Figure 9-9.-A manometer.
runs into the turbine and causes the rotor to turn. After
it has passed through the turbine, it still exerts a back
pressure against the blades. If the back pressure were
not reduced, it would build until it became as great as
that of the incoming steam and prevent the turbine from
turning at all. Therefore, the exhaust steam is run
through pipes surrounded by cold sea water to reduce
the back pressure as much as possible. The cold
temperature causes the steam in the pipes to condense
into water, and the pressure drops well below
The engineer needs to know the pressure in the
condensers at all times. To measure this reduced
pressure, or partial vacuum, the engineer uses a gauge
called a manometer. As shown in figure 9-9, it consists
of a U-shaped tube. One end is connected to the
low-pressure condenser, and the other end is open to the
air. The tube is partially filled with colored water. The
normal air pressure against the colored water is greater
than the low pressure of the steam from the condenser.
Therefore, the colored water is forced part of the way
into the left arm of the tube. A scale between the two
arms of the U indicates the difference in the height of
the two columns of water. This difference tells the
engineer the degree of vacuum-or how much below
atmospheric pressure the pressure within the condenser
You should remember seven points about force and
A force is a push or a pull exerted on or by an object.
You measure force in pounds.
Pressure is the force per unit area exerted on an
object or exerted by an object. You measure it
in pounds per square inch (psi).
You calculate pressure by the formula P = f.
Spring scales and lever balances are familiar
instruments you use for measuring forces.
Bourdon gauges, barometers, and manometers
are instruments for the measurement of
The normal pressure of the air is 14.7 psi at sea level.
Pressure is generally relative; that is, it is sometimes
greatersometimes lessthan normal air
pressure. Pressure that is less than the normal
air pressure is called a vacuum.