MAPLE SIRUP PRODUCERS MANUAL 



119 



boiling sirup reaches the desired temperature 

 above that for boiling water at that location. 

 The operation is not completely automatic, 

 since the thermoswitch must be handset as 



PN-)K23 



Figure 127.— Where coal is inexpensive, high-pressure 

 steam boilers may be used to evaporate sap to sirup. 



PN-4824 



Figure i2^. — Interior of a multiple-pan, all-steam central 

 evaporator plant at Stoystown, Pa. 



many as three or four times a day to compen- 

 sate for fluctuations in barometric pressure. 

 The U.S. Department of Agriculture has devel- 

 oped a new controller that will automatically 

 compensate for changes in the boiling point of 

 water due to changes in barometric pressure 

 (15). 



In some installations, the partly concentrated 

 sirup is not supplied to the finishing pan by 

 gravity feed. Instead, an electric pump, acti- 

 vated by an electrically operated liquid level 

 sensing device in the finishing pan, removes the 

 sap from the last flue pan or semifinishing pan 

 when it reaches the desired concentration — any 

 point between 2(f and 60? Brbc. 



If the Brix value of the sirup supplied to the 

 finishing pan is above 5(f , essentially all of the 

 sugar sand will have been formed and will be in 

 suspension. Its viscosity will be very low (see 

 table 13). It is advantageously filtered at this 

 point. The filtered sirup is then pumped into 

 the finishing pan. When it reaches the desired 

 density, it is automatically drawn by means of 

 solenoid valves and thermoswitch, and piped to 

 the holding or canning supply tank. With this 

 procedure, little or no sugar sand is formed in 

 the finishing pan. A cartridge-type filter can be 

 installed in this line to iwlish the sirup (that is, 

 remove the cloudlike precipitated sugar sand). 

 If the sirup is not prefiltered, it can be piped 

 from the finishing pan to a pressure filter such 

 as a plate-and-frame type and then to the 

 holding tank. Either method is desirable, since 

 once the sirup reaches standard density, it is 

 kept in a closed system so that it cannot evapo- 

 rate further. 



To reduce holdup time, it is good practice to 

 keep the liquid level as low as possible in each 

 evaporator. There is, of course, always a danger 

 that because of some failure, insufficient liquid 

 will be fed to each pan and the pan will be 

 ruined by burning. This can be prevented by 

 connecting a hose to the raw-sap feed line by 

 which sap can be added quickly to any location 

 in any one of the pans. 



Although a gas-fired finishing pan is satisfac- 

 tory for smaller plants, it is advisable to use 

 high-pressure steam for the finishing pan in 

 plants that make as much as 15 gallons of sirup 

 per hour. The steam permits finishing the sirup 

 without danger of burning it. The steam is best 



