56 



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



PN-4780 



Figure 8J,. — High-pressure steam boilers are economical 

 when low-priced fuel is available. Approximately 20 

 gallons of finished sirup per hour can be made in the 

 two 100-h.p. boilers shown. 



Figure 86. — This evaporator consists of several units 

 connected in series with the partly evaporated sap 

 moving successively to the next evaporator by means 

 of a float control to prevent intermixing of concen- 

 trated sap with less concentrated sap. 



easier to keep clean. (6) The high-pressure 

 steam can be used as the source of heat in 

 making a variety of maple products. (7) Where 

 soft coal can be obtained cheaply, high-pressure 

 steam is economical. 



The disadvantages of steam evaporators are: 

 (1) A license may be required to operate a 

 steam boiler. (2) The boiler needs periodic in- 

 spection and overhauling. (.3) In some areas the 

 water is not suitable for use in a steam boiler. 

 (4) The initial cost of the steam boiler may not 

 be justified. 



The approximate size of a steam boiler (boiler 

 horsepower, b.h.p.) required to evaporate sap to 

 sirup can be calculated, since 1 b.h.p. will evapo- 

 rate approximately 3.25 gallons of water (sap) 

 per hour. The value 3.25 varies slightly, depend- 

 ing on the temperature of the sap as it enters 

 the evaporator and the operating pressure of 

 the boiler. As indicated earlier, 33.25 gallons of 

 water must be evaporated from sap with an 

 initial Brix value of 2.5° to produce 1 gallon of 



/ 33.25 * 

 sirup. Approximately 10 b.h.p. 



3.25 



will be 



PN-nsi 

 Figure 85. — A converted evaporator that uses high-pres- 

 sure steam coils with steam generated by two 100-h.p. 

 boilers. 



required to produce 1 gallon of sirup per hour. 



A system that is proving successful is the 

 combination of oil and steam. In this two-stage 

 system, oil is used to concentrate the sap to 

 about 30° or 40° Brix in flue pans, and steam is 



