EVAPORATION 331 



With engines of reasonable efficiency, and with the elimination of small 

 isolated units, especially of direct action steam-driven pumps, a consumption 

 of 30 Ibs. steam per indicated horse-power-hour may be obtained ; of this 

 quantity 25 Ibs. should appear in the exhaust and 5 Ibs. will disappear in 

 cylinder condensation, etc. The steam actually used then in power is 

 1,650 X 5, or 8,250 Ibs. or 4-1 per cent, on cane. 



It is apparent that this estimate refers only to the stipulated conditions, 

 and that under other circumstances a different total consumption and different 

 distribution will result. 



Heating and Evaporation. The heat efficiencies of heating and evapora- 

 tion have been determined by Kerr, 10 the writer and others. From a study 

 of the results it is conservative to accept as a basis of design a consumption 

 105 per cent, of that computed with no loss of heat ; the steam required 

 at the different stations may then be determined as follows : 



Juice Heaters. Let the juice have a specific heat of 0-9 and let steam 

 at 5 Ibs. gauge be employed to heat the juice ; then the consumption per 



i Ib. per i F. is * X ' 9 = 0-000986 Ib. steam. 

 960-1 X 0-95 



Evaporators. With juice entering at the temperature of ebullition in 

 the first cell, the consumption of steam per Ib. of water evaporated will be 

 chiefly controlled by the temperature at which the syrup leaves, by the 

 system adopted for evacuating the condensed water, and by the percentage 

 of evaporation. For initial steam at 5 Ibs. gauge, syrup leaving at 130 F., 

 and water evacuated from each cell at the temperature of the steam entering 

 that cell, and 95 per cent, heat efficiency, i Ib. of steam on computation 

 will be found to evaporate n + a Ibs. of water, where n is the number of 

 effects and a is small. For convenience of calculation and to add a further 

 margin of safety a is neglected. 



Pans. In the process of pan boiling the syrup becomes diluted with the 

 washings from the tanks, with water used at the centrifugals, and with water 

 used to dilute the molasses before rebelling. All these additions of water 

 are estimated as being 2 per cent, on cane. In addition the syrup from the 

 multiple, when held over, cools to some extent, and the molasses will nearly 

 cool down -to the temperature of the atmosphere, all of which causes tend 

 to make a computation rather uncertain. Actually, with a steam pressure 

 of 40 Ibs. gauge, and a temperature in the pan of 140 F., i Ib. of steam will 



evaporate at 95 per cent, efficiency ' -^- = 0-85 water; allowing, 



however, for the sources of extra consumption mentioned above, an allowance 

 of 0-7 Ib. water per Ib. of steam is all that can be counted on, including 

 in this estimate all the sources of steam consumption indicated in this 

 paragraph. 



Let the factory, the steam consumption of which is to be analysed, work 

 up 100 tons cane with n per cent, fibre per hour and obtain 120 tons dilute 

 juice, which, with washings from tanks and filter cake washings, become, 

 as delivered to the first cell of the evaporator, 130 tons at 13 Brix. Then 

 for the first heating of the juice from, say, 82 F. to 212 F., there will be 

 required 130 X 120 X 0-000986 = 15-4 steam per cent, on cane. 



For the reheating of the juice from, say, 190 F. to 212 F. there will 

 be required 130 X 22 X 0-000985 =2-8 steam per cent, on cane. 



