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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
generator. In the generator the weak absorbent is heated to drive
(evaporate) the refrigerant out of the absorbent and restore the
strong absorbent. The strong absorbent then passes through the
heat exchanger, where it gives up some heat to the weak
absorbent, and then returns to the spray heads in the absorber
completing the cycle for the absorbent.
NOTE: Industry standards reverse the definitions of strong
absorbent and weak absorbent when ammonia is the
refrigerant and water the absorbent.
The refrigerant vapor migrates from the generator to the
condenser where it is cooled until it condenses to a liquid. The
liquid refrigerant flows to the evaporator where the refrigerant
pump sprays the liquid over the chilled water coils. The heat
from the chilled water evaporates the liquid. The resulting vapor
migrates to the absorber where it is absorbed by the strong
absorbent and pumped to the generator to complete the
refrigerant cycle.
REFRIGERANT VAPOR
GENERATOR
STEAM
HEAT
EXCHANGER
ABSORBER
CONDENSER
WATER
GENERATOR PUMP
KEY:
WEAK ABSORBENT
STRONG ABSORBENT
Fig. 7. Absorption Chiller Operating Cycle Schematic.
Figure 8 is a typical water-lithium bromide absorption cycle
chiller. Lithium bromide is the absorbent and water is the
refrigerant. Use of water as a refrigerant requires that the system
be sealed, all the air removed, and an absolute pressure of
0.847 kPa be maintained. Under these conditions the refrigerant
(water) boils at 4.5 C which allows the refrigerant to cool the
chilled water to 6.5 C.
CONDENSER
REFRIGERANT
VAPOR
EVAPORATOR
CHILLED
WATER
REFRIGERANT PUMP
REFIRGERANT (LIQUID)
REFIRGERANT (VAPOR)
C2579
CONDENSER
WATER IN
GENERATOR
ABSORBER
HEAT EXCHANGER
SOLUTION PUMP
KEY:
WEAK ABSORBENT
STRONG ABSORBENT
Reprinted by permission from the ASHRAE
Handbook–1994 Refrigeration
Fig. 8. Diagram of Two-Shell Lithium
Bromide Cycle Water Chiller.
ABSORPTION CHILLER
Capacity control of a water-lithium bromide absorption chiller
is modulated by changing the concentration of the strong absorbent
by varying the heat input to the process in the generator, controlling
condenser water flow, or controlling flow of the strong absorber.
Heat sources may be hot water, high temperature hot water, steam,
or a gas flame (direct fired). Light loads require a reduced
concentration of strong absorbent (absorbent retains more
refrigerant) or less flow of the strong absorbent. The amount of
heat required for a given cooling load is proportional to the
temperature difference between condensing water and chilled
water (refrigerant pressure). It is also proportional to temperature
lift (chilled water temperature difference).
Some absorption chillers require the condensing water be
kept constant at the design temperature. To improve seasonal
operating efficiency some designs accept condensing water
temperatures below design down to 7 C. This requires an
ENGINEERING MANUAL OF AUTOMATIC CONTROL
300
CONDENSER
CONDENSER
WATER OUT
EVAPROATOR
REFRIGERANT PUMP
REFIRGERANT (LIQUID)
REFIRGERANT (VAPOR)
C2580
STEAM
OUT
CHILLED
WATER
IN
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