Environmental Engineering Reference
In-Depth Information
varying from 20 to 100 pounds of clothes or fabrics. Clothes are completely soaked and tumbled in
detergent-charged solvent for 8-12 min at a temperature ranging from 70-85°F. A typical machine
may pump perchloroethylene into the rotating basket i lled with soiled clothing at a rate of 1500
gallons per hour; for an 8-min cleaning cycle, soiled clothes would be soaked in 200 gallons of
solvent (Cantin, 1992). The detergent-charged solvent dissolves greases and oily stains and is con-
tinually i ltered during the wash cycle. Soil-laden solvent is i rst pumped through a button trap to
remove pins and buttons from the solvent to prevent damage to the pump. Charged solvent is then
pumped through either a disposable cartridge i lter, a spin-disk i lter, or a regenerative or l ex-tube
i lter to remove soil, lint, and other insoluble materials (CARB, 1996).
A typical dry-cleaning machine has one or two perchloroethylene solvent tanks: one for the pure
solvent and the other for the “charged” solvent, made up of distilled solvent with a small amount of
detergent and water to help clean water-soluble stains. Solvent is recycled through continuous distil-
lation. Fresh solvent is added to the solvent tanks to replace the solvent lost to emissions, i lters, and
water separation.
To extract perchloroethylene from washed clothes, the solvent bath is drained; then clothes
are spun at high speed to wring out excess solvent. Both transfer and dry-to-dry machines combine
the washer-extractor functions in a single unit. In transfer machines, an operator moves clothes
from the washer-extractor to the dryer-reclaimer. In the solvent-reclaiming dryer, heated air is blown
over the clothes in a closed, recirculating loop to vaporize the solvent, which is recovered in a water-
cooled condenser. After a timed drying step lasting 15-20 min, fresh air is blown over the clothes
for approximately 5 min to complete the drying cycle and aerate the clothes. The air stream and
perchloroethylene vapors are vented to a carbon adsorption system (USEPA, 1980).
In the second wash, clothing is rinsed in fresh, pure solvent for 4-5 min to remove detergent resi-
due. Like the charged solvent, the pure solvent is i ltered continuously by one of the three i ltration
systems described in Section 1.1.1.4.9 . Rinsed clothing is rapidly spun to extract the solvent. The
spent solvent is drained into the charged solvent tank and is used for the i rst wash for the next load
of soiled clothing.
Freshly cleaned clothing is heated to about 160°F for approximately 40 min. The remaining
perchloroethylene fumes and solvent are vaporized by warm air and then condensed over cooling
coils to recover the solvent. The residual amount of perchloroethylene removed from clothing
following the second wash ranges from 30 to 60 pounds for 100 pounds of clothing (Cantin, 1992).
Most of the perchloroethylene is removed from the clothing via the last two extraction and drying
steps; however, trace levels of perchloroethylene may remain in dry-cleaned clothing.
1.1.1.4.2 First- Generation Dry- Cleaning Machines
Transfer machines i rst appeared on the market in the 1930s and were used exclusively until the
1960s (Schmidt et al., 2001). In 1995, 34% of the dry-cleaning machines in the United States were
transfer machines. Transfer machines were banned in California on October 1, 1998 (Izzo, 1992).
Transfer machines usually included a washer-extraction unit and a separate dryer unit. Clothing and
solvent are agitated in the drum, which is then spun at high speed to extract the solvent. An atten-
dant must manually transfer the clothing to a dryer. The clothing is tumbled in recirculating warm
air to vaporize the residual solvent. After the drying cycle, cool air is circulated through the clothing
to reduce wrinkling. Some transfer machines comprised a washer unit, a separate spin-dry extrac-
tion unit, and a tumbler dryer or reclaimer to vaporize the remaining solvent and fumes.
Transfer machines allowed increased production because a new load of clothing can be washed
while the previous load is drying. Compared with dry-to-dry machines, maintenance of transfer
machines was easier because of their basic construction with less automation. There was also less
fabric damage because the drum remains cool after the previous load is removed (USEPA, 1995).
The disadvantages of transfer machines included additional labor needed to manually transfer the
solvent-soaked clothing from the washer-extractor to the dryer, worker exposure to solvent vapors
during transfer of clothing, and loss of solvent during transfer.
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