338 



T?are 



CHAPTER XVIII 



Magnesia 



lOO 



1170 -o 



120 -6 



94 '2 



85-2 



132-6 



119-4 



100 -2 



1284 -O 



104 



95 

 147 

 132 

 III 



200 

 1380 -o 

 142-8 

 III -6 

 102 -6 

 156 -o 



142 -2 

 119 -4 



250 



151-8 

 II8-2 

 108 -6 



166 -2 



150 O 

 126 -6 



It will be noted that while the loss with bare pipes increases rapidly 

 as the pressure rises, the loss is subject to small variation with covered pipes ; 

 that is to say, the dominant factor is the resistance of the covering, precisely 

 as the scale in evaporator tubes dominates the rate of heat transference 

 there. 



A quadruple effect evaporator will expose about 0-3 sq. ft. area for each 

 I sq. ft. of heating surface : the temperatures of each unit may be taken 

 as 215° F., 200° F., 180° F., 130° F., so that if the external air be 80° F. 

 the temperature differences are 135° F., 120° F., 100° F., and 50° F. From 

 the data on page 337 one sq. ft. of bare pipe at 212° F. loses per hour 350 

 B.T.U. With external air at 72° F., the loss per sq. ft. per hour in the 

 evaporator will then be in each cell of the order 350, 300, 250, 125 

 B.T.U. 



If a computation of the water evaporated per lb. of steam supplied 

 be made on the lines indicated earlier in this chapter, a difference of about 

 0-15 lbs. water per lb. of steam supplied will be found. If i lb. steam enter 

 the apparatus for every 5 lbs. of cane, the loss indicated is, steam 3 per cent, 

 on cane ; but this loss is recoverable at quadruple effect, and is hence reduced 

 to 0*75 per cent, on cane. If, however, owing to radiation losses, the evapor- 

 ator is unable to deliver syrup of the proper density, the loss has to be 

 recovered at single effect and remains as before of the order 3 per cent, 

 on cane. 



A sugar factory will have about 50 sq. ft. pipe area in both live and exhaust 

 lines per ton-cane-hour. When protected, each sq. ft. in live and exhaust 

 will lose as an average about 100 B.T.U. per hour, or 10,000 B.T.U. per ton 

 of cane, or roughly o - 5 steam per cent, on cane. This loss should be regarded 

 as a minimum. With unprotected pipes the exhaust line will lose about 

 400 B.T.U. per hour, or steam i per cent, on cane, and the live steam lines 

 will lose about 1,200 B.T.U. per hour, or steam 3 per cent, on cane. 



Juices entering the settling tanks at 212° F. will cool down to 170° F, 

 in bare tanks, and not below 200° F. in well protected tanks. The un- 

 necessary loss of 30° F. represents, with juice equal in weight to cane, a loss 

 of steam from 2-5 per cent, to 3 per cent, on cane. 



The feed water to the boilers can be returned through a closed system, 

 the temperature of which depends on the pressures of steam used in pans 

 and evaporators. Nearly always an open system is found, and an unneces- 

 sary loss of 50° F. is quite common. With a total production of steam 

 50 per cent, on cane, this loss amounts to about 2 per cent, to 2-5 per cent, 

 steam on cane. 



The Working Capacity of Evaporators. — Koppeschaar,^^ bringing together 

 the observations of Jelinek, Claassen, Brunnings and himself, gives the follow- 

 ing as average working conditions in quadruple effect evaporators ; his 

 figures are here transposed into British units and the writer's nomenclature : 



