100 MISC. PUBLICATION 5 4 0, IT. S. DEPT. OF AGRICULTURE 



able as an air-, electric-, or direct vapor-operated instrument. Fea- 

 tures such as indicating and recording devices can be included and are 

 desirable for use on vegetable dehydrators. 



The modulating controller is more complicated than the off-on type 

 and is available as an air- or electric-operated unit. Because of its 

 comparative ease of adjustment and its ability to transmit a modulat- 

 ing force to the motorized valve, the air-operated instrument 

 dominates. 



In modulating controllers, action is imparted to the motorized valve 

 in such a way that the valve-stem movement is proportional to the 

 difference between the actual tunnel temperature and the set tempera- 

 ture on the controller. Instruments vary in sensitivity — that is, they 

 vary in the amount of movement of the indicating pointer required 

 to cause the motorized valve to change from a fully opened to a fully 

 closed position. Sensitivity is measured in degrees required for maxi- 



AIR-OPERATED TEMPERATURE 



CONTROLLER 

 AIR-OPERATED MOTORIZED VALVE, 



FLAME-FAILURE THERMO- 

 STATIC SHUT-OFF VALVE 



SHUT-OFF VALVE 

 FUEL SUPPLY 



m 



\r 



-o^ 



THERMAL ELEMENT OR BULB 

 LOCATED IN DEHYDRATER 



mrrrr 



MANUALLY OPERATED 

 GLOBE VALVE FOR 

 LOW-FIRE ADJUSTMENT 





FLAME- FAILURE 

 THERMOSTATIC ELEMENT 



Figure 55. — Typical simplified burner control diagram. 



mum valve opening. Thus, if the temperature required for full open- 

 ing of the valve is at wide variance with the set temperature, then 

 the instrument is said to have low sensitivity. Conversely, if the vari- 

 ance is close, the instrument is said to have high sensitivity. If the 

 sensitivity of an instrument were carried to its ultimate, it would 

 become an off- on instrument. 



Sensitivity should not be confused with instrument response. Re- 

 sponse of an instrument is the ability of the thermal element to trans- 

 mit temperature variations to the controller. It is assumed in sub- 

 sequent discussion that the thermal-element temperature response is 

 the same regardless of the type of controller used. Most instruments 

 incorporate means to permit change of sensitivity. The need for this 

 variable-sensitivity device will be discussed in more detail. 



Every heating system, whether it is for a tunnel or a cabinet dehy- 

 drator, a bin finisher, or a lye vat, has an inherent characteristic that 

 must be fully recognized before an automatic temperature controller 

 can be properly selected. Generally, if the temperature response of 

 the system is rapid, the controller should be of the modulating type ; 

 if it is sluggish, the controller can be either the off-on or sensitive 

 modulating type. In the latter case, the off-on controller, being the 

 less costly of the two, is generally used. In order to illustrate these 

 general rules, the temperature-control problems of several different 

 heating systems will be analyzed below : 



Suppose that a counterflow tunnel is being used as a finishing drier 

 for onions and that it is being heated by an open gas flame. What 

 type of controller should be provided? Temperature regulation for 

 onions is critical. Excess temperatures may cause discoloration of the 



