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5.3. Partial optimization of standard parameters
5.3.4 Closest Approach Temperature Differences (∆T min )
In this section, the effect of the closest approach temperature difference
∆T min of the various heat exchangers will be investigated. In theory
∆T min can approach 0 but in praxis there will always be undesired
losses, mixing, resistances and heat transmission in the direction of the
tubes/plates. Hence the real values will be above zero. Since heat
transmission in a liquid is generally better than in a gas, the standard
closest approach temperature has been set differently for the different
heat exchanger groups (depending on the phase of the fluid). Thus each
group of heat exchanger will be investigated separately in the following.
Water-Water Heat exchangers
This group includes the evaporator, absorber, desorber2 10 and the
internal LiBr solution heat exchangers. These are all liquid-liquid with
a standard ∆T min of 5 ◦ C. All of them are now investigated in the range
from 0 to 16 ◦ C: As expected, the system performs better if more efficient
exchangers (lower ∆T min ) are used. The tendency is, however, not as
pronounced as one might have expected. From figure 5.21 it is seen that
the evaporator, absorber and SHEX1 are the most important of the waterwater
heat exchangers. The cooling power output increases around 3-
4% when ∆T min is reduced from 5 ◦ C to 0 ◦ C. If ∆T min is increased, the
absorber suddenly becomes the most sensitive, and above 10 ◦ C it heavily
decreases the COP.
When it comes to value for money, it looks like it would be beneficial
to buy a more efficient SHEX1 and a less efficient SHEX2, since COP
is about three times more sensitive to SHEX1 than SHEX2. Of course
SHEX1 is almost twice as big as SHEX2, since it contains the flow of
both desorbers, so decreasing its ∆T min will probably be about twice as
expensive as for SHEX2 per degree Celsius, but the gain would also be
three times as large.
CATD of DES2 is seen to have very limited effect on the COP, so it
doesn’t really matter if the water circuit (point 41 to 43) is used or if
WGHEX1 and DES2 were instead integrated.
10 ∆ T,min,DES1 is not examined since WGHEX2 and COND2 are assumed always to
be integrated. ∆ T,min,DES2 on the other hand could be relevant, if the absorption cooling
unit was originally indirect fired.
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