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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