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FORM 155.32-ICOM1.EN.GB
ISSUE DATE: 10/13/2017
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-heater. The refriger-
ant vapor passes through the mist eliminators and into
the absorber section of the chiller-heater. As the liquid
refrigerant passes down through the bundle of evapo-
rator tubes, more and more of the refrigerant vapor-
izes. The refrigerant remaining in a liquid state at the
bottom drains into the refrigerant tank and is pumped
back up the top of the tube bundle where the process
is repeated.
ABSORBER
Concentrated LiBr solution enters the absorber section
of the chiller-heater 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 the LiBr solution
as the refrigerant (water) is absorbed into the solution.
This dilution process generates heat and, if not cooled,
would eventually stop as the solution temperature
would rise with a corresponding rise in vapor pressure.
This would be similar to closing the vanes or slowing
down a centrifugal compressor on a centrifugal 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 pro-
cess to continue and the solution becomes more diluted
as it absorbs more refrigerant vapors. When the LiBr
solution reaches the bottom of the absorber section, it
goes into the suction of the solution pump. The liquid
is then pumped to the generators.
The following section describes the unique 2-step
evaporator-absorber design of the YHAU-CG direct
fired absorption chiller-heater.
JOHNSON CONTROLS
SECTION 2 – PRODUCT DESCRIPTION
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-heater arrangement along with the parallel flow
cycle, enables lower LiBr solution concentrations. This
reduces pressure, the potential for corrosion, and the
risk of crystallization as well as improves efficiency
in conjunction with other advanced components de-
scribed 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
two 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 re-
quired temperature and pressure in the lower evapora-
tor.
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.
2
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