Environmental Engineering Reference
In-Depth Information
Duhring's rule for the solution field, where the internal temperatures of the generator
T
G
, condenser
T
C
, absorber
T
A
and evaporator
T
E
are combined:
(
T
G
−
T
A
)
(
T
C
−
T
E
)
B
=
(6.2)
For single effect water/lithiumbromide absorption chillers theDuhring factor ranges
between 1.1 and 1.2 for normal operating conditions. For a single effect ACM or
DACM with the ammonia/water, working pair the Duhring factor is between 1.6 and
2.4.
The cooling power
Q
E
delivered by the evaporator of the machine is then a simple
function of the double temperature difference
t
Q
E
=
s
(
t
−
t
min
)
(6.3)
The slope
s
in Equation 6.4 contains the transferred power
UA
between external
and internal circuits and the enthalpy differences in each component related to the
specific evaporation enthalpy. For example, for the condenser
C
E
is obtained from the
enthalpy difference between the incoming refrigerant vapour and the exiting liquid
refrigerant:
C
E
=
h
exit,cond
)
/q
evaporation
. Likewise in the generator,
G
E
is calculated using the enthalpy difference between the expelled refrigerant and the
solution outlet and
A
E
with the enthalpy difference between the incoming refrigerant
vapour and the incoming weak solution for the absorber. The intersection
t
min
is
given by the performance of the solution heat exchanger:
(
h
entry,cond
−
G
E
UA
G
+
B
C
E
−
1
A
E
UA
A
+
1
UA
E
s
=
UA
C
+
(6.4)
Q
gx
UA
G
+
Q
ax
UA
A
t
min
=
(6.5)
The solution heat loss
Q
gx
describes the enthalpy difference between generator
outlet and solution inlet, that is after the heat exchanger:
m
sr
h
solution,outlet
−
h
solution,inlet
Q
gx
=
(6.6)
Likewise
Q
ax
contains the enthalpy difference between the solution entering the ab-
sorber, that is returning from the heat exchanger, and the solution exiting the absorber,
multiplied by the rich solution mass flow. The generator power is also a function of
s
and the double temperature difference
t
. In addition the solution heat loss
Q
gx
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