The energy efficiency of absorption can be
improved by recovering some of the heat normally rejected
to the cooling tower circuit. A two-stage or two-effect absorption
chiller accomplishes this by taking vapors driven off by heating
the first stage concentrator (or generator) to drive off more
water in a second stage. Many absorption chiller manufacturers
offer this higher efficiency alternative.
Notice that two separate shells are used. The smaller is
the first stage concentrator. The second shell is essentially
the single stage absorption chiller from before, containing
the concentrator, condenser, evaporator, and absorption
chiller. The temperatures, pressures, and solution concentrations
within the larger shell are similar to the single-stage
absorption chiller as well.
Steam at pressures typically in the l25 - 150 psig range
is brought into the stainless steel tubes of the first stage
concentrator causing the solution there to boil. The pressure
at which boiling occurs and the pressure of the released
refrigerant vapor is approximately 5 psig (20 psia). The
partially concentrated solution from this first stage flows
through the high temperature heat exchanger where it is
cooled by the lower temperature dilute solution returning
from the concentrator. This concentrate then passes into
the lower pressure second stage concentrator where the vapors
from the first stage take it to the final desired concentration
levels. This second stage operates at a pressure of 0.1
atmosphere (1.5 psia).
The reuse of the vapors from the first stage generator
makes this machine more efficient than single stage absorption
chillers, typically by about 30%.
Two-stage absorption chillers are typically driven by high-pressure
(60 to 130 psig) steam, direct-fired with natural gas or
#2 fuel oil, or using hot exhaust gas from combustion engines.
|