Page 464 - Advanced thermodynamics for engineers

19.3 STEP 2: INTERVAL HEAT BALANCES 455

mC Q

310 205 4.28 449.4 MJ/h
3
Hot streams 5 280 220 95 2.84 426.0 MJ/h
245

65
2.38
7
511.7 MJ/h
205 95
1 2.88 316.8 MJ/h
511.7 MJ/h
40
218
220
Cold streams 205 150 2 2.88 408.7 MJ/h
518.4 MJ/h
4
7.43
140 354.0 MJ/h 65
6 4.72 354.0 MJ/h
FIGURE 19.9
Initial heat transfer: heat transfer from streams 5 to 6, and 7 to 2.

mC Q

310
Hot streams 3 245 300 205 4.28 449.4 MJ/h
220
2.84
72.0 MJ/h
5
205 120 72.0 MJ/h 95
1 2.88 316.8 MJ/h
Cold streams 205 408.7 MJ/h 150 2 7.43 408.7 MJ/h
220
218
2.88
6.7 MJ/h
4

FIGURE 19.10
Removing the residuals for streams 2 and 5.

By this stage, it is necessary to consider adding the heat from the hot utility. In this case, the
temperature at which this energy is added is relatively arbitrary, and the heat should be transferred at as
low a temperature as possible. This is indicated by the 210.8 MJ heat transfer in Fig. 19.11.