Environmental Engineering Reference
In-Depth Information
T
ARGETING
U
SING
A
LGEBRAIC
M
ETHOD
Information needed
Given a unit with a list of hot streams to be cooled and cold streams to be heated
Table 2. Data for Algebraic Method
FCp (kW/ ºC)
Supply Temp
(º C)
Target Temp.
(º C)
10
520
330
5
380
300
10
300
550
5
320
380
1. Constructing Temperature Iinterval Diagram
1.1.
Draw two temperature scales one for the hot streams and another for the cold streams
1.2.
Select a reasonable minimum temperature approach between the hot streams and the
cold stream, (for instance, 10 ºC)
1.3.
Draw all the hot streams (in the table hot section) to be cooled according to the hot
steam scale as arrows that start at the supply temperatures and end at the target
temperatures
1.4.
Repeat step 1.3 for all cold streams in the cold section of the table
1.5.
Start at the highest temperature of any hot stream in the hot section and draw a
horizontal line that spans across the two sections of the table, the hot and the cold.
1.6.
Draw horizontal lines again at the start and the end of any arrow representing the hot
streams in the hot section of the table
1.7.
Repeat step 1.6 for any arrow representing cold stream in the cold section (at the start
and the end of any arrow)
1.8.
Count the number of segments generated and number them starting at the highest
temperature (they are called temperature intervals)
1.9.
Make sure that each temperature interval has now temperature value on both the hot
temperature scale and cold temperature scale. The difference is the desired minimum
temperature approach (for instance the 10 ºC used in this example)
These procedures are depicted in the figure below
Note:
This structure means that within any temperature interval it is thermodynamically
feasible to transfer heat from the hot streams to cold streams. It is also feasible to transfer heat
from a hot stream in an interval “x” to any cold stream which lies in an interval below.
Note:
The temperature symbol
T*
'(Figure 6) is the interval inlet temperature used later
on constructing what is know as grand composite curve for selecting the suitable energy
utility mix. To calculate T* we take the average interval inlet temperature of the hot and cold
temperature scale.
