PRINCIPLES OF NAVAL ENGINEERING 



DATUM LEVEL TUBE 



HORIZONTAL RUN OF 



AT LEAST 2 FEET 



REOUIREDv 



DATUM COLUMN 

 SHUTOFF VALVE 



I [XF^TO 



I BOILER 



DATUM 

 COLUMN 



HORIZONTAL RUN OF AT 



LEAST 2 FEET 



REQUIRED, 





NWL MARK 

 NORMAL WATER 



LEVEL 



DATUM COLUMN 

 SHUTOFF VALVE. 



TO BOILER 



TO WASTE C 



GAGE SHUTOFF -J S. Y- 



VALVE ' ^ ^ 



J TO WASTE 

 GAGE SHUTOFF VALVE 

 EQUALIZING VALVE 



SOLID FLOAT 

 MERCURY 

 MAGNETIC FINGERS 



GAGE 



98.98 



Figure 11-34.— Jerguson remote water level indicator. 



performed by a control system. Taken together, 

 they constitute a closed loop of action and 

 counteraction by which some quantity or con- 

 dition is measured and controlled. The closed 

 control loop is often called a feedback loop , 

 since it requires a feedback signal from some- 

 thing on the output side of the process to some- 

 thing on the input side of the process. The closed 

 loop concept is illustrated in figure 11-38. 



Now let us examine a simple automatic con- 

 trol system such as the one shown in figure 11- 

 39. The process being controlled is a heat 

 exchange process in which steam is used to heat 

 cold water. The two inputs, steam and cold 

 water, produce one output— hot water at some 

 desired temperature. Such a process could be 

 controlled by various kinds of automatic con- 

 trol systems— electrical, hydraulic, pneumatic, 



308 



