Page 144 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.8 Data extraction 141

stream. With the same logic, a safely linearised cold composite curve should not allow the temperature
to be below the actual stream temperature.
To summarise
• The actual hot stream must be hotter than the linearised hot stream
• The actual cold stream must be colder than the linearised cold stream
Fig. 5.11 shows the safe linearization results for the composite curves.

T

Actual
Linearised

H
FIGURE 5.11
Stream linearisation to remain on the safe side.

5.8.2 Avoid mixing of streams at different temperatures
Sometimes mixing of process streams at different temperatures is shown on flowsheets where the
mixing junction appears as a direct contact heat exchanger. Let us consider streams 1 and 2 at tem-
peratures T 1 and T 2 shown on a flow sheet to mix and result in temperature T 3 (T 1 > T 3 > T 2 ) before
being utilised. Then the data extraction should consider the start and target temperature of the streams
to be (T 1 , T 3 ) and (T 2 , T 3 ), respectively, and isothermal mixing of the streams should be contemplated
at T 3 . In addition, if any of the streams has passed through one or more exchangers before attaining T 1
or T 2 , then the temperature of the streams upstream of the exchanger(s) should be considered as its start
temperature.

5.8.3 Use effective temperatures
As an example, for a cold stream changing phase from sub-cooled state, the effective temperature
driving force must consider the driving force due to sensible as well as latent heat transfer.