Page 204 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

202 Chapter 6 Evaporators

T con,4 ¼ saturation temperature of steam at pressure P 3 ¼ 50 C

L 3 ¼ Fx F /x 3 ¼ 72.92; V total ¼ F L 3 ¼ 760.41;
Trial 1: V 1 , V 2 , V 3 are assumed equal to one third of V total
V 1 ¼ V total /3 ¼ 253.47; V 2 ¼ 253.47; V 3 ¼ 253.47;

Step e A
L 1 ¼ FeV 1 ¼ 579.86; L 2 ¼ L 1 e V 2 ¼ 326.39;
x 1 ¼ Fx F /L 1 ¼ 0.1007; x 2 ¼ Fx F /L 2 ¼ 0.1787

Step e B
Roughly estimating BPE in effects from BPE table, C: 0.5, 1 and 16.2.

Total BPE ¼ 17.7

Total DT av ¼ 100.6 e 50 ¼ 50.6 C
DT eff, total ¼ 50.6 e 17.7 ¼ 32.9 C

Estimating individual contribution by assuming same q and A i in all effects
DT eff,1 ¼ 32.9 (1/2350)/(1/2350 þ 1/1250 þ 1/1120) ¼ 6.61
DT eff,2 ¼ 32.9 (1/1250)/(1/2350 þ 1/1250 þ 1/1120) ¼ 12.42
DT eff,3 ¼ 32.9 e 7.89 11.82 ¼ 13.87
T 3 ¼ T con,4 þ BPE 3 ¼ 50 þ 16.2 ¼ 66.2
T con,3 ¼ 66.2 þ 13.87 ¼ 80.07
T 2 ¼ T con,3 þ BPE 2 ¼ 80.07 þ 1 ¼ 81.07
T con,2 ¼ 81.07 þ 12.42 ¼ 93.49
T 1 ¼ T con,2 þ BPE 1 ¼ 93.49 þ 0.5 ¼ 93.99

T con,1 ¼ saturation temperature of steam ¼ 100.6 C
% Checking if assumed BPE values match x and T con,2 , T con,3 , T con,4 values
Recalculated BPE values e
BPE 1 ¼ BPE(93.49,0.1007) ¼ 0.6412 against 0.5
BPE 2 ¼ BPE(80.07,0.1787) ¼ 1.21 against 1
BPE 3 ¼ BPE(50,.80) ¼ 16.2 against 16.2
Check if for each effect, the recalculated BPE value differ from the previous value by more than

0.1 C. If the difference is more that 0.1 C, new assumed values are the recalculated values and cal-
culations from Step B are repeated till the previous and values differ by less than 0.1 C.

Further reading
1. Badger, W. L., & Banchero, J. (1997). Introduction to chemical engineering. Bombay: Tata McGraw-Hill
Edition.
2. Kern, D. Q. (1990). Process heat transfer. New York: McGraw-Hill.