52 



AGRICULTURE HANDBOOK 134, U.S. DEPT. OF AGRICULTURE 



AUTOMATIC SIRUP DRAW OFF 

 MECHANICAL ASSEMBLY 



EVAPORATOR PAN 



\ 



THERMO-i 

 SWITCH 



5 MICROFARAD 600 VOLT 



OIL-FILLED CONDENSER 



OR TYPE 8A5PS5 (ITT) FEDERAL 



CONTACT PROTECTOR 



FOR 117 V AC OR EQUAL 



Chart 10. — Automatic sirup drawoff. 



or finishing pan, must be sensitive to changes 

 of ±0.1° F. The thermoregulator must be recaH- 

 brated three times a day. 



When the thermoregulator is operated on 110 

 V. a.c, a condenser must be shunted across the 

 line (see chart 10) to protect the contact points 

 against the surge of heavy current that is set 

 up each time the solenoid coil operates. To 

 avoid this, it is better to operate the thermo- 

 switch on low d.c. voltage and use a high- 

 capacity mercury relay to operate the solenoid 

 valve. 



Another type of temperature regulator uses 

 thermistor probes as the sensing element. A 

 sensitive electrical relay must be used w^ith this 

 type of regulator, and it is recommended for use 

 with the thermoregulator. 



The thermistor probes in the boiling sirup 

 must be kept free of sugar-sand deposits. De- 

 posits of sugar sand on the probes will change 

 their heat-transmitting properties and cause 

 error in their response to the sirup boiling 

 temperatures. The probes can be kept free of 

 sugar sand by soaking them in 10-percent sul- 

 famic acid between runs. This same precaution 

 applies to the bulb of the thermometer. 



A further advance in controlling automatic 

 drawoff has been made at the Eastern Regional 

 Research Center (15). A new thermoregulator 



PN-n78 



Figure 82. — The automatic drawoff is especially well 

 suited for removing sirup from the evaporator. When a 

 finishing pan is used, the evaporator functions as a 

 semifinishing pan. 



containing a Wheatstone bridge, thermistor 

 probes, and a meter relay automatically com- 

 pensates for changes in boiling point caused by 

 changes in barometric pressure (fig. 83). The 

 instrument employs two thermistor probes. The 

 water probe continually responds to changes in 

 the temperature of boiling water. The sirup 

 probe causes the valve to open whenever the 

 temperature of the boiling sirup reaches a pre- 

 determined number of degrees above the tem- 

 perature of the boiling water — T F. for stand- 

 ard-density sirup and slightly more for heavier 

 sirup. 



End uf an Eva|K>i'ation 



When the evaporation of a run of sap has 

 been completed, care must be taken or the pans 

 may be burned. If water is available, it can be 

 run into the storage tank as the last of the sap 

 is being withdrawn. Little sap will be lost, and 

 the pans can be flooded with .3 to 5 inches of 

 water before the fire is extinguished. This pre- 

 caution is necessai-y when either wood or oil is 

 used because enough heat will remain in the 



