338 CHAPTER XVIII 



Bare 



Sponge-filled asbestos, i in. 



2 in. 



3 in. 



Magnesia 



1 in. 

 ii in. 



2 in. 



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

 as the pressure rises, the toss 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 Ib. of steam supplied 

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

 0*15 Ibs. water per Ib. of steam supplied will be found. If I Ib. steam enter 

 the apparatus for every 5 Ibs. 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 o 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, 13 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 : 



