Page 145 - Chemical engineering design

The spreadsheet AS-EASY-AS (TRIUS Inc) was used to develop the tables for this
problem. FUNDAMENTALS OF ENERGY BALANCES 125
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First calculate the interval temperatures, for T min D 20 C
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hot streams T int D T act 10 C
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cold streams T int D T act C 10 C
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Actual temp. C Interval temp. C
Stream Source Target Source Target
1 180 40 170 30
2 150 60 140 50
3 30 180 40 190
4 80 160 90 170

Next rank the interval temperatures, ignoring any duplicated values. Show which
streams occur in each interval to aid in the calculation of the combined stream heat
capacities:

190°C
Interval 1 1 180°C 180°C
170
2 2 150°C 160°C
140
3
90
4 80°C
4 60°C
50
5
40
3 30°C
6 40°C
30
Figure 3.30. Intervals and streams
Now set out the problem table:
Interval T CP c ð CP h H
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Interval temp. C Ž C kW/ C kW Cascade
190 0 2900
1 170 20 60.0 1200 1200 1700
2 140 30 40.0 1200 2400 500
3 90 50 10.0 500 2900 0
4 50 40 10.0 400 2500 400
5 40 10 20.0 200 2700 200
6 30 10 40.0 400 2300 600
In the last column 2900 kW of heat have been added to eliminate the negative values
in the previous column.
So, the hot utility requirement is 2900 kW and the cold, the bottom value in the column,
is 600 kW.
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The pinch occurs where the heat transferred is zero, that is at interval number 3, 90 C.