Figure 9-9.-A manometer.runs into the turbine and causes the rotor to turn. Afterit has passed through the turbine, it still exerts a backpressure against the blades. If the back pressure werenot reduced, it would build until it became as great asthat of the incoming steam and prevent the turbine fromturning at all. Therefore, the exhaust steam is runthrough pipes surrounded by cold sea water to reducethe back pressure as much as possible. The coldtemperature causes the steam in the pipes to condenseinto water, and the pressure drops well belowatmospheric pressure.The engineer needs to know the pressure in thecondensers at all times. To measure this reducedpressure, or partial vacuum, the engineer uses a gaugecalled a manometer. As shown in figure 9-9, it consistsof a U-shaped tube. One end is connected to thelow-pressure condenser, and the other end is open to theair. The tube is partially filled with colored water. Thenormal air pressure against the colored water is greaterthan the low pressure of the steam from the condenser.Therefore, the colored water is forced part of the wayinto the left arm of the tube. A scale between the twoarms of the U indicates the difference in the height ofthe two columns of water. This difference tells theengineer the degree of vacuum-or how much belowatmospheric pressure the pressure within the condenseris.SUMMARYYou should remember seven points about force andpressure: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 anobject or exerted by an object. You measure itin pounds per square inch (psi).You calculate pressure by the formula P = f.Spring scales and lever balances are familiarinstruments you use for measuring forces.Bourdon gauges, barometers, and manometersare instruments for the measurement ofpressure.The normal pressure of the air is 14.7 psi at sea level.Pressure is generally relative; that is, it is sometimesgreater—sometimes less—than normal airpressure. Pressure that is less than the normalair pressure is called a vacuum.9-7
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