102 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



heating coil will produce a dela} T ed heating action and the tunnel 

 temperature may exceed the set temperature. If the steam-supply 

 piping and controls were similar to those shown in figure 56, the man- 

 ually operated valve might again be used to throttle the steam in 

 order to maintain a temperature level within close but safe approach 

 to the set temperature. If we assume that 125° F. is the safe tempera- 

 ture level for a set temperature of 135° under this condition of opera- 

 tion, the sensitivity of the modulating controller must be adjusted 

 so that at a given temperature, for example 120°, the motorized 

 valve would be fully opened. At 135° the valve would be fully closed. 

 It will be seen, then, that at some point between 125° and the set 



air-operated temperature .— ■ 



controller' 



air-operated motorized valve 



THERMAL ELEMENT OR BULB 

 LOCATED IN DEHYDRATER 



SHUT-OFF VALVE 

 STEAM SUPPLY. 



' 1 



MANUALLY OPERATEO--^ * HEATING COIL 



GLOBE VALVE FOR 

 BYPASS 



Figure 56. — Typical simplified steam heater control diagram. 



temperature, the system would come to equilibrium, even though this 

 temperature is obviously lower than the set temperature. The opera- 

 tor must recognize this fact, and readjust the controller indicating 

 point and recording arm so that they indicate and record the new 

 equilibrium temperature; otherwise his records and the true tunnel 

 temperatures will be at variance. If the modulating controller is of 

 the simple type, this action must be carried out as often as the load 

 in the tunnel changes. Instruments are available, however, which 

 perform this function automatically. When this feature is included 

 on a modulating controller, it is said to have re-set features. 



Other special features are obtainable on temperature-control in- 

 struments. One type deserving special mention is the cam-operated 

 time-temperature controller. This unit includes a clock-driven cam 

 that moves the control arm in accordance with some prearranged 

 temperature schedule. The controller can be of either the off-on or 

 modulating type. This type is especially suited for use on a cabinet 

 dehydrator in reproducing scheduled temperature variations. 



In addition to the control of dry-bulb temperature, specially adapted 

 sensitive elements fitted with wicks or other wetting devices can be 

 used for indicating, controlling, and recording wet-bulb tempera- 

 tures. An important matter is the care of wicks and wet-bulb devices. 

 Care should be taken to keep these surfaces clean. It is preferable 

 that the water used in the appliance be distilled, since water contain- 

 ing calcium and other salts will leave a residue after the water has 

 evaporated. These salts contaminate the wick and cause the instru- 

 ment to give a false reading. If hydrant water is used in wetting the 

 bulb, frequent attention should be given to cleaning or renewing the 

 wick. Multiple-pen units can be obtained for such purposes as indi- 

 cating, controlling, and recording dry-bulb temperatures and wet- 

 bulb temperatures simultaneously. Many motor-operated devices, 

 besides valves, are available for use with temperature controllers. 

 These can bemused for the operation of dampers and other devices 

 necessary in the automatic operation of a dehydration unit. 



