Principles of naval engineering - Historic Naval Ships Association

^IPRINCIPLES OF NAVAL ENGINEERINGDATUMLEVEL TUBEHORIZONTAL RUN OFAT LEAST 2 FEETREOUIREDvIDATUM COLUMNSHUTOFF VALVE[XF^TOBOILERDATUMCOLUMNNWL MARKNORMAL WATERLEVELHORIZONTAL RUN OF ATLEAST 2 FEETREQUIRED,DATUM COLUMNSHUTOFF VALVE.TO BOILERTO WASTE C^GAGE SHUTOFF S. Y--J'VALVEJTO WASTEGAGE SHUTOFF VALVEEQUALIZING VALVESOLID FLOATMERCURYMAGNETIC FINGERSGAGEFigure 11-34.— Jerguson remote water level indicator.98.98performed by a control system. Taken together,they constitute a closed loop of action andcounteraction by which some quantity or conditionis measured and controlled. The closedcontrol loop is often called a feedback loop ,since it requires a feedback signal from somethingon the output side of the process to somethingon the input side of the process. The closedloop concept is illustrated in figure 11-38.Now let us examine a simple automatic controlsystem such as the one shown in figure 11-39. The process being controlled is a heatexchange process in which steam is used to heatcold water. The two inputs, steam and coldwater, produce one output— hot water at somedesired temperature. Such a process could becontrolled by various kinds of automatic controlsystems— electrical, hydraulic, pneumatic,308