CHAPTER 3
BASIC STEAM CYCLE
To understand steam generation, you must
know what happens to the steam after it leaves
the boiler. A good way to learn the steam plant
on your ship is to trace the path of steam and
water throughout its entire cycle of operation. In
each cycle, the water and the steam flow through
the entire system without ever being exposed to
the atmosphere. The four areas of operation in
a main steam system are generation, expansion,
condensation, and feed. After studying this
chapter, you will have the knowledge and ablity
to describe the main steam cycle and the functions
of the auxiliary steam systems.
MAIN STEAM SYSTEM
The movement of a ship through the water is
the result of a number of energy transformations.
Although these transformations were mentioned
in the last chapter, we will now discuss these
transformations as they occur. Figure 3-1 shows
the four major areas of operation in the basic
steam cycle and the major energy transformations
that take place. These areas are A—generation,
B—expansion, C—condensation, and D—feed.
GENERATION—The first energy transfor-
mation occurs in the boiler furnace when fuel
oil burns. By the process of combustion,
the chemical energy stored in the fuel oil
is transformed into thermal energy. Thermal
energy flows from the burning fuel to the
water and generates steam. The thermal energy
is now stored as internal energy in steam,
as we can tell from the increased pressure
and temperature of the steam.
EXPANSION—When steam enters the turbines
and expands, the thermal energy of the steam
converts to mechanical energy, which turns the
shaft and drives the ship.
For the remainder of the cycle, energy is
returned to the water (CONDENSATION and
FEED) and back to the boiler where it is again
heated and changed into steam. The energy used
for this purpose is the thermal energy of the
auxiliary steam.
The following paragraphs will explain the four
major areas of operation in the basic steam cycle
shown in figure 3-1.
GENERATION
When a liquid boils, it generates a vapor.
Some or all of the liquid changes its physical state
from liquid to gas (or vapor). As long as the vapor
is in contact with the liquid from which it is
being generated,
it remains at the same
temperature as the boiling liquid. In this
condition, the liquid and its vapors are in
equilibrium contact with each other. Area A of
figure 3-1 shows the GENERATION area of the
basic steam cycle.
The temperature at which a boiling liquid
and its vapors may exist in equilibrium contact
depends on the pressure under which the
process takes place. As the pressure increases,
the boiling temperature increases. As the
pressure decreases,
the boiling temperature
decreases. Determining the boiling point depends
on the pressure.
When a liquid is boiling and generating
vapor, the liquid is a SATURATED LIQUID and
the vapor is a SATURATED VAPOR. The
temperature at which a liquid boils under a
given pressure is the SATURATION TEM-
PERATURE, and the corresponding pressure is
the SATURATION PRESSURE. Each pressure
has a corresponding saturation temperature, and
each temperature has a corresponding saturation
3-1
