Technologies: Thermally Activated Absorption Chillers
proven absorption cooling systems, ranging in size from 3 to 1,700 tons, are readily
available today. These systems come as stand-alone chillers or as chillers with
integral heating systems.
The basic operating principle of an absorption chiller is the same as that of
a conventional vapor compression chiller, namely, that cooling is provided by
evaporating a refrigerant. However, large absorption systems are different in
rather than standard refrigerants.
low pressure/vacuum conditions, rather than at moderate to high pressure.
Use heat rather
than a compressor as their driving force.
activated absorption chillers operate on the principle that some materials will
absorb other materials even when both are in liquid form. For example, ordinary
table salt pulls water vapor out of the air, absorbing it and making it difficult
to pour. Lithium bromide water solution is a liquid substance that absorbs water.
One of the
major differences between a
conventional vapor compression cycle and an absorption cycle is the refrigerant
used. Chlorofluorocarbons, or CFCs, have been the most popular refrigerants for
mechanical refrigeration systems; however, distilled water is used as the refrigerant
in most large commercial absorption systems.
mechanical compression systems, absorption cycles need a second fluid, the lithium
bromide water solution, which is nontoxic. Because lithium bromide (the absorbent)
does not boil, water (the refrigerant) is easily separated from it by adding heat.
absorption systems feature several advantages over conventional electric systems.
1. Lower Operating
2. No Ozone-Damaging Refrigerants
3. Safer, Quieter Operation
4. Smaller Total Space Requirements Compared to an Electric Chiller with Separate
5. High Reliability
6. Low Maintenance
absorption systems on the market today use water as the refrigerant and a lithium
bromide solution as the absorbent material. A typical air-cooled absorption chiller
uses ammonia as the refrigerant and water as the absorbent material.
They Work From a refrigeration standpoint, the absorption cycle shown in Figure
2 is identical to the vapor compression cycle (Figure 1). The only differences
are the components contained in the dotted box on each figure and the refrigerants
used. The dotted box shown in the vapor compression cycle (Figure 1) is drawn
around the compressor. In the absorption cycle (Figure 2), this dotted box is
drawn around a group of components that are sometimes referred to collectively
as the thermal compressor, because they serve the same purpose as a compressor,
namely, they take in low-pressure refrigerant and create high-pressure refrigerant.
cycle begins when the refrigerant leaves the condenser as a high pressure liquid.
On the way to the evaporator, the refrigerant flows through an expansion valve
that drastically lowers the operating pressure. Once inside the evaporator (step
1), heat is absorbed and the low-pressure liquid "boils" and is vaporized.
process uses vaporization to produce a cooling effect, but the work restoring
the refrigerant is done differently than the vapor compression cooling cycle.
The vapor returning to the absorber, (step 2) is absorbed by a liquid (an "absorbent")
just as alcohol absorbs water. The resulting solution can then be pressurized
by a simple motor-driven pump. Then, by using a gas-fueled generator (boiler)
(step 3) to heat the solution, the two fluids can be separated. The liquid absorbent
is cycled back to pick up more refrigerant. The high pressure refrigerant vapor
is condensed to a liquid releasing its heat to the outdoors (step 4) and sent
back to the evaporator to produce more cooling.
Single-Effect Absorption Cycle A single-effect absorption cycle, which has just been described on the
previous page, can be direct-fired with natural gas or fuel oil, or it can be
indirectly powered by hot water or steam. The cycle can be driven by a relatively
low-grade heat source, so it is very effective for heat-recovery applications.
below is a schematic representation of a single-effect chiller. This diagram better
illustrates how an absorption unit actually looks. Heat exchangers for large commercial
units are usually tube bundles contained in tubular-shaped pressure vessels.
Double-Effect Absorption Cycle The figure below shows the double-effect absorption cycle. Double-effect
absorption machines add a second generator and condenser that operate at a higher
temperature. The higher-temperature generator (1) is called the first stage-generator.
Refrigerant vapor is recovered from the first-stage generator in the higher-temperature
condenser (2). The refrigerant vapor is then condensed at a higher temperature
and the heat from this condensation process is used to desorb additional refrigerant
(water) from a lower-temperature, second-stage generator (3).
refrigerant increases the cooling effect in the evaporator for the same heat input
because the mass flow of refrigerant through the evaporator has increased. This
increased refrigerant flow results in a 40% performance increase. Another way
of thinking of this process is that the double-effect cycle can produce the same
refrigerant flow rate, or cooling effect, as the single-effect cycle for a fraction
of the heat input. It is clear why the coefficient of performance (COP) of absorption
chillers ranges from 0.60 to 0.70 for indirect-fired single-effect systems, to
about 1.20 for indirect-fired double-effect units.
There are three
types of commercially available double-effect absorption machines. The series
flow cycle gets its name from the fact that the lithium bromide water solution
flows in series, first to the primary generator and then to the secondary generator.
The other two double-effect cycles are the parallel-flow cycle and the reverse-flow
cycle. The parallel-flow cycle gets its name from the fact that the solution stream
is split after the low-temperature heat exchanger and the solution flows to the
two generators through parallel paths. The reverse cycle sends the weak solution
to the secondary generator before proceeding on to the primary generator.
Absorption Technology Generator absorber heat exchanger (GAX) technology, a new entry into
the HVAC market for residential and light-commercial applications, differs from
conventional electric technologies because a thermal compressor, consisting of
a generator and an absorber, replaces the motor/compressor. GAX uses water and
ammonia as working fluids to avoid harmful CFCs and HCFCs. The GAX cycle heats
or cools the conditioned space; in the heating mode, it is less sensitive to ambient
temperatures than electric systems and therefore requires less back-up heat at
low ambience. Driven by a gas burner, GAX reduces the use of expensive peak-load
electricity. Also, the GAX should require no more maintenance than the occasional
burner cleanings required for all furnaces.
Processes: Thermoplastics Molding, Equipment Cooling