Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 19

To minimize worker exposure to perchloroethylene vapors during transfer of clothing from the
washer to the dryer, a hamper enclosure made of impervious plastic was used to enclose the clothing
when the clothing was transferred from the washer to the dryer. In a second approach, a room enclo-
sure, composed of a metal frame covered with an impervious plastic, surrounded both the washer
and dryer units and drew solvent vapors through a device to control vapor emissions. First-generation
machines consumed and emitted more solvent than later-generation machines. Perchloroethylene
consumption in i rst-generation machines was typically in the range of 78-100 kg per 1000 kg of
clothes cleaned, depending on the primary control technology as well as on operator and mainte-
nance practices (CARB, 1996).
1.1.1.4.3 Second- Generation Vented Dry-to-Dry Machines
Second-generation dry-cleaning machines were the vented dry-to-dry machines introduced in the
late 1960s. Washing, extracting, and drying were done in one unit, thereby avoiding the transfer of
solvent-soaked clothing from the washer to the dryer. Second-generation machines were originally
designed to release residual vapors to the atmosphere, but they could be retroi tted with a vapor-
control device such as carbon absorbers or refrigerated condensers. In 1995, of U.S. dry cleaners,
66% had vapor-control devices consisting of refrigeration units, and 32% had vented units without
vapor-control devices (USEPA, 1995).
Carbon adsorbers were retroi tted to both transfer and vented dry-to-dry machines to improve
solvent recovery. Instead of venting solvent vapors directly to the atmosphere, retroi tted machines
recover solvent by directing the solvent-laden vapors through an activated carbon bed that adsorbs
the solvent. The adsorbed solvent is recovered by passing low-pressure steam or hot air through the
carbon bed; then the mixed steam and solvent vapor are passed through a water-cooled condenser
and collected in a phase separator. The desorbed activated carbon bed is dried and reused, and the
recovered solvent is returned to the solvent tank. The removal efi ciency is about 85% compared to
noncontrolled vented dry-to-dry machines (USEPA, 1995).
1.1.1.4.4 Third-Generation Closed-Loop (Nonvented) Dry-to-Dry Machines
Third-generation or closed-loop dry-to-dry machines with built-in refrigerated condensers came on
the market in the late 1970s. A third-generation machine is essentially a second-generation machine
retroi tted with a refrigerated condenser. Closed-loop machines do not vent solvent vapors to the
atmosphere; instead they recycle it continually throughout the dry-cleaning cycle. In 1995, 34% of
dry-cleaning facilities used third-generation machines.
1.1.1.4.5 Fourth-Generation Closed-Loop (Nonvented) Dry-to-Dry Machines
In the early 1990s, fourth-generation machines came on the market. They feature a nonvented
closed-loop dry-to-dry approach and have an additional internal vapor-recovery device, usually a
built-in carbon adsorber installed in series with the refrigerated condenser. Another common fea-
ture of fourth-generation machines is secondary containment below the machine to capture acci-
dental releases of perchloroethylene. Increased vapor controls in fourth-generation machines
provided nominally higher solvent mileage than third-generation machines (CARB, 1996).
1.1.1.4.6 Fifth-Generation Closed-Loop (Nonvented) Dry-to-Dry Machines
Fifth-generation machines were introduced in the late 1990s. These machines are i tted with a
single-beam infrared photometer to monitor the perchloroethylene concentration in the machine
cylinder. An interlock on the machine door prevents the attendant from opening the door until
perchloroethylene concentration in the cylinder falls below 290 ppm.
While fourth- or i fth-generation machines can achieve a factory solvent-mileage design rating
of 10 kg of solvent per 1000 kg of clothes, operator and maintenance practices are critically impor-
tant for achieving the design performance. During sustained commercial operation, however, an
average solvent mileage of 20 kg of perchloroethylene per 1000 kg of clothing cleaned is a reasonable