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Absorption Cycle The absorption cycle uses a heat-driven concentration difference to move
refrigerant vapors (usually water) from the evaporator to the condenser. The
high concentration side of the cycle absorbs refrigerant vapors (which, of
course, dilutes that material). Heat is then used to drive off these
refrigerant vapors thereby increasing the concentration again. Lithium
bromide is the most common absorbent used in commercial cooling equipment,
with water used as the refrigerant. Smaller absorption chillers sometimes
use water as the absorbent and ammonia as the refrigerant. As you can
probably guess, the absorption chiller must operate at very low pressures
(about l/l00th of normal atmospheric pressure) for the water to vaporize at
a cold enough temperature (e.g., at ~ 40°F) to produce 44°F chilled water.
The simplified diagram here
illustrates the overall flow path. Starting with the evaporator, water at
about 40°F is evaporating off the chilled water tubes, thereby bringing the
temperature down from the 54°F being returned from the air handlers to the
required 44°F chilled water supply temperature. One ton of cooling
evaporates about 12 pounds of water per hour in this step.
This water vapor is absorbed by the
concentrated lithium bromide solution due to its hygroscopic
characteristics. The heat of vaporization and the heat of solution are
removed using cooling water at this step. The solution is then pumped to the
concentrator at a higher pressure where heat is applied (using steam or hot
water) to drive off the water and thereby re-concentrate the lithium
bromide.
The water driven off by the heat input step
is then condensed (using cooling tower water), collected, and then flashed
to the required low temperature (40°F in our illustration) to complete the
cycle. Since water is moving the heat from the evaporator to the condenser,
it serves as the refrigerant in this cycle. There are also absorption
chillers in use (e.g. in motor homes) that use ammonia as the refrigerant in
the same cycle.
The absorbent is the material that is used
to maintain the concentration difference in the machine. Most commercial
absorption chillers use lithium bromide. Lithium bromide has a very high
affinity for water, is relatively inexpensive and non-toxic. However, it can
be highly corrosive and disposal is closely controlled. Water of course is
extremely low cost and safety simply isn't an issue.
Absorption Chillers
Absorption chillers are available in two
types:
- Single Effect (Stage) Units using low
pressure (20 psig or less) as the driving force. These units typically
have a COP of 0.7 and require about 18pph per ton of 9 psig steam at the
generator flange (after control valve) at ARI standard rating conditions.
- Double Effect (2-Stage) Units are
available as gas-fired (either direct gas firing, or hot exhaust gas from
a gas-turbine or engine) or steam-driven with high pressure steam (40 to
140 psig). These units typically have a COP of 1.0 to 1.2. Steam driven
units require about 9 to 10 pph per ton of 114 psig input steam at ARI
standard rating conditions. Gas-fired units require an input of about
10,000 to 12,000 Btuh HHV per ton of cooling at ARI standard rating
conditions. To achieve this improved performance they have a second
generator in the cycle and require a higher temperature energy source.
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