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38 TRG-TRC011-EN
notes
period three
Capacity Control
By plotting the single-effect absorption refrigeration cycle on the equilibrium
chart, it is apparent that crystallization is most likely to occur in the heat
exchanger. At this particular condition, the 65% concentrated solution (') is
cooled to 135°F [57.2°C] (() as it passes through the heat exchanger. As noted
previously, the saturation temperature of 65% solution is 123°F [50.6°C] so
there is no danger of crystallization.
Consider, however, if the solution was instead 66% concentrated and cooled to
the same 135°F [57.2°C] temperature. The saturation temperature for 66%
concentrated solution is approximately 143°F [61.7°C]. The result would be a
deposit of salt crystals inside the heat exchanger. Prolonged operation at this
condition could result in a buildup of salt that would eventually block the
passages through the heat exchanger, interrupting the operation of the chiller.
Once a chiller is crystallized, operation can only be resumed after the solution
temperature is raised above its saturation temperature, above 143°F [61.7°C] in
this example. At this higher temperature, the salt crystals would return to the
solution, allowing the chiller to operate again.
With the advent of microelectronic controls, modern absorption water chillers
are designed to monitor and control solution concentrations and temperatures,
allowing the chiller to operate over a broad range of conditions without danger
of crystallization. In addition, safety controls are available to avoid
crystallization and even to de-crystallize the chiller if necessary. Therefore,
crystallization is not the serious problem that it once was with absorption
chillers.
6
5
6
5
6
0
6
0
5
5
5
5
5
0
5
0
4
0
4
0
50°F
50°F
[10°C]
[10°C]
100°F
100°F
[37.8°C]
[37.8°C]
150°F
150°F
[65.6°C]
[65.6°C]
200°F
200°F
[93.3°C]
[93.3°C]
$
$
%
%
&
&
'
'
(
(
)
)
15
15
psia
psia
[103.4
[103.4
kPa
kPa
]
]
5
5
psia
psia
[34.5
[34.5
kPa
kPa
]
]
1
1
psia
psia
[6.9
[6.9
kPa
kPa
]
]
0.1
0.1
psia
psia
[0.69
[0.69
kPa
kPa
]
]
LiBr
LiBr
solution
solution
crystallization
line
crystallization
line
v
a
p
o
r
p
r
e
s
s
u
r
e
v
a
p
o
r
p
r
e
s
s
u
r
e
concentration
concentration
concentration
solution temperature
solution temperature
Figure 45