Page 198 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

196 Chapter 6 Evaporators

N
P
viii) SBPE ¼ BPEðx i Þ
i¼1
ix) DT ov ¼ T s (P steam ) T s (P N )
and for all the effects put together DT eff,total ¼ DT ov SBPE
N 1
P
x) Assume: DT eff,i , i ¼ 1, .,N 1 and estimate DT eff;N ¼ DT eff;total DT eff ;i ;
i¼1
Initial guess can be from assuming same heat transfer rate in each effect which gives
DT eff ;total
DT eff;i ¼ ð1 =U i Þ
N
P
ð1=U i Þ
i¼1
xi) T N ¼ T s (P N )þBPE(x N );
T con;1 ¼ T s ðP steam Þ
T con;i ¼ T s ðP i 1 Þ¼ T i þ DT eff;i and T i 1 ¼ T con;i þ BPEðx i 1 Þ; i ¼ N; .; 2

xii) S ¼ {V 1 h V,1 (T 1 )þh con,steam F h F (T F )}/h steam (T steam , P steam )

h F ¼ h F ðT F Þ

h V;i ¼ h s T con;iþ1 þ C P;steam T i T con;iþ1 ; i ¼ 1; .; N 1

h V;N ¼ h s T con;N 1 þ C P;steam ðT N T s ðP N ÞÞ

h con,i ¼ h w (T con,i ),i ¼ 1, .,N 1, [h w (T con,i ) stands for saturated water enthalpy at T con,i
obtained from steam table.]
h L,i ¼ h L (x i ,T i ),i ¼ 1, .,N, [Enthalpy of solution with solute concentration x i at T i from table/
chart similar to Fig. 6.22 applicable for the system.]
xiii) Calculate heat load and area required for each effect

q 1 ¼ S h steam ðT steam ; P steam Þ T con;1

q i ¼ V i 1 h V T con;i ; i ¼ 2; .N

A i ¼ q i = U i DT eff ;i
N
If max ABS A i S ðA i Þ = N > 0:05; i ¼ 1; .; N; then
i¼1
re-estimate

DT eff
DT eff;i ¼ ðq i = U i Þ; go to step xi
N
P
ð1=U i Þ
i¼1
xiv) Recalculate V from the following set of N equations formed by energy balance around each
effect e 1st Effect

V 1 ¼ F h F h L;1 þ S h steam ðT steam ; P steam Þ h con;1 = h V;1 h L;1