TABLE OF CONTENTS Initially, we can estimate a value of ? T min; let us choose 20 C, which is similar to the values used in the earlier case studies.
If we have software capable of handling streams with variable CP, we can calculate the exact energy targets immediately. However, if we only have simple software (such as the spreadsheet supplied with this book) we initially calculate the targets and pinch temperature using a constant CP of 25. This is not "safe-side" linearisation, as the effect is to shift more of the cold stream heat load to lower temperatures and overestimate possible heat recovery. The targets come out as 4,635 kW hot utility and 1,135 kW cold utility, with the pinch at 113 C shifted temperature, corresponding to 123 C for hot streams and 103 C for cold streams. Now, we divide the crude stream into two segments at the pinch and recalculate. The new data for the two segments are illustrated in Table 3.4.
| Stream name | Stream type | Initial temperature ( C) | Target temperature ( C) | CP (mean) (kW/K) | New heat flowrate (kW) | Old heat flowrate (kW) |
|---|---|---|---|---|---|---|
| Crude feed | Cold 1A | 20 | 103 | 23.075 | -1,915 | -2,075 |
| Crude feed | Cold 1B | 103 | 180 | 27.075 | -2,095 | -1,925 |
Recalculating, we get energy targets of 4,795 kW hot utility and 1,295 kW cold utility, and a pinch at 113 C. Note that although the targets have changed by 160 kW...
