A triple-effect evaporator of the long-tube type is to be used to concentrate 35.000 gal/h of a 17 percent solution of dissolved solids to 38 percent dissolved solids. The feed enters at 60°F and passes through three tube-and-shell heaters, a, b, and c. in series and then through the three effects in order II, III, I. Heater a is heated by vapor taken from the vapor line between the third effect and the condenser, heater b with vapor from the vapor line between the second and third effects, and heater c with vapor from the line between the first effect and the second. In each heater the warm-end temperature approach is 10°F. Other data are given below and in Table 16.5.

Table 16.5

Steam to I, 230°F, dry and saturated. 

Vacuum on III, 28 in., referred to a 30-in. barometer. 

Condensates leave steam chests at condensing temperatures. 

Boiling-point elevations: 1°F in II, 5°F in III, 15°F in l. 

Coefficients, in Btu/ft²·h·°F, corrected for boiling-point elevation. 450 in l. 700 in II, 500 in III. 

All effects have equal areas of heating surface. Calculate 

(a) The steam required in pounds per hour,

(b) The heating surface per effect,

(c) The economy in pounds per pound of steam,

(d) The latent heat to be removed in the condenser.

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TABLE 16.5 Concentration solids, % 10 20 30 35 40 Specific gravity 1.02 1.05 1.10 1.16 1.25 Specific heat, Btu/lb-F 0.98 0.94 0.87 0.82 0.75