Chapter 11-BOILER FITTINGS AND CONTROLS 



CANTILEVER 

 ARM 



ADJUSTER FOR <T^ 

 SETTING ALARM ^[^ 

 TEMPERATURE «^|) 



61.31X 

 Figure 11-35.— Superheater temperature alarm. 



or some combination of these. A simple pneu- 

 matic control system has been selected because 

 boiler control systems are, almost without ex- 

 ception, pneumatic. 



To perform the control functions of measure- 

 ment, comparison, computation, and correction, 

 the automatic control system must have a meas- 

 uring means, a controlling means, and some 

 arrangement for comparison and computation. 



The measuring means in the system shown 

 in figure 11-39 consists of a thermometer bulb, 

 a Bourdon tube, and connecting capillary tubing. 

 A change in the controlled variable— that is, the 

 temperature of the hot water— leads to a change 

 in the pressure transmitted to the Bourdon tube 

 and thus leads to a change in the position of the 

 Bourdon tube. Through a series of mechanical 

 linkages, the position of the Bourdon tube affects 



BULL'S-EYE 

 LENS 



LAMP 



LAMP UNIT 



REFLECTOR 

 UNIT 



VISION UNIT 



Figure 11-36.— Smoke indicator. 



38.60 



309 



