58 



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



(2) The vacuum evaporator, which is Hmited to 

 large-scale or central-plant operation, is 

 used to complete the evaporation of sap that 

 has been partly concentrated on the farm. 



The equipment used usually is idle milk- 

 evaporation equipment. The sirup produced 

 has essentially no maple flavor, but it is 

 excellent for use in making high-flavored 

 sirup. 



FUEL 



Wood 



The modern flue-type evaporator was de- 

 signed for burning wood. A wood fire carries a 

 luminous flame throughout the entire length of 

 the arch. The flue area of the evaporator and 

 the part that lies over the firebox are heated 

 both by radiant and by convection heat liber- 

 ated by the burning gases. The wood may be 

 sound cordwood, defective trees removed in 

 improvement cuttings, or sawmill wastes — 

 either culls or slab (69). 



In the evaporation process, the object is to 

 evaporate the water in the shortest possible 

 time. Therefore, it is essential to use only diy, 

 sound wood that will produce a hot fire. Wet or 

 green wood will not produce as much heat as 

 will the same volume of dry wood. Poor burning 

 fuel results in a slower boiling rate. This, in 

 turn, causes the sap to be held in the evapora- 

 tor for a longer time and results in a darker 

 sirup. 



A steady fire shortens the boiling time. The 

 best results are obtained by charging the fire- 

 box first on one side and then on the other, 

 keeping the fuel in the firebox at almost con- 

 stant volume (fig. 87). The fire doors should be 

 closed immediately after each charging to re- 

 duce the intake of cold air which cools the 

 underside of the pans. When this happens, the 

 boiling rate of the sap decreases and holdup 

 time in the evaporator increases. Likewise, ash- 

 pit draft doors that are open too wide will admit 

 more air than is required for combustion, and 

 the excess air has a cooling effect. Introduction 

 of cold air beneath the evaporator pan in either 

 the firebox or the flue area not only reduces the 

 boiling rate but also tends to set up counter 

 currents in the flowing sirup in the different 

 channels of the evaporator. This also contrib- 

 utes to the production of a darker sirup. 



Based on $25 per cord of wood, the fuel to 

 produce a gallon of sirup would cost about $L 



This represents about 10 percent of the cost of 

 sirup production (5, 113). The heating values of 

 different wood fuels expressed in British ther- 

 mal units (B.t.u.'s) for a standard 4- by 4- by 8- 

 foot cord are maple, 22,800,000; beech, 

 20,900,000; and hickory, 24,800,000. 



oa 



The advantages offered by using oil as the 

 heat source for evaporating maple sap to sirup 

 are numerous (lOi). Chief among these are (1) it 

 is automatic; therefore, it does not require the 

 services of a fireman; (2) it provides a steady 

 uniform heat, which is desirable for producing 

 high-quality sirup; (3) it is clean and therefore 

 aids in better housekeeping and sanitation in 

 the evaporator house; and (4) in terms of Brit- 

 ish thermal units, the cost of oil at 35 cents per 



PN^783 



Figure S7.— When both doors are opened for firing, the 

 excess air admitted chills the pan. Boiling stops; sap 

 and partly evaporated sirup intermix; and then, when 

 the fuel is again burning briskly, the evaporator must 

 equilibrate itself. 



