Environmental Engineering Reference
In-Depth Information
The characteristic equation for the evaporator cooling capacity
Q
E
of the DACM
now includes the cooling losses in the auxiliary gas circuit
Q
AUX
.
Q
E
=
s
E
(
t
−
t
min
)
Q
AUX
−
(6.8)
The intersection
t
min
is determined by Equation 6.5 using the efficiency of the
solution heat exchanger and thus the dissipated energy which results from the solution
circulation between the absorber
Q
ax
and the generator
Q
gx
. The slope
s
E
again
contains the enthalpy differences of each component and the heat transfer coefficient
UA
between the external and internal circuits and includes as additional components
the dephlegmator and the auxiliary gas circuit (generator
G
, condenser
C
, absorber
A
, evaporator
E
):
G
E
+
G
E,deph
UA
G
B
C
E
−
1
A
E
+
A
E,aux
UA
A
1
UA
E
s
E
=
+
+
UA
C
+
(6.9)
Also, the pumping performance of the bubble pump as a function of generator
temperature has to be taken into account. The rich solution mass flow
m
Sr
(kg h
−
1
)
of the generator/bubble pump performance has been empirically determined from
various series of measurements depending on the external generator inlet temperature
and external mass flow heating circuit as follows:
2
.
901 426
301
.
499 335 1
m
g
10
−
2
t
g,in
−
m
Sr
=
×
5
.
413 764 26
t
g,in
+
(6.10)
The load behaviour of the DACMcan be determined when changing the characteris-
tic temperature difference. For given cold water temperature, the cooling capacity and
the COP mainly depend on the generator heating and the cooling water temperatures
for the absorption and condensation processes. In contrast to the LiBr/H
2
O absorption
systems, the NH
3
/H
2
O diffusion absorption machine has decreasing COPs at increas-
ing generator temperatures. This is due to the rising heat losses in the rectifier, if more
water is driven out of the solution at high generator temperatures. Figure 6.4 shows
a comparison of measured and simulated evaporator cooling power and the COP for
the current prototype. The variable enthalpy model fits the experimental data points
better than the constant enthalpy model.
The main optimization potential of the prototype is the falling film evaporator,
which has an insufficient distribution capacity for the liquid ammonia. The evaporation
efficiency is related to the surface wetting factor, which in this case is only 0.5 (Jakob,
2006). Also, the heat transfer power of both evaporator and absorber is too low.
A parameter study was carried out to determine how to improve the performance
of the DACM at different evaporator inlet and cooling water temperatures, together
with evaporator surface wetting factors and GHX recovery factors. The COP and the
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