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FORM 155.31-ICOM2.EN.UL
ISSUE DATE: 12/21/2018
EVAPORATOR
Liquid refrigerant enters the evaporator and is distrib-
uted over the top of the tube bundle. As the refrigerant
droplets cover the outside surface of the tubes, the heat
from the returning chilled water passing through the
tubes causes the refrigerant to flash from a liquid to a
vapor. The temperature at which this happens depends
on the evaporator shell pressure which is dictated by
the absorber section of the chiller. The refrigerant va-
por passes through the mist eliminators and into the
absorber section of the chiller. As the liquid refrigerant
passes down through the bundle of evaporator tubes,
more and more refrigerant vaporizes. The refrigerant
remaining in a liquid state at the bottom drains into the
refrigerant tank and is pumped back up to the top of the
tube bundle where the process is repeated.
ABSORBER
Concentrated LiBr solution enters the absorber sec-
tion of the chiller and is sprayed over the absorber
tube bundle. Because the vapor pressure of the con-
centrated solution is very low, the refrigerant (water)
vapor from the evaporator flows into the absorber and
is absorbed into the LiBr solution. This mass transfer
process lowers the concentration of LiBr solution as
the refrigerant (water) is absorbed into the solution.
This dilution process generates heat and if not cooled
will eventually stop as the solution temperature rises
with a corresponding increase in vapor pressure. This
would be similar to closing the vanes or slowing down
a centrifugal compressor on a chiller where the load
was constant. The water flowing inside the absorber
tube bundle comes from the cooling tower and serves
to cool the LiBr solution as it flows down over the tube
bundle. This allows the absorption process to continue
and the solution becomes more diluted as it absorbs
more refrigerant vapors. When the LiBr solution reach-
es the bottom of the absorber section and goes into the
suction of the solution pump where it is pumped back
into the generators.
The following section describes the unique 2-step
evaporator-absorber design of the YHAU-CW steam
chiller.
TWO-STEP EVAPORATOR-ABSORBER
The evaporator as well as the absorber is split into two
sections. This design, similar to a series-counter-flow
chiller arrangement along with the parallel flow cycle,
enables lower lithium bromide solution concentrations.
This reduces crystallization risk, reduces pressure and
JOHNSON CONTROLS
SECTION 2 – PRODUCT DESCRIPTION
the potential for corrosion, as well as improves effi-
ciency in conjunction with other advanced components
described later in this section.
The two evaporators are in series with respect to the
chilled water flow through the tubes. In other words,
the chilled water flows through the lower evaporator
tubes first and then to the upper evaporator tubes. Each
evaporator operates at a slightly different temperature
and pressure. The refrigerant in the lower evaporator
boils at a slightly higher temperature than in the upper
evaporator, consequently cooling the chilled water in
2 steps.
The two absorber sections are split as well, with the
strong solution first entering the top of the uppermost
absorber and flowing down through the top absorber
bundle. It then flows into the top of the lower absorber
section. The strong solution entering the upper absorb-
er takes advantage of its lower vapor pressure allowing
the upper Evaporator to operate at a lower pressure and
temperature.
When the LiBr solution enters the lower absorber sec-
tion it is somewhat diluted from the refrigerant vapor
that boiled off in the upper evaporator. At this lower
concentration the solution vapor pressure would nor-
mally not be sufficient to provide an evaporator pres-
sure low enough to satisfy the leaving chilled water
design. However, the lower evaporator is the first step
of the chilled water cooling cycle, and the dilute solu-
tion's vapor pressure is adequate to maintain the lower
evaporator at the required temperature and pressure in
the lower evaporator.
The cooling tower water enters the lower absorber sec-
tion first, keeping the vapor pressure of the weaker so-
lution as low as possible.
Both the refrigerant (water) and LiBr dispersion sys-
tem are gravity fed and made of stainless steel.
Refrigerant
Chilled
Water
Out
Evaporator
(Upper)
Chilled
Evaporator
(Lower)
Water
In
LiBr
Absorber
(Upper)
Absorber
(Lower)
Cooling
Water
In
LD19871b1
17
2
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