118 



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



CENTRAL SAP EVAPORATOR PLANT 



Chart 23. — Flow diagram: Oil-and-steani plant. 



Coal is used in some areas, particularly 

 where it is cheap. It is best used to generate 

 high-pressure steam which, in turn, is used to 

 evaporate the sap (fig. 127). 



As with oil-fired evaporators, a series of pans 

 is used. These pans, like the oil-fii-ed pans, are 

 mounted stepwise, as shown in figure 128. The 

 pans are heated with 80 to 110 p.s.i.g. steam in 

 coils or manifolds of ^/4-inch brass tubing 

 mounted at the bottom of the flat pans. 



The specifications for a small plant (about 

 8,800 gal. per season) described by Pasto and 

 Taylor {86) are as follows: 



Sap gallons per hour. 



Water evaporated do 



Sirup produced do 



Tapholes number.,. 



Fuel consumption gallons 



Capital investment dollars. _. 



Floorspace square feet 



Sap storage gallons... 



Capital investment and cost for depreciation 

 and repairs for a small plant, as reported by 

 Pasto and Taylor {86), are given in table 18. 



Smaller plants are in operation and are prov- 

 ing highly successful. Usually these plants are 

 small initially, but they are built so that they 

 can be enlarged after 2 or 3 years' operation. 

 Typical of these is the central evaporator plant 

 established in 1962 at Ogema, Wis. (fig. 124). 

 This plant has one 6- x 20-foot evaporator and a 

 separate finishing pan. Sap is supplied from 

 9,500 tapholes on 16 farms. 



Operation 



The sap supplied to the evaporator from the 

 storage tanks is fed to the first flue pan. Since 

 the flue pans are connected in series, the sap 

 flows successively through each pan to the 

 next. The sap is conducted between pans 

 through large-diameter, heat-resistant tubes or 

 pipe at least IV2 inches in diameter. The pans 

 can be installed in a stepwise manner to insure 

 no backward flow of sap from pans of higher 

 concentration to pans of lower concentration 

 and to better control the depth of the liquid 

 level in each pan. Since the elevation between 

 pans is only 6 to 8 inches, there is only a small 

 hydrostatic pressure in each interconnecting 

 feed line. Feed lines and valves must be large 

 enough to supply sap to the pans rapidly 

 enough under this low pressure to replace the 

 vast quantities of water being removed by 

 evaporation. 



The liquid level in the evaporator pans is 

 maintained at a fixed depth by means of a 

 mechanical float valve or by an electrically 

 operated liquid level sensing element and sole- 

 noid valve. Whichever mechanism is used, it 

 must be sensitive to minute changes in liquid 

 level and must operate instantaneously. In 

 principle, when the finishing pan requires more 

 liquid to maintain its depth of sap, the sap is 

 obtained from the third pan of a 3-flue pan 

 installation, which in turn obtains more sap 

 from the second pan, and so on back to the 

 storage tank. The sirup is removed from the 

 finishing pan when it reaches standard density 

 (66.(f Brix) or slightly higher. 



The operation can be automated by use of a 

 thermoswitch and solenoid valve. The thermo- 

 switch is adjusted to open the valve when the 



