermodynamic cycles may be classified as being open cycles or closed
cycles.
a.
A closed cycle is one in which
the working fluid is reused. Steam
plants and refrigeration cycles are closed cycles.
In a steam plant, the water undergoes a series of processes that change
the state of the water. Eventually the water returns to its original state and
is ready to begin the cycle again.
b.
An open cycle is one in which
the working fluid is not reused. Open
cycles typically use the atmosphere as a working fluid.
An internal combustion engine represents a typical open cycle.
Air is drawn into the engine, combusted in the cylinders, and exhausted
back to the atmosphere. Fresh air
is drawn into the engine to begin the cycle again.
B.
Heat Addition and Temperature
1.
When heat is added to a material, one of two things will occur: the
material will change temperature or the material will change state. When a substance is below the temperature at a given
pressure required to change state, the addition of sensible
heat will raise the temperature of the substance. Sensible heat applied to a pot of water will raise its
temperature until it boils. Once
the substance reaches the necessary temperature at a given pressure to change
state, the addition of latent heat
causes the substance to change state. Adding
latent heat to the boiling water does not get the water any hotter, but changes
the liquid (water) into a gas (steam).
2.
One can state that a certain amount of heat is required to raise the
temperature of a substance 1°F. This
energy is called the specific heat
capacity. The specific heat
capacity of a substance depends upon the volume and pressure of the material. For water, the specific heat capacity is 1 BTU/lbm-°F and
remains constant. This means that
if we add 1 BTU of heat to 1 lbm of water, the temperature will rise 1°F.
C.
Introduction to Steam Tables
1.
When a teapot of water is placed on a hot burner, sensible heat begins to
heat the water. The energy added to
the water raises its internal energy and its temperature. When the water reaches 212°F, the temperature no longer
rises as latent heat begins to change the water from a liquid to a vapor.
The mass inside the teapot is slowly changing from a 100% water / 0%
steam mixture into a 0% water / 100% steam mixture.
If we add only half the necessary latent heat, then only half the water
will boil into steam. The result
would be a 50% water / 50% steam mixture at 212°F.
If we add all the latent heat necessary, then the water at 212°F changes
completely into steam at 212°F. Continuing
to add heat to the 212°F steam results in a temperature increase
(superheating), and we are again raising the temperature by adding sensible
heat. Refer to figure 3.2-1 (sensible/latent heat and enthalpy).
2.
While the properties of water at atmospheric pressure are commonly known,
water under different pressures will exhibit different properties.
When water is boiled at pressures higher than atmospheric, the same
events occur as described above with two exceptions.
First, the boiling temperature will be higher than 212°F.
Second, less latent heat is required to be added to change the water
completely into steam. If water
were to be boiled at a pressure lower than atmospheric pressure, then we would
find that the boiling temperature would be less than 212°F and a larger amount
of latent heat would be required to change the water completely into steam.
Refer to figure 3.2-2 (temperature vs. latent heat).
a.
When water is below the boiling point, the addition of heat is seen as
sensible heat. This water is said
to be a subcooled liquid.
When enough sensible heat is added so that the temperature of the water
approaches saturation temperature but no steam has yet been formed, the water is
said to be a saturated liquid.
b.
As the water is transformed from a saturated liquid to saturated steam,
boiling is occurring. As latent
heat is added, the temperature of the water remains the same but the saturated
liquid is being changed into a saturated vapor.
During this period the water is referred to as a liquid/vapor
mixture. When enough latent
heat is added so that all of the liquid is converted into vapor, the water
becomes a saturated vapor.
Note that the saturated vapor is 100% vapor and exists at the same
temperature as the saturated liquid. Above
the saturated steam point, vapor exists at a temperature higher than saturation
temperature. This is the superheated vapor region.
c.
Steam tables are a useful tool for determining the properties of steam
and water at various temperatures and pressures.
The steam tables are broken into three tables.
D.
Mollier Diagram
1.
The Mollier diagram is a small portion of data from the steam tables
graphed onto enthalpy-entropy coordinates.
It presents the region that is commonly found in propulsion plant steam
systems. Examine the last section
of the steam tables for a representation of a Mollier diagram.
2.
Locating information off the Mollier diagram is done as follows: The
horizontal axis is entropy (s) in BTU/lbm-°R.
The vertical axis is enthalpy (h) in BTU/lbm.
The dark line across the middle of the chart is called a “steam dome”
because of its shape. Above this
line, the data is for superheated steam. Below
this line, the data is for a steam-water mixture.
The data directly on the line is for saturated steam.
3.
To find data in the steam-water mixture region of the chart, enter the
chart using the absolute pressure and %-moisture (y).
Once you find the intersection of these two parameters, read off the
number directly across from the intersection point for enthalpy.
Read off the number directly below the intersection point for entropy.
To find data in the
superheated region of the chart, enter the chart using the measured temperature
and pressure of the steam. Again,
find the intersection point of these two parameters and read off the values for
entropy and enthalpy. Notice that
moisture does not plot in the superheat region.
This is because moisture is a parameter which only exists in saturated
conditions.
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