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JOHNSON CONTROLS
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SECTION 2 – PRODUCT DESCRIPTION
FORM 155.35-ICOM2.EN.CE/GB (1018)
DATE: 10/30/2018
2
CRYSTALLIZATION
All absorption chillers that use lithium bromide and
water as the solution and refrigerant pair are subject to
crystallization. This is due to the fact that some areas of
the unit operate with solution liquid concentration lev-
els that are only possible at temperatures higher than
the normal ambient temperature surrounding the unit.
For example, the solution concentration in the genera-
tor of a single effect absorption unit is typically 64.3%
lithium bromide by weight. That concentration begins
to crystallize at 43.3 °C.
Crystallization happens when the LiBr solution tem-
perature drops too low or the concentration is too high.
The LiBr solution becomes like slush. At this point, the
LiBr solution cannot absorb any more water and will
start to solidify (crystallize).
Crystallization occurs in the solution heat exchanger.
It may also happen in the generator. Pipes that are not
well insulated and are located in rooms where the tem-
perature can affect the solution as it moves through the
pipes are also subject to crystallization.
You can prevent crystallization by making sure you
keep the solution temperature high and the solution
concentration at the optimum percentage (64%).
Since the solution temperature in the generator is typi-
cally high enough, no crystallization will occur as long
as the higher temperature is maintained. Before the
unit is shut down, make sure the solution is sufficiently
diluted in all areas of the unit to prevent crystallization
during the off cycle. Remember, the solution tempera-
ture will eventually become equal to the ambient tem-
perature of the room.
All units employ some sort of dilution cycle, which ful-
fills this requirement. As long as the unit is allowed to
dilute itself during an orderly shutdown sequence, the
unit should be able to sit idle at fairly low plant room
ambient temperatures for extended periods of time
without any threat of crystallization. Typically, after
a dilution cycle, the average solution concentration in
the chiller-heater will be below 45% lithium bromide
by weight. Although the crystallization line on Figure
41 on page 111 or Figure 42 on page 112 does not
extend that far, you can see that the solution at a 45%
concentration should not crystallize at normal ambient
temperatures.
Why Does Crystallization Occur?
The most common reason for crystallization is power
failure. If a chiller-heater is running at full load and
power is interrupted for a long time, the concentrated
solution in the high side of the unit eventually cools
down. Since no dilution cycle was performed, the solu-
tion concentration in some areas of the unit will still be
high. If the temperature of this concentrated solution is
allowed to fall low enough, the solution will reach its
crystallization point. Plant room temperature, insula-
tion quality and the solution concentration all play a
part in the determination of how long it will take before
the unit will crystallize. See SECTION 7 – MAINTE-
NANCE for information on water quality control and
crystallization. The Duhring Diagram (PTX Chart)
shows the specific temperatures and pressures of the
crystallization area. See Figure 41 on page 111 and
Figure 42 on page 112 .