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5. SIMULATION AND RESULTS
Efficiency vs NTU for counterflow HEX
ε
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
C_ratio = 0
C_ratio = 0,25
C_ratio = 0,50
C_ratio = 1,00
0 1 2 3 4 5
NTU
Figure 5.30: The figure shows the relation between ɛ (effectiveness) and NTU (Number of
Transfer Units) for a counterflow heat exchanger.
The correlation between NTU and ɛ for a counter flow heat exchanger
can be seen in figure 5.30. (GGHEX4 and the SHEXes are assumed to be
counterflow, and the EVAP and ABSO has a heat capacity flow ratio of 0
which means that the flow ɛ-NTU relation is unaffected by the flow configuration
and only depends on NTU).
It is seen that the SHEXes have the largest relative sizes (a large NTU
means that the HEX is large relative to the (smallest) heat capacity flow).
Hence it is easiest (and cheapest) to improve the heat exchangers which
have a low NTU number. So the SHEXes will not be improved further,
whereas the GGHEX, EVAP and ABSO can be improved. The list below
show the values used in the optimization.
1. GGHEX4: 8,7% (∆T min =9,4 ◦ C, ɛ=0,9, NTU=5,0) Ċ r atio = 0,78
2. SHEX1: 2,4% (∆T min =5 ◦ C, ɛ=0,87, NTU=5,0 Ċ r atio = 0,88)
3. EVAP: 1,6% (∆T min =2,7 ◦ C, ɛ=0,65, NTU=1,0)Ċ r atio = 0
4. ABSO: 1,6% (∆T min =1,8 ◦ C, ɛ=0,65, NTU=1,0) Ċ r atio = 0
5. SHEX2: 1,5% (∆T min =5 ◦ C, ɛ=0,87, NTU=5,0) Ċ r atio = 0,89
Since GGHEX4 is very important, it has been decided that a very good
HEX should be used (NTU=5). For the evaporator and absorber the NTU
150